Patent Application
20200239964
Deoxyribonucleic acid (DNA) methylation plays an important role in regulating gene expression. Aberrant DNA methylation has been implicated in many disease processes, including cancer. DNA methylation profiling using methylation sequencing (e.g., whole genome bisulfite sequencing (WGBS)) is increasingly recognized as a valuable diagnostic tool for detection, diagnosis, and/or monitoring of cancer. For example, specific patterns of differentially methylated regions and/or allele specific methylation patterns may be useful as molecular markers for non-invasive diagnostics using circulating cell-free (cf) DNA. However, there remains a need in the art for improved methods for analyzing methylation sequencing data from cell-free DNA for the detection, diagnosis, and/or monitoring of diseases, such as cancer.
Early detection of cancer in subjects is important as it allows for earlier treatment and therefore a greater chance for survival. Sequencing of DNA fragments in a cell-free (cf) DNA sample to determine methylation states of various dinucleotides of cytosine and guanine (known as CpG sites) in the fragments provides insight into whether a subject may have cancer, and further insight on what type of cancer the subject may have. Towards that end, this description includes systems and methods for analyzing methylation states of CpG sites of DNA fragments for determining a subject's likelihood of having cancer.
An analytics system processes a multitude of DNA fragments from a test sample. The analytics system first creates a methylation state vector for each sequenced DNA fragment. The methylation state vector contains the CpG sites in a DNA fragment as well as a methylation state for each CpG site—methylated or unmethylated. The analytics system determines whether each DNA fragment is an anomalous fragment, that is, whether the DNA fragment has anomalous methylation of CpG sites in the DNA fragment.
In one embodiment, anomalous fragments are identified using a probabilistic analysis and the control group data structure to identify the unexpectedness of observing a given fragment (or portion thereof) having the observed methylation states at the CpG sites in the fragment. This is accomplished by enumerating the alternate possibilities of methylation state vectors having a same length (in sites) and position within the reference genome as a given fragment (or portion thereof), and uses the counts from the data structure to determine the probability for each such possibility. After calculating probabilities for each possibility of methylation state vector, the analytics system generates a p-value score for the DNA fragment by summing those probabilities for possibilities of methylation state vectors smaller than the probability for the possibility matching the test methylation state vector. The analytics system compares the generated p-value against a threshold to identify DNA fragments that are anomalously methylated relative to the control group. In some further embodiments, the analytics system may filter out DNA fragments that are not anomalously methylated from use in analyses downstream in the workflow.
In another embodiment, the analytics system identifies fragments as DNA fragments with at least some number of CpG sites that have over some threshold percentage of methylation or unmethylation as hypermethylation and hypomethylation, respectively. For example, the analytics system may define a DNA fragment as hypermethylated if the DNA fragment has more than 5 CpG sites with more than 80% of the CpG sites having a methylated state. The analytics system may alternatively filter out fragments that are not hypermethylated or hypomethylated selectively using anomalous fragments that are hypermethylated or hypomethylated. In one embodiment, the hypermethylated and hypomethylated fragments are anomalous fragments, as described herein.
With the fragments (or alternatively the anomalous fragments), the analytics system is able to train and deploy a cancer classifier for generating a cancer prediction for a test sample. The analytics system selects a plurality of CpG sites for consideration in the cancer classifier. In some embodiments, the analytics system computes an information gain for each of an initial set of CpG sites to select informative CpG sites for use in the cancer classifier.
Regarding which training samples are used to train the cancer classifier, the analytics uses training samples that have already been identified and labeled as having one or a number of cancer types, as well as training samples that are from healthy individuals that are labeled as non-cancer. Each training sample includes a set of fragments. For each training sample, the analytics system generates a feature vector by assigning a score to each of the identified CpG sites based on the fragments. The analytics system may group the training samples into sets of one or more training samples for iterative training of the cancer classifier. The analytics system inputs each set of feature vectors into the cancer classifier and adjusts classification parameters in the cancer classifier such that a function of the cancer classifier calculates cancer predictions that accurately predict the labels of the training samples in the set based on the feature vectors and the classification parameters. Training of the cancer classifier may conclude after iterating the above steps through each set of training samples. In one embodiment, each training sample includes a set of anomalous fragments and the assigned score is an anomaly score.
During deployment, the analytics system generates a feature vector for a test sample in a similar manner to the training samples, i.e., by assigning a score (or an anomaly score) to each of the identified CpG sites based on the fragments in the test sample. Then the analytics system inputs the feature vector for the test sample into the cancer classifier which returns a cancer prediction. In one embodiment, the cancer classifier may be configured as a binary classifier to return a cancer prediction of a likelihood of having or not having cancer or a particular type of cancer. In another embodiment, the cancer classifier may be configured as a multiclass classifier to return a cancer prediction with a prediction value representative of a likelihood of having or not having each of a plurality of cancer types corresponding to the multiclass classifier.
In a further embodiment, the classification parameters of the trained model are trained on information comprising a plurality of training samples, each training sample corresponding to a cancer type and comprising a set of fragments; and a plurality of training feature vectors for the training samples, each training feature vector comprising, for each of the CpG sites, a score based on whether one or more of the fragments of the training sample overlaps the CpG site.
In a further embodiment, each fragment is an anomalous fragment and each fragment includes at least a threshold number of CpG sites with more than a threshold percentage of the CpG sites being methylated or with more than the threshold percentage of the CpG sites being unmethylated.
In a further embodiment, each fragment is an anomalous fragment determined by filtering an initial set of fragments with p-value filtering to generate the set of anomalous fragments, the filtering comprising removing fragments from the initial set having below a threshold p-value with respect to others to achieve the set of anomalous fragments.
In a further embodiment, the score for a corresponding CpG site is a binary value indicating whether one or more of the fragments overlaps that CpG site.
In a further embodiment, the score for a corresponding CpG site is based on a count of the fragments overlapping that CpG site.
In a further embodiment, each feature vector is normalized based on a coverage of the training or test sample, the coverage representing a measure of depth over all CpG sites covered by the fragments comprising the training or the test sample, respectively.
In a further embodiment, the measure of depth is one of: a median depth and an average depth.
In a further embodiment, the cancer types include a breast cancer type, a colorectal cancer type, an esophageal cancer type, a head/neck cancer type, a hepatobiliary cancer type, a lung cancer type, a lymphoma cancer type, an ovarian cancer type, a pancreas cancer type an anorectal cancer type, a cervical cancer type, a gastric cancer type, a leukemia cancer type, a multiple myeloma cancer type, a prostate cancer type, a renal cancer type, a thyroid cancer type, a uterine cancer type, a brain cancer type, a sarcoma cancer type, a neuroendocrine cancer type.
In a further embodiment, the function is a logistic regression.
In a further embodiment, the function is a multinomial regression.
In a further embodiment, the function is a non-linear regression.
In a further embodiment, the CpG sites used in the trained model are selected from an initial set of CpG sites according to a computed information gain for each CpG site of the initial set of CpG sites.
In a further embodiment, the CpG sites used in the trained model are selected by ranking the initial set of CpG sites based on the computed information gain, and wherein selecting the CpG sites used in the trained model is based on the ranking of the initial set of CpG sites.
In a further embodiment, the CpG sites used in the trained model are selected so as to be at least a threshold number of base pairs away from the other CpG sites used in the trained model.
In a further embodiment, determining a cancer type in a test sample from a test subject comprising a set of fragments of deoxyribonucleic acid (DNA) comprises steps of generating a test feature vector comprising for each of a plurality of CpG sites from a reference genome, generating a score based on whether one or more of the fragments overlaps the CpG site; inputting the test feature vector into a first trained model to generate a first cancer prediction for the test sample, the first cancer prediction describing a likelihood the test sample has cancer or likely does not have cancer, the first trained model comprising a first set of classification parameters and a first function representing a relation between the test feature vector received as input and the first cancer prediction generated as output based on the test feature vector and the first set of classification parameters; determining whether the test sample is likely to have cancer according to the first cancer prediction; responsive to determining that the test sample is likely to have cancer, inputting the test feature vector into a second trained model to generate a second cancer prediction, the second cancer prediction describing a likelihood the test sample has a cancer type of a plurality of cancer types, the second trained model comprising a second set of classification parameters and a second function representing a relation between the test feature vector received as input and the second cancer prediction generated as output based on the test feature vector and the second set of classification parameters; and returning one or more of the first cancer prediction and the second cancer prediction. It should be noted that, as used herein, “responsive to” in some embodiments may infer the performance of a step in a process when a specified condition or criteria is satisfied, while in other embodiments may infer a chronological order between steps in a process.
In a further embodiment, a non-transitory computer-readable storage medium stores executable instructions that, when executed by a processor, cause the processor to implement a classifier to detect or diagnose cancer, wherein the classifier is generated by the process comprising: obtaining sequence reads of a set of fragments for each of a plurality of cancer samples sourced from subjects with cancer and sequence reads of a set of fragments for each of a plurality of non-cancer samples sourced from individuals without cancer, wherein each cancer sample is of a cancer type from a plurality of cancer types; for each fragment, determining whether the fragment has an anomalous methylation pattern, thereby obtaining a set of anomalously methylated fragments for each sample; for each anomalously methylated fragment, determining if the anomalously methylated fragment is hypomethylated or hypermethylated, wherein hypomethylated and hypermethylated fragments comprise at least a threshold number of CpG sites with at least a threshold percentage of the CpG sites being unmethylated or methylated, respectively; for each sample, generating a sample feature vector by generating for each of a plurality of CpG sites in a reference genome a score based on whether one or more hypomethylated fragments or hypermethylated fragments from the sample overlaps the CpG site; training a diagnostic model (or “predictive model”) based on the generated feature vectors for the cancer samples and the generated features vectors for the non-cancer samples, the diagnostic model configured to receive a test feature vector generated from a test sample sourced from a test subject and to output a cancer prediction based on the test feature vector, the cancer prediction comprising a cancer prediction value for each of the plurality of cancer types describing a likelihood the test sample is of that particular cancer type; and storing a set of parameters representative of the diagnostic model on the non-transitory computer-readable storage medium.
In a further embodiment, a non-transitory computer-readable storage medium stores executable instructions that, when executed by a processor, cause the processor to implement a classifier to diagnose cancer, wherein the classifier is generated by the process comprising: obtaining sequence reads of a set of fragments for each of a plurality of cancer samples sourced from subjects with cancer and sequence reads of a set of fragments for each of a plurality of non-cancer samples sourced from individuals without cancer, wherein each cancer sample is of a cancer type from a plurality of cancer types; for each fragment, determining whether the fragment has an anomalous methylation pattern, thereby obtaining a set of anomalously methylated fragments for each sample; for each anomalously methylated fragment, determining if the anomalously methylated fragment is hypomethylated or hypermethylated, wherein hypomethylated and hypermethylated fragments comprise at least a threshold number of CpG sites with at least a threshold percentage of the CpG sites being unmethylated or methylated, respectively; for each sample, generating a sample feature vector by generating for each of a plurality of CpG sites in a reference genome a score based on whether one or more hypomethylated fragments or hypermethylated fragments from the sample overlaps the CpG site; training a first diagnostic model based on the generated feature vectors for the cancer samples and the generated features vectors for the non-cancer samples, the first diagnostic model configured to receive a test feature vector generated from a test sample sourced from a test subject and to output a first cancer prediction based on the test feature vector, the first cancer prediction describing a likelihood the test sample has cancer; storing a first set of parameters representative of the first diagnostic model on the non-transitory computer-readable storage medium; training a second diagnostic model based on the generated feature vectors for the cancer samples, the second diagnostic model configured to receive a test feature vector with a first cancer prediction above a threshold likelihood and to output a second cancer prediction based on the test feature vector, the second cancer prediction cancer prediction comprising a cancer prediction value for each of the plurality of cancer types describing a likelihood the test sample is of that particular cancer type; and storing a second set of parameters representative of the second diagnostic model on the non-transitory computer-readable storage medium.
The figures depict various embodiments for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein.
In accordance with the present description, cfDNA fragments from an individual are treated, for example by converting unmethylated cytosines to uracils, sequenced and the sequence reads compared to a reference genome to identify the methylation states at specific CpG sites within the DNA fragments. Each CpG site may be methylated or unmethylated. Identification of anomalously methylated fragments, in comparison to healthy individuals, may provide insight into a subject's cancer status. As is well known in the art, DNA methylation anomalies (compared to healthy controls) can cause different effects, which may contribute to cancer. Various challenges arise in the identification of anomalously methylated cfDNA fragments. First off, determining a DNA fragment to be anomalously methylated only holds weight in comparison with a group of control individuals, such that if the control group is small in number, the determination loses confidence due to statistical variability within the smaller size of the control group. Additionally, among a group of control individuals, methylation status can vary which can be difficult to account for when determining a subject's DNA fragments to be anomalously methylated. On another note, methylation of a cytosine at a CpG site causally influences methylation at a subsequent CpG site. To encapsulate this dependency is another challenge in itself.
Methylation typically occurs in deoxyribonucleic acid (DNA) when a hydrogen atom on the pyrimidine ring of a cytosine base is converted to a methyl group, forming 5-methylcytosine. In particular, methylation tends to occur at dinucleotides of cytosine and guanine referred to herein as “CpG sites”. In other instances, methylation may occur at a cytosine not part of a CpG site or at another nucleotide that is not cytosine; however, these are rarer occurrences. In this present disclosure, methylation is discussed in reference to CpG sites for the sake of clarity. Anomalous DNA methylation can be identified as hypermethylation or hypomethylation, both of which may be indicative of cancer status. Throughout this disclosure, hypermethylation and hypomethylation is characterized for a DNA fragment, if the DNA fragment comprises more than a threshold number of CpG sites with more than a threshold percentage of those CpG sites being methylated or unmethylated.
Those of skill in the art will appreciate that the principles described herein are equally applicable for the detection of methylation in a non-CpG context, including non-cytosine methylation. In such embodiments, the wet laboratory assay used to detect methylation may vary from those described herein. Further, the methylation state vectors discussed herein may contain elements that are generally sites where methylation has or has not occurred (even if those sites are not CpG sites specifically). With that substitution, the remainder of the processes described herein are the same, and consequently the inventive concepts described herein are applicable to those other forms of methylation.
The term “individual” refers to a human individual. The term “healthy individual” refers to an individual presumed to not have a cancer or disease. The term “subject” refers to an individual who is known to have, or potentially has, a cancer or disease.
The term “cell free nucleic acid” or “cfNA” refers to nucleic acid fragments that circulate in an individual's body (e.g., blood) and originate from one or more healthy cells and/or from one or more cancer cells. The term “cell free DNA,” or “cfDNA” refers to deoxyribonucleic acid fragments that circulate in an individual's body (e.g., blood). Additionally cfNAs or cfDNA in an individual's body may come from other non-human sources.
The term “genomic nucleic acid,” “genomic DNA,” or “gDNA” refers to nucleic acid molecules or deoxyribonucleic acid molecules obtained from one or more cells. In various embodiments, gDNA can be extracted from healthy cells (e.g., non-tumor cells) or from tumor cells (e.g., a biopsy sample). In some embodiments, gDNA can be extracted from a cell derived from a blood cell lineage, such as a white blood cell.
The term “circulating tumor DNA” or “ctDNA” refers to nucleic acid fragments that originate from tumor cells or other types of cancer cells, and which may be released into a bodily fluid of an individual (e.g., blood, sweat, urine, or saliva) as result of biological processes such as apoptosis or necrosis of dying cells or actively released by viable tumor cells.
The term “DNA fragment,” or “fragment” may generally refer to any portion of a deoxyribonucleic acid molecule, i.e., cfDNA, gDNA, ctDNA, etc. For example, a DNA molecule can be broken up, or fragmented into, a plurality of segments, either through natural processes, as is the case with, e.g., cfDNA fragments that can naturally occur within a biological sample, or through in vitro manipulation (e.g., known chemical, mechanical or enzymatic fragmentation methods). In some embodiments, as one of skill in the art would readily appreciate, and as described herein, methylation status at one or more methylation sites (e.g., CpG sites) in a fragment can be determined, or inferred, from one or more sequence reads derived from the fragment. For example, the nucleotide base sequence of a DNA fragment or molecule can be determined from sequence reads derived from the DNA fragment, and thus, methylation status at one or more methylation sites (e.g., CpG sites) in the original fragment determined or inferred. Accordingly, “fragment” and “sequence read” can be used interchangeably herein.
The term “sequence read,” “sequence reads,” or “reads,” used interchangeably herein, refer to a nucleotide sequence produced by any sequencing process described herein or known in the art. Reads can be generated from one end of nucleic acid fragments (“single-end reads”), or generated from both ends of nucleic acid fragments (e.g., paired-end reads, double-end reads). Sequence reads can be obtained through various methods known in the art. As described herein, the nucleotide base sequence of a DNA fragment or molecule can be determined, or inferred, from sequence reads derived from the DNA fragment or molecule, and thus, “fragment” and “sequence read” can be used interchangeably in various embodiments described herein.
The term “sequencing depth” or “depth” refers to a total number of sequence reads or read segments at a given genomic location or loci from a test sample from an individual.
The term “anomalous fragment,” “anomalously methylated fragment,” or “fragment with an anomalous methylation pattern” refers to a fragment that has anomalous methylation of CpG sites. Anomalous methylation of a fragment may be determined using probabilistic models to identify unexpectedness of observing a fragment's methylation pattern in a control group.
The term “unusual fragment with extreme methylation” or “UFXM” refers to a hypomethylated fragment or a hypermethylated fragment. A hypomethylated fragment and a hypermethylated fragment refers to a fragment with at least some number of CpG sites (e.g., 5) that have over some threshold percentage (e.g., 90%) of methylation or unmethylation, respectively.
The term “anomaly score” refers to a score for a CpG site based on a number of anomalous fragments (or, in some embodiments, UFXMs) from a sample overlaps that CpG site. The anomaly score is used in context of featurization of a sample for classification.
From the sample, the analytics system isolates each cfDNA molecule. The cfDNA molecules are treated to convert unmethylated cytosines to uracils. In one embodiment, the method uses a bisulfite treatment of the DNA which converts the unmethylated cytosines to uracils without converting the methylated cytosines. For example, a commercial kit such as the EZ DNA Methylation™—Gold, EZ DNA Methylation™—Direct or an EZ DNA Methylation™—Lightning kit (available from Zymo Research Corp (Irvine, Calif.)) is used for the bisulfite conversion. In another embodiment, the conversion of unmethylated cytosines to uracils is accomplished using an enzymatic reaction. For example, the conversion can use a commercially available kit for conversion of unmethylated cytosines to uracils, such as APOBEC-Seq (NEBiolabs, Ipswich, Mass.).
From the converted cfDNA molecules, a sequencing library is prepared 130. Optionally, the sequencing library may be enriched 135 for cfDNA molecules, or genomic regions, that are informative for cancer status using a plurality of hybridization probes. The hybridization probes are short oligonucleotides capable of hybridizing to particularly specified cfDNA molecules, or targeted regions, and enriching for those fragments or regions for subsequent sequencing and analysis. Hybridization probes may be used to perform a targeted, high-depth analysis of a set of specified CpG sites of interest to the researcher. In one embodiment, the hybridization probes are designed to enrich for DNA molecules that have been treated (e.g., using bisulfite) for conversion of unmethylated cytosines to uracils. Once prepared, the sequencing library or a portion thereof can be sequenced to obtain a plurality of sequence reads. The sequence reads may be in a computer-readable, digital format for processing and interpretation by computer software.
From the sequence reads, the analytics system determines 150 a location and methylation state for each CpG site based on alignment to a reference genome. The analytics system generates 160 a methylation state vector for each fragment specifying a location of the fragment in the reference genome (e.g., as specified by the position of the first CpG site in each fragment, or another similar metric), a number of CpG sites in the fragment, and the methylation state of each CpG site in the fragment whether methylated (e.g., denoted as M), unmethylated (e.g., denoted as U), or indeterminate (e.g., denoted as I). Observed states are states of methylated and unmethylated; whereas, an unobserved state is indeterminate. Indeterminate methylation states may originate from sequencing errors and/or disagreements between methylation states of a DNA fragment's complementary strands. The methylation state vectors may be stored in temporary or persistent computer memory for later use and processing. Further, the analytics system may remove duplicate reads or duplicate methylation state vectors from a single sample. The analytics system may determine that a certain fragment with one or more CpG sites has an indeterminate methylation status over a threshold number or percentage, and may exclude such fragments or selectively include such fragments but build a model accounting for such indeterminate methylation statuses; one such model will be described below in conjunction with
After conversion, a sequencing library 130 is prepared and sequenced 140 generating a sequence read 142. The analytics system aligns 150 the sequence read 142 to a reference genome 144. The reference genome 144 provides the context as to what position in a human genome the fragment cfDNA originates from. In this simplified example, the analytics system aligns 150 the sequence read 142 such that the three CpG sites correlate to CpG sites 23, 24, and 25 (arbitrary reference identifiers used for convenience of description). The analytics system thus generates information both on methylation status of all CpG sites on the cfDNA molecule 112 and the position in the human genome that the CpG sites map to. As shown, the CpG sites on sequence read 142 which were methylated are read as cytosines. In this example, the cytosines appear in the sequence read 142 only in the first and third CpG site which allows one to infer that the first and third CpG sites in the original cfDNA molecule were methylated. Whereas, the second CpG site is read as a thymine (U is converted to T during the sequencing process), and thus, one can infer that the second CpG site was unmethylated in the original cfDNA molecule. With these two pieces of information, the methylation status and location, the analytics system generates 160 a methylation state vector 152 for the fragment cfDNA 112. In this example, the resulting methylation state vector 152 is <M23, U24, M25>, wherein M corresponds to a methylated CpG site, U corresponds to an unmethylated CpG site, and the subscript number corresponds to a position of each CpG site in the reference genome.
The analytics system determines anomalous fragments for a sample using the sample's methylation state vectors. For each fragment in a sample, the analytics system determines whether the fragment is an anomalous fragment using the methylation state vector corresponding to the fragment. In one embodiment, the analytics system calculates a p-value score for each methylation state vector describing a probability of observing that methylation state vector or other methylation state vectors even less probable in the healthy control group. The process for calculating a p-value score will be further discussed below in Section II.B.i. P-Value Filtering. The analytics system may determine fragments with a methylation state vector having below a threshold p-value score as anomalous fragments. In another embodiment, the analytics system further labels fragments with at least some number of CpG sites that have over some threshold percentage of methylation or unmethylation as hypermethylated and hypomethylated fragments, respectively. A hypermethylated fragment or a hypomethylated fragment may also be referred to as an unusual fragment with extreme methylation (UFXM). In other embodiments, the analytics system may implement various other probabilistic models for determining anomalous fragments. Examples of other probabilistic models include a mixture model, a deep probabilistic model, etc. In some embodiments, the analytics system may use any combination of the processes described below for identifying anomalous fragments. With the identified anomalous fragments, the analytics system may filter the set of methylation state vectors for a sample for use in other processes, e.g., for use in training and deploying a cancer classifier.
In one embodiment, the analytics system calculates a p-value score for each methylation state vector compared to methylation state vectors from fragments in a healthy control group. The p-value score describes a probability of observing the methylation status matching that methylation state vector or other methylation state vectors even less probable in the healthy control group. In order to determine a DNA fragment to be anomalously methylated, the analytics system uses a healthy control group with a majority of fragments that are normally methylated. When conducting this probabilistic analysis for determining anomalous fragments, the determination holds weight in comparison with the group of control subjects that make up the healthy control group. To ensure robustness in the healthy control group, the analytics system may select some threshold number of healthy individuals to source samples including DNA fragments.
With each fragment's methylation state vector, the analytics system subdivides 205 the methylation state vector into strings of CpG sites. In one embodiment, the analytics system subdivides 205 the methylation state vector such that the resulting strings are all less than a given length. For example, a methylation state vector of length 11 may be subdivided into strings of length less than or equal to 3 would result in 9 strings of length 3, 10 strings of length 2, and 11 strings of length 1. In another example, a methylation state vector of length 7 being subdivided into strings of length less than or equal to 4 would result in 4 strings of length 4, 5 strings of length 3, 6 strings of length 2, and 7 strings of length 1. If a methylation state vector is shorter than or the same length as the specified string length, then the methylation state vector may be converted into a single string containing all of the CpG sites of the vector.
The analytics system tallies 210 the strings by counting, for each possible CpG site and possibility of methylation states in the vector, the number of strings present in the control group having the specified CpG site as the first CpG site in the string and having that possibility of methylation states. For example, at a given CpG site and considering string lengths of 3, there are 2{circumflex over ( )}3 or 8 possible string configurations. At that given CpG site, for each of the 8 possible string configurations, the analytics system tallies 210 how many occurrences of each methylation state vector possibility come up in the control group. Continuing this example, this may involve tallying the following quantities: <Mx, Mx+1, Mx+2>, <Mx, Mx+1, Ux+2>, . . . <Ux, Ux+1, Ux+2> for each starting CpG site x in the reference genome. The analytics system creates 215 the data structure storing the tallied counts for each starting CpG site and string possibility.
There are several benefits to setting an upper limit on string length. First, depending on the maximum length for a string, the size of the data structure created by the analytics system can dramatically increase in size. For instance, maximum string length of 4 means that every CpG site has at the very least 2{circumflex over ( )}4 numbers to tally for strings of length 4. Increasing the maximum string length to 5 means that every CpG site has an additional 2{circumflex over ( )}4 or 16 numbers to tally, doubling the numbers to tally (and computer memory required) compared to the prior string length. Reducing string size helps keep the data structure creation and performance (e.g., use for later accessing as described below), in terms of computational and storage, reasonable. Second, a statistical consideration to limiting the maximum string length is to avoid over-fitting downstream models that use the string counts. If long strings of CpG sites do not, biologically, have a strong effect on the outcome (e.g., predictions of anomalousness that predictive of the presence of cancer), calculating probabilities based on large strings of CpG sites can be problematic as it requires a significant amount of data that may not be available, and thus would be too sparse for a model to perform appropriately. For example, calculating a probability of anomalousness/cancer conditioned on the prior 100 CpG sites would require counts of strings in the data structure of length 100, ideally some matching exactly the prior 100 methylation states. If only sparse counts of strings of length 100 are available, there will be insufficient data to determine whether a given string of length of 100 in a test sample is anomalous or not.
For a given methylation state vector, the analytics system enumerates 230 all possibilities of methylation state vectors having the same starting CpG site and same length (i.e., set of CpG sites) in the methylation state vector. As each methylation state is generally either methylated or unmethylated there are effectively two possible states at each CpG site, and thus the count of distinct possibilities of methylation state vectors depends on a power of 2, such that a methylation state vector of length n would be associated with 2n possibilities of methylation state vectors. With methylation state vectors inclusive of indeterminate states for one or more CpG sites, the analytics system may enumerate 230 possibilities of methylation state vectors considering only CpG sites that have observed states.
The analytics system calculates 240 the probability of observing each possibility of methylation state vector for the identified starting CpG site and methylation state vector length by accessing the healthy control group data structure. In one embodiment, calculating the probability of observing a given possibility uses a Markov chain probability to model the joint probability calculation. In other embodiments, calculation methods other than Markov chain probabilities are used to determine the probability of observing each possibility of methylation state vector.
The analytics system calculates 250 a p-value score for the methylation state vector using the calculated probabilities for each possibility. In one embodiment, this includes identifying the calculated probability corresponding to the possibility that matches the methylation state vector in question. Specifically, this is the possibility having the same set of CpG sites, or similarly the same starting CpG site and length as the methylation state vector. The analytics system sums the calculated probabilities of any possibilities having probabilities less than or equal to the identified probability to generate the p-value score.
This p-value represents the probability of observing the methylation state vector of the fragment or other methylation state vectors even less probable in the healthy control group. A low p-value score, thereby, generally corresponds to a methylation state vector which is rare in a healthy individual, and which causes the fragment to be labeled anomalously methylated, relative to the healthy control group. A high p-value score generally relates to a methylation state vector is expected to be present, in a relative sense, in a healthy individual. If the healthy control group is a non-cancerous group, for example, a low p-value indicates that the fragment is anomalous methylated relative to the non-cancer group, and therefore possibly indicative of the presence of cancer in the test subject.
As above, the analytics system calculates p-value scores for each of a plurality of methylation state vectors, each representing a cfDNA fragment in the test sample. To identify which of the fragments are anomalously methylated, the analytics system may filter 260 the set of methylation state vectors based on their p-value scores. In one embodiment, filtering is performed by comparing the p-values scores against a threshold and keeping only those fragments below the threshold. This threshold p-value score could be on the order of 0.1, 0.01, 0.001, 0.0001, or similar.
According to example results from the process 400, the analytics system yields a median (range) of 2,800 (1,500-12,000) fragments with anomalous methylation patterns for participants without cancer in training, and a median (range) of 3,000 (1,200-220,000) fragments with anomalous methylation patterns for participants with cancer in training. These filtered sets of fragments with anomalous methylation patterns may be used for the downstream analyses as described below in Section III.
In one embodiment, the analytics system uses 255 a sliding window to determine possibilities of methylation state vectors and calculate p-values. Rather than enumerating possibilities and calculating p-values for entire methylation state vectors, the analytics system enumerates possibilities and calculates p-values for only a window of sequential CpG sites, where the window is shorter in length (of CpG sites) than at least some fragments (otherwise, the window would serve no purpose). The window length may be static, user determined, dynamic, or otherwise selected.
In calculating p-values for a methylation state vector larger than the window, the window identifies the sequential set of CpG sites from the vector within the window starting from the first CpG site in the vector. The analytic system calculates a p-value score for the window including the first CpG site. The analytics system then “slides” the window to the second CpG site in the vector, and calculates another p-value score for the second window. Thus, for a window size l and methylation vector length m, each methylation state vector will generate m−l+1 p-value scores. After completing the p-value calculations for each portion of the vector, the lowest p-value score from all sliding windows is taken as the overall p-value score for the methylation state vector. In another embodiment, the analytics system aggregates the p-value scores for the methylation state vectors to generate an overall p-value score.
Using the sliding window helps to reduce the number of enumerated possibilities of methylation state vectors and their corresponding probability calculations that would otherwise need to be performed. To give a realistic example, it is possible for fragments to have upwards of 54 CpG sites. Instead of computing probabilities for 2{circumflex over ( )}54 (˜1.8×10{circumflex over ( )}16) possibilities to generate a single p-score, the analytics system can instead use a window of size 5 (for example) which results in 50 p-value calculations for each of the 50 windows of the methylation state vector for that fragment. Each of the 50 calculations enumerates 2{circumflex over ( )}5 (32) possibilities of methylation state vectors, which total results in 50×2{circumflex over ( )}5 (1.6×10{circumflex over ( )}3) probability calculations. This results in a vast reduction of calculations to be performed, with no meaningful hit to the accurate identification of anomalous fragments.
In embodiments with indeterminate states, the analytics system may calculate a p-value score summing out CpG sites with indeterminates states in a fragment's methylation state vector. The analytics system identifies all possibilities that have consensus with the all methylation states of the methylation state vector excluding the indeterminate states. The analytics system may assign the probability to the methylation state vector as a sum of the probabilities of the identified possibilities. As an example, the analytics system calculates a probability of a methylation state vector of <M1, I2, U3> as a sum of the probabilities for the possibilities of methylation state vectors of <M1, M2, U3> and <M1, U2, U3> since methylation states for CpG sites 1 and 3 are observed and in consensus with the fragment's methylation states at CpG sites 1 and 3. This method of summing out CpG sites with indeterminate states uses calculations of probabilities of possibilities up to 2{circumflex over ( )}i, wherein i denotes the number of indeterminate states in the methylation state vector. In additional embodiments, a dynamic programming algorithm may be implemented to calculate the probability of a methylation state vector with one or more indeterminate states. Advantageously, the dynamic programming algorithm operates in linear computational time.
In one embodiment, the computational burden of calculating probabilities and/or p-value scores may be further reduced by caching at least some calculations. For example, the analytic system may cache in transitory or persistent memory calculations of probabilities for possibilities of methylation state vectors (or windows thereof). If other fragments have the same CpG sites, caching the possibility probabilities allows for efficient calculation of p-score values without needing to re-calculate the underlying possibility probabilities. Equivalently, the analytics system may calculate p-value scores for each of the possibilities of methylation state vectors associated with a set of CpG sites from vector (or window thereof). The analytics system may cache the p-value scores for use in determining the p-value scores of other fragments including the same CpG sites. Generally, the p-value scores of possibilities of methylation state vectors having the same CpG sites may be used to determine the p-value score of a different one of the possibilities from the same set of CpG sites.
In another embodiment, the analytics system determines anomalous fragments as fragments with over a threshold number of CpG sites and either with over a threshold percentage of the CpG sites methylated or with over a threshold percentage of CpG sites unmethylated; the analytics system identifies such fragments as hypermethylated fragments or hypomethylated fragments. Example thresholds for length of fragments (or CpG sites) include more than 3, 4, 5, 6, 7, 8, 9, 10, etc. Example percentage thresholds of methylation or unmethylation include more than 80%, 85%, 90%, or 95%, or any other percentage within the range of 50%-100%.
In various embodiments, the sequencer 420 receives an enriched nucleic acid sample 410. As shown in
In some embodiments, the sequencer 420 is communicatively coupled with the analytics system 400. The analytics system 400 includes some number of computing devices used for processing the sequence reads for various applications such as assessing methylation status at one or more CpG sites, variant calling or quality control. The sequencer 420 may provide the sequence reads in a BAM file format to the analytics system 400. The analytics system 400 can be communicatively coupled to the sequencer 420 through a wireless, wired, or a combination of wireless and wired communication technologies. Generally, the analytics system 400 is configured with a processor and non-transitory computer-readable storage medium storing computer instructions that, when executed by the processor, cause the processor to process the sequence reads or to perform one or more steps of any of the methods or processes disclosed herein.
In some embodiments, the sequence reads may be aligned to a reference genome using known methods in the art to determine alignment position information, e.g., via step 140 of the process 100 in
In various embodiments, for example when a paired-end sequencing process is used, a sequence read is comprised of a read pair denoted as R_1 and R_2. For example, the first read R_1 may be sequenced from a first end of a double-stranded DNA (dsDNA) molecule whereas the second read R_2 may be sequenced from the second end of the double-stranded DNA (dsDNA). Therefore, nucleotide base pairs of the first read R_1 and second read R_2 may be aligned consistently (e.g., in opposite orientations) with nucleotide bases of the reference genome. Alignment position information derived from the read pair R_1 and R_2 may include a beginning position in the reference genome that corresponds to an end of a first read (e.g., R_1) and an end position in the reference genome that corresponds to an end of a second read (e.g., R_2). In other words, the beginning position and end position in the reference genome represent the likely location within the reference genome to which the nucleic acid fragment corresponds. An output file having SAM (sequence alignment map) format or BAM (binary) format may be generated and output for further analysis.
Referring now to
The sequence processor 440 generates methylation state vectors for fragments from a sample. At each CpG site on a fragment, the sequence processor 440 generates a methylation state vector for each fragment specifying a location of the fragment in the reference genome, a number of CpG sites in the fragment, and the methylation state of each CpG site in the fragment whether methylated, unmethylated, or indeterminate via the process 100 of
Further, multiple different models 450 may be stored in the model database 455 or retrieved for use with test samples. In one example, a model is a trained cancer classifier for determining a cancer prediction for a test sample using a feature vector derived from anomalous fragments. The training and use of the cancer classifier will be further discussed in conjunction with Section III. Cancer Classifier for Determining Cancer. The analytics system 400 may train the one or more models 450 and store various trained parameters in the parameter database 465. The analytics system 400 stores the models 450 along with functions in the model database 455.
During inference, the score engine 460 uses the one or more models 450 to return outputs. The score engine 460 accesses the models 450 in the model database 455, such as a cancer prediction model, along with trained parameters from the parameter database 465, such as anomalous fragments derived from training fragments. The score engine 460 applies an accessed model to data representative of anomalous fragments within a test sample, and the model produces an output representative of a likelihood that the test sample is associated with a disease state based on the data representative of the anomalous fragments. As noted herein, the disease state can be a presence or absence of cancer generally, a presence or absence of a particular type of cancer, or a presence or absence of a non-cancer disease or human condition. In some use cases, the score engine 460 further calculates metrics correlating to a confidence in the outputs produced by the accessed model. In other use cases, the score engine 460 calculates other intermediary values for use in the model.
The cancer classifier is trained to receive a feature vector for a test sample and determine whether the test sample is from a test subject that has cancer or, more specifically, a particular cancer type. The cancer classifier comprises a plurality of classification parameters and a function representing a relation between the feature vector as input and the cancer prediction as output determined by the function operating on the input feature vector with the classification parameters. In one embodiment, the feature vectors input into the cancer classifier are based on set of anomalous fragments determined from the test sample. The anomalous fragments may be determined via the process 220 in
The analytics system determines 320, for each training sample, a feature vector based on the set of anomalous fragments of the training sample. The analytics system calculates an anomaly score for each CpG site in an initial set of CpG sites. The initial set of CpG sites may be all CpG sites in the human genome or some portion thereof—which may be on the order of 104, 105, 106, 107, 108, etc. In one embodiment, the analytics system defines the anomaly score for the feature vector with a binary scoring based on whether there is an anomalous fragment in the set of anomalous fragments that encompasses the CpG site. In another embodiment, the analytics system defines the anomaly score based on a count of anomalous fragments overlapping the CpG site. In one example, the analytics system may use a trinary scoring assigning a first score for lack of presence of anomalous fragments, a second score for presence of a few anomalous fragments (e.g., greater than zero but less than a threshold number of anomalous fragments), and a third score for presence of more than a few anomalous fragments (e.g., greater than the threshold number of anomalous fragments). For example, the analytics system counts 5 anomalous fragment in a sample that overlap the CpG site and calculates an anomaly score based on the count of 5.
Once all anomaly scores are determined for a training sample, the analytics system determines the feature vector as a vector of elements including, for each element, one of the anomaly scores associated with one of the CpG sites in an initial set. The analytics system normalizes the anomaly scores of the feature vector based on a coverage of the sample. Here, coverage refers to a median or average sequencing depth over all CpG sites covered by the initial set of CpG sites used in the classifier, or based on the set of anomalous fragments for a given training sample.
As an example, reference is now made to
The analytics system may further limit the CpG sites considered for use in the cancer classifier. The analytics system computes 330, for each CpG site in the initial set of CpG sites, an information gain based on the feature vectors of the training samples. From step 320, each training sample has a feature vector that may contain an anomaly score all CpG sites in the initial set of CpG sites which could include up to all CpG sites in the human genome. However, some CpG sites in the initial set of CpG sites may not be as informative as others in distinguishing between cancer types, or may be duplicative with other CpG sites.
In one embodiment, the analytics system computes 330 an information gain for each cancer type and for each CpG site in the initial set to determine whether to include that CpG site in the classifier. The information gain is computed for training samples with a given cancer type compared to all other samples. For example, two random variables ‘anomalous fragment’ (‘AF’) and ‘cancer type’ (‘CT’) are used. In one embodiment, AF is a binary variable indicating whether there is an anomalous fragment overlapping a given CpG site in one or more given samples as determined for the anomaly score/feature vector above. CT is a random variable indicating whether the cancer is of a particular type. The analytics system computes the mutual information with respect to CT given AF. That is, how many bits of information about the cancer type are gained if it is known whether there is an anomalous fragment overlapping a particular CpG site.
For a given cancer type, the analytics system uses this information to rank CpG sites based on how cancer specific they are. This procedure is repeated for all cancer types under consideration. If a particular region is commonly anomalously methylated in training samples of a given cancer but not in training samples of other cancer types or in healthy training samples, then CpG sites overlapped by those anomalous fragments will tend to have high information gains for the given cancer type. The ranked CpG sites for each cancer type are greedily added (selected) 340 to a selected set of CpG sites based on their rank for use in the cancer classifier.
In additional embodiments, the analytics system may consider other selection criteria for selecting informative CpG sites to be used in the cancer classifier. One selection criterion may be that the selected CpG sites are above a threshold separation from other selected CpG sites. For example, the selected CpG sites are to be over a threshold number of base pairs away from any other selected CpG site (e.g., 100 base pairs), such that CpG sites that are within the threshold separation are not both selected for consideration in the cancer classifier.
In one embodiment, according to the selected set of CpG sites from the initial set, the analytics system may modify 350 the feature vectors of the training samples as needed. For example, the analytics system may truncate feature vectors to remove anomaly scores corresponding to CpG sites not in the selected set of CpG sites.
With the feature vectors of the training samples, the analytics system may train the cancer classifier in any of a number of ways. The feature vectors may correspond to the initial set of CpG sites from step 320 or to the selected set of CpG sites from step 350. In one embodiment, the analytics system trains 360 a binary cancer classifier to distinguish between a cancer classification and a non-cancer classification based on the feature vectors of the training samples. In this manner, the analytics system uses training samples that include both non-cancer samples from healthy individuals and cancer samples from individuals with cancer. Each training sample has one of the two labels “cancer” or “non-cancer.” In this embodiment, the classifier outputs a cancer prediction indicating the likelihood of the presence or absence of cancer.
In another embodiment, the analytics system trains 450 a multiclass cancer classifier to distinguish between many cancer types. The possible set of cancer types may include one or more cancers and may also include a non-cancer type. Likewise, the set of cancer types may also include any additional other diseases or genetic disorders, etc. To do so, the analytics system uses the cancer type cohorts and may also include or not include a non-cancer type cohort. In this multi-cancer classifier embodiment, the cancer classifier is trained to determine a cancer prediction that comprises a prediction value for each of the cancer types being classified. The prediction values may correspond to a likelihood that a given training sample (and during inference, a test sample) has each of the cancer types.
In one implementation, the prediction values are scored between 0 and 100, wherein the cumulation of the prediction values equals 100. For example, the cancer classifier returns a cancer prediction including a prediction value for breast cancer, lung cancer, and non-cancer. In this example, the classifier can return a cancer prediction that a test sample is 65% likelihood of breast cancer, 25% likelihood of lung cancer, and 10% likelihood of non-cancer. The analytics system may further process the prediction values to generate a single cancer determination. Continuing with the example above and given the percentages, in this example the system may determine that the sample has breast cancer given that breast cancer has the highest likelihood. It should also be noted that the multi-cancer classifier can classify a test sample and produce a score for each of the types of cancer associated with the multi-cancer classifier such that the scores are independent of each other (and thus do not necessarily add up to 100). In this embodiment, the classifier may output a 90% likelihood of breast cancer and an 80% likelihood of lung cancer, indicating that the individual associated with the test sample has more than one type of cancer (or has a cancer that has metastasized to a different location).
In both embodiments, the analytics system trains the cancer classifier by inputting sets of training samples with their feature vectors into the cancer classifier and adjusting classification parameters so that a function of the classifier accurately relates the training feature vectors to their corresponding label. The analytics system may group the training samples into sets of one or more training samples for iterative batch training of the cancer classifier. After inputting all sets of training samples including their training feature vectors and adjusting the classification parameters, the cancer classifier is sufficiently trained to label test samples according to their feature vector within some margin of error. The analytics system may train the cancer classifier according to any one of a number of methods. As an example, the binary cancer classifier may be a L2-regularized logistic regression classifier that is trained using a log-loss function. As another example, the multi-cancer classifier may be a multinomial logistic regression. In practice either type of cancer classifier may be trained using other techniques. These techniques are numerous including potential use of kernel methods, random forest classifier, a mixture model, an autoencoder model, machine learning algorithms such as multilayer neural networks, etc.
During use of the cancer classifier, the analytics system obtains a test sample from a subject of unknown cancer type. The analytics system may process the test sample comprised of DNA molecules with any combination of the processes 100, 200, and 220 to achieve a set of anomalous fragments. The analytics system determines a test feature vector for use by the cancer classifier according to similar principles discussed in the process 300. The analytics system calculates an anomaly score for each CpG site in a plurality of CpG sites in use by the cancer classifier. For example, the cancer classifier receives as input feature vectors inclusive of anomaly scores for 1,000 selected CpG sites. The analytics system thus determines a test feature vector inclusive of anomaly scores for the 1,000 selected CpG sites based on the set of anomalous fragments. The analytics system calculates the anomaly scores in a same manner as the training samples. In one embodiment, the analytics system defines the anomaly score as a binary score based on whether there is a hypermethylated or hypomethylated fragment in the set of anomalous fragments that encompasses the CpG site.
The analytics system then inputs the test feature vector into the cancer classifier. The cancer classifier, when applied to the test feature vector, generates a cancer prediction based on the classification parameters trained in the process 300 and the test feature vector. In the first manner, the cancer prediction is binary and selected from a group consisting of “cancer” or non-cancer;” in the second manner, the cancer prediction is selected from a group of many cancer types and “non-cancer.” In additional embodiments, the cancer prediction has predictions values for each of the many cancer types. Moreover, the analytics system may determine that the test sample is most likely to be of one of the cancer types. Following the example above with the cancer prediction for a test sample as 65% likelihood of breast cancer, 25% likelihood of lung cancer, and 10% likelihood of non-cancer, the analytics system may determine that the test sample is most likely to have breast cancer. In another example, where the cancer prediction is binary as 60% likelihood of non-cancer and 40% likelihood of cancer, the analytics system determines that the test sample is most likely not to have cancer. In additional embodiments, the cancer type prediction with the highest likelihood may still be compared against a threshold (e.g., 40%, 50%, 60%, 70%) in order to classify the test subject as having that cancer type. If the cancer prediction with the highest likelihood does not surpass that threshold, the analytics system may return an inconclusive result.
In additional embodiments, the analytics system chains a cancer classifier trained in step 360 of the process 300 with another cancer classifier trained in step 370 or the process 300. The analytics system inputs the test feature vector into the cancer classifier trained as a binary classifier in step 360 of the process 300. The analytics system receives an output of a cancer prediction. The cancer prediction may be binary, indicating whether the test subject likely has or likely does not have cancer. In other implementations, the cancer prediction includes prediction values that describe likelihood of cancer and likelihood of non-cancer. For example, the cancer prediction has a cancer prediction value of 85% and the non-cancer prediction value of 15%. The analytics system may determine the test subject to likely have cancer. Once the analytics system determines a test subject is likely to have cancer, the analytics system may input the test feature vector into a multiclass cancer classifier trained to distinguish between different cancer types. The multiclass cancer classifier receives the test feature vector and returns a cancer prediction of a cancer type of the plurality of cancer types. For example, the multiclass cancer classifier provides a cancer prediction specifying that the test subject is most likely to have ovarian cancer. In another implementation, the multiclass cancer classifier provides a prediction value for each cancer type of the plurality of cancer types. For example, a cancer prediction may include a breast cancer type prediction value of 40%, a colorectal cancer type prediction value of 15%, and a liver cancer prediction value of 45%.
In some embodiments, in response to the cancer classifier outputting a cancer prediction for a test sample (e.g., either the likelihood of the presence or absence of cancer generally, or the likelihood of the presence or absence of a particular type of cancer), the prediction can be clinically verified. For instance, an individual predicted to have lung cancer can be diagnosed as having lung cancer or not having lung cancer by a physician, or an individual predicted to be cancer-free can be diagnosed with cancer by a physician. In response to the verification or contradiction of the cancer prediction outputted by the cancer classifier, the feature vector associated with the test sample can be added to the training sample set with a label representative of the verification or contradiction (e.g., the feature vector can be labeled “lung cancer,” “non-cancer”, and the like). The classifier can then be retrained using the updated training sample set in order to improve the performance of the classifier in subsequent applications.
In some embodiments, the methods, analytic systems and/or classifier of the present invention can be used to detect the presence of cancer, monitor cancer progression or recurrence, monitor therapeutic response or effectiveness, determine a presence or monitor minimum residual disease (MRD), or any combination thereof. For example, as described herein, a classifier can be used to generate a probability score (e.g., from 0 to 100) describing a likelihood that a test feature vector is from a subject with cancer. In some embodiments, the probability score is compared to a threshold probability to determine whether or not the subject has cancer. In other embodiments, the likelihood or probability score can be assessed at multiple different time points (e.g., before or after treatment) to monitor disease progression or to monitor treatment effectiveness (e.g., therapeutic efficacy). In still other embodiments, the likelihood or probability score can be used to make or influence a clinical decision (e.g., diagnosis of cancer, treatment selection, assessment of treatment effectiveness, etc.). For example, in one embodiment, if the probability score exceeds a threshold, a physician can prescribe an appropriate treatment.
In some embodiments, the methods and/or classifier of the present invention are used to detect the presence or absence of cancer in a subject suspected of having cancer. For example, a classifier (e.g., as described above in Section III and exampled in Section V) can be used to determine a cancer prediction describing a likelihood that a test feature vector is from a subject that has cancer.
In one embodiment, a cancer prediction is a likelihood (e.g., scored between 0 and 100) for whether the test sample has cancer (i.e. binary classification). Thus, the analytics system may determine a threshold for determining whether a test subject has cancer. For example, a cancer prediction of greater than or equal to 60 can indicate that the subject has cancer. In still other embodiments, a cancer prediction greater than or equal to 65, greater than or equal to 70, greater than or equal to 75, greater than or equal to 80, greater than or equal to 85, greater than or equal to 90, or greater than or equal to 95 indicates that the subject has cancer. In other embodiments, the cancer prediction can indicate the severity of disease. For example, a cancer prediction of 80 may indicate a more severe form, or later stage, of cancer compared to a cancer prediction below 80 (e.g., a probability score of 70). Similarly, an increase in the cancer prediction over time (e.g., determined by classifying test feature vectors from multiple samples from the same subject taken at two or more time points) can indicate disease progression or a decrease in the cancer prediction over time can indicate successful treatment.
In another embodiment, a cancer prediction comprises many prediction values, wherein each of a plurality of cancer types being classified (i.e. multiclass classification) for has a prediction value (e.g., scored between 0 and 100). The prediction values may correspond to a likelihood that a given training sample (and during inference, training sample) has each of the cancer types. The analytics system may identify the cancer type that has the highest prediction value and indicate that the test subject likely has that cancer type. In other embodiments, the analytics system further compares the highest prediction value to a threshold value (e.g., 50, 55, 60, 65, 70, 75, 80, 85, etc.) to determine that the test subject likely has that cancer type. In other embodiments, a prediction value can also indicate the severity of disease. For example, a prediction value greater than 80 may indicate a more severe form, or later stage, of cancer compared to a prediction value of 60. Similarly, an increase in the prediction value over time (e.g., determined by classifying test feature vectors from multiple samples from the same subject taken at two or more time points) can indicate disease progression or a decrease in the prediction value over time can indicate successful treatment.
According to aspects of the invention, the methods and systems of the present invention can be trained to detect or classify multiple cancer indications. For example, the methods, systems and classifiers of the present invention can be used to detect the presence of one or more, two or more, three or more, five or more, ten or more, fifteen or more, or twenty or more different types of cancer.
Examples of cancers that can be detected using the methods, systems and classifiers of the present invention include carcinoma, lymphoma, blastoma, sarcoma, and leukemia or lymphoid malignancies. More particular examples of such cancers include, but are not limited to, squamous cell cancer (e.g., epithelial squamous cell cancer), skin carcinoma, melanoma, lung cancer, including small-cell lung cancer, non-small cell lung cancer (“NSCLC”), adenocarcinoma of the lung and squamous carcinoma of the lung, cancer of the peritoneum, gastric or stomach cancer including gastrointestinal cancer, pancreatic cancer (e.g., pancreatic ductal adenocarcinoma), cervical cancer, ovarian cancer (e.g., high grade serous ovarian carcinoma), liver cancer (e.g., hepatocellular carcinoma (HCC)), hepatoma, hepatic carcinoma, bladder cancer (e.g., urothelial bladder cancer), testicular (germ cell tumor) cancer, breast cancer (e.g., HER2 positive, HER2 negative, and triple negative breast cancer), brain cancer (e.g., astrocytoma, glioma (e.g., glioblastoma)), colon cancer, rectal cancer, colorectal cancer, endometrial or uterine carcinoma, salivary gland carcinoma, kidney or renal cancer (e.g., renal cell carcinoma, nephroblastoma or Wilms' tumor), prostate cancer, vulval cancer, thyroid cancer, anal carcinoma, penile carcinoma, head and neck cancer, esophageal carcinoma, and nasopharyngeal carcinoma (NPC). Additional examples of cancers include, without limitation, retinoblastoma, thecoma, arrhenoblastoma, hematologic malignancies, including but not limited to non-Hodgkin's lymphoma (NHL), multiple myeloma and acute hematologic malignancies, endometriosis, fibrosarcoma, choriocarcinoma, laryngeal carcinomas, Kaposi's sarcoma, Schwannoma, oligodendroglioma, neuroblastomas, rhabdomyosarcoma, osteogenic sarcoma, leiomyosarcoma, and urinary tract carcinomas.
In some embodiments, the cancer is one or more of anorectal cancer, bladder cancer, breast cancer, cervical cancer, colorectal cancer, esophageal cancer, gastric cancer, head & neck cancer, hepatobiliary cancer, leukemia, lung cancer, lymphoma, melanoma, multiple myeloma, ovarian cancer, pancreatic cancer, prostate cancer, renal cancer, thyroid cancer, uterine cancer, or any combination thereof.
In some embodiments, the one or more cancer can be a “high-signal” cancer (defined as cancers with greater than 50% 5-year cancer-specific mortality), such as anorectal, colorectal, esophageal, head & neck, hepatobiliary, lung, ovarian, and pancreatic cancers, as well as lymphoma and multiple myeloma. High-signal cancers tend to be more aggressive and typically have an above-average cell-free nucleic acid concentration in test samples obtained from a patient.
In some embodiments, the cancer prediction can be assessed at multiple different time points (e.g., or before or after treatment) to monitor disease progression or to monitor treatment effectiveness (e.g., therapeutic efficacy). For example, the present invention include methods that involve obtaining a first sample (e.g., a first plasma cfDNA sample) from a cancer patient at a first time point, determining a first cancer prediction therefrom (as described herein), obtaining a second test sample (e.g., a second plasma cfDNA sample) from the cancer patient at a second time point, and determining a second cancer prediction therefrom (as described herein).
In certain embodiments, the first time point is before a cancer treatment (e.g., before a resection surgery or a therapeutic intervention), and the second time point is after a cancer treatment (e.g., after a resection surgery or therapeutic intervention), and the classifier is utilized to monitor the effectiveness of the treatment. For example, if the second cancer prediction decreases compared to the first cancer prediction, then the treatment is considered to have been successful. However, if the second cancer prediction increases compared to the first cancer prediction, then the treatment is considered to have not been successful. In other embodiments, both the first and second time points are before a cancer treatment (e.g., before a resection surgery or a therapeutic intervention). In still other embodiments, both the first and the second time points are after a cancer treatment (e.g., after a resection surgery or a therapeutic intervention). In still other embodiments, cfDNA samples may be obtained from a cancer patient at a first and second time point and analyzed. e.g., to monitor cancer progression, to determine if a cancer is in remission (e.g., after treatment), to monitor or detect residual disease or recurrence of disease, or to monitor treatment (e.g., therapeutic) efficacy.
Those of skill in the art will readily appreciate that test samples can be obtained from a cancer patient over any desired set of time points and analyzed in accordance with the methods of the invention to monitor a cancer state in the patient. In some embodiments, the first and second time points are separated by an amount of time that ranges from about 15 minutes up to about 30 years, such as about 30 minutes, such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or about 24 hours, such as about 1, 2, 3, 4, 5, 10, 15, 20, 25 or about 30 days, or such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 months, or such as about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, 25, 25.5, 26, 26.5, 27, 27.5, 28, 28.5, 29, 29.5 or about 30 years. In other embodiments, test samples can be obtained from the patient at least once every 3 months, at least once every 6 months, at least once a year, at least once every 2 years, at least once every 3 years, at least once every 4 years, or at least once every 5 years.
In still another embodiment, the cancer prediction can be used to make or influence a clinical decision (e.g., diagnosis of cancer, treatment selection, assessment of treatment effectiveness, etc.). For example, in one embodiment, if the cancer prediction (e.g., for cancer or for a particular cancer type) exceeds a threshold, a physician can prescribe an appropriate treatment (e.g., a resection surgery, radiation therapy, chemotherapy, and/or immunotherapy).
A classifier (as described herein) can be used to determine a cancer prediction that a sample feature vector is from a subject that has cancer. In one embodiment, an appropriate treatment (e.g., resection surgery or therapeutic) is prescribed when the cancer prediction exceeds a threshold. For example, in one embodiment, if the cancer prediction is greater than or equal to 60 one or more appropriate treatments are prescribed. In another embodiments, if the cancer prediction is greater than or equal to 65, greater than or equal to 70, greater than or equal to 75, greater than or equal to 80, greater than or equal to 85, greater than or equal to 90, or greater than or equal to 95, one or more appropriate treatments are prescribed. In other embodiments, the cancer prediction can indicate the severity of disease. An appropriate treatment matching the severity of the disease may then be prescribed.
In some embodiments, the treatment is one or more cancer therapeutic agents selected from the group consisting of a chemotherapy agent, a targeted cancer therapy agent, a differentiating therapy agent, a hormone therapy agent, and an immunotherapy agent. For example, the treatment can be one or more chemotherapy agents selected from the group consisting of alkylating agents, antimetabolites, anthracyclines, anti-tumor antibiotics, cytoskeletal disruptors (taxans), topoisomerase inhibitors, mitotic inhibitors, corticosteroids, kinase inhibitors, nucleotide analogs, platinum-based agents and any combination thereof. In some embodiments, the treatment is one or more targeted cancer therapy agents selected from the group consisting of signal transduction inhibitors (e.g. tyrosine kinase and growth factor receptor inhibitors), histone deacetylase (HDAC) inhibitors, retinoic receptor agonists, proteosome inhibitors, angiogenesis inhibitors, and monoclonal antibody conjugates. In some embodiments, the treatment is one or more differentiating therapy agents including retinoids, such as tretinoin, alitretinoin and bexarotene. In some embodiments, the treatment is one or more hormone therapy agents selected from the group consisting of anti-estrogens, aromatase inhibitors, progestins, estrogens, anti-androgens, and GnRH agonists or analogs. In one embodiment, the treatment is one or more immunotherapy agents selected from the group comprising monoclonal antibody therapies such as rituximab (RITUXAN) and alemtuzumab (CAMPATH), non-specific immunotherapies and adjuvants, such as BCG, interleukin-2 (IL-2), and interferon-alfa, immunomodulating drugs, for instance, thalidomide and lenalidomide (REVLIMID). It is within the capabilities of a skilled physician or oncologist to select an appropriate cancer therapeutic agent based on characteristics such as the type of tumor, cancer stage, previous exposure to cancer treatment or therapeutic agent, and other characteristics of the cancer.
Study design and samples: CCGA (NCT02889978) is a prospective, multi-center, case-control, observational study with longitudinal follow-up. De-identified biospecimens were collected from approximately 15,000 participants from 142 sites. Samples were divided into training (1,785) and test (1,015) sets; samples were selected to ensure a prespecified distribution of cancer types and non-cancers across sites in each cohort, and cancer and non-cancer samples were frequency age-matched by gender.
Whole-genome bisulfite sequencing: cfDNA was isolated from plasma, and whole-genome bisulfite sequencing (WGBS; 30× depth) was employed for analysis of cfDNA. cfDNA was extracted from two tubes of plasma (up to a combined volume of 10 ml) per patient using a modified QIAamp Circulating Nucleic Acid kit (Qiagen; Germantown, Md.). Up to 75 ng of plasma cfDNA was subjected to bisulfite conversion using the EZ-96 DNA Methylation Kit (Zymo Research, D5003). Converted cfDNA was used to prepare dual indexed sequencing libraries using Accel-NGS Methyl-Seq DNA library preparation kits (Swift BioSciences; Ann Arbor, Mich.) and constructed libraries were quantified using KAPA Library Quantification Kit for Illumina Platforms (Kapa Biosystems; Wilmington, Mass.). Four libraries along with 10% PhiX v3 library (Illumina, FC-110-3001) were pooled and clustered on an Illumina NovaSeq 6000 S2 flow cell followed by 150-bp paired-end sequencing (30×).
For each sample, the WGBS fragment set was reduced to a small subset of fragments having an anomalous methylation pattern. Additionally, hyper or hypomethylated cfDNA fragments were selected. cfDNA fragments selected for having an anomalous methylation pattern and being hyper or hypermethylated, i.e., UFXM. Fragments occurring at high frequency in individuals without cancer, or that have unstable methylation, are unlikely to produce highly discriminatory features for classification of cancer status. We therefore produced a statistical model and a data structure of typical fragments using an independent reference set of 108 non-smoking participants without cancer (age: 58±14 years, 79 [73%] women) (i.e., a reference genome) from the CCGA study. These samples were used to train a Markov-chain model (order 3) estimating the likelihood of a given sequence of CpG methylation statuses within a fragment as described above in Section II.B. This model was demonstrated to be calibrated within the normal fragment range (p-value>0.001) and was used to reject fragments with a p-value from the Markov model as >=0.001 as insufficiently unusual.
As described above, further data reduction step selected only fragments with at least 5 CpGs covered, and average methylation either >0.9 (hyper methylated) or <0.1 (hypo-methylated). This procedure resulted in a median (range) of 2,800 (1,500-12,000) UFXM fragments for participants without cancer in training, and a median (range) of 3,000 (1,200-220,000) UFXM fragments for participants with cancer in training. As this data reduction procedure only used reference set data, this stage was only required to be applied to each sample once.
The analytics system selects CpG sites to be considered in the cancer classifier. The information gain is computed for training samples with a given cancer type compared to all other samples. For example, two random variables ‘anomalous fragment’ (‘AF’) and ‘cancer type’ (CT′) are used. CT is a random variable indicating whether the cancer is of a particular type. The analytics system computes the mutual information with respect to CT given AF. That is, how many bits of information about the cancer type are gained if it is known whether there is an anomalous fragment overlapping a particular CpG site. For a given cancer type, the analytics system uses this information to rank CpG sites based on how cancer specific they are. This procedure is repeated for all cancer types under consideration. The ranked CpG sites for each cancer type are greedily added (e.g., to achieve approximately 3,000 CpG sites) for use in the cancer classifier.
For featurization of samples, the analytics system identifies fragments in each sample with anomalous methylation patterns and furthermore UFXM fragments. For one sample, the analytics system calculates an anomaly score for each selected CpG site for consideration (˜3,000). The analytics system defines the anomaly score with a binary scoring based on whether the sample has a UFXM fragment that encompasses the CpG site.
For the breast cancer cohort, the colorectal cancer cohort, and the lung cancer cohort, the cancer classifier was gradually more accurate in accurately predicting the cancer type in subsequent stages of cancer. For the head/neck cohort, ovarian cohort, and pancreas cohort, the cancer classifier had accuracy increases in the latter stage, i.e., Stage III and/or Stage IV. For the esophageal cohort and the hepatobiliary cohort, the cancer classifier also had latter stage accuracy, i.e., Stage III and Stage IV. With the non-cancer cohort, the cancer classifier was perfectly accurate in predicting the non-cancer samples to not likely have cancer. Last but not least, the lymphoma cohort had success throughout varying stages with a peak success in accurately predicting samples in Stage II of cancer.
In comparison to the example in
The confusion matrix shows agreement between cancer types having samples with known cancer tissue of origin (along x-axis) and predicted cancer tissue of origin (along y-axis). To validate performance of the trained KLR classifier, a cohort of samples (indicated in parentheses along the y-axis for each cancer type) for each cancer type was classified with the KLR classifier. The x-axis indicates how many samples from each cohort was classified under each cancer type. For example, with the lung cancer cohort having 25 samples with known lung cancer, the KLR classifier predicted one sample to have ovarian cancer, nineteen samples to have lung cancer, two samples to have head/neck cancer, one sample to have pancreas cancer, one sample to have breast cancer, and one sample to be labeled as other cancer type. Notably, for all cancer types except other cancer type, the KLR classifier accurately predicted more than half of each cohort with particularly high accuracy for the cancer types of multiple myeloma (2/2 or 100%), colorectal (18/20 or 90%), lymphoma (8/9 or 88.8%), ovarian (4/5 or 80%), lung (19/25 or 76%), and head/neck (3/4 or 75%). These results demonstrate the predictive accuracy of the KLR classifier.
By way of example,
The structure of a computing machine described in
By way of example, a computing machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a personal digital assistant (PDA), a cellular telephone, a smartphone, a web appliance, a network router, an internet of things (IoT) device, a switch or bridge, or any machine capable of executing instructions 824 that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” and “computer” may also be taken to include any collection of machines that individually or jointly execute instructions 824 to perform any one or more of the methodologies discussed herein.
The example computer system 800 includes one or more processors 802 such as a CPU (central processing unit), a GPU (graphics processing unit), a TPU (tensor processing unit), a DSP (digital signal processor), a system on a chip (SOC), a controller, a state equipment, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or any combination of these. Parts of the computing system 800 may also include a memory 804 that store computer code including instructions 824 that may cause the processors 802 to perform certain actions when the instructions are executed, directly or indirectly by the processors 802. Instructions can be any directions, commands, or orders that may be stored in different forms, such as equipment-readable instructions, programming instructions including source code, and other communication signals and orders. Instructions may be used in a general sense and are not limited to machine-readable codes.
One or more methods described herein improve the operation speed of the processors 802 and reduces the space required for the memory 804. For example, the machine learning methods described herein reduces the complexity of the computation of the processors 802 by applying one or more novel techniques that simplify the steps in training, reaching convergence, and generating results of the processors 802. The algorithms described herein also may reduce the size of the models and datasets to reduce the storage space requirement for memory 804.
The performance of certain of the operations may be distributed among the more than one processors, not only residing within a single machine, but deployed across a number of machines. In some example embodiments, the one or more processors or processor-implemented modules may be located in a single geographic location (e.g., within a home environment, an office environment, or a server farm). In other example embodiments, the one or more processors or processor-implemented modules may be distributed across a number of geographic locations. Even though in the specification or the claims may refer some processes to be performed by a processor, this should be construed to include a joint operation of multiple distributed processors.
The computer system 800 may include a main memory 804, and a static memory 806, which are configured to communicate with each other via a bus 808. The computer system 800 may further include a graphics display unit 810 (e.g., a plasma display panel (PDP), a liquid crystal display (LCD), a projector, or a cathode ray tube (CRT)). The graphics display unit 810, controlled by the processors 802, displays a graphical user interface (GUI) to display one or more results and data generated by the processes described herein. The computer system 800 may also include alphanumeric input device 812 (e.g., a keyboard), a cursor control device 814 (e.g., a mouse, a trackball, a joystick, a motion sensor, or other pointing instrument), a storage unit 816 (a hard drive, a solid state drive, a hybrid drive, a memory disk, etc.), a signal generation device 818 (e.g., a speaker), and a network interface device 820, which also are configured to communicate via the bus 808.
The storage unit 816 includes a computer-readable medium 822 on which is stored instructions 824 embodying any one or more of the methodologies or functions described herein. The instructions 824 may also reside, completely or at least partially, within the main memory 804 or within the processor 802 (e.g., within a processor's cache memory) during execution thereof by the computer system 800, the main memory 804 and the processor 802 also constituting computer-readable media. The instructions 824 may be transmitted or received over a network 826 via the network interface device 820.
While computer-readable medium 822 is shown in an example embodiment to be a single medium, the term “computer-readable medium” should be taken to include a single non-transitory medium or multiple media (e.g., a centralized or distributed database, or associated caches and servers) able to store instructions (e.g., instructions 824). The computer-readable medium may include any medium that is capable of storing instructions (e.g., instructions 824) for execution by the processors (e.g., processors 802) and that cause the processors to perform any one or more of the methodologies disclosed herein. The computer-readable medium may include, but not be limited to, data repositories in the form of solid-state memories, optical media, and magnetic media.
The foregoing detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments of the present disclosure. Other embodiments having different structures and operations do not depart from the scope of the present disclosure. The term “the invention” or the like is used with reference to certain specific examples of the many alternative aspects or embodiments of the applicants' invention set forth in this specification, and neither its use nor its absence is intended to limit the scope of the applicants' invention or the scope of the claims.
Embodiments of the invention may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability.
Any of the steps, operations, or processes described herein as being performed by the analytics system may be performed or implemented with one or more hardware or software modules of the apparatus, alone or in combination with other computing devices. In one embodiment, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described.
This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/784,355, filed on Dec. 21, 2018, and entitled “Cancer Classification Based on Methylation Fragments in Cell-Free DNA Samples,” the contents of which is herein incorporated by reference in its entirety. This application also claims the benefit of and priority to U.S. Provisional Patent Application No. 62/899,919, filed on Sep. 13, 2019, and entitled “Anomalous Fragment Detection and Classification”, the contents of which is herein incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62784355 | Dec 2018 | US | |
| 62899919 | Sep 2019 | US |
