Patent Application
20190169685
The present invention relates generally to the detection, monitoring, and treatment of cancer and more specifically to determining the MSI status of a patient by liquid biopsy.
Cancer causes more than a half a million deaths each year in the United States alone. The success of current treatments depends on the type of cancer and the stage at which it is detected. Many treatments include costly and painful surgeries and chemotherapies, and are often unsuccessful. Early and accurate detection of mutations is essential for effective cancer therapy.
Many cancers involve the accumulation of mutations that results from failure of the DNA mismatch-repair (MMR). One important marker of MMR deficiency is microsatellite instability (MSI), a polymorphism of tandem nucleotide repeat lengths ubiquitously distributed throughout the genome. The presence of MMR-deficiency or MSI may serve as a marker for immunotherapy response with checkpoint inhibition. Knowledge of MSI status is thus important and valuable for the treatment of cancer. While it may be possible to determine MSI status by sequencing DNA from a tumor sample, such as a formalin-fixed paraffin-embedded (FFPE) tumor tissue specimen, there are patients for whom tumor material is not readily obtained.
Absent a fixed tissue specimen, a potential source for tumor information is through the analysis of circulating tumor DNA (ctDNA). ctDNA is released from tumor tissue into the blood and can be analyzed by liquid biopsy. Liquid biopsies potentially allow for the detection and characterization of cancer. However, liquid biopsies present their own inherent challenges associated with low circulating tumor DNA (ctDNA) levels as well as problems with faithfully amplifying and sequencing regions of DNA characterized by tracts of mononucleotide repeats.
The present invention is based on the seminal discovery that a circulating tumor DNA based approach is useful for the detection of high tumor mutation burden and microsatellite instability in cancer patients with advanced disease and can be used to predict responders to immune checkpoint blockade.
The invention provides methods for determining the MSI status of a patient by liquid biopsy. Methods include a sample preparation using hybrid capture and non-unique barcodes. The sample preparation both compensates for errors such as sequencing artifacts and polymerase slippage and provides for the successful capture of target DNA even when present only at a very low fraction of total DNA. Methods include sequencing tracts of mononucleotide repeats within captured sample and modelling the distribution of lengths of those tracts. A peak-finding operation evaluates peaks in the modelled distribution and reveals MSI in the patient when the peaks deviate from a reference distribution (e.g., such as by indicating that the tracts of mononucleotide repeats in the patient's DNA are markedly shorter than in healthy DNA).
Methods of the disclosure are amenable to implementation in conjunction with other genomic screenings such as screening panels of markers, genes, or whole genomes to report mutations or mutational burden. Methods may be implemented by including MSI markers within any suitable liquid-biopsy based sequencing assay and may evaluate MSI status by interrogating MSI markers such as BAT-25, BAT-26, MONO-27, NR-21, and NR-24, BAT-40, TGFβ RII, IGFIIR, hMSH3, BAX and dinucleotide D2S123, D9S283, D9S1851 and D18S58 loci, by way of example, or by modeling distributions of lengths of any other suitable set(s) of repeats in the genome.
In certain aspects, the invention provides a method of detecting microsatellite instability (MSI). The method includes obtaining cell-free DNA (cfDNA) from a sample of plasma from a patient and sequencing portions of the cfDNA to obtain sequences of a plurality of tracts of nucleotide repeats in the cfDNA. A report is provided describing an MSI status in the patient when a distribution of lengths of the plurality of tracts has peaks that deviate significantly from peaks in a reference distribution. Obtaining the cfDNA may include capturing target portions of DNA with probes, fragmenting the target portions to yield fragments, and attaching barcodes to the fragments. In preferred embodiments, the barcodes are non-unique barcodes that include duplicates such that different ones of the fragments are attached to identical barcodes.
The method may include amplifying the fragments to produce amplicons that include barcode information and copies of the fragments, wherein the sequencing step comprises sequencing the amplicons. In one aspect, the sequencing is next-generation, short-read sequencing. The obtained sequences may include a plurality of sequence reads and the method may include aligning the sequence reads to a reference, and identifying groups of sequence reads that originated from a unique segment of the cfDNA by means of the barcode information and position or content of the sequence reads.
The use of the non-unique barcodes to identify groups of sequence reads that originated from a unique segment of the cfDNA allows for the lengths of the plurality of tracts to be determined correctly by correcting for errors introduced by sequencing artifacts or polymerase slippage during the amplifying step.
Preferably, the target portions are markers for MSI such as one or more of BAT25, BAT26, MON027, NR21, NR24, Penta C, and Penta D. For example, the markers may include all of BAT25, BAT26, MON027, NR21, and NR24. In certain embodiments, each of the microsatellite markers is selected from the group consisting of BAT-25, BAT-26, MONO-27, NR-21, NR-24, Penta C, and Penta D, BAT-40, TGFβ RII, IGFIIR, hMSH3, BAX and dinucleotide D2S123, D9S283, D9S1851 and D18S58 loci, by way of example.
In some embodiments, the method includes recommending a treatment for the patient based on the MSI status. Where the MSI status indicates that the patient is microsatellite instable, the treatment may include an immune checkpoint inhibitor. In certain embodiments, the method includes administering the treatment (e.g., the immune checkpoint inhibitor) to the patient. The immune checkpoint inhibitor may be, for example, an antibody such as an anti-PD-1 antibody; an anti-IDO antibody; anti-CTLA-4 antibody; an anti-PD-L1 antibody; or an anti-LAG-3 antibody.
Related aspects provide a method of detecting microsatellite instability (MSI) that includes obtaining a sample comprising fragments of cell-free DNA from a patient; attaching barcodes to the fragments, wherein at least some of the barcodes are not unique; sequencing the barcodes to obtain sequences of a plurality of markers in the DNA; determining a distribution of lengths of the plurality of markers; and providing a report describing MSI in the patient when peaks in the distribution deviate significantly from expected peaks in a modeled healthy distribution.
The present invention relates to the discovery that microsatellite instability (MSI) and high tumor mutation burden (TMB-High) are pan-tumor biomarkers used to select patients for treatment with immune checkpoint blockade. The present invention shows a plasma-based approach for detection of MSI and TMB-High in patients with advanced cancer. To detect sequence alterations across a 98 kilobase panel, including those in microsatellite regions, the inventors developed an error correction approach with specificities >99% (n=163) and sensitivities of 75% (n=12) and 60% (n=10), respectively, for MSI and TMB-High. For patients treated with PD-1 blockade, the data demonstrate that MSI and TMB-High in pre-treatment plasma predicted progression-free survival (hazard ratios 0.2 and 0.12, p=0.01 and 0.004, respectively). The data shows the results when plasma during therapy was analyzed in order to develop a prognostic signature for patients who achieved durable response to PD-1 blockade. These analyses demonstrate the feasibility of non-invasive pan-cancer screening and monitoring of patients who exhibit MSI or TMB-High and have a high likelihood of responding to immune checkpoint blockade.
The disclosure provides for the detection of MSI by liquid biopsy. While plasma is the illustrative example provided herein, it is understood that a liquid biopsy can be performed with a biological sample including blood, plasma, saliva, urine, feces, tears, mucosal secretions and other biological fluids.
In particular, methods of the disclosure provide and include the analytical validation of an integrated NGS-based liquid biopsy approach for the detection of microsatellite instability associated with cancers such as pancreatic, colon, gastric, endometrial, cholangiocarcinoma, breast, lung, head and neck, kidney, bladder, or prostate cancer, as well as hematopoietic cancers, among others. Failure of the DNA mismatch repair (MMR) pathway during DNA replication in cancer leads to the increased accumulation of somatic mutations. One important marker of MMR deficiency is microsatellite instability (MSI), which presents as polymorphism of tandem nucleotide repeat lengths ubiquitously distributed throughout the genome. Methods of the disclosure are offered to assay for and detect those markers via liquid biopsy. Additionally, since the presence of MMR-deficiency or MSI may serve as a marker for immunotherapy response with checkpoint inhibition, methods may be used to determine a course of treatment such as immunotherapy or the administration of a checkpoint inhibitor.
Microsatellite instability (MSI) and mismatch repair (MMR) deficiency have recently been demonstrated to predict immune checkpoint blockade response. The checkpoint inhibitor pembrolizumab is now indicated for the treatment of adult and pediatric patients with any unresectable or metastatic solid tumors identified as having either of these biomarkers. This indication covers patients with solid tumors that have progressed following prior treatment and have no satisfactory alternative treatment options.
Cancer is characterized by the accumulation of somatic mutations that have the potential to result in the expression of neoantigens, which may elicit T-cell-dependent immune responses against tumors. MMR is a mechanism by which post-replicative mismatches in daughter DNA strands are repaired and replaced with the correct DNA sequence. MMR deficiency results in both MSI and high tumor mutation burden (TMB-High), which increases the likelihood that acquired somatic mutations may be transcribed and translated into proteins that are recognized as immunogenic neoantigens. Historically, testing for MSI has been restricted to screening for Hereditary Non-Polyposis Colorectal Cancer (HNPCC), which is often characterized by early age onset colorectal cancer and endometrial cancer, as well as other extracolonic tumors. HNPCC, commonly referred to as Lynch Syndrome, is caused by mutations in the DNA mismatch repair genes (MLH1, MSH2, MSH6 and PMS2), as well as the more recently described, EPCAM(16). In addition to familial conditions, MSI can occur sporadically in cancer, and both hereditary and sporadic MSI patients respond to immune checkpoint blockade(1,2). A recent study, conducted across 39 tumor types and 11,139 patients to determine the landscape of MSI prevalence, concluded that 3.8% of these cancers across 27 tumor types displayed MSI, including 31.4% of uterine/endometrial carcinoma, 19.7% of colon adenocarcinoma, and 19.1% of stomach adenocarcinoma.
MSI can be detected through alterations in the length of microsatellite sequences typically due to deletions of repeating units of DNA to create novel allele lengths in tumor-derived DNA when compared to a matched-normal or a reference population. Current methods for MSI testing, using tissue biopsies and resection specimens, include PCR-based amplification followed by capillary electrophoresis, and more recently, next-generation sequencing (NGS) based approaches, which are used to quantify microsatellite allele lengths. The challenge associated with application of the former approach are polymerase induced errors (stutter bands), particularly in samples with low tumor purity, such as cell-free DNA (cfDNA), which can mask true biological alleles exhibiting MSI. In the case of NGS based approaches, sensitivity is typically limited by the accuracy for determination of homopolymer lengths. A novel method was recently described for determination of MSI using pre-PCR elimination of wild-type DNA homopolymers in liquid biopsies. However, given the low prevalence of MSI across cancer, it would be preferable to develop an NGS profiling approach which can include other clinically actionable alterations in cancer, including TMB, sequence mutations, copy number alterations, and translocations.
In addition to the technical challenges associated with MSI detection, it is often not possible to readily obtain biopsy or resection tissue for genetic testing due to insufficient material (biopsy size and tumor cellularity), exhaustion of the limited material available after prior therapeutic stratification, logistical considerations for tumor and normal sample acquisition after initial diagnosis, or safety concerns related to additional tissue biopsy interventions(26). In contrast, plasma-based approaches offer the unique opportunity to obtain a rapid and real-time view of the primary tumor and metastatic lesions along with associated response to therapy. Circulating tumor DNA can be used to monitor and assess residual disease in response to clinical intervention, such as surgery or chemotherapy(27-33), which can directly impact patient care. To determine the clinical impact of identifying tumors that harbor MSI or TMB-High using cfDNA, we developed and applied a 98 kb 58-gene targeted panel to cancer patients with advanced disease treated with PD-1 blockade.
Briefly, cell-free DNA may be extracted from cell line or blood or plasma specimens and prepared into a genomic library suitable for next-generation sequencing with oligonucleotide barcodes through end-repair, A-tailing and adapter ligation. An in-solution hybrid capture, utilizing for example, 120 base-pair (bp) RNA oligonucleotides may be performed.
In one embodiment, at least about 10-100 ng, such as 50 ng of DNA in 100 microliters of TE is fragmented in a sonicator to a size of about 150-450 bp. To remove fragments smaller than 150 bp, DNA may be purified using Agencourt AMPure XP beads (Beckman Coulter, IN) in a ratio of 1.0 to 0.9 of PCR product to beads twice and, e.g., washed using 70% ethanol per the manufacturer's instructions. Purified, fragmented DNA is mixed with H2O, End Repair Reaction Buffer, End Repair Enzyme Mix (cat# E6050, NEB, Ipswich, Mass.). The mixture is incubated then purified using Agencourt AMPure XP beads (Beckman Coulter, IN) in a ratio of 1.0 to 1.25 of PCR product to beads and washed using 70% ethanol per the manufacturer's instructions. To A-tail, end-repaired DNA is mixed with Tailing Reaction Buffer and Klenow (exo-) (cat# E6053, NEB, Ipswich, Mass.). The mixture is incubated at 37 degree C. for 30 min and purified using Agencourt AMPure XP beads (Beckman Coulter, IN) in a ratio of 1.0 to 1.0 of PCR product to beads and washed using 70% ethanol per the manufacturer's instructions. For adaptor ligation, A-tailed DNA is mixed with H2O, PE-adaptor (Illumina), Ligation buffer and Quick T4 DNA ligase (cat# E6056, NEB, Ipswich, Mass.). The ligation mixture was incubated, then amplified.
Exonic or targeted regions were captured in solution using the Agilent SureSelect v.4 kit according to the manufacturer's instructions (Agilent, Santa Clara, Calif.). The captured library was then purified with a Qiagen MinElute column purification kit. To purify PCR products, a NucleoSpin Extract II purification kit (Macherey-Nagel, PA) may be used before sequencing.
Targeted sequencing is performed. Two technical challenges to implementing these approaches in the form of a liquid biopsy include the limited amount of DNA obtained and the low mutant allele frequency associated with the MSI markers. It may be that as few as several thousand genomic equivalents are obtained per milliliter of plasma, and the mutant allele frequency can range from <0.01% to >50% total cfDNA. see Bettegowda, 2014, Detection of circulating tumor DNA in early- and late-stage human malignancies, Sci Trans Med 6(224):224ra24, incorporated by reference. The disclosed techniques overcome such problems and improve test sensitivity, optimized methods for conversion of cell-free DNA into a genomic library, and digital sequencing approaches to improve the specificity of next-generation sequencing approaches.
Methods may include extracting and isolating cell-free DNA from a blood or plasma sample and assigning an exogenous barcode to each fragment to generate a DNA library. The exogenous barcodes are from a limited pool of non-unique barcodes, for example 8 different barcodes. The barcoded fragments are differentiated based on the combination of their exogenous barcode and the information about the reads that results from sequencing such as the sequence of the reads (effectively, an endogenous barcode) or position information (e.g., stop and/or start position) of the read mapped to a reference. The DNA library is redundantly sequenced 115 and the sequences with matching barcodes are reconciled. The reconciled sequences may be aligned to a human genome reference.
The invention recognizes that completely unique barcode sequences are unnecessary. Instead, a combination of predefined set of non-unique sequences together with the endogenous barcodes can provide the same level of sensitivity and specificity that unique barcodes could for biologically relevant DNA amounts and can, in-fact, correct for sequencing artifacts or polymerase slippage. A limited pool of barcodes is more robust than a conventional unique set and easier to create and use. Methods include obtaining a sample comprising nucleic acid fragments, providing a plurality of sets of non-unique barcodes, and tagging 111 the nucleic acid fragments with the barcodes to generate a genomic library, wherein each nucleic acid fragment is tagged with the same barcode as another different nucleic acid fragment in the genomic library.
In embodiments, the plurality of sets is limited to twenty or fewer unique barcodes. In other embodiments, the plurality of sets is limited to ten or fewer unique barcodes.
According to the present invention, a small pool of non-unique exogenous barcodes can be used to provide a robust assay that achieves levels of sensitivity that are comparable to traditional, more complex barcoding schemes, while vastly reducing cost and complication.
After processing steps such as those described above, nucleic acids can be sequenced. Sequencing may be by any method known in the art. DNA sequencing techniques include classic dideoxy sequencing reactions (Sanger method) using labeled terminators or primers and gel separation in slab or capillary, and next generation sequencing methods such as sequencing by synthesis using reversibly terminated labeled nucleotides, pyrosequencing, 454 sequencing, Illumina/Solexa sequencing, allele specific hybridization to a library of labeled oligonucleotide probes, sequencing by synthesis using allele specific hybridization to a library of labeled clones that is followed by ligation, real time monitoring of the incorporation of labeled nucleotides during a polymerization step, polony sequencing, and SOLiD sequencing. Separated molecules may be sequenced by sequential or single extension reactions using polymerases or ligases as well as by single or sequential differential hybridizations with libraries of probes.
A sequencing technique that can be used includes, for example, use of sequencing-by-synthesis systems sold under the trademarks GS JUNIOR, GS FLX+ and 454 SEQUENCING by 454 Life Sciences, a Roche company (Branford, Conn.), and described by Margulies, M. et al., Genome sequencing in micro-fabricated high-density picotiter reactors, Nature, 437:376-380 (2005); U.S. Pat. No. 5,583,024; U.S. Pat. No. 5,674,713; and U.S. Pat. No. 5,700,673, the contents of which are incorporated by reference herein in their entirety.
Other examples of DNA sequencing techniques include SOLiD technology by Applied Biosystems from Life Technologies Corporation (Carlsbad, Calif.) and ion semiconductor sequencing using, for example, a system sold under the trademark ION TORRENT by Ion Torrent by Life Technologies (South San Francisco, Calif.). Ion semiconductor sequencing is described, for example, in Rothberg, et al., An integrated semiconductor device enabling non-optical genome sequencing, Nature 475:348-352 (2011); U.S. Pub. 2010/0304982; U.S. Pub. 2010/0301398; U.S. Pub. 2010/0300895; U.S. Pub. 2010/0300559; and U.S. Pub. 2009/0026082, the contents of each of which are incorporated by reference in their entirety.
Another example of a sequencing technology that can be used is Illumina sequencing. Illumina sequencing is based on the amplification of DNA on a solid surface using fold-back PCR and anchored primers. Adapters are added to the 5′ and 3′ ends of DNA that is either naturally or experimentally fragmented. DNA fragments that are attached to the surface of flow cell channels are extended and bridge amplified. The fragments become double stranded, and the double stranded molecules are denatured. Multiple cycles of the solid-phase amplification followed by denaturation can create several million clusters of approximately 1,000 copies of single-stranded DNA molecules of the same template in each channel of the flow cell. Primers, DNA polymerase and four fluorophore-labeled, reversibly terminating nucleotides are used to perform sequential sequencing. After nucleotide incorporation, a laser is used to excite the fluorophores, and an image is captured and the identity of the first base is recorded. The 3′ terminators and fluorophores from each incorporated base are removed and the incorporation, detection and identification steps are repeated. Sequencing according to this technology is described in U.S. Pat. No. 7,960,120; U.S. Pat. No. 7,835,871; U.S. Pat. No. 7,232,656; U.S. Pat. No. 7,598,035; U.S. Pat. No. 6,911,345; U.S. Pat. No. 6,833,246; U.S. Pat. No. 6,828,100; U.S. Pat. No. 6,306,597; U.S. Pat. No. 6,210,891; U.S. Pub. 2011/0009278; U.S. Pub. 2007/0114362; U.S. Pub. 2006/0292611; and U.S. Pub. 2006/0024681, each of which are incorporated by reference in their entirety.
Preferably sequencing is done redundantly for deep coverage, preferably at least 30× coverage or 100×. DNA libraries may be sequenced using paired-end 111umina HiSeq 2500 sequencing chemistry to an average target total coverage of either >20,000-fold or >5,000-fold coverage for each targeted base. Sequence data may be mapped to the reference human genome. Preferably, the sequencing is next-generation, short-read sequencing. The obtained sequences may include a plurality of sequence reads and the method may include aligning the sequence reads to a reference, and identifying groups of sequence reads that originated from a unique segment of the cfDNA by means of the barcode information and position or content of the sequence reads. Primary processing of sequence data may be performed using Illumina CASAVA software (v1.8), including masking of adapter sequences. Sequence reads may bealigned against the human reference genome (version hg18) using ELAND with additional realignment of select regions using the Needleman-Wunsch method.
In some embodiments, the barcodes are non-unique barcodes that include duplicates such that different ones of the fragments are attached to identical barcodes. The high clinical efficacy of MSI status now requires a fast, objective, highly sensitive screening method, particularly in late-stage patients where tumor material may not be readily obtained. However, to extend this approach to a liquid biopsy panel requires technological advances to both overcome the inherent challenges associated with low circulating tumor DNA (ctDNA) levels which is compounded by polymerase slippage in mononucleotide repeat regions during PCR amplification as well as other sequencing artifacts.
To overcome these limitations, we applied error correction approach using molecular barcoding together with high sequencing depth and a novel peak finding algorithm to more accurately identify the specific mononucleotide sequences in cell-free DNA (cfDNA) analyses of a 64 gene panel, by way of illustration. The MSI markers can be sequenced in conjunction with such 64 gene panel, or in isolation (e.g,. just sequence the markers) or in conjunction with any other gene panel (e.g., >300 genes) or with whole genome or whole exome sequencing.
The method may include amplifying the fragments to produce amplicons that include barcode information and copies of the fragments, wherein the sequencing step comprises sequencing the amplicons.
The use of the non-unique barcodes to identify groups of sequence reads that originated from a unique segment of the cfDNA allows for the lengths of the plurality of tracts to be determined correctly by correcting for errors introduced by sequencing artifacts or polymerase slippage during the amplifying step. By eliminating a significant majority of sequencing errors and polymerase slippage artifacts, we were able to reduce background error rates by >90%. Combined with implementation of a distribution modeling and a peak finding algorithm, we were able to accurately sequence the mononucleotide tracts to minimize false discovery rates for cfDNA analyses.
MSI may be assayed by hybrid capture and NGS to address such markers as mononucleotide repeat markers such as BAT25, BAT26, MON027, NR21, and NR24. See U.S. Pub. 2017/0267760, incorporated by reference. Knowledge of MSI status is important and valuable in the treatment of many cancers, and there are patients for whom tumor material is not readily obtained. Tumors deficient in mismatch repair are particularly susceptible to a particular form of immunotherapy because this phenotype results in ongoing accumulation of mutations at a high frequency. Methods may include recommending or administering treatment for cancer patients that display the microsatellite instability phenotype or other high mutational burden. The treatment involves an inhibitory antibody for an immune checkpoint. Such checkpoints include PD-1, IDO, CTLA-4, PD-L1, and LAG-3 by way of example. Other immune checkpoints can be used as well. Antibodies can be administered by any means that is convenient, including but not limited to intravenous infusion, oral administration, subcutaneous administration, sublingual administration, ocular administration, nasal administration, and the like.
Preferably, the method 101 includes providing 125 a report with MSI status.
In some embodiments, the method includes recommending a treatment for the patient based on the MSI status. Where the MSI status indicates that the patient is microsatellite instable, the treatment may include an immune checkpoint inhibitor. In certain embodiments, the method includes administering the treatment (e.g., the immune checkpoint inhibitor) to the patient. The immune checkpoint inhibitor may be, for example, an antibody such as an anti-PD-1 antibody; an anti-IDO antibody; anti-CTLA-4 antibody; an anti-PD-L1 antibody; or an anti-LAG-3 antibody. Types of antibodies which can be used include any that are developed for the immune checkpoint inhibitors. These can be monoclonal or polyclonal. They may be single chain fragments or other fragments of full antibodies, including those made by enzymatic cleavage or recombinant DNA techniques. They may be of any isotype, including but not limited to IgG, IgM, IgE. The antibodies may be of any species source, including human, goat, rabbit, mouse, cow, chimpanzee. The antibodies may be humanized or chimeric. The antibodies may be conjugated or engineered to be attached to another moiety, whether a therapeutic molecule or a tracer molecule. The therapeutic molecule may be a toxin, for example. The present invention is more particularly described in the following examples which are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. The following examples are intended to illustrate but not limit the invention.
Patients and Sample Collection
Formalin fixed paraffin embedded (FFPE) tumor and matched normal buffy coat specimens (n=61) from individuals with cancer were obtained after surgical resection through commercial biorepositories from BioIVT (Hicksville, N.Y., USA), Indivumed (Hamburg, Germany), and iSpecimen (Lexington, Mass., USA). Plasma samples from healthy individuals (n=163) were procured through BioIVT (Hicksville, N.Y., USA) during routine screening with negative results and no prior history of cancer. Human cells from previously characterized MSI cell lines were obtained from ATCC (Manassas, Va., USA) (n=5; LS180, LS411N, SNU-C2B, RKO, and SNU-C2A). Finally, baseline and serial plasma samples from cancer patients with progressive metastatic carcinoma (n=16; 11 colorectal, 3 ampullary, and 2 small intestine) were obtained while patients were enrolled in a phase 2 clinical trial to evaluate immune checkpoint blockade with pembrolizumab(1,2). Radiographic and serum protein biomarker data for CEA and CA19-9 were collected as a part of routine clinical care. All samples were obtained under Institutional Review Board approved protocols with informed consent for research.
Orthogonal Testing of FFPE Tissue for MSI Status
The Promega MSI analysis system (Madison, Wis., USA) was used to assess MSI status in DNA derived from FFPE tumor tissue together with matched normal buffy coat by multiplex PCR and fluorescent capillary electrophoresis. Tumors were classified as MSI if two or more of the five mononucleotide markers (BAT25, BAT26, MONO27, NR21, and NR24) had significant length differences compared to the matched normal allele lengths. Additionally, 2-pentanucleotide repeat loci (PentaC and PentaD) were used to confirm case identity between normal and tumor samples.
Sample Preparation and Next-Generation Sequencing
FFPE Tumor and Normal Analyses
Sample processing from tissue or buffy coat, library preparation, hybrid capture, and sequencing were performed as previously described at Personal Genome Diagnostics (Baltimore, Md.)(34,36). Briefly, DNA was extracted from FFPE tissue and matched normal buffy coat cells using the Qiagen FFPE Tissue Kit and DNA Blood Mini Kit, respectively (Qiagen, Hilden, Germany). Genomic DNA was sheared using a Covaris sonicator (Woburn, Mass., USA) to a size range of 150-450 bp, and subsequently used to generate a genomic library using the New England Biolabs (Ipswich, Mass., USA) end-repair, A-tailing, and adapter ligation modules. Finally, genomic libraries were amplified and captured using the Agilent SureSelect XT in-solution hybrid capture system with a custom 120 bp RNA panel targeting the pre-defined regions of interest across 125 genes (Table 1). Captured libraries were sequenced on the Illumina HiSeq 2000 or 2500 (Illumina, San Diego, Calif., USA) with 100 bp paired end reads.
Plasma Analyses
Sample processing from plasma, library preparation, hybrid capture, and sequencing were performed as previously described at Personal Genome Diagnostics (Baltimore, Md.)(34). Briefly, blood was collected in EDTA tubes and centrifuged at 800 g for 10 minutes at 4° C. to separate plasma from white blood cells. Cell-free DNA was extracted from plasma using the QIAamp Circulating Nucleic Acid Kit (Qiagen, Hilden, Germany). Libraries were prepared with 5-250 ng of cfDNA using the NEBNext DNA Library Prep Kit (New England Biolabs, Ipswich, Mass., USA). After end repair and a-tailing, a pool of eight unique Illumina dual index adapters with 8 bp barcodes were ligated to cfDNA to allow for accurate error correction of duplicate reads, followed by 12 cycles of amplification. Targeted hybrid capture was performed using Agilent SureSelect XT in-solution hybrid capture system with a custom 120 bp RNA panel targeting the pre-defined regions of interest across 58 genes (Table 4) according to the manufacturer protocol (Agilent Technologies, Santa Clara, Calif., USA). Captured libraries were sequenced on the Illumina HiSeq 2000 or 2500 (Illumina, San Diego, Calif., USA) with 100 bp paired end reads.
Sequence data were aligned to the human reference genome assembly (hg19) using BWA-MEM(37). Reads mapping to microsatellites were excised using Samtools(38) and analyzed for insertion and deletion events (indels). In most cases, alignment and variant calling did not generate accurate indel calls in repeated regions due to low quality bases surrounding the microsatellites. Therefore, a secondary local realignment and indel quantitation was performed. Reads were considered for an expanded indel analysis if (i) the mononucleotide repeat was contained to more than eight bases inside of the start and end of the read, (ii) the indel length was ≤12 bases from the reference length, (iii) there were no single base changes found within the repeat region, (iv) the read had a mapping score of 60, and (v) ≤20 bases of the read were soft clipped for alignment. After read specific mononucleotide length analysis, error correction was performed to allow for an aggregated and accurate quantitation among duplicated fragments using molecular barcoding. Reads were aggregated into barcode families by using the ordered and combined read 1 and read 2 alignment positions with the molecular barcode. Barcode families were considered for downstream analysis if they comprised of at least 2 reads and >50% of reads had consistent mononucleotide lengths. The error corrected mononucleotide length distribution was subjected to a peak finding algorithm where local maxima were required to be greater than the error corrected distinct fragment counts of the adjacent lengths ±2 bp. Identified peaks were further filtered to only include those which had >3 error corrected distinct fragments at ≥1% of the absolute coverage. The shortest identified mononucleotide allele length was compared to the hg19 reference length. If the allele length was ≥3 bp shorter than the reference length, the given mononucleotide loci was classified as exhibiting instability. This approach was applied across all mononucleotide loci. Samples were classified as MSI-H if ≥20% of loci were MSI. In the targeted 58 gene plasma panel, BAT25, BAT26, MONO27, NR21, and NR24 mononucleotide loci were for the determination of MSI status. In the targeted 125 gene targeted tissue panel, an additional 65 microsatellite regions were used for MSI classification.
Next generation sequencing data were processed and variants were identified using the VariantDx custom software as previously described(34). A final set of candidate somatic mutations were selected for tumor mutational burden analyses based on: (i) variants enriched due to sequencing or alignment error were removed (≤5 observations or <0.30% mutant allele fraction), (ii) nonsynonymous and synonymous variants were included, but variants arising in non-coding regions were removed, (iii) hotspot variants annotated in COSMIC (version 72) were not included to reduce bias toward driver alterations, (iv) common germline SNPs found in dbSNP (version 138) were removed as well as variants deemed private germline variants based on the variant allele frequency, and (v) variants associated with clonal hematopoietic expansion were not included in the candidate variant set(39).
In Silico TCGA Analyses
In order to evaluate the accuracy of the 98 kb targeted panel for prediction of TMB, a comparison to whole-exome sequencing data derived from The Cancer Genome Atlas (TCGA)(35) was performed by considering synonymous and nonsynonymous alterations, excluding known hotspot mutations which may not be representative of TMB in the tumor. The cutoff for consideration as TMB-High was set to 5 candidate variants (50.8 mutations/Mbp sequenced) based on in silico analyses utilizing the TCGA data to achieve >95% accuracy (>36 mutations/Mbp).
Statistical Analyses
Due to small sample size, Firth's Penalized Likelihood was used to evaluate significant differences between Kaplan-Meier curves for progression free survival and overall survival with the classifiers baseline MSI status, baseline TMB status, two consecutive timepoints with >80% reduction in baseline protein biomarker levels, two consecutive timepoints with 0% residual MSI alleles on treatment, and two consecutive timepoints with >90% reduction in baseline TMB levels. Pearson correlations were used to evaluate significant association between TMB in the 58 gene targeted panel compared to whole-exome analyses, progression free and overall survival compared to residual protein biomarker levels, and progression free and overall survival compared to residual MSI and TMB allele levels. A student t-test was used to evaluate significant differences between the mean TMB level in TMB-High and TMB-Low patients. Response rate was calculated as the number of patients exhibiting a complete or partial response as a proportion of the total patients considered, and then evaluated using a Fisher's exact test.
To identify MSI in tumor-derived cfDNA, a method to detect length polymorphisms in mononucleotide tract alleles in circulating tumor DNA (ctDNA), which occur at low frequency in plasma, is needed. To overcome this issue, we developed a highly sensitive error-correction approach incorporating the commonly-used mononucleotide tracts BAT25, BAT26, MONO27, NR21, and NR24 for the determination of MSI status in tissue and plasma specimens using NGS. DNA was converted into an NGS compatible library using molecular barcoding, after which these targeted microsatellite loci were enriched using in-solution hybrid capture chemistry together with the regions associated with other clinically relevant genomic alterations.
To address the technical challenges associated with detection of low level allele length polymorphisms obtained from NGS, we combined an error correction approach for accurate determination of insertions and deletions (indels) present in the cfDNA fragments, together with a digital peak finding (DPF) method for quantification of MSI and MSS alleles. Redundant sequencing of each cfDNA fragment was performed, and reads were aligned to the five microsatellite loci contained in the human reference genome (hg19). cfDNA sequences were then analyzed for indels through a secondary local alignment at these five microsatellite loci to more accurately determine the indel length. To perform the error correction, duplicated reads associated with each cfDNA molecule were consolidated, only recognizing indels present throughout barcoded DNA fragment replicates obtained through redundant sequencing. Finally, the DPF approach was applied across the error corrected distribution of indels to identify high confidence alleles which exhibit microsatellite instability (
To demonstrate the capability of this approach, we first evaluated the performance of the method for detection of MSI in formalin fixed, paraffin embedded (FFPE) tumor tissue specimens obtained from 31 MSI-High (MSI-H) and 30 microsatellite stable (MSS) tumors previously characterized with the PCR-based Promega MSI analysis system. In addition to these five mononucleotide markers, we sequenced 125 selected cancer genes which harbor clinically actionable genetic alterations consisting of sequence mutations (single base substitutions and indels), copy number alterations, and gene rearrangements in cancer (Table 1). Analyses of these 61 colorectal tumors yielded 193 Gb of total sequence data, corresponding to 832-fold distinct coverage on average across the 979 kb panel (Table 2). Analysis of these five mononucleotide loci, together with 65 additional microsatellite regions contained within the 125 gene panel resulted in 100% sensitivity (31/31) and 100% specificity (30/30) for determination of MSI status using the patient-matched tumor and normal samples (Table 3) Similarly, analysis of tumor NGS data using the DPF approach without the patient-matched normal sample yielded 100% concordance (61/61).
Next, we evaluated the signal-to-noise ratio in homopolymer regions from next-generation sequencing data obtained using cfDNA extracted from plasma. Together with the five mononucleotide loci, we developed a 98 kb, 58 gene panel for sequence mutation (single base substitutions and indels) analyses of clinically actionable genetic alterations in cancer (Table 4). To demonstrate the specificity of this approach for direct detection of MSI, we first obtained plasma from healthy donors (n=163), all of which would be expected to be tumor-free and MSS. These analyses yielded over 1.2 Tb of total sequence data, corresponding to 2,600-fold distinct coverage on average across the 98 kb targeted panel, and resulted in a per-patient specificity of 99.4% (162/163) for determination of MSI status (
Because ctDNA, even in patients with advanced cancer, may be present at mutant allele fractions (MAFs) less than 5%, we characterized the ability of DPF for sensitive and reproducible detection of MSI at low MAFs. Five previously characterized MSI cell line samples obtained from ATCC (LS180, LS411N, SNU-C2B, RKO, and SNU-C2A) were sheared to a fragment profile simulating cfDNA and diluted with normal DNA to yield a total of 25 ng evaluated at 1% MAF. Additionally, three of these cell lines (LS180, LS411N, and SNU-C2B) were evaluated at 1% MAF in triplicate within, and triplicate across library preparation and sequencing runs (Table 5). Based on the MAF observed in the parental cell line, the cases detected as MSI were computationally confirmed to contain MSI allele MAFs of 0.35%-1.87%, with a median MSI allele MAF of 0.92%. In total, MSI was detected in 90% (18/20) of samples and demonstrated 93.3% (14/15) repeatability and reproducibility within and across runs (Table 6). For one case which was not detected as MSI, one MSI allele was identified at 0.33% MAF and for the other case, no MSI alleles were detected.
To evaluate the analytical and clinical performance of this approach for determination of MSI in cfDNA from patients with late-stage cancers, we obtained baseline and serial plasma from patients with metastatic cancers (including 11 colorectal, 3 ampullary, and 2 small intestine), with or without MMR deficiency, while enrolled in a clinical trial to evaluate immune checkpoint blockade with the PD-1 blocking antibody, pembrolizumab(1,2)(Table 7). In total, 12 MSI-H cases and 4 MSS cases, determined through archival tissue-based analyses, were evaluated across at least two timepoints, including baseline, and after approximately 2 weeks, 10 weeks, 20 weeks, and >100 weeks.
Patients with MSI tumors as determined by archival tissue analyses had improved progression-free survival (hazard ratio, 0.25; p=0.05, likelihood ratio test) and overall survival (hazard ratio, 0.24; p=0.041, likelihood ratio test) (
We then evaluated pre-treatment MSI status in ctDNA to predict response and clinical outcome to treatment with PD-1 blockade. We assessed radiographic response, progression-free and overall survival to predict clinical outcome. When compared to progression free survival, direct detection of MSI in baseline cfDNA could be used to predict response to immune checkpoint blockade (hazard ratio, 0.2; p=0.01, likelihood ratio test) (
Estimating Tumor Mutation Burden in ctDNA
In addition to MSI status, we also evaluated the ability of our cfDNA panel to predict TMB across a range of tumor types, using whole exome sequencing data derived from The Cancer Genome Atlas (TCGA)(35). We considered synonymous and nonsynonymous alterations identified by TCGA and excluded known hotspot mutations which may not be representative of TMB in the tumor. These analyses demonstrated a positive correlation between predicted TMB from our targeted 58 gene plasma panel compared to the TCGA whole exome analyses (r=0.91, p<0.0001; Pearson correlation) (
Patients with TMB-High tumors as determined by archival tissue analyses (≥10 mutations/Mbp) had improved progression-free survival (hazard ratio, 0.19; p=0.041, likelihood ratio test) and overall survival (hazard ratio, 0.18; p=0.047, likelihood ratio test) (
In addition to baseline plasma analyses, we also hypothesized that the molecular remission, as measured by ctDNA during treatment, would be predictive of long term durable response to immune checkpoint blockade. We first evaluated the utility of monitoring serum tumor protein biomarkers CEA or CA19-9 for determination of response and found that multiple consecutive timepoints with a >80% reduction in the baseline protein biomarker level resulted in improved overall and progression free survival (hazard ratio, 0.05; p=0.01 and hazard ratio, 0.05; p=0.01, likelihood ratio test, respectively) (
Additionally, for three patients (CS97, CS98, and CS00) with a complete response to immune checkpoint blockade, and one patient (CS05) without a response to immune checkpoint blockade, circulating protein biomarkers (CEA, ng/mL or CA19-9, units/mL) and residual alleles exhibiting MSI and TMB were evaluated over time during treatment (
Patient CS97 demonstrated a partial radiographic response at 10.6 months, however, achieved a 100% reduction in residual MSI and TMB levels at 2.8 months. CS97 then went on to a complete radiographic response at 20.2 months (Table 7). A different patient, CS98, appeared to develop new liver lesions at 20 weeks suggestive of progressive disease (
The checkpoint inhibitor pembrolizumab is now indicated for the treatment of adult and pediatric patients with unresectable or metastatic solid tumors identified as having MSI or MMR deficiency(1,2). This represents the first pan-cancer biomarker indication, and now covers patients with solid tumors that have progressed following prior treatment and have no satisfactory alternative treatment options, as well as patients with colorectal cancer that have progressed following treatment with certain chemotherapy drugs. However, it is often not possible to readily obtain biopsy or resection tissue for genetic testing due to insufficient material, exhaustion of the limited material available after prior therapeutic stratification, logistical considerations for tumor and normal sample acquisition after initial diagnosis, or safety concerns related to additional tissue biopsy interventions(26).
We have described the development of a method for simultaneous detection of MSI and TMB-High directly from cfDNA and demonstrated proof of concept for the clinical utility afforded through these analyses for the prediction of response to immune checkpoint blockade. Additionally, given the concordance with circulating protein biomarker data while these patients were on treatment, these data suggest that the residual MSI allele burden and TMB prognostic signature could be applied to other tumor types where standardized protein biomarkers do not exist and may be an earlier predictor of response than radiographic imaging.
These methods described herein provide feasibility for a viable diagnostic approach for screening and monitoring of patients who exhibit MSI or TMB-High and may respond to immune checkpoint blockade.
Any and all references and citations to other documents, such as patents, patent applications, patent publications, journals, books, papers, web contents, that have been made throughout this disclosure are hereby incorporated herein by reference in their entirety for all purposes.
Although the present invention has been described with reference to specific details of certain embodiments thereof in the above examples, it will be understood that modifications and variations are encompassed within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims.
This application claims benefit of priority under 35 U.S.C. § 119(e) of U.S. Ser. No. 62/593,664 filed Dec. 1, 2017, and of U.S. Ser. No. 62/741,448 filed Oct. 4, 2018, the entire contents of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62593664 | Dec 2017 | US | |
| 62741448 | Oct 2018 | US |
