Patent Application
20210222155
The Sequence Listing submitted as a text file named “YU_6997_PCT_ST25.txt,” created on Apr. 27, 2017, and having a size of 50,495 bytes is hereby incorporated by reference pursuant to 37 C.F.R. § 1.52(e)(5).
This application is generally in the field of gene modification, and more specifically methods of modifying a genome without breaking the backbone of the target DNA.
Most eukaryotic genome editing technologies—zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR-associated endonuclease Cas9 (CRISPR-Cas9)—generate DNA double-strand breaks (DSBs) at targeted loci to introduce genomic modifications (Doudna and Charpentier, 2014; Gaj et al., 2013). Although ZFNs and TALENs recognize specific DNA sequences through protein-DNA interactions and use the FokI nuclease domain to introduce DSBs at genomic loci, construction of functional ZFNs and TALENs with desired DNA specificity remains laborious, costly, and primarily limited to modifications at a single genetic locus. CRISPR-Cas9 has been broadly adopted for multiplexed targeting of genomic modifications because the CRISPR nuclease Cas9 uses a short guide RNA (gRNA) to recognize the target DNA via Watson-Crick base-pairing and has been shown to function in many organisms (Cong, L., et al., Science, 339:819-823 (2013); Jinek, et al., Elife 2, e00471 (2013); Mali, et al., Science, 339:823-826 (2013)). In this regard, CRISPR-Cas9 is highly suited for gene disruption applications by non-homologous end joining (NHEJ) (Yang, et al., Science, 350:1101-1104 (2015)), and gene editing at one or more sites with homology directed repair (HDR) (Doudna, et al., Science, 346:1258096 (2014)).
For applications that require multisite editing or precise base-pair level genome modifications by HDR, the DSB mechanism is limiting for three key reasons. First, cleaving the genome is cytotoxic, and cell lethality is magnified when DSBs are introduced across multiple target sites (Jakociunas, et al., Metab Eng, 28:213-222 (2015)). Second, in eukaryotes most single base-pair HDR changes introduced by DSB repair are subject to additional unwanted insertions or deletions (indels) resulting from NHEJ (Inui, et al., Sci Rep, 4:5396 (2014)). These additional mutations result from high tolerance of the targeted nuclease (i.e., Cas9:gRNA) to mismatches in the DSB target which can lead to additional cleavage even after HDR editing has occurred (Fu, et al., Nature Biotechnology, 31:822-826 (2013)). For single base-pair HDR, the inclusion of blocking mutations in the donor DNA is typically required to mask the genomic target site from further cutting by the Cas9:Grna (Horwitz, et al., Cell Syst, 1:88-96 (2015); Paquet, et al., Nature, 533:125-129 (2016)). For many types of genetic elements (e.g., promoters, ncRNAs), the exact DNA sequence dictates function such that additional blocking mutations are often prohibitive.
Despite the highly improved stringencies of engineered Cas9 variants, mismatches are still tolerated for many nonstandard target sites with repetitive regions (Tsai, et al., Nat Rev Genet, 17:300-312 (2016)). Third, the inefficiency of generating targeted single base-pair edits with DSBs limits the ability to simultaneously modify many loci in a single cell or across a population to produce combinatorial genetic diversity for exploration of vast genomic landscapes. Efficient DNA base editing without a DSB has been reported using Cas9-guided deamination, but this technique is limited to specific C→T or G→A mutations in an imprecise window of several base-pairs (Komor, et al., Nature (2016)). Thus, creating precise edits at single base-pair resolution in a single-step at any genomic sequence remains a defining challenge for eukaryotic genome engineering technologies.
For the application of creating targeted genomic diversity with high DNA sequence resolution, ablation of the target-site sequence is often not feasible without perturbing surrounding sequence context. Therefore developing high efficiency genome mutagenic methods that do not rely on breaking the DNA backbone could serve as a powerful alternative to generate multi-site modifications of the genome. Such a tool would be useful for modification of genetic elements where precise sequence context is important such as promoters, open reading frames with strong codon biases, terminator elements, splice sites, enhancers, and other types of global regulatory elements. Furthermore, precise editing of eukaryotic genomes across many positions at once is a challenge that if solved would allow for exploration of genomic sequence space at time-scales feasible for laboratory studies.
Prior work in Escherichia coli demonstrated that targeted chromosomal modifications could be introduced without DSBs using synthetic ssDNA oligodeoxynucleotides (ssODNs) complementary to the lagging strand of the replicating chromosome at high efficiencies (>10%) (Costantino, et al., Proceedings of the National Academy of Sciences of the United States of America, 100:15748-15753 (2003); Li, et al., Nucleic Acids Research, 31:6674-6687 (2003)). With the advent of multiplex automated genome engineering (MAGE), this approach was enhanced to generate multi-site gene modifications with base-pair precision at increased efficiencies (>30%) and used for pathway diversification (Wang, et al., Nature, 460:894-898 (2009)), whole genomic recoding (Lajoie, et al., Science, 342:357-360 (2013)), and molecular evolution of proteins (Amiram, et al., Nature Biotechnology, 33:1272-1279 (2015)). Although homologous recombination (HR) of ssODNs was developed ˜30 years ago in S. cerevisiae (Moerschell, et al., Methods in Enzymology, 194:362-369 (1991); Moerschell, et al., J Biol Chem, 265:19638-19643 (1990); Moerschell, et al., Proceedings of the National Academy of Sciences of the United States of America, 85:524-528 (1988); Yamamoto, et al., Yeast, 8:935-948 (1992a); Yamamoto, et al., Genetics, 131:811-819 (1992b)), low gene targeting efficiencies (˜0.0001-0.001%) limited the scope of applications to single locus modifications requiring counter-selectable markers (Storici, et al., Genet Eng (NY), 25:189-207 (2003)). The mechanism of ssODN incorporation in eukaryotic cells is less precisely defined than in E. coli and likely involves several parameters, which include direction of DNA replication (Rodriguez, et al., PloS One, 7:e42905 (2012); Yamamoto, et al., Genetics, 131:811-819 (1992b)), cell-cycle phase (Engstrom, et al., BMC Mol Biol, 8:9 (2007)), transcription (Brachman, et al., J Cell Sci, 117:3867-3874 (2004)), DNA mismatch repair (MMR) (Kow, et al., Proceedings of the National Academy of Sciences of the United States of America, 104:11352-11357 (2007)), and HR (Liu, et al., Nucleic Acids Research, 32:2093-2101 (2004)). Efforts to develop an analogous MAGE technology in S. cerevisiae have focused on overexpression of HR factors Rad51 and Rad54 in MMR deficient strains and resulted in moderately enhanced allelic replacement frequencies (ARF) (˜0.1-2%) (DiCarlo, et al., ACS Synth Biol, 2:741-749 (2013); Liu, et al., Nucleic Acids Research, 32:2093-2101 (2004)). Thus, no clear method has been established in eukaryotes for precise, multisite genome modification with ssODNs that approaches efficiencies attained in E. coli (>30%).
Thus, it is an object of the invention to provide compositions and methods for genome modification without breaking the backbone of the target DNA.
It is a further object of the invention to provide compositions and methods of genome modification suitable for large-scale genomic modifications.
It is also an object of the invention provide compositions and methods for genome modification that are suitable for use in eukaryotes.
Compositions and methods for gene editing are provided. The methods employ an oligo-based annealing mechanism that is rooted in the process of DNA replication rather than homologous recombination (HR). Oligo incorporation efficiencies are comparable and often exceed those of CRISPR/cas9 editing without the need for double strand breaks (DSBs). By relying on the multiplex annealing of oligos rather than DSBs the process is highly scalable across a genomic region of interest and can generate many scar-less modifications of a chromosome simultaneously. Combinatorial genomic diversity can be generated across a population of cells in a single transformation event; genomic landscapes can be traversed through successive iterations of the process, and genome-wide changes can be massively parallelized and amplified through systematic strain mating.
The Examples below exemplify the strategy by highly multiplexed combinatorial genome engineering of the model eukaryote and industrial chassis organism S. cerevisiae. After showing that the method can incorporate single oligos at high efficiencies, an unprecedented ability to incorporate multiple oligos at once was demonstrated, and a cycling protocol to iteratively introduce oligonucleotides for generating genetic diversity at genetic regions of interest was developed. Many sites can be targeted simultaneously on a chromosome. In the experiments described below, cells with up 42 designed mutations in a single chromosome after only 3 cycles. Many of the specific types of mutations designed and observed were extremely high resolution mutations (single base-pair changes) that could not have been made using the CRISPR/cas9 system without the introduction of additional sequences to destroy the target sites. Thus, the method provides highly precise editing at many sites without introducing unwanted additional scar sequences.
The methods induce genome modification by annealing oligos at the replication fork in which synthetic single stranded DNA (ssDNA) oligonucleotides are incorporated into the genome, by a method/process developed for achieving 100-fold higher efficiencies than the current state of the art of oligonucleotide-based genome editing technologies in eukaryotes, and can be carried out without DSBs. Mutation efficacy can be increased by reducing RAD51 expression to drive the mechanism away from strand invasion/homologous recombination, deletion/impairment of DNA repair proteins such as MSH2, or a combination thereof
For example, a method for preparing a library of mutant eukaryotic cells can include (i) transfecting or transforming a population of host cells with (a) an oligonucleotide that can introduce one or more mutations into a selectable marker when incorporated into a cell's genome by replication fork annealing and (b) one or more oligonucleotides that can introduce one or more mutations into a target region when incorporated into a cell's genome by replication fork annealing; and (ii) selecting mutant cells that have a mutation in the selectable marker.
The target region and the selectable marker can be separated by, for example, 4, 3, 2, 1, or 0 origins of replication. The target region can range from, for example, 1 base pair to 1 million, 2 million, 5 million, 10 million, 100 million, or more base pairs in either direction from the origin of replication closest to the selectable marker.
The selectable marker can be a counter-selectable marker (e.g., URA3) and mutant cells can be selected by culturing the transfected cells in presences of a compound that kills cells expressing the un-mutated selectable marker (e.g., 5FOA). The one or more oligonucleotides of step (b) can introduce mutations into a gene regulatory region, an open reading frame, an intron, or a combination thereof. Any of the mutations can be insertions, deletions, substitutions, or a combination thereof. Any of the oligonucleotides can be about 30 to about 120 nucleotides in length. The oligonucleotides of (b) can a pool of oligonucleotides each having 1 or more mutations.
The method can include a step (iii) of selecting mutant cells that have a mutation in the target region. The selection of mutant cells with a mutation in the target region can include phenotypic or genotypic screening.
The host cells can be any eukaryotic cells, for example, yeast cells, fungal cells, mammalian cells, or plant cells. The host cells can be deleted or otherwise treated or altered to reduce expression of RAD51 or a homolog thereof, alone or in combination with RAD52. The host cells can be deleted or otherwise treated or altered to reduce expression of one or more DNA mismatch repair enzymes. The DNA mismatch repair enzyme can be selected from the group consisting of MSH2, MSH6, MLH1, PMS1, homologs thereof, and combinations thereof. In some embodiments, expression of RAD59 or another ssDNA protein or recombinase (e.g., Beta recombinase), or a homolog thereof is increased in the host cells.
Host cells can also include a selectable marker adjacent to an origin of replication.
The method can include treating host cells prior to and/or during step (i) to reduce replication fork speed. For example, the cells can be treated with hydroxyurea.
In some embodiments, steps (i) and (ii) are repeated for two or more cycles using the same or different oligonucleotides for steps (a) and (b). The target region can include a biosynthetic pathway having two or more genes and oligonucleotides of step (b) can target two or more of the genes in the biosynthetic pathway.
Libraries of mutant cells prepared according to any the disclosed methods, and well as single mutant cells and clonal colonies thereof, are also provided.
The sequences in
As used herein, the term “eukaryote” or “eukaryotic” refers to organisms or cells or tissues derived from these organisms belonging to the phylogenetic domain Eukarya such as animals (e.g., mammals, insects, reptiles, and birds), ciliates, plants (e.g., monocots, dicots, and algae), fungi, yeasts, flagellates, microsporidia, and protists.
As used herein, the terms “oligonucleotide,” “nucleic acid oligomers,” and “polynucleotide” refers to a natural or synthetic molecule including two or more linked nucleotides.
As used herein, the term “gene” refers to a DNA sequence that encodes through its template or messenger RNA a sequence of amino acids characteristic of a specific peptide, polypeptide, or protein. The term “gene” also refers to a DNA sequence that encodes an RNA product. The term gene as used herein with reference to genomic DNA includes intervening, non-coding regions as well as regulatory regions and can include 5′ and 3′ ends.
As used herein, the term “vector” refers to a polynucleotide capable of transporting into a cell another polynucleotide to which the vector sequence has been linked. The term “expression vector” includes any vector, (e.g., a plasmid, cosmid or phage chromosome) containing a gene construct in a form suitable for expression by a cell (e.g., linked to a transcriptional control element). “Plasmid” and “vector” are used interchangeably, as a plasmid is a commonly used form of vector.
As used herein, the terms “transformation” and “transfection” refer to the introduction of a polynucleotide, e.g., an expression vector, or single stranded oligonucleotide into a recipient cell including introduction of a polynucleotide to the chromosomal DNA of the cell.
As used herein, the term “isolated” is meant to describe a compound of interest (e.g., nucleic acids) that is in an environment different from that in which the compound naturally occurs, e.g., separated from its natural milieu such as by concentrating a peptide to a concentration at which it is not found in nature. “Isolated” is meant to include compounds that are within samples that are substantially enriched for the compound of interest and/or in which the compound of interest is partially or substantially purified. Isolated nucleic acids are at least 60% free, preferably 75% free, and most preferably 90% free from other associated components.
As used herein, the terms “host,” “parent,” “parental,” “progenitor,” and “background,” when used to describe a cell, population of cells, or strain of cells, refer to the cell type or strain type that is subjected to the disclosed methods of editing eukaryotic genomes.
RAD51 dependent methods of gene editing exhibit low efficiency. RAD51-independent methods of gene editing are provided. In the disclosed methods, eukaryotic genome editing is carried out by targeting modification at the replication fork. Unlike most prokaryotes, eukaryotic genomes are typically arranged in linear chromosomes with multiple origins of replication that are known to fire stochastically throughout S-phase. Targeting synthetic oligonucleotides to the DNA replication fork has been a major technical challenge compared to prokaryotes.
At any given time a subset of cells in a given population have a replication fork active at the genomic site of interest. Thus methods for selecting these cells from the population to enrich for cells that can be highly edited by synthetic oligonucleotides using a replication fork annealing mechanism are provided.
The disclosed methods typically include one or more rounds of replication fork annealing-induced genome modification, followed by selection for cells likely to carry a modification in the target region. Typically at least one oligonucleotide is designed to mutate a selectable marker and at least one oligonucleotide is designed to mutate a target region near, preferably downstream of, the marker on the same chromosome. Cells are selected based on mutation of the selectable marker and can be screened for desirable mutations in the target region. The method can be harnessed to induce single or multiple mutations into one or more target regions during each round of transformation and selection.
A. Selectable Markers
The disclosed methods of genome editing are typically carried out in cells including a selectable marker. To improve selection of genetically modified mutants the marker should be adjacent, preferably directly adjacent, to one or more origins of replication near the region that is the target of genomic modification. In the most preferred embodiments, the cells include a marker adjacent, preferably directly adjacent, to the origin of replication that is closest to the region that is the target of genomic modification. Selected cells that incorporate the mutagenic oligonucleotide at the selectable marker gene are primed for incorporating other oligonucleotides in the target region at the same replication fork.
In the most preferred embodiments, there is only one or zero origins of replication between the selectable marker and the target region, though in some cases there can be more, particularly if the oligos are targeted to the appropriate strand (see, e.g.,
In yeast the average distance between origins of replication is approximately 30 kb, while in mammals the origin distances can be 1-2 million bases. Thus in some embodiments the target region can be from 1 base pair to up to about 2 million or more (as many as 100 million bp) base pairs in either direction from the origin of replication closest to the selectable marker.
The selectable marker is typically adjacent to an origin of replication. In some embodiments, the selectable marker can be, or begin, within 500 bp, 1 kb, 1.5 kb, 5 kb, 10 kb, 15 kb, or 20 kb of the origin or replication. In some embodiments, the distance from the origin of replication is measured by genetic markers. For example, the selectable marker can be placed between the origin of replication and 0, 1, 2, 3, 4, 5, 10, 15, 20, 25 or more intact endogenous or heterologous genes in either direction from the origin.
The target region and the selectable marker can be separated by, for example, 4, 3, 2, 1, or 0, preferably 0 or 1, origins of replication. The target region can range from, for example, 1 base pair to 1 million, 2 million, 5 million, 10 million, 100 million, or more base pairs in either direction from the origin of replication closest to the selectable marker. The selectable marker can be placed such that 0, 1, 2, 3, 4, 5, 10, 15, 20, 25 or more genes are between it and the target gene or target region.
Preferably, the target region is closer to the origin of replication closest to the selectable marker than the next origin of replication, as the strand biases can change as the target region gets closer to the origin that is replicating from the other direction.
The selectable marker can be any marker that can drive a selection. The selectable marker can be a counter-selectable marker. A counter-selectable marker is a gene that can be selected in its presence and absence by altering specific chemicals in the cellular growth media. For example, under appropriate growth conditions, a counter-selectable gene promotes the death of the microorganisms harboring it (Reyrat, et al., Infect Immun. 1998 September; 66(9): 4011-4017). Transformants which have integrated a suicide vector containing a counterselectable marker (e.g., by a single event of homologous or illegitimate recombination) retain a copy of the counterselectable marker in the chromosome and are therefore eliminated in the presence of the counterselective compound. Consequently, counterselectable markers have been used for the positive selection of mutants that have undergone defined genetic alterations leading to the loss of the marker. The most-used counterselectable markers are the genes that confer sucrose, streptomycin, or fusaric acid sensitivity. An example is thymidine kinase, which makes the host sensitive to ganciclovir selection. Exemplary counter-selectable markers suitable for use in eukaryotes include, but are not limited to, hygromycin and zeocin. Another counter-selection marker for higher order eukaryotes is HPRT (Fukagawa, et al., Nucleic Acids Research, 27(9):1966-1969 (2000)
Positive selection markers are markers that confer selective advantage to the host organism, for example antibiotic resistance, which allows the host organism to survive antibiotic selection, or an enzyme that can complement an auxotrophy.
Positive and negative selectable markers can serve as both a positive and a negative marker by conferring an advantage to the host under one condition, but inhibits growth under a different condition. An example is an enzyme that can complement an auxotrophy (positive selection) and be able to convert a chemical to a toxic compound (negative selection). URA3, an orotidine-5′ phosphate decarboxylase from yeast is a positive and negative selectable marker. It is required for uracil biosynthesis and can complement ura3 mutants that are auxotrophic for uracil (positive selection). The enzyme URA3 also converts 5-fluoroorotic acid (5FOA) into the toxic compound 5-fluorouracil, so any cells carrying the URA3 gene will be killed in the presence of 5FOA (negative selection).
A marker does not need to be a gene. It can also be a sequence of DNA base pairs in the chromosome that serve as the target site for a targeted nuclease. In this case, cells that acquire a targeted mutation in this “marker” region from the oligo are “immune” to the nuclease cleavage at the “marker” site. Cells that do not contain the mutation at the “marker” site will undergo nuclease-driven cleavage and present a growth disadvantage in the cell population.
As used in the disclosed methods, the marker is designed to be selectable following mutation by replication fork annealing of a mutagenic oligonucleotide. Thus in some embodiments, the marker is a counter-selectable marker that confers sensitivity to a compound and mutation alleviates sensitivity to the compound (e.g. URA3). Thus cells with a wildtype copy of the marker are killed when cultured in the presence of the compound, while cells with a mutated copy, induced by the mutagenic oligonucleotide, survive. In some embodiments, the marker is a mutated positive selectable marker, a wildtype copy of which is needed for the cells to survive in the presence of a compound. Thus cells with the mutant copy of the marker are killed when cultured in the presence of the compound, while cells with a copy of the marker reverted to wildtype by the mutagenic oligonucleotide, survive.
High levels of combinatorial genomic diversity can be achieved through iterative cycling of oligo incorporation. In the Examples below, cycling was achieved by the counter-selectable capability of URA3 through liquid selection of the population after each transformation. The odd cycle oligo targeted a nonsense mutation to URA3 for 5-FOA selections, and the even cycle oligo restored the functional URA3 gene for auxotrophic selection. A similar strategy of repeating cycles of introducing a mutation followed by reversing the mutation, or reversing a mutation followed by introducing a mutation, can likewise be applied to other selectable markers and marker strategies as discussed herein.
A string of several tandem unidirectional selection markers would also allow for many rounds of selection if the targeted marker was changed each round.
Other strategies include, for example, a GFP gene harboring a null mutation, which could be used for selecting cells via FACS if the “selectable” oligo restores the GFP, YFP, or any other fluorescent marker. It could be used in the opposite logic as well to sort out clones lacking GFP.
Other exemplary markers that can be used for positive and/or negative selection include, but are not limited to, lacZ gene, which encodes β-galactosidase, dihydrofolate reductase (DHFR), thymidine kinase, and antibiotics such as neomycin, neomycin analog G418, hydromycin, chlorophenicol, zeocin, blasticidin, KanR, geneticin, and puromycin.
Suitable auxotrophic markers are also known in the art. See, for example, ade1-14, ade1-101, ade2-1, ade2-101, ade2-BglII, can1-100, his3delta200, his3delta1, his3-11,15, his3delta, leu2delta1, leu2-3,112, leu2delta, lys2-801, lys2delta202, lys2delta, trp1delta1, trp1delta63, trp1-1, trp1-1, trp1delta, trp1-289, trp5delta, ura3-52, ura3-1, ura3delta, ura4delta, ade2delta::hisG, leu2delta0, lys2delta0, met15delta0, and ura3delta0, which are discussed in SGD Wiki, “Commonly Used Auxotrophic Markers.”
In some embodiments, the selectable marker is an essential gene. The essential gene can be endogenous essential gene. In some embodiments the selectable marker is an endogenous essential gene that has been moved from its endogenous loci to be adjacent to an origin of replication. In some embodiments, the selectable marker is not an endogenous essential gene.
The Examples below illustrate oligos targeting four possible strand combinations for URA3 (selectable marker) and ADE2 (region targeted for genomic modification). In some embodiments, a selectable marker is present both adjacent to the origin of replication and as part of, or adjacent to, the region targeted for genomic modification. Thus in some embodiments, there is a marker that allows for direct selection of modifications at the region targeted for genomic modification.
The foregoing marker strategies are intended to by illustrative, and it will be appreciated that numerous marker strategies can be employed provided they allow the user to select for cells that have a mutation in a selectable marker adjacent to an origin of replication near a target region of interest. The Examples below illustrate that when cells were first selected based on mutation of a selectable marker and subsequently screened for additional mutations in target site locus, a dramatic 100-fold increase in the frequency of oligonucleotides incorporated at the adjacent target locus downstream of the marker were observed. Thus in some embodiments, including a selection step increases the frequency of oligonucleotides incorporated at the adjacent target locus 10, 25, 50, 75, 100, 150 or more —fold over the same method carried out absent selection based on mutation of a selectable marker adjacent to the target origin of replication.
The Examples below also illustrate that the observed targeting efficiencies at the target site increased from approximately 0.2% to 20% of colonies containing the desired mutations. Thus in some embodiments, the observed targeting efficiencies at the target site increases to 1%, 5%, 10%, 15%, 20%, 25%, 50%, or more of colonies containing the desired mutations when selection based on mutation of a selectable marker adjacent to the target origin of replication is included in the method.
In some embodiments, the cells include two or more selectable markers adjacent to the same origin of replication. In some embodiments, the cells include one or more selectable markers adjacent to two or more different origins of replication. The selectable markers can be the same or different. In this way, target regions at two adjacent to two or more origins of replication, on the same or different chromosomes, can be targeted at one time.
The selectable marker can be inserted into genome of the cells using traditional cloning methods, DSB, CRISPR/Cas9, or other methods that are known in the art.
B. Oligonucleotide Design
As introduced above, typically at least one oligonucleotide is designed to mutate the selectable marker and at least one oligonucleotide is designed to mutate a target region near, preferably downstream of, the marker on the same chromosome. Although it will be appreciated that mutation of the selectable marker is a genomic modification, as used herein, “target region,” “target site of genomic modification,” etc., typically refers to one or more additional sites, which is not part of the selectable marker, at which genomic modification is desired. The target region(s) can be one or more genetic elements including, but not limited to, regulatory elements such as promoters and terminators, open reading frames, or other elements within the genome.
The oligonucleotides are typically single stranded oligonucleotides designed to hybridize to single stranded template strands of genomic DNA during replication. The oligonucleotide includes one or more mutations relative to the genome of the host cell, which is incorporated into the newly synthesized strand during replication. The selectable marker can be encoded on the same or opposite strand as the region targeted for genomic modification, or a combination thereof. The selectable maker can be on the same or opposite side of the origin of replication as the region targeted for genomic modification, or a combination thereof.
The Examples below illustrate oligos targeting four possible strand combinations for URA3 (selectable marker) and ADE2 (region targeted for genomic modification) arrangement were tested allelic replacement frequencies (ARF) is dependent on URA3/ADE2 orientation with respect to origin of replication (ARS1516). Targeting the lagging-lagging strand combination for URA3 and ADE2 was the most efficient when URA3 was located adjacent to ADE2. When the selectable marker and target region of genomic modification were divided by the origin of replication, targeting the URA3 leading strand and ADE2 lagging strand was most efficient, which is likely due to the two oligos targeting the same chromosomal strand. Thus in preferred embodiments, the selectable maker is on the same side of the selectable marker as the target region, and in particularly preferred embodiments, the oligos are designed to hybridize to the lagging strand of both the selectable marker and the target region. Thus in preferred methods, incorporation of the selectable marker and design of the oligonucleotides are coordinated such that oligonucleotides are incorporated as Okazaki fragments.
Although oligonucleotides greater than 120 or even those greater than 150 nucleotides in length can be used and should be suitable, typically synthetic oligonucleotides are prepared in the ranges of about 30 to about 120 nucleotides. In particular embodiments, the oligonucleotides are 30-90 nucleotides, 40-90 nucleotides, 50-90 nucleotides, 70-90 nucleotides, 30-120 nucleotides, or 40-120 nucleotides. The oligonucleotides contain at least one mutated, inserted or deleted nucleotide relative to the target DNA sequence. Target sequences can be within the coding DNA sequence of a gene or within introns. Target sequences can also be within DNA sequences which regulate expression of the target gene, including promoter or enhancer sequences or sequences that regulate RNA splicing.
The oligonucleotides can contain a variety of mutations relative to the target sequence. Representative types of mutations include, but are not limited to, point mutations, deletions and insertions. Deletions and insertions can result in frameshift mutations or deletions. Point mutations can cause missense or nonsense mutations. These mutations may disrupt, reduce, stop, increase, improve, or otherwise alter the expression of the target gene.
The methods can include use of one or more oligos targeting the selectable marker and one or more oligos targeting one or more target regions. The oligonucleotides can be administered in a single transfection, or sequential transfections, though a single transfection is preferred.
The methods can include introducing large pools of oligos with different sequences. In some embodiments one or a few oligos target the selectable marker and a much larger number of oligos target one or more genetic loci (i.e., region targeted for genomic modification).
Oligonucleotides can be DNA oligonucleotides, composed of the principal naturally-occurring nucleotides (uracil, thymine, cytosine, adenine and guanine) as the heterocyclic bases, deoxyribose as the sugar moiety, and phosphate ester linkages. Oligonucleotides can include modifications to nucleobases, sugar moieties, or backbone/linkages, depending on the desired structure of the replacement sequence at the target site or to provide some resistance to degradation by nucleases, to enhance stability introduced during chemical synthesis or subsequent enzymatic modification or polymerase copying. These modifications include, but are not limited to, the inclusion of one or more alkylated nucleic acids, locked nucleic acids (LNAs), peptide nucleic acids (PNAs), phosphonates, phosphothioates, and the like in the oligomer. Examples of modified nucleotides include, but are not limited to 5-fluorouracil, 5-bromouracil, 5-chlorouracil, 5-iodouracil, hypoxanthine, ÿanthine, 4-acetylcytosine, 5-(carboxyhydroxylmethyl)uracil, 5-carboxymethylaminomethyl-2-thiouridine, 5-carboxymethylaminomethyluracil, dihydrouracil, beta-D-galactosylqueosine, inosine, N6-isopentenyladenine, 1-methylguanine, 1-methylinosine, 2,2-dimethylguanine, 2-methyladenine, 2-methylguanine, 3-methylcytosine, 5-methylcytosine, N6-adenine, 7-methylguanine, 5-methylaminomethyluracil, 5-methoxyaminomethyl-2-thiouracil, beta-D-mannosylqueosine, 5′-methoxycarboxymethyluracil, 5-methoxyuracil, 2-methylthio-D46-isopentenyladenine, uracil-5-oxyacetic acid (v), wybutoxosine, pseudouracil, queosine, 2-thiocytosine, 5-methyl-2-thiouracil, 2-thiouracil, 4-thiouracil, 5-methyluracil, uracil-5-oxyacetic acid methylester, uracil-5-oxyacetic acid (v), 5-methyl-2-thiouracil, 3-(3-amino-3-N-2-carboxypropyl) uracil, (acp3)w, 2,6-diaminopurine and the like. Nucleic acid molecules may also be modified at the base moiety, sugar moiety or phosphate backbone.
C. Mutagenesis
1. Methods of Mutagenesis
The disclosed methods can be used to introduce one or more nucleic acid sequences into a cell. The methods typically include introducing oligonucleotides including the desired nucleic acid sequence into cells by transformation or transfection.
The oligos can be introduced by methods known in the art, for example infection, transfection, transformation or transconjugation. Techniques for transferring DNA into such organisms are widely known and provided in references such as Sambrook, et al. (2012) Molecular Cloning: A Laboratory Manual, 4th ed., vol. 1-3, Cold Spring Harbor Press, Plainview N.Y. Exemplary preferred methods include electroporation, lipofection, calcium phosphate, or calcium chloride co-precipitation, DEAE dextran, or other suitable transfection method. For example, the cells can be transformed or transfected using transformation medium or transfection medium including at least one nucleic acid oligomer containing one or more mutations, replacing the transformation medium or transfection medium with growth medium, incubating the cell in the growth medium, and repeating the steps if necessary or desired until nucleic acid sequences have been introduced into the cell.
In some embodiments, the one or more nucleic acid oligomers is a pool of oligomers having a diversity of different random or non-random mutations at the location(s) of desired mutagenesis. Cells can be transfected with a variety of combinations of oligonucleotides leading to the formation of a diverse genomic library of mutants. The diversity of the library can be increased by increasing the number of cycles. Thus in some embodiments, multiple mutations are generated in a chromosome or in a genome. The oligomers can be single-stranded DNA.
Genetic diversity can be tuned by selecting the number and/or diversity of the oligonucleotides introduced during any step of the mutagenesis processes. It will be appreciated that the number of oligonucleotides can be increased, that the oligonucleotides can include one or multiple mutations per oligonucleotide and therefore target multiple position (e.g., amino acid positions encoded by the target DNA, etc.); that the oligonucleotides can introduce various types of mutations (mismatches, insertions, deletions and with varying degrees of degeneracy (4N—A, T, G, C, 2 selected therefrom, or 3 selected therefrom) or specificity (N equals specific nt).
As exemplified in the experiments below, the disclosed methods are amenable to transfection with very large numbers of mutagenic oligonucleotides, in the range, for example of 1 to 1,000,000,000,000 oligonucleotides. This number can be very high because the oligos can be designed to have degenerate positions and thus quickly expand the complexity of the oligo pool, i.e., the number of oligos. The experiment reported in
Genetic diversity of the mutants can also be tuned by the number of cycles of mutagenesis. For example, increasing the number of cycles of mutagenesis generally increases the diversity of the library. In particular embodiments, a library is prepared by one or more cycles or transformation and selection, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15. In a particular embodiment, a library of mutants is prepared by, for example, between 1 and 50, between 3 and 15, between 5 and 9 cycles. In some embodiments, one or more round of transformation is carried out without intervening rounds of selection. The methods can also be modified to include additional or alternative steps to improve genetic diversity. See, for example, Carr, et al., Nucleic Acids Research, 1; 40(17):e132, 12 pages (2012), and Gregg, et al., Nucleic Acids Research, 42(7):4779-90 (2014).
The disclosed methods allow for multiplex automated genome engineering (MAGE) or eukaryotic cells (also referred to as “eMAGE” and “eukaryotic MAGE”). This approach was can be used in a similar fashion to bacterial MAGE to create generate multi-site gene modifications with base-pair precision at increased efficiencies. The methods can be used for pathway diversification, whole genomic recoding, and molecular evolution of proteins.
In general, the experiments can be divided into three classes, characterized by varying degrees of scale and complexity: (i) many target sites, single genetic mutations; (ii) single target site, many genetic mutations; and (iii) many target sites, many genetic mutations. For example, in the first class, the methods can be used to recode all instances of a stop codon (e.g., TAG) with a synonymous codon (e.g., TAA) to produce a genetically recoded organism with a ‘blank’ codon available. The ‘blank’ codon can be reassigned, for example, for site-specific incorporation of nonstandard amino acids.
In the second class, the methods can be used to explore the effects of all possible amino acid substitutions at a single target locus. In such an experiment, it is possible, for example, to use a single degenerate ssDNA containing the NNN triplet at its center to introduce all possible amino acid substitutions.
In the third class, the method can be used to construct diverse cell populations containing combinations of alleles across many loci involved in the biosynthesic pathways. This method is exemplified in the experiments below using a beta carotene biosynthetic pathway. In this implementation, discrete oligos designed to knockout competing pathways by deletion can be mixed with degenerate oligos designed to randomize target positions in the coding sequence or regulatory regions of key pathway enzymes. The highly diverse population resulting from use of the provided methods can be used downstream to screen or select for mutants with a prescribed phenotype (e.g., overproduction of a metabolite or small molecule).
2. Methods of Selection
As discussed above, in some embodiments, such as those discussed above, a diverse population is the desired result and no further selection or analysis is needed. However, in some embodiments, one or more particular phenotypes or genotypes are desired. Thus, in some embodiments, coincident with or after a preferred population of prospective mutants is prepared by one or more rounds of transfection and selection for modification of the selectable marker at the desire origin of replication, a subset or individual cells can be selected based on a desirable phenotype or genotype.
Selection can include one or more cycles of negative selection, one or more cycles of positive selection, or a combination thereof. Selection can be integrated in between cycles of mutagenesis, reserved until after mutagenesis is complete, or a combination thereof. Negative selection can be before or after positive selection, or a combination thereof. Positive selection can be before or after negative selection, or a combination thereof. Therefore, selection can include any combination of iterative rounds of positive and/or negative selection.
Negative selection generally refers to a process of reducing undesirable mutants from the library of mutants. Positive selection generally refers to a process of choosing desirable mutants from the library of mutants. Selection can include, for example, in some embodiments, the mutagenesis creates or corrects auxotrophy, creates or destroys sensitivity to a compound or a nuclease, or creates, alters or destroys another biochemical, morphological, or phenotypic characteristic. In some embodiments, the individuals are additionally or alternatively screened or selected by genotyping, such can include genomic sequencing, as well as hybridization techniques that allow the practitioner to select the desire cell(s) without actually sequencing the cell's genome. By way of non-limiting illustration, the target region of some of the experiments below was the ADE2 gene, and red/white phenotype of ADE2 was used as a screening assay for ade2 mutants.
D. Tuning Replication Fork Speed and Other Considerations
Mutagenesis can be improved by reducing the speed of the replication fork. Cells can be cultured or otherwise treated with a compound, such as hydroxyurea, that transiently slows the DNA replication fork in an effective amount to allow for even higher efficiencies of oligonucleotide incorporation. In some embodiments, oligonucleotide incorporation is improved 10, 25, 30, 35, 40, 45, 50, 55, 60, 70, 75, 80, 85, 90, 95, 100 percent or more. The Examples below utilize hydroxyurea in range of about 200 nM to about 800 nM, with the greatest changes in oligonucleotide incorporation observed in the range of about 300 nM to about 700 nM, and about 400 nM to about 600 nM. In the non-limiting experiments below, a short exposure time of 30 minutes prior to transformation was utilized to minimize any mutagenic effects of HU. In some embodiments, the exposure to hydroxyurea or another agent that reduces replication fork speed, is between about “x” minutes and about “y” minutes, wherein “x” is an integer between 1 and 359, and “y” is an integer between 2 and 360 that is greater than “x”.
The Examples also show that efficiencies of replication fork oligonucleotide incorporation were decreased by high levels of RAD51 and the efficiencies increased in the rad51 knockout conditions. In some embodiments, the cells utilized in the disclosed methods are mutated or deleted at rad51 and/or rad52 (or their homologs in another species). These data indicate that replication fork annealing is a competing alternative mechanism to the canonical RAD51 ssDNA strand invasion mechanism used in other methods.
RAD59 appears to play a role in replication fork annealing. Thus in some embodiments, the rad59 gene (or its homologs in another species) is intact and/or expression is expressed or supplemented using compositions and methods known in the art. For example, cells can be genetically modified to increase (or constitutively express RAD59), cells can be transfected with a rad59 expression construct, or mRNA or protein can be transiently transfected into the cells. Additionally or alternatively expression of another ssDNA annealing or binding protein or recombinase (e.g., Beta recombinase) or a homolog thereof, is increased. A heterologous recombinase (beta) can complement rad59 deletion. Examples of ssDNA recombinases include, but are not limited to
E. coli
E. faecalis
L. pneumophila
V. cholerae
P. luminescens
E. coli
L. monocytogenes
L. lactis
B. subtilis
M. smegmatis
S. aureus
Other examples of recombinases are known in the art and can be found in the Remote Homology Detection of Viral Protein Families (VIRFAM) website.
In some embodiments, the cells are mutated or deleted at one or more DNA mismatch repair (MMR) genes. Exemplary genes include msh2, msh6, mlh1, and pms1 in yeast, and their homologous in other species.
In some embodiments, mutations are amplified by mating two cells. For example, in the experiments below, haploid yeast mutant libraries were prepared targeting mutations to different target regions, and haploids carrying varying mutations were subsequently mated to compound the mutations.
E. Cells
The disclosed methods are designed for use in eukaryotic systems. The methods are particularly suitable for use with eukaryotic genomes arranged in linear chromosomes with multiple origins of replication that fire stochastically throughout S-phase. Typically, the host, parent, parental, progenitor, or background cell, population of cells, or strain of cells is one that has one or more modifications, typically genetic modifications, which improve its use or application in the disclosed methods. For example, the host, parent, parental, progenitor, or background cell, population of cells, or strain of cells can be the one(s) that is transformed or transfected with one or more single-stranded oligonucleotides for selectable marker modification, target region modification, or a combination thereof or a predecessor cell thereof
1. Eukaryotic Cells
Yeasts useful as host cells include, but are not limited to, those from the genus Saccharomyces, Pichia, K. Actinomycetes, Kluyveromyces, and Yarrowia (e.g., Yarrowia lipolytica). The methods can include other standard molecular biological techniques, and may utilize, for example, vectors. Yeast vectors will often contain an origin of replication sequence, an autonomously replicating sequence (ARS), a promoter region, sequences for polyadenylation, sequences for transcription termination, and a selectable marker gene. Suitable promoter sequences for yeast vectors include, among others, promoters for metallothionein, 3-phosphoglycerate kinase (Hitzeman et al., J. Biol. Chem. 255:2073, (1980)) or other glycolytic enzymes (Holland et al., Biochem. 17:4900, (1978)) such as enolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, and glucokinase. Other suitable vectors and promoters for use in yeast expression are further described in Fleer et al., Gene, 107:285-195 (1991), in Li, et al., Lett Appl Microbiol. 40(5):347-52 (2005), Jansen, et al., Gene 344:43-51 (2005) and Daly and Hearn, J. Mol. Recognit. 18(2):119-38 (2005). Other suitable promoters and vectors for yeast and yeast transformation protocols are well known in the art.
In some embodiments, the cells are mammalian, fungal, insect, or plant cells. Suitable host cell culture systems are well known in the art. Commonly used promoter sequences and enhancer sequences are derived from Polyoma virus, Adenovirus 2, Simian Virus 40 (SV40), and human cytomegalovirus. DNA sequences derived from the SV40 viral genome may be used to provide other genetic elements for expression of a structural gene sequence in a mammalian host cell, e.g., SV40 origin, early and late promoter, enhancer, splice, and polyadenylation sites. Viral early and late promoters are particularly useful because both are easily obtained from a viral genome as a fragment which may also contain a viral origin of replication. Exemplary expression vectors for use in mammalian host cells are well known in the art.
The disclosed methods are exemplified below using a yeast system, however, most or all of the steps should be adaptable for use in other eukaryotic cells. Small changes may be employed to utilize molecular biological techniques that are particularly advantageous or disadvantageous in one system over another. For example, the physical method of oligo delivery might be dictated by the system: Lipofection might be used as the most optimal delivery method in mammalian cells compared to electroporation in yeast. However once inside the cell, the oligo will encounter structurally the same replication fork and the same principles of fork slowing with hydroxyurea (shown to work the same way mammalian cells), and annealing to the lagging strand as an Okazaki fragment mimic would apply, etc. Other eukaryotes can also be cultured for several generations so the general principles of cycling protocol to generate targeted genomic diversity also apply, and the relevant replication, recombination, and repair proteins are conserved in eukaryotes from yeast upwards to human.
2. Parent Cells
Cells for use as the host, parent, parental, progenitor, or background cell, population of cells, or strain of cells in the disclosed methods are also provided. As introduced above, preferred cells can be predisposed, genetically modified, or otherwise inclined to increase, enhance, or favor replication fork annealing, enhance introduction of genetic mutations, or a combination thereof. For example, as discussed above, RAD59 appears to play a role in replication fork annealing. The rad59 gene (or its homologs in another species) can be intact and/or expression is expressed or supplemented using compositions and methods known in the art and discussed above. Additionally or alternatively expression of another ssDNA annealing or binding protein or recombinase (e.g., Beta recombinase) or a homolog thereof, is increased or supplemented as discussed above.
The parent or progenitor cells can have pre-existing modification(s) that reduce the effectiveness of intrinsic DNA damage prevention, DNA damage repair, or a combination thereof. The cells include those wherein the ability to carry out a DNA mismatch repair pathway is reduced or prevented. For example, the cells can be mutated or deleted at rad51 and/or rad52 (or their homologs in another species). Other pathways include the post-replicative DNA mismatch repair system (MMR), which in eukaryotic systems including human can include two different heterodimers: MutS alpha (MSH2-MSH6 heterodimer) and MutS beta (MSH2-MSH3 heterodimer) which binds to DNA mismatches thereby initiating DNA repair. Base-pair mismatches are recognized by MutS alpha and beta heterodimers. The complex of MutS and mismatched DNA recruits MutL (related genes include: MLH1, MLH2, MLH3, MLH4, PMS1, PMS2, and others) leading to subsequent strand separation and degradation by a variety of other factors. Cells can have mutation or deletion in one or more msh, mlh, or pms genes that can reduce the function or effectiveness of MMR. In the examples below, some of the yeast strains have a msh2 deletion.
The parent or progenitor cells can also be predisposed, genetically modified, or otherwise inclined to increase, enhance, or favor selection of the desirable progeny or clones after the parent or progenitor cells are subjected to the genome modification methods disclosed herein. For example, the parent or progenitor cell can be genetically modified to include a selectable marker adjacent, preferably directly adjacent, to an origin of replication near the region that is the target of genomic modification. Suitable selectable markers are discussed in more detail above and exemplified in the experiments described below.
In some embodiments, the cells have modified to express an exogenous or heterologous single-stranded annealing protein (SSAP), or recombinase, or overexpress an endogenous SSAP or recombinase system, or any combination thereof. In some embodiments, the cells have been modified to exclude a non-essential endogenous single-stranded annealing protein (SSAP) or recombinase, have not been modified to include an exogenous or heterologous SSAP or recombinase, or any combination thereof. Additionally or alternatively, the expression can be modulated up or down transiently with mRNA (increased expression) or inhibitory nucleic acids (e.g. siRNA) (decreased expression).
Examples of single-stranded annealing protein (SSAP) systems and recombinase systems include, but are not limited to, beta protein of λ Red, RecT protein of the RecET system, and/or proteins like Rad52-like, Rad51-like, or Gp2.5-like superfamilies, or a combination thereof. Single-stranded annealing protein (SSAP) systems or recombinase systems are discussed in WO 2015/017866. The SSAPs can be modified to include specific targeting sequences, such as a nuclear localization sequence (NLS) to promote trafficking to the eukaryotic nucleus.
Other exemplary genetic modifications for parental or progenitor cells are discussed in more detail in the examples below. See, for example, Table 1, which provides various parental yeast strains utilized in the experiments disclosed below. Any of the genetic modifications and strains provided in Table 1 can be used. The synonymous or homologous modification(s) can also be reproduced in other eukaryotes including, but not limited to, human cells.
The genetic background for the host cell can be selected based on the application. For example, the practitioner may desire to evolve a cell with one or more specific properties but these are agnostic to the genome sequence of the resultant organism. In this case, the MMR deficient background is useful in generating a large library of variants at the targeted sites with potential additional mutations that might contribute beneficially to the desired phenotype in an unforeseen manner. In some embodiments, the user might want to create one or more specific genetic mutants to test. In this case a preferred host cell would contain an intact MMR system or a transiently disabled MMR system to reduce the likelihood of additional mutations elsewhere in the genome.
Host cells can also include a selectable marker adjacent to an origin of replication.
3. Genetically Modified Cells
Cells resulting from the disclosed methods of eukaryotic genome modification are also provided. Such a cell is typically one (or a population thereof) that has one or more genomic modifications relative to its parent. The genomic modification is typically induced according to a disclosed method. Such cells can also be referred to as genetically modified cells, child or children cells, progeny, etc., relative to its parent cell. Most typically the genetically modified cells have one or more genetic modifications in one or more target regions or genes as discussed herein. In some embodiments, genetically modified cells, child or children cells, progeny, etc., may be further modified to remove or replace modifications that reduced the effectiveness of intrinsic DNA damage prevention, DNA damage repair, selectable markers, or a combination thereof, including those discussed above with respect to parent cells. These modifications may no longer be needed, or are even undesirable once a preferred child cells has been identified and selected. Additionally, or alternatively, mating techniques or other genetic manipulations can be used to compound the genetic modifications in two or more different child cells.
Additionally or alternatively RNAi or CRISPRi can be used as a knockdown strategy to transiently repress target changes. Also, those same genes and others can be modulated by expression from tunable promoters induced by chemicals, temperature or light.
The disclosed methods can be used for deeper exploration of genome sequence space and ultimately drive the ability to study and engineer new phenotypes for both fundamental research and industrial purposes. By altering multiple sites in the genome in a programmable fashion without cytotoxic effects, the methods and the cells made therewith can be utilized in every key area of eukaryotic biology. For example, the compositions and methods can be used in production of natural products from heterologous pathways (building off beta-carotene example discussed below), production of specialty chemicals, hydrocarbons (alkanes, alkenes), biofuels, drugs, and remediation (e.g., bioremediation).
Specific exemplary applications include, but are not limited to,
Furthermore, the disclosed methods are complementary and distinct with CRISPR/cas9 editing technology. Although recent advances in CRISPR/cas (or other nuclease-mediated technologies, e.g., Zinc Finger Nucleases, TALENs) have demonstrated new capabilities to introduce targeted gene editing modifications, the ability to generate high density, diverse and combinatorial modifications in eukaryotic genomes is limited by current technologies that require double-stranded DNA cleavage. The fundamental mechanism of CRISPR and its sister technologies will prevent it alone from achieving a comparable goal. The disclosed methods satisfy a complementary need of the CRISPR technology and expand the scaling of targeted genome modification across many loci, namely precise editing at many positions of the genome simultaneously, the ability to generate combinatorial genomic variation and a facile ability to iteratively introduce genome modifications from a complex oligonucleotide pool. The approach employs a fundamentally different mechanism than the CRISPR technology, which makes advantageous for editing genomes at high resolution and at many genomic sites at once.
Strain Construction
A complete list of strains used in this study can be found in Table 1. Strain BY4741 (MATa his3Δ1 leu2Δ0 met15Δ0 ura3Δ0) was chosen due to its common use as a laboratory strain and for its use in the Saccharomyces Genome Deletion Project. MMR knockout strains from the Saccharomyces Genome Deletion Project were purchased (Open Biosystems, Thermo Scientific). Strains harboring the URA3 marker for coupled ssODN selection (eMAGE) were constructed via homologous recombination with a dsDNA URA3 PCR product and selection on CSM-Ura plates. The β-carotene pathway is derived from Xanthophyllomyces dendrorhous and was PCR-amplified from plasmid pJC178, then incorporated at ARS1516 adjacent to the URA3 marker or at the indicated ARS location. Strains harboring the β-carotene biosynthetic pathway were constructed using a CRISPR-Cas9 genome integration step using gRNA target site sequence ‘CTTGTTGCATGGCTACGAAC’ located at chrXV 566360.
Media
For general strain manipulation cells were grown in YPADU liquid medium, which consists of YPD (10 g/L Yeast Extract, 20 g/L Peptone, 20 g/L Dextrose), supplemented with 40 mg/L adenine hemisulfate, and 40 mg/L uracil. For URA3-coupled ssODN experiments (eMAGE), strains were grown in CSM-Ura medium during odd numbered cycles and CSM-Ura+5-FOA (1 g/L)+uracil (50 mg/L) medium during even numbered cycles. For HR gene overexpression experiments, CEN/ARS plasmids were maintained with the hygromycin B resistance marker. After electroporation, cells were allowed to recover in YPADU/0.5M Sorbitol (Recovery Medium).
Plasmid Assembly for Overexpression of HR Genes
To clone pTEF1 expression plasmids for HR genes, all HR gene ORFs were PCR amplified from BY4741 genomic DNA prepared via a standard glass bead yeast genomic preparation protocol. Forward primers for each ORF contained 40 bases of 5′ overhang with sequence identity to 3′ end of the TEF1 promoter. Reverse primers for each ORF contained 5′ overhang with 40 bases of cyc1T terminator identity. Gibson assembly cloning was used to assemble a hygromycin B resistant (hphMX) CEN/ARS plasmid backbone (pRCVS6H) for each ORF to generate (pRCVS6H-pTEF1-ORF-CycT-hphMX plasmids). All HR plasmids were sequence verified.
Yeast ssODN Electroporation with Rad51-Dependent HR
A 2 mL culture was inoculated with a single colony and grown to saturation overnight in YPADU or YPADU+Hygromycin B (200 ug/mL) for plasmid maintenance during HR overexpression experiments. The next day a 10 mL culture was inoculated at OD600˜0.1 and grown for 6 hours in a roller drum at 30° C. until OD600˜1.0 (˜3×107 cells/mL). Cells were pelleted at 2,900×g for 3 minutes and washed twice with 40 mL of room temperature dH2O. Cells were pre-treated with 1 mL of TE pH 8 containing 500 mM Lithium Acetate/25 mM DTT (Pretreatment Buffer) for 30 minutes in the roller drum at 30° C. Cells were washed 1× with 1 mL ice cold dH2O and 1× with 1 mL ice cold 1M sorbitol. Cells were gently suspended in 200 uL of 1M sorbitol+2 uM of total ssODN for each transformation, and added to a pre-chilled electroporation cuvette (0.2 cm) on ice. 2 uM of ssODN was previously determined to be optimal for Rad51-dependent HR (DiCarlo et al., 2013). Electroporation was performed with the following parameters: 1500V, 25 uF, 200Ω. Immediately after pulsing, the cells were recovered in 6 mL of Recovery Medium for 12 hours in the roller drum at 30° C.
ssODN Transformations for eMAGE
A list of ssODNs used for
Recombinase proteins (e.g., Rad51) catalyze the pairing and exchange of homologous DNA sequences (San Filippo, et al., Annual Review of Biochemistry, 77:229-257 (2008)). In S. cerevisiae additional factors, including Rad52, Rad54, Rad55, Rad57, and Rad59 participate in HR by promoting the formation and stabilization of Rad51-ssDNA filaments or through annealing of ssDNA (San Filippo, et al., Annual Review of Biochemistry, 77:229-257 (2008); Sung, J Biol Chem, 272: 28194-28197 (1997)). Prior work proposed that ssODNs are incorporated in the yeast genome through Rad51-mediated HR (
Experiments were designed to determine if Rad51-dependent ssODN recombination and incorporation of ssODNs at the DNA replication fork are two distinct pathways in eukaryotes (
To assess the impact of HR factors on the proposed replication fork annealing mechanism, a set of knockout and overexpression strains for HR genes were created and ssODN incorporation at the Ori-URA3-ADE2 experimental locus was assayed. Unlike the observation for targeting the RPL28-ADE2 interchromosomal pair, overexpression of Rad51 decreased ARF to ˜5% (four-fold) and deletion of Rad51 increased ARF to ˜30% (
To further elucidate the mechanism of ssODN incorporation at the replication fork, the effect of targeting the leading versus lagging strands by placing URA3 was tested in two orientations with respect to ADE2 (
Target Distance Efficiency Determination
Target mutation distance is reported as the distance between the URA3 marker mutation incorporated by the selection-ssODN and the mutation incorporated by the target-ssODN. For target distances of 1 and 2 kb, ssODNs ADE290RC and ADE2_Mult10 were used within the ADE2 gene and the ARFs for these sites were determined by red/white phenotype screening. For targets at 5, 10, 15, and 20 kb distances from the Ori-URA3, target sites were chosen that differ by only a single base-pair from an EcoRI restriction site. The target ssODN was designed to incorporate a base-pair change to create the EcoRI restriction site. For each target assayed, 96 clones were analyzed by colony PCR (Primers for the amplicons analyzed are listed in Table 3) coupled to EcoRI (NEB) digestion of the amplicon for 1 hr at 37° C. The percentage of amplicons cut vs. uncut is represented as the ARF for each distance site. Each distance experiment was performed in triplicate+/−HU treatment. ARF values represent mean+/−SD.
Hydroxyurea (HU) has been shown to slow the rate of DNA replication fork progression and increase the availability of ssDNA at the lagging strand (Alvino, et al., Mol Cell Biol, 27: 6396-6406 (2007)). Experiments were designed to determine if transient treatment with HU would increase the efficiency for ssODN annealing at the replication fork. A 30 minute treatment with 500 mM HU increased the ARF by ˜two-fold compared to the untreated condition. Seven different ssODNs containing a single base-pair mismatch (ssODNs 1,2,5-7), insertion (ssODN #4), or deletion (ssODN #3) in ADE2 were tested and an average ARF increase of 56% (
To determine the ability to introduce diverse genetic mutations at the replication fork, a set of ssODNs containing mismatches, insertions, and deletions within ADE2 (msh2Δ strain) were tested. ARFs were inversely correlated to the number of nucleotides targeted for modification. More specifically, a single base-pair mismatch, insertion, or deletion was incorporated with an ARF>40% (
Materials and Methods
Multiplex Incorporation of ssODNs and Cycling
For multiplex eMAGE experiments targeting ADE2, red clones that grew on 5-FOA plates were assayed via yeast colony PCR and Sanger sequencing (Genewiz). Insertions were chosen for easy sequence detection since degenerate mismatches would contain WT sequence positions. For cycling experiments, cells were recovered in recovery media after electroporation as described above. After recovery, the population was subjected to liquid selection in 5-FOA for odd cycles and CSM-Ura for even numbered cycles to enrich for edited clones. Selections were performed for 500 uL of recovery culture seeded into 50 mL of selection medium and grown to saturation at 30° C. (˜2 days).
After the first 1:100 selection the population was diluted 1:50 in selection media and grown for 6 hours for the next electroporation step. The process was repeated for 3 cycles. A total of 100 red clones were sequenced after each cycle. For cycles 1 and 2, 200 unique genotypes were observed out of 200 sequenced, but for cycle 3, 76 unique genotypes were observed out of the 100 clones sequenced. Given that the degeneracy of the ssODN pool largely out-scales the number of clones assayed redundant genotypes would not be expected to arise for independent clones in any cycle assayed. The 24 redundant clones observed in cycle 3 were comprised of 5 genotypes. These clones observed in cycle 3 are due to an enrichment of those genotypes that occurred from selection after cycle 2. For future experiments improved selection capabilities are highly desired in order to ensure maintenance of high population diversity between multiple cycles. For the purposes of this small-scale demonstration the liquid selections were seeded with ˜103-104 edited genotypes between each cycle. For applications requiring large library sizes, larger scale selections (Liters) could be employed to maintain the population complexity generated after electroporation.
To determine if multiple ssODNs can be introduced via annealing at the replication fork, 10 loci across the ADE2 gene in WT and msh2Δ strains (
To further increase the rate and accumulation of genetic diversity, a strategy was developed for continuous diversification of a yeast cell population by cyclical introduction of a complex pool of ssODNs coupled to positive and negative URA3 selections (
The msh2Δ strain was shown to have a background mutation rate that is elevated ˜225-fold greater than the WT yeast strain (Lang, et al., G3 (Bethesda), 3:1453-1465 (2013)). Throughout the process of eMAGE cells can be grown for as many as hundreds of generations and are exposed to relatively higher stress conditions such as DTT-HU treatment, electroporation, and osmotic stress. In order to understand the effect of the eMAGE protocol on the background mutation rate whole genome sequencing was performed for 12 clones from each eMAGE cycle. In accordance with the previously reported mutational rate from (Lang, et al., G3 (Bethesda), 3:1453-1465 (2013)) (7.1×10−8) for the msh2Δ strain, an average mutation rate of 8.3×10−8 mutations per base pair per generation was observed. The majority of mutations were indels (80%) compared to SNPs (20%). The average SNP rate across the three cycles (1.7×10−8) was approximately four-fold greater than the published rate (4.0×10−9).
Generation of Mutant Population for NGS Diversity Analysis
Approximately 3×108 cells were electroporated in a 20 uM solution containing ssODNs designed to introduce insertions at targeted sites. Three ssODNs each encoding five insertions were pooled together. After electroporation and recovery the entire population was then seeded in 1 L of liquid 5-FOA selection media and grown to saturation over 2 days. 10 mL of this selected population was used for the genome prep and bug or approximately 5×109 genomic copies were seeded into the PCR reaction.
NGS Diversity Sample Preparation and Sequencing
A 307-322 bp region (depending on the number of insertions introduced) including the bases targeted for mutagenesis was PCR amplified with primers that added five degenerate base pairs on each end. The addition of degenerate bases aided in initial base calling during sequencing and reduced the need to add an increased fraction of phiX DNA (Tewhey et al 2016) The number of PCR cycles was limited to 12 in order to reduce the introduction of errors and bias. The PCR product was then gel purified and sent to the Yale Center for Genome Analysis for adaptor ligation and 2×250 paired end sequencing on an Illumina HiSeq4000. This allowed for coverage of the entire amplicon and sufficient overlap between the paired end reads to assemble phased sequences for each observed variant.
NGS Diversity Filtering and Processing of Paired End Reads into Merged Sequences
The sequencing process generated 49,714,782 paired end reads. Trimmomatic was used to remove the first five degenerate bases added by the primers and trim reads with low quality bases (Bolger, A. M., Lohse, M., & Usadel, B. (2014). A sliding window requiring an average quality score of either 20, 25, or 30 over two bases was used to trim the ends of lower quality reads. This resulted in 49,384,456, 48,711,289, and 47,505,196 paired end reads respectively passing quality control. BBMerge was then used to assemble the overlapping paired end reads into full length amplicons. The “strict” stringency setting was used during this step. This resulted in 37,227,893, 24,116,204, and 12,065,464 fully assembled amplicons of which 16,704,731, 12,952,187, and 9,048,602 were wild type length of 307 bp. BBMerge seeks to reduce the error rate in the assembled amplicons by considering the quality score of the bases in the overlapping sequences during merging and when there is a mismatch between the two reads selecting the base with the higher quality score. Similar numbers of reads pass the quality trimming step at Q20 and Q30, but the number of assembled amplicons is dramatically different between the two cutoffs. This indicates that while a large number of reads are passing quality control at Q30 they are being trimmed to a greater extent and this is resulting in read pairs that no longer overlap and are unable to be assembled.
NGS Diversity Computational Analysis
The merged reads were then arranged in the same orientation and aligned to a wildtype copy of the edited genomic locus using the BWA mem algorithm (Li, et al., Nucleic Acids Research, 31:6674-6687 (2003)). Picard tools were used to calculate the experimental substitution error rate. Custom scripts then utilized the CIGAR and MD strings in the resulting SAM file to extract the position and base introduced when insertions occurred at the targeted sites. Calculations of the distribution of the number of insertions introduced and the positional insertion distribution were then performed on these vectors containing the base introduced by each targeted insertion in an amplicon.
In order to quantify the genetic diversity generated by an eMAGE experiment, Next-Generation Sequencing (NGS) was performed on a diversified cell population. To a starting population of approximately 3×108 cells were transformed with a pool of three ssODNs each encoding five degenerate insertions at ADE2. After electroporation and recovery in nonselective media the entire population was seeded in liquid 5-FOA selection media and grown to saturation. A genomic prep from the 5-FOA selected population served as the template for a PCR reaction of the ADE2 target region. High-throughput sequencing reads of the target amplicon were analyzed with a computational pipeline including rarefaction analysis (Amiram, et al., Nature Biotechnology, 33:1272-1279 (2015)). The number of unique variants detected at the read limit was ˜1.59×105 and ˜6.70×105 for read quality score cutoffs of Q30 and Q20, respectively. In line with the Sanger sequencing data a distribution of insertions per ssODN and a 3′ position insertion bias were observed.
HPLC Characterization of Carotenoids
Each of the analyzed clones was grown for 3 days at 30° C. in 5 mL YPADU media. Carotenoids were harvested from 1 mL of cell culture. 1 mL of cells was pelleted via centrifugation and washed twice with water. The resulting pellet was extracted with 200 uL of hexane using glass bead disruption with the Beadbeater cell homogenizer (3×45 s at 7,000 rpm). After centrifugation, 120 uL of the hexane carotenoid mixture was transferred to a glass vial and dried with a speedvac machine for 1 hour. The sample was then resuspended in 50/50 Hexane/Ethyl Acetate and filtered before HPLC analysis. 20 uL of sample was injected for HPLC using an Agilent Poroshell 120 EC-C18 2.7 um 3.0×50 mm column. Peaks were detected using an isocratic elution with 50/50 Methanol/Acetonitrile (containing 0.1% Formic Acid). Analytical standards were used for quantification of β-carotene (detected at 475 nm), phytoene (286 nm), and lycopene (475 nm). Carotenoid quantifications were calculated in relation to dry cell weight (DCW) for 100 uL of cell culture dried for 2 days and weighed. Additional carotenoid peaks were observed beyond the three carotenoid peaks analyzed, which likely contribute to clone color in some cases (i.e., M30, M35). HPLC experiments were carried out in technical triplicate for all clones.
To further study the ability to generate multi-site combinatorial genetic variation with base-pair-level precision, a heterologous β-carotene pathway was targeted for the creation of diverse genotype-phenotype variants. The pathway consists of four constitutively expressed genes (crtE, crtI, crtYB, and tHMG1), and was previously engineered in S. cerevisiae to convert farnesyl diphosphate (FPP) to β-carotene through a series of enzymatic steps (
To understand the phenotypic contribution from each of the four pathway genes, a complete set of the 15 possible combinatorial gene knockouts were generated in a single transformation. The lack of broad phenotypic diversity in the knockout set indicates that precise modifications to the pathway sequence might result in gene expression variants with color changes unlike the gene knockouts. Diverse clones with colorimetric phenotypes were observed to deviate from the ancestral strain after a single cycle of diversification (
Whether targeted edits in genes located on different chromosomes could be generated across haploids in parallel and then combined through mating was also tested. A MATα haploid were constructed containing the crtE gene adjacent to a URA3 cassette at Ori ARS510 on chromosome V, and a MATa haploid containing the crtI, crtYB, and tHMG1 genes at Ori ARS1516 on chromosome XV. Control mating was used to demonstrate that the resultant diploids showed the yellow phenotype indicative of the presence of all four genes of the wildtype β-carotene pathway (
Since ssODN annealing at the replication fork can enact a wide range of sequence modification, experiments were designed to determine whether ssODNs can be used to seamlessly replace promoter elements such as TFBS to change the transcriptional logic of constitutive promoters. A set of ssODNs was designed to precisely replace native TFBS of the β-carotene pathway with the 18 nucleotide galactose-inducible Gal4 binding sequence. Cells were transformed with the set of Gal4 ssODNs and an ssODN containing the crtYB-D52G mutation. Clones were identified with altered color phenotypes on glucose plates and these clones were inspected for phenotypic changes when spotted to plates containing galactose. Several clones with altered color phenotypes (G2-G6) and one clone (G1) with impaired growth on galactose were observed. To confirm that the different phenotypes observed on glucose and galactose corresponded to altered gene expression at the Gal4 engineered gene, RT-qPCR of the four pathway genes (
In this study, a eukaryotic genome engineering technology was developed that targets annealing of ssODNs at the DNA replication fork and avoids the creation of DSBs in the genome to enact base-pair precision and combinatorial genome editing across many genetic loci. Although the mechanism is independent of Rad51, other HR factors could be involved in ssODN incorporation. In addition to its role as a mediator for Rad51 (Sung, J Biol Chem, 272: 28194-28197 (1997)), Rad52 is also implicated in replication fork processing of Okazaki fragments with Rad59 (Lee, et al., J Biol Chem, 289:15064-15079 (2014)). Thus, it is possible that Rad52 participates in ssODN annealing at the replication fork. The lack of transformants in the Rad59 knockout indicated that it enhances ssODN incorporation at the replication fork whereas overexpression of Rad59 showed a decrease in ARF (
For single base-pair editing similar ARFs were observed in WT and msh2Δ strains, however, the data showed that MMR inhibits multiplex gene editing and therefore decreases the generation of genome complexity across the population. Although MMR mutants are not always desirable when trying to maintain genome stability, directed evolution and pathway engineering applications, as shown here, could benefit from an elevated mutation rate. Alternatively, transient disabling of MMR through small molecule inhibitors or CRISPRi (Gilbert, et al., Cell, 154:442-451 (2013)) approaches could be used to achieve desired genetic modifications during a transient relaxed genomic state (disabled MMR) followed by rapid return to a stabilized genomic state (intact MMR).
The approach to diversify the β-carotene pathway is also a blueprint that could be applied to metabolic engineering applications and natural product biosynthesis to optimize the production of high-value molecules from heterologous genetic sources currently produced in yeast (Galanie, et al., Science, 349:1095-1100 (2015); Krivoruchko, et al., Curr Opin Biotechnol, 35: 7-15 (2007); Mitchell, et al., Nucleic Acids Research, 43:6620-6630 (2015); Montiel, et al., Proceedings of the National Academy of Sciences of the United States of America, 112:8953-8958 (2015)). Large microbial gene clusters are often genetically silent and require modulation of pathway expression dynamics or refactoring to produce the desired secondary metabolites (Smanski, et al., Nature Biotechnology, 32:1241-1249 (2014)). Current methods to tune the expression of biosynthetic pathways in yeast rely on the use of combinatorial promoter swapping strategies (Mitchell, et al., Nucleic Acids Research, 43:6620-6630 (2015); Montiel, et al., Proceedings of the National Academy of Sciences of the United States of America, 112:8953-8958 (2015); Wingler, et al., Proceedings of the National Academy of Sciences of the United States of America, 108:15135-15140 (2011)). While promoter library approaches are effective, the possible modes of gene expression are limited to quantal units of transcription dictated by the promoter library. Precision editing with ssODNs in live cells enables rapid exploration of diverse gene expression states. Since CRISPR-Cas9 can efficiently introduce large genetic fragments into the genome and eMAGE can enact many precise combinatorial edits with ssODNs, the two approaches could be used in concert to recombine and diversify heterologous pathways for discovery and production of secondary metabolites and valuable compounds in yeast.
Eukaryotic MAGE can also uniquely enable the construction of targeted sets of genetic variants that can be functionally studied to elucidate causal links between genotype and phenotype. The diversification and mating studies established here could be automated (Wang, et al., Nature, 460:894-898 (2009)), employed to uncover many types of precise allelic interactions between specific sets of genes complementary to those explored by synthetic genetic arrays (Costanzo, et al., Science, 353 (2016); Tong, et al., Methods Mol Biol, 313:171-192 (2006)), and used to hierarchically construct highly modified chimeric genomes from multiple strains. Moreover, the approach used for S. cerevisiae targets conserved mechanisms and establishes a framework for developing efficient multiplex ssODN annealing methods in multicellular eukaryotes, including plants and animals. eMAGE capabilities could be included in future synthetic eukaryotic chromosome projects (Annaluru, et al., Science, 344: 55-58 (2014); Boeke, et al., Science, 353: 126-127 (2016)) to enable efficient single base-pair precision editing of designer genomes.
Table 4: List of ssODNs targeting the Beta Carotene pathway. Each targeted sequence is indicated with its WT sequence (Column 3) and the ssODN mutation design (Column 4), the type of sequence (Column 5), and notes on the mutation outcome (Column 6). Abbreviations: Transcription factor binding site (TFBS), Transcription start site (TSS). Mutation annotations for oligos: ‘N’=mixed bases A,T,G,C; ‘W’=mixed bases A,T; ‘Y’=mixed bases C,T; ‘R’=mixed bases A,G; ‘K’=mixed bases G,T; ‘M’=mixed bases A,C.
Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference.
Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.
This application claims the benefit of and priority to U.S. Ser. No. 62/328,507 filed Apr. 27, 2016 and which is incorporated by reference in its entirety.
This invention was made with government support under 1122492 awarded by National Science Foundation and N66001-12-C-4020 awarded by Defense Advanced Research Projects Agency. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/029922 | 4/27/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62328507 | Apr 2016 | US |
