Increased efficiency of oligonucleotide-mediated gene repair through slowing replication fork progression

Mono- and biallelic replication-coupled gene editing discriminates dominant-negative and loss-of-function variants of DNA mismatch repair genes

Replication-coupled gene editing using locked-nucleic-acid-modified single-stranded oligodeoxyribonucleotides (LMOs) can genetically engineer mammalian cells with high precision at single nucleotide resolution. Based on this method, we developed oligonucleotide-directed mutation screening (ODMS) to determine whether variants of uncertain clinical significance of DNA mismatch-repair (MMR) genes can cause Lynch syndrome. In ODMS, the appearance of 6-thioguanine (6TG)-resistant colonies upon introduction of the variant is indicative for defective MMR and hence pathogenicity. Whereas previously mouse embryonic stem cells (mESCs) hemizygous for DNA mismatch-repair (MMR) genes were used, we now show that ODMS can also be applied in wild-type mESCs carrying two functional alleles of each MMR gene. 6TG resistance can result from two possible events: first, the mutation is present in only one allele, which is indicative for dominant-negative activity of the variant; second, both alleles contain the planned modification, which is indicative for a regular loss-of-function variant. Thus, ODMS in wild-type mESCs can discriminate fully disruptive and dominant-negative MMR variants. The feasibility of biallelic targeting suggested that the efficiency of LMO-mediated gene targeting at a non-selectable locus may be enriched in cells that had undergone a simultaneous selectable LMO targeting event. This turned out to be the case and provided a protocol to improve recovery of LMO-mediated gene modification events.

On the Origins of Homology Directed Repair in Mammalian Cells

Over the course of the last five years, expectations surrounding our capacity to selectively modify the human genome have never been higher. The reduction to practice site-specific nucleases designed to cleave at a unique site within the DNA is now centerstage in the development of effective molecular therapies. Once viewed as being impossible, this technology now has great potential and, while cellular and molecular barriers persist to clinical implementations, there is little doubt that these barriers will be crossed, and human beings will soon be treated with gene editing tools. The most ambitious of these desires is the correction of genetic mutations resident within the human genome that are responsible for oncogenesis and a wide range of inherited diseases. The process by which gene editing activity could act to reverse these mutations to wild-type and restore normal protein function has been generally categorized as homology directed repair. This is a catch-all basket term that includes the insertion of short fragments of DNA, the replacement of long fragments of DNA, and the surgical exchange of single bases in the correction of point mutations. The foundation of homology directed repair lies in pioneering work that unravel the mystery surrounding genetic exchange using single-stranded DNA oligonucleotides as the sole gene editing agent. Single agent gene editing has provided guidance on how to build combinatorial approaches to human gene editing using the remarkable programmable nuclease complexes known as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and their closely associated (Cas) nucleases. In this manuscript, we outline the historical pathway that has helped evolve the current molecular toolbox being utilized for the genetic re-engineering of the human genome.

Gene Correction of Point Mutations Using PolyPurine Reverse Hoogsteen Hairpins Technology

Monogenic disorders are often the result of single point mutations in specific genes, leading to the production of non-functional proteins. Different blood disorders such as ß-thalassemia, sickle cell disease, hereditary spherocytosis, Fanconi anemia, and Hemophilia A and B are usually caused by point mutations. Gene editing tools including TALENs, ZFNs, or CRISPR/Cas platforms have been developed to correct mutations responsible for different diseases. However, alternative molecular tools such as triplex-forming oligonucleotides and their derivatives (e.g., peptide nucleic acids), not relying on nuclease activity, have also demonstrated their ability to correct mutations in the DNA. Here, we review the Repair-PolyPurine Reverse Hoogsteen hairpins (PPRHs) technology, which can represent an alternative gene editing tool within this field. Repair-PPRHs are non-modified single-stranded DNA molecules formed by two polypurine mirror repeat sequences linked by a five-thymidine bridge, followed by an extended sequence at one end of the molecule which is homologous to the DNA sequence to be repaired but containing the corrected nucleotide. The two polypurine arms of the PPRH are bound by intramolecular reverse-Hoogsteen bonds between the purines, thus forming a hairpin structure. This hairpin core binds to polypyrimidine tracts located relatively near the target mutation in the dsDNA in a sequence-specific manner by Watson-Crick bonds, thus producing a triplex structure which stimulates recombination. This technology has been successfully employed to repair a collection of mutants of the dhfr and aprt genes within their endogenous loci in mammalian cells and could be suitable for the correction of mutations responsible for blood disorders.

Bacterial Genetic Engineering by Means of Recombineering for Reverse Genetics

Serving a robust platform for reverse genetics enabling the in vivo study of gene functions primarily in enterobacteriaceae, recombineering -or recombination-mediated genetic engineering-represents a powerful and relative straightforward genetic engineering tool. Catalyzed by components of bacteriophage-encoded homologous recombination systems and only requiring short ∼40–50 base homologies, the targeted and precise introduction of modifications (e.g., deletions, knockouts, insertions and point mutations) into the chromosome and other episomal replicons is empowered. Furthermore, by its ability to make use of both double- and single-stranded linear DNA editing substrates (e.g., PCR products or oligonucleotides, respectively), lengthy subcloning of specific DNA sequences is circumvented. Further, the more recent implementation of CRISPR-associated endonucleases has allowed for more efficient screening of successful recombinants by the selective purging of non-edited cells, as well as the creation of markerless and scarless mutants. In this review we discuss various recombineering strategies to promote different types of gene modifications, how they are best applied, and their possible pitfalls.

Targeting ideal oral vaccine vectors based on probiotics: a systematical view

Probiotics have great potential to be engineered into oral vaccine delivery systems, which can facilitate elicitation of mucosal immunity without latent risks of pathogenicity. Combined with the progressive understanding of probiotics and the mucosal immune system as well as the advanced biotechniques of genetic engineering, the development of promising oral vaccine vectors based on probiotics is available while complicated and demanding. Therefore, a systematical view on the design of practical probiotic vectors is necessary, which will help to logically analyze and resolve the problems that might be neglected during our exploration. Here, we attempt to systematically summarize several fundamental issues vital to the effectiveness of the vector of probiotics, including the stability of the engineered vectors, the optimization of antigen expression, the improvement of colonization, and the enhancement of immunoreactivity. We also compared the existent strategies and some developing ones, attempting to figure out an optimal strategy that might deserve to be referred in the future development of oral vaccine vectors based on probiotics.

Genome-wide Mapping of Off-Target Events in Single-Stranded Oligodeoxynucleotide-Mediated Gene Repair Experiments

Short single-stranded oligodeoxynucleotides are versatile molecular tools used in different applications. They enable gene repair and genome editing, and they are central to the antisense technology. Because the usability of single-stranded oligodeoxynucleotides depends on their efficiencies, as well as their specificities, analyzing their genotoxic off-target activities is important. Thus, we have developed a protocol that follows the fate of a biotin-labeled single-stranded oligodeoxynucleotide in human cells based on its physical incorporation into the targeted genome. Affected chromosomal fragments are enriched and preferably sequenced by nanopore sequencing. This protocol was validated in gene repair experiments without intentionally inducing a DNA double-strand break. For a 21-nucleotide-long phosphorothioate-modified oligodeoxynucleotide, we compiled a broad array of error-free incorporations, point mutations, indels, and structural rearrangements from actively dividing HEK293-derived cells. Additionally, we demonstrated the usefulness of this approach for primary cells by treating human CD34⁺ hematopoietic stem and progenitor cells with a 100-nucleotide-long unmodified oligodeoxynucleotide directed against the endogenous CYBB locus. This work should pave the way for future genotoxicity analyses. Concerning genome engineering approaches based on nuclease-induced DNA double-strand breaks, this protocol could aid in detecting the unwanted effects caused by the donor fragments themselves.

Mechanisms of precise genome editing using oligonucleotide donors

The use of programmable meganucleases is transforming genome editing and functional genomics. CRISPR/Cas9 was developed such that targeted genomic lesions could be introduced in vivo with unprecedented ease. In the presence of homology donors, these lesions facilitate high-efficiency precise genome editing (PGE) via homology-directed repair (HDR) pathways. However, the identity and hierarchy of the HDR (sub)pathways leading to the formation of PGE products remain elusive. Here, we established a green to blue fluorescent protein conversion system to systematically characterize oligodeoxynucleotide (ODN)-mediated PGE using Cas9 and its nickase variants in human cells. We demonstrate that, unlike double-stranded DNA (dsDNA) donors with central heterologies, ODNs generated short conversion tracts with Gaussian-like distributions. Interestingly, single-nick–induced PGE using ODN donors produced conversion tracts biased either mostly uni- or bidirectional depending on the relative strandedness of the ODNs and the nick. Moreover, the ODNs were physically incorporated into the genome only in the bidirectional, but not in the unidirectional, conversion pathway. In the presence of double-stranded genomic lesions, the unidirectional conversion pathway was preferentially utilized even though the knock-in mutation could theoretically have been converted by both pathways. Collectively, our results suggest that ODN-mediated PGE utilizes synthesis-dependent strand annealing and single-stranded DNA incorporation pathways. Both of these pathways generate short conversion tracts with Gaussian-like distributions. Although synthesis-dependent strand annealing is preferentially utilized, our work unequivocally establishes the existence of a single-stranded DNA incorporation pathway in human cells. This work extends the paradigms of HDR-mediated gene conversion and establishes guidelines for PGE in human cells.

Regulation of Gene Editing Activity Directed by Single-Stranded Oligonucleotides and CRISPR/Cas9 Systems

Single-stranded DNA oligonucleotides (ssODNs) can direct the repair of a single base mutation in human genes. While the regulation of this gene editing reaction has been partially elucidated, the low frequency with which repair occurs has hampered development toward clinical application. In this work a CRISPR/Cas9 complex is employed to induce double strand DNA breakage at specific sites surrounding the nucleotide designated for exchange. The result is a significant elevation in ssODN-directed gene repair, validated by a phenotypic readout. By analysing reaction parameters, we have uncovered restrictions on gene editing activity involving CRISPR/Cas9 complexes. First, ssODNs that hybridize to the non-transcribed strand direct a higher level of gene repair than those that hybridize to the transcribed strand. Second, cleavage must be proximal to the targeted mutant base to enable higher levels of gene editing. Third, DNA cleavage enables a higher level of gene editing activity as compared to single-stranded DNA nicks, created by modified Cas9 (Nickases). Fourth, we calculated the hybridization potential and free energy levels of ssODNs that are complementary to the guide RNA sequences of CRISPRs used in this study. We find a correlation between free energy potential and the capacity of single-stranded oligonucleotides to inhibit specific DNA cleavage activity, thereby indirectly reducing gene editing activity. Our data provide novel information that might be taken into consideration in the design and usage of CRISPR/Cas9 systems with ssODNs for gene editing.

Replicative DNA polymerase δ but not ε proofreads errors in Cis and in Trans

It is now well established that in yeast, and likely most eukaryotic organisms, initial DNA replication of the leading strand is by DNA polymerase ε and of the lagging strand by DNA polymerase δ. However, the role of Pol δ in replication of the leading strand is uncertain. In this work, we use a reporter system in Saccharomyces cerevisiae to measure mutation rates at specific base pairs in order to determine the effect of heterozygous or homozygous proofreading-defective mutants of either Pol ε or Pol δ in diploid strains. We find that wild-type Pol ε molecules cannot proofread errors created by proofreading-defective Pol ε molecules, whereas Pol δ can not only proofread errors created by proofreading-defective Pol δ molecules, but can also proofread errors created by Pol ε-defective molecules. These results suggest that any interruption in DNA synthesis on the leading strand is likely to result in completion by Pol δ and also explain the higher mutation rates observed in Pol δ-proofreading mutants compared to Pol ε-proofreading defective mutants. For strains reverting via AT→GC, TA→GC, CG→AT, and GC→AT mutations, we find in addition a strong effect of gene orientation on mutation rate in proofreading-defective strains and demonstrate that much of this orientation dependence is due to differential efficiencies of mispair elongation. We also find that a 3′-terminal 8 oxoG, unlike a 3′-terminal G, is efficiently extended opposite an A and is not subject to proofreading. Proofreading mutations have been shown to result in tumor formation in both mice and humans; the results presented here can help explain the properties exhibited by those proofreading mutants.

Many DNA polymerases are able to proofread their errors: after incorporation of a wrong base, the resulting mispair invokes an exonuclease activity of the polymerase that removes the mispaired base and allows replication to continue. Elimination of the proofreading activity thus results in much higher mutation rates. We demonstrate that the two major replicative DNA polymerases in yeast, Pol δ and Pol ε, have different proofreading abilities. In diploid cells, Pol ε is not able to proofread errors created by other Pol ε molecules, whereas Pol δ can proofread not only errors created by other Pol δ molecules but also errors created by Pol ε molecules. We also find that mispaired bases not corrected by proofreading have much different likelihoods of being extended, depending on the particular base-base mismatch. In humans, defects in Pol δ or Pol ε proofreading can lead to cancer, and these results help explain the formation of those tumors and the finding that Pol ε mutants seem to be found as frequently, or more so, in human tumors as Pol δ mutants.

Stimulation of oligonucleotide-directed gene correction by Redβ expression and MSH2 depletion in human HT1080 cells

The correction of disease-causing mutations by single-strand oligonucleotide-templated DNA repair (ssOR) is an attractive approach to gene therapy, but major improvements in ssOR efficiency and consistency are needed. The mechanism of ssOR is poorly understood but may involve annealing of oligonucleotides to transiently exposed single-stranded regions in the target duplex. In bacteria and yeast it has been shown that ssOR is promoted by expression of Redβ, a single-strand DNA annealing protein from bacteriophage lambda. Here we show that Redβ expression is well tolerated in a human cell line where it consistently promotes ssOR. By use of short interfering RNA, we also show that ssOR is stimulated by the transient depletion of the endogenous DNA mismatch repair protein MSH2. Furthermore, we find that the effects of Redβ expression and MSH2 depletion on ssOR can be combined with a degree of cooperativity. These results suggest that oligonucleotide annealing and mismatch recognition are distinct but interdependent events in ssOR that can be usefully modulated in gene correction strategies.

Combinatorial gene editing in mammalian cells using ssODNs and TALENs

The regulation of gene editing is being elucidated in mammalian cells and its potential as well as its limitations are becoming evident. ssODNs carry out gene editing by annealing to their complimentary sequence at the target site and acting as primers for replication fork extension. To effect a genetic change, a large amount of ssODN molecules must be introduced into cells and as such induce a Reduced Proliferation Phenotype (RPP), a phenomenon in which corrected cells do not proliferate. To overcome this limitation, we have used TAL-Effector Nucleases (TALENs) to increase the frequency, while reducing the amount of ssODN required to direct gene correction. This strategy resolves the problem and averts the serious effects of RPP. The efficiency of gene editing can be increased significantly if cells are targeted while they progress through S phase. Our studies define new reaction parameters that will help guide experimental strategies of gene editing.

Oligonucleotide delivery by nucleofection does not rescue the reduced proliferation phenotype of gene-edited cells

Gene editing using single-stranded oligonucleotides (ODNs) can be used to reverse or create a single base mutation in mammalian cells. This approach could be used to treat genetic diseases caused, at least in part, by a nucleotide substitution. The technique could also be used as a tool to establish single base polymorphisms at multiple sites and thus aid in creating cell lines that can be used to define the basis for drug resistance in human cells. A troubling outcome of the gene-editing reaction is the effect on normal growth of cells that have undergone nucleotide exchange. In this work, we attempt to overcome this reduced proliferation phenotype by changing the method by which the ODN is introduced into the target cell. Using a series of assays that measure gene editing, DNA damage response, and cell viability, we report that chemically modified ODNs, the most active form of ODN for gene editing, can be used successfully if introduced into the cell by the method of nucleofection. Unlike electroporation, which has been used as the standard mode of ODN delivery, one new result is that nucleofection does not induce a dramatic loss of viability within the first 24 hours after the start of gene editing. In addition, and importantly, ODNs introduced to the cell by nucleofection do not activate the DNA damage response pathway as dramatically as ODNs introduced by electroporation. These 2 novel findings are encouraging for the application of gene editing in other systems. However, reduced proliferation phenotype is still observed when the population of corrected cells is monitored out to 8 days, and thus, delivery by nucleofection does not solve the proliferation problem encountered by cells bearing an edited gene.

Stable gene targeting in human cells using single-strand oligonucleotides with modified bases

Recent advances allow multiplexed genome engineering in E. coli, employing easily designed oligonucleotides to edit multiple loci simultaneously. A similar technology in human cells would greatly expedite functional genomics, both by enhancing our ability to test how individual variants such as single nucleotide polymorphisms (SNPs) are related to specific phenotypes, and potentially allowing simultaneous mutation of multiple loci. However, oligo-mediated targeting of human cells is currently limited by low targeting efficiencies and low survival of modified cells. Using a HeLa-based EGFP-rescue reporter system we show that use of modified base analogs can increase targeting efficiency, in part by avoiding the mismatch repair machinery. We investigate the effects of oligonucleotide toxicity and find a strong correlation between the number of phosphorothioate bonds and toxicity. Stably EGFP-corrected cells were generated at a frequency of ~0.05% with an optimized oligonucleotide design combining modified bases and reduced number of phosphorothioate bonds. We provide evidence from comparative RNA-seq analysis suggesting cellular immunity induced by the oligonucleotides might contribute to the low viability of oligo-corrected cells. Further optimization of this method should allow rapid and scalable genome engineering in human cells.

DNA damage response pathway and replication fork stress during oligonucleotide directed gene editing

Single-stranded DNA oligonucleotides (ODNs) can be used to direct the exchange of nucleotides in the genome of mammalian cells in a process known as gene editing. Once refined, gene editing should become a viable option for gene therapy and molecular medicine. Gene editing is regulated by a number of DNA recombination and repair pathways whose natural activities often lead to single- and double-stranded DNA breaks. It has been previously shown that introduction of a phosphorotioated ODN, designed to direct a gene-editing event, into cells results in the activation of γH2AX, a well-recognized protein biomarker for double-stranded DNA breakage. Using a single copy, integrated mutant enhanced green fluorescent protein (eGFP) gene as our target, we now demonstrate that several types of ODNs, capable of directing gene editing, also activate the DNA damage response and the post-translational modification of proliferating cell nuclear antigen (PCNA), a signature modification of replication stress. We find that the gene editing reaction itself leads to transient DNA breakage, perhaps through replication fork collapse. Unmodified specific ODNs elicit a lesser degree of replication stress than their chemically modified counterparts, but are also less active in gene editing. Modified phosphothioate oligonucleotides (PTOs) are detrimental irrespective of the DNA sequence. Such collateral damage may prove problematic for proliferation of human cells genetically modified by gene editing.

Transient suppression of MLH1 allows effective single-nucleotide substitution by single-stranded DNA oligonucleotides

Short synthetic single-stranded oligodeoxyribonucleotides (ssODNs) can be used to introduce subtle modifications into the genome of mouse embryonic stem cells (ESCs). We have previously shown that effective application of ssODN-mediated gene targeting in ESC requires (transient) suppression of DNA mismatch repair (MMR). However, whereas transient down-regulation of the mismatch recognition protein MSH2 allowed substitution of 3 or 4 nucleotides, 1 or 2 nucleotide substitutions were still suppressed. We now demonstrate that single- or dinucleotide substitution can effectively be achieved by transient down-regulation of the downstream MMR protein MLH1. By exploiting highly specific real-time PCR, we demonstrate the feasibility of substituting a single basepair in a non-selectable gene. However, disabling the MMR machinery may lead to inadvertent mutations. To obtain insight into the mutation rate associated with transient MMR suppression, we have compared the impact of transient and constitutive MMR deficiency on the repair of frameshift intermediates at mono- and dinucleotide repeats. Repair at these repeats relied on the substrate specificity and functional redundancy of the MSH2/MSH6 and MSH2/MSH3 MMR complexes. MLH1 knockdown increased the level of spontaneous mutagenesis, but modified ESCs remained germ line competent. Thus, transient MLH1 suppression provides a valuable extension of the MSH2 knockdown strategy, allowing rapid generation of mice carrying single basepair alterations in their genome.

Oligo/polynucleotide-based gene modification: strategies and therapeutic potential

Oligonucleotide- and polynucleotide-based gene modification strategies were developed as an alternative to transgene-based and classical gene targeting-based gene therapy approaches for treatment of genetic disorders. Unlike the transgene-based strategies, oligo/polynucleotide gene targeting approaches maintain gene integrity and the relationship between the protein coding and gene-specific regulatory sequences. Oligo/polynucleotide-based gene modification also has several advantages over classical vector-based homologous recombination approaches. These include essentially complete homology to the target sequence and the potential to rapidly engineer patient-specific oligo/polynucleotide gene modification reagents. Several oligo/polynucleotide-based approaches have been shown to successfully mediate sequence-specific modification of genomic DNA in mammalian cells. The strategies involve the use of polynucleotide small DNA fragments, triplex-forming oligonucleotides, and single-stranded oligodeoxynucleotides to mediate homologous exchange. The primary focus of this review will be on the mechanistic aspects of the small fragment homologous replacement, triplex-forming oligonucleotide-mediated, and single-stranded oligodeoxynucleotide-mediated gene modification strategies as it relates to their therapeutic potential.

Strand bias influences the mechanism of gene editing directed by single-stranded DNA oligonucleotides

Gene editing directed by modified single-stranded DNA oligonucleotides has been used to alter a single base pair in a variety of biological systems. It is likely that gene editing is facilitated by the direct incorporation of the oligonucleotides via replication and/or by direct conversion, most likely through the DNA mismatch repair pathway. The phenomenon of strand bias, however, as well as its importance to the gene editing reaction itself, has yet to be elucidated in terms of mechanism. We have taken a reductionist approach by using a genetic readout in Eschericha coli and a plasmid-based selectable system to evaluate the influence of strand bias on the mechanism of gene editing. We show that oligonucleotides (ODNs) designed to anneal to the lagging strand generate 100-fold greater 'editing' efficiency than 'those that anneal to' the leading strand. The majority of editing events (∼70%) occur by the incorporation of the ODN during replication within the lagging strand. Conversely, ODNs that anneal to the leading strand generate fewer editing events although this event may follow either the incorporation or direct conversion pathway. In general, the influence of DNA replication is independent of which ODN is used suggesting that the importance of strand bias is a reflection of the underlying mechanism used to carry out gene editing.

Gene editing activity on extrachromosomal arrays in C. elegans transgenics

Gene editing by modified single-stranded oligonucleotides is a strategy aimed at inducing single base changes into the genome, generating a permanent genetic change. The work presented here explores gene editing capabilities in the model organism Caenorhabditis elegans. Current approaches to gene mutagenesis in C. elegans have been plagued by non-specificity and thus the ability to induce precise, directed alterations within the genome of C. elegans would offer a platform upon which structure/function analyses can be carried out. As such, several in vivo assay systems were developed to evaluate gene editing capabilities in C. elegans. Fluorescence was chosen as the selectable endpoint as fluorescence can be easily detected through the transparent worm body even from minimal expression. Two tissue specific fluorescent expression vectors containing either a GFP or mCherry transgene were mutagenized to create a single nonsense mutation within the open reading frame of each respective fluorescent gene. These served as the target site to evaluate the frequency of gene editing on extrachromosomal array transgenic lines. Extrachromosomal arrays can carry hundreds of copies of the transgene, therefore low frequency events (like those in the gene editing reaction) may be detected. Delivery of the oligonucleotide was accomplished by microinjection into the gonads of young adult worms in an effort to induce repair of the mutated fluorescent gene in the F1 progeny. Despite many microinjections on the transgenic strains with varying concentrations of ODNs, no gene editing events were detected. This result is consistent with the previous research, demonstrating the difficulties encountered in targeting embryonic stem cells and the pronuclei of single-celled embryos.

Multiplexed genome engineering and genotyping methods applications for synthetic biology and metabolic engineering

Engineering at the scale of whole genomes requires fundamentally new molecular biology tools. Recent advances in recombineering using synthetic oligonucleotides enable the rapid generation of mutants at high efficiency and specificity and can be implemented at the genome scale. With these techniques, libraries of mutants can be generated, from which individuals with functionally useful phenotypes can be isolated. Furthermore, populations of cells can be evolved in situ by directed evolution using complex pools of oligonucleotides. Here, we discuss ways to utilize these multiplexed genome engineering methods, with special emphasis on experimental design and implementation.

Progress and prospects: oligonucleotide-directed gene modification in mouse embryonic stem cells: a route to therapeutic application

Gene targeting by single-stranded oligodeoxyribonucleotides (ssODNs) is a promising technique for introducing site-specific sequence alterations without affecting the genomic organization of the target locus. Here, we discuss the significant progress that has been made over the last 5 years in unraveling the mechanisms and reaction parameters underlying ssODN-mediated gene targeting. We will specifically focus on ssODN-mediated gene targeting in murine embryonic stem cells (ESCs) and the impact of the DNA mismatch repair (MMR) system on the targeting process. Implications of novel findings for routine application of ssODN-mediated gene targeting and challenges that need to be overcome for future therapeutic applications are highlighted.

Subtle gene modification in mouse ES cells: evidence for incorporation of unmodified oligonucleotides without induction of DNA damage

Gene targeting by single-stranded oligodeoxyribonucleotides (ssODNs) is a promising tool for site-specific gene modification in mouse embryonic stem cells (ESCs). We have developed an ESC line carrying a mutant EGFP reporter gene to monitor gene correction events shortly after exposure to ssODNs. We used this system to compare the appearance and fate of cells corrected by sense or anti-sense ssODNs. The slower appearance of green fluorescent cells with sense ssODNs as compared to anti-sense ssODNs is consistent with physical incorporation of the ssODN into the genome. Thus, the supremacy of anti-sense ssODNs, previously reported by others, may be an artefact of early readout of the EGFP reporter. Importantly, gene correction by unmodified ssODNs only mildly affected the viability of targeted cells and did not induce genomic DNA double-stranded breaks (DSBs). In contrast, ssODNs that were end-protected by phosphorothioate (PTO) linkages caused increased H2AX phosphorylation and impaired cell cycle progression in both corrected and non-corrected cells due to induction of genomic DSBs. Our results demonstrate that the use of unmodified rather than PTO end-protected ssODNs allows stable gene modification without compromising the genomic integrity of the cell, which is crucial for application of ssODN-mediated gene targeting in (embryonic) stem cells.

Site-directed gene repair of the dystrophin gene mediated by PNA-ssODNs

Permanent correction of gene defects is an appealing approach to the treatment of genetic disorders. The use of single-stranded oligodeoxynucleotides (ssODNs) has been demonstrated to induce single-point mutations in the dystrophin gene and to restore dystrophin expression in the skeletal muscle of models of Duchenne muscular dystrophy (DMD). Here we show that ssODNs made of peptide nucleic acids (PNA-ssODNs) can achieve gene repair frequencies more than 10-fold higher than those obtained using an older generation of targeting oligonucleotides. Correction was demonstrated in muscles cells isolated from mdx(5cv) mice and was stably inherited over time. Direct intramuscular injection of PNA-ssODNs targeting the mdx(5cv) mutation resulted in a significant increase in dystrophin-positive fibers when compared with muscles that received the ssODNs designed to correct the dystrophin gene but made of unmodified bases. Correction was demonstrated at both the mRNA and the DNA levels using quantitative PCR and was confirmed by direct sequencing of amplification products. Analysis at the protein level demonstrated expression of full-length dystrophin in vitro as well as in vivo. These results demonstrate that oligonucleotides promoting strand invasion in the DNA double helix can significantly enhance gene repair frequencies of the dystrophin gene. The use of PNA-ssODNs has important implications in terms of both efficacy and duration of the repair process in muscles and may have a role in advancing the treatment of DMD.

Adeno-associated virus gene repair corrects a mouse model of hereditary tyrosinemia in vivo

Adeno-associated virus (AAV) vectors are ideal for performing gene repair due to their ability to target multiple different genomic loci, low immunogenicity, capability to achieve targeted and stable expression through integration, and low mutagenic and oncogenic potential. However, many handicaps to gene repair therapy remain. Most notable is the low frequency of correction in vivo. To date, this frequency is too low to be of therapeutic value for any disease. To address this, a point-mutation-based mouse model of the metabolic disease hereditary tyrosinemia type I was used to test whether targeted AAV integration by homologous recombination could achieve high-level stable gene repair in vivo. Both neonatal and adult mice were treated with AAV serotypes 2 and 8 carrying a wild-type genomic sequence for repairing the mutated Fah (fumarylacetoacetate hydrolase) gene. Hepatic gene repair was quantified by immunohistochemistry and supported with reverse transcription polymerase chain reaction and serology for functional correction parameters. Successful gene repair was observed with both serotypes but was more efficient with AAV8. Correction frequencies of up to 10(-3) were achieved and highly reproducible within typical dose ranges. In this model, repaired hepatocytes have a selective growth advantage and are thus able to proliferate to efficiently repopulate mutant livers and cure the underlying metabolic disease.

CONCLUSION

AAV-mediated gene repair is feasible in vivo and can functionally correct an appropriate selection-based metabolic liver disease in both adults and neonates.

Cell death caused by single-stranded oligodeoxynucleotide-mediated targeted genomic sequence modification

Targeted gene repair directed by single-stranded oligodeoxynucleotides (ssODNs) offers a promising tool for biotechnology and gene therapy. However, the methodology is currently limited by its low frequency of repair events, variability, and low viability of "corrected" cells. In this study, we showed that during ssODN-mediated gene repair reaction, a significant population of corrected cells failed to divide, and were much more prone to undergo apoptosis, as marked by processing of caspases and PARP-1. In addition, we found that apoptotic cell death triggered by ssODN-mediated gene repair was largely independent of the ATM/ATR kinase. Furthermore, we examined the potential involvement of the mismatch repair (MMR) proteins in this "correction reaction-induced" cell death. Result showed that while defective MMR greatly enhanced the efficiency of gene correction, compromising the MMR system did not yield any viable corrected clone, indicating that the MMR machinery, although plays a critical role in determining ssODN-directed repair, was not involved in the observed cellular genotoxic responses.

Specific targeted gene repair using single-stranded DNA oligonucleotides at an endogenous locus in mammalian cells uses homologous recombination

The feasibility of introducing point mutations in vivo using single-stranded DNA oligonucleotides (ssON) has been demonstrated but the efficiency and mechanism remain elusive and potential side effects have not been fully evaluated. Understanding the mechanism behind this potential therapy may help its development. Here, we demonstrate the specific repair of an endogenous non-functional hprt gene by a ssON in mammalian cells, and show that the frequency of such an event is enhanced when cells are in S-phase of the cell cycle. A potential barrier in using ssONs as gene therapy could be non-targeted mutations or gene rearrangements triggered by the ssON. Both the non-specific mutation frequencies and the frequency of gene rearrangements were largely unaffected by ssONs. Furthermore, we find that the introduction of a mutation causing the loss of a functional endogenous hprt gene by a ssON occurred at a similarly low but statistically significant frequency in wild type cells and in cells deficient in single strand break repair, nucleotide excision repair and mismatch repair. However, this mutation was not induced in XRCC3 mutant cells deficient in homologous recombination. Thus, our data suggest ssON-mediated targeted gene repair is more efficient in S-phase and involves homologous recombination.

Parameters of oligonucleotide-mediated gene modification in mouse ES cells

Gene targeting by single-stranded oligodeoxyribonucleotides (ssODNs) is emerging as a powerful tool for the introduction of subtle gene modifications in mouse embryonic stem (ES) cells and the generation of mutant mice. Here, we have studied the role of ssODN composition, transcription and replication of the target locus, and DNA repair pathways to gain more insight into the parameters governing ssODN-mediated gene targeting in mouse ES cells. We demonstrated that unmodified ssODNs of 35–40 nt were most efficient in correcting a chromosomally integrated mutant neomycin reporter gene. Addition of chemical modifications did not further enhance the efficacy of these ssODNs. The observed strand bias was not affected by transcriptional activity and may rather be caused by the different accessibility of the DNA strands during DNA replication. Consistently, targeting frequencies were enhanced when cells were treated with hydroxyurea to reduce the rate of replication fork progression. Transient down-regulation of various DNA repair genes by RNAi had no effect on the targeting frequency. Taken together, our data suggest that ssODN-mediated gene targeting occurs within the context of a replication fork. This implies that any given genomic sequence, irrespective of transcriptional status, should be amenable to ssODN-mediated gene targeting. The ability of ES cells to differentiate into various cell types after ssODN-mediated gene targeting may offer opportunities for future therapeutic applications.

DNA breakage associated with targeted gene alteration directed by DNA oligonucleotides

Understanding the mechanism by which single-stranded oligonucleotides (ODNs) elicit targeted nucleotide exchange (TNE) is imperative to achieving optimal correction efficiencies and medical applicability. It has been previously shown that introduction of an ODN into cells results in the activation of DNA damage response pathways, but there has been no evaluation of the damage created at the level of the DNA. The activation of H2AX, a hallmark protein of DNA breakage, suggests that a double-strand break (DSB) could be occurring during the targeted gene alteration (TGA) reaction. Using the human HCT116 cell line with a single integrated mutant eGFP gene as our model system, we demonstrate that the DNA strand breakage occurs when a specific ODN, designed to direct TGA, is transfected into the cells. Both single- and double-stranded DNA cleavage is observed dependent on the level of ODN added to the reaction. Possible mechanisms of ODN-dependent DSB formation, as a function of TGA, are discussed herein.

A comparison of synthetic oligodeoxynucleotides, DNA fragments and AAV-1 for targeted episomal and chromosomal gene repair

Background

Current strategies for gene therapy of inherited diseases consist in adding functional copies of the gene that is defective. An attractive alternative to these approaches would be to correct the endogenous mutated gene in the affected individual. This study presents a quantitative comparison of the repair efficiency using different forms of donor nucleic acids, including synthetic DNA oligonucleotides, double stranded DNA fragments with sizes ranging from 200 to 2200 bp and sequences carried by a recombinant adeno-associated virus (rAAV-1). Evaluation of each gene repair strategy was carried out using two different reporter systems, a mutated eGFP gene or a dual construct with a functional eGFP and an inactive luciferase gene, in several different cell systems. Gene targeting events were scored either following transient co-transfection of reporter plasmids and donor DNAs, or in a system where a reporter construct was stably integrated into the chromosome.

Results

In both episomal and chromosomal assays, DNA fragments were more efficient at gene repair than oligonucleotides or rAAV-1. Furthermore, the gene targeting frequency could be significantly increased by using DNA repair stimulating drugs such as doxorubicin and phleomycin.

Conclusion

Our results show that it is possible to obtain repair frequencies of 1% of the transfected cell population under optimized transfection protocols when cells were pretreated with phleomycin using rAAV-1 and dsDNA fragments.

Targeted correction of a thalassemia-associated beta-globin mutation induced by pseudo-complementary peptide nucleic acids

β-Thalassemia is a genetic disorder caused by mutations in the β-globin gene. Triplex-forming oligonucleotides and triplex-forming peptide nucleic acids (PNAs) have been shown to stimulate recombination in mammalian cells via site-specific binding and creation of altered helical structures that provoke DNA repair. However, the use of these molecules for gene targeting requires homopurine tracts to facilitate triple helix formation. Alternatively, to achieve binding to mixed-sequence target sites for the induced gene correction, we have used pseudo-complementary PNAs (pcPNAs). Due to steric hindrance, pcPNAs are unable to form pcPNA–pcPNA duplexes but can bind to complementary DNA sequences via double duplex-invasion complexes. We demonstrate here that pcPNAs, when co-transfected with donor DNA fragments, can promote single base pair modification at the start of the second intron of the beta-globin gene. This was detected by the restoration of proper splicing of transcripts produced from a green fluorescent protein-beta globin fusion gene. We also demonstrate that pcPNAs are effective in stimulating recombination in human fibroblast cells in a manner dependent on the nucleotide excision repair factor, XPA. These results suggest that pcPNAs can be effective tools to induce heritable, site-specific modification of disease-related genes in human cells without purine sequence restriction.

Regulation of targeted gene repair by intrinsic cellular processes

Targeted gene alteration (TGA) is a strategy for correcting single base mutations in the DNA of human cells that cause inherited disorders. TGA aims to reverse a phenotype by repairing the mutant base within the chromosome itself, avoiding the introduction of exogenous genes. The process of how to accurately repair a genetic mutation is elucidated through the use of single-stranded DNA oligonucleotides (ODNs) that can enter the cell and migrate to the nucleus. These specifically designed ODNs hybridize to the target sequence and act as a beacon for nucleotide exchange. The key to this reaction is the frequency with which the base is corrected; this will determine whether the approach becomes clinically relevant or not. Over the course of the last five years, workers have been uncovering the role played by the cells in regulating the gene repair process. In this essay, we discuss how the impact of the cell on TGA has evolved through the years and illustrate ways that inherent cellular pathways could be used to enhance TGA activity. We also describe the cost to cell metabolism and survival when certain processes are altered to achieve a higher frequency of repair.

Improvement of SSO-mediated gene repair efficiency by nonspecific oligonucleotides

Targeted gene repair mediated by single-stranded DNA oligonucleotides (SSOs) is a promising method to correct the mutant gene precisely in prokaryotic and eukaryotic systems. We used a HeLa cell line, which was stably integrated with mutant enhanced green fluorescence protein gene (mEGFP) in the genome, to test the efficiency of SSO-mediated gene repair. We found that the mEGFP gene was successfully repaired by specific SSOs, but the efficiency was only approximately 0.1%. Then we synthesized a series of nonspecific oligonucleotides, which were single-stranded DNA with different lengths and no significant similarity with the SSOs. We found the efficiency of SSO-mediated gene repair was increased by 6-fold in nonspecific oligonucleotides-treated cells. And this improvement in repair frequency correlated with the doses of the nonspecific oligonucleotides, instead of the lengths. Our evidence suggested that this increased repair efficiency was achieved by the transient alterations of the cellular proteome. We also found the obvious strand bias that antisense SSOs were much more effective than sense SSOs in the repair experiments with nonspecific oligonucleotides. These results provide a fresh clue into the mechanism of SSO-mediated targeted gene repair in mammalian cells.

Endothelium-specific overexpression of class III deacetylase SIRT1 decreases atherosclerosis in apolipoprotein E-deficient mice

AIMS

Hazardous environmental and genetic factors can damage endothelial cells to induce atherosclerotic vascular disease. Recent studies suggest that class III deacetylase SIRT1 may promote cell survival via novel antioxidative mechanisms. The current study tested the hypothesis that SIRT1, specifically overexpressed in the endothelium, is atheroprotective.

METHODS AND RESULTS

Human umbilical vein endothelial cells (HUVECs) were used to study the effects of oxidized low-density lipoprotein (LDL) on SIRT1 expression. Endothelial cell-specific SIRT1 transgenic (SIRT1-Tg) mice were used to study the effects of SIRT1 on aortic vascular tone. SIRT1-Tg mice were crossed with apolipoprotein E null (apoE(-/-)) mice to obtain SIRT1-Tg/apoE(-/-) mice for the analysis of atherogenesis in the presence of endothelial overexpression of SIRT1. SIRT1 expression in HUVECs was increased by the treatment with oxidative LDL. Adenoviral-mediated overexpression of SIRT1 was protective of apoptosis of HUVECs. Calorie restriction increased, whereas high-fat diet decreased, the SIRT1 expression in mouse aortas. In SIRT1-Tg mice, high fat-induced impairment in endothelium-dependent vasorelaxation was improved compared with that of wild-type littermates. This was accompanied by an upregualtion of aortic endothelial nitric oxide synthase expression in the SIRT1-Tg mice. The SIRT1-Tg/apoE(-/-) mice had less atherosclerotic lesions compared with apoE(-/-) controls, without affecting blood lipids and glucose levels.

CONCLUSION

These results suggest that endothelium-specific SIRT1 overexpression likely suppresses atherogenesis via improving endothelial cell survival and function.

Double-stranded break can be repaired by single-stranded oligonucleotides via the ATM/ATR pathway in mammalian cells

Single-stranded oligonucleotide (SSO)-mediated gene modification is a newly developed tool for site-specific gene repair in mammalian cells; however, the corrected cells always show G2/M arrest and cannot divide to form colonies. This phenomenon and the unclear mechanism seriously challenge the future application of this technique. In this study, we developed an efficient SSO-mediated DNA repair system based on double-stranded break (DSB) induction. We generated a mutant EGFP gene with insertions of 24 bp to 1.6 kb in length as a reporter integrated in mammalian cell lines. SSOs were successfully used to delete the insertion fragments upon DSB induction at a site near the insertion. We demonstrated that this process is dependent on the ATM/ATR pathway. Importantly, repaired cell clones were viable. Effects of deletion length, SSO length, strand bias, and SSO modification on gene repair frequency were also investigated.

Simultaneous targeted exchange of two nucleotides by single-stranded oligonucleotides clusters within a region of about fourteen nucleotides

Background

Transfection of cells with gene-specific, single-stranded oligonucleotides can induce the targeted exchange of one or two nucleotides in the targeted gene. To characterize the features of the DNA-repair mechanisms involved, we examined the maximal distance for the simultaneous exchange of two nucleotides by a single-stranded oligonucleotide. The chosen experimental system was the correction of a hprt-point mutation in a hamster cell line, the generation of an additional nucleotide exchange at a variable distance from the first exchange position and the investigation of the rate of simultaneous nucleotide exchanges.

Results

The smaller the distance between the two exchange positions, the higher was the probability of a simultaneous exchange. The detected simultaneous nucleotide exchanges were found to cluster in a region of about fourteen nucleotides upstream and downstream from the first exchange position.

Conclusion

We suggest that the mechanism involved in the repair of the targeted DNA strand utilizes only a short sequence of the single-stranded oligonucleotide, which may be physically incorporated into the DNA or be used as a matrix for a repair process.

Targeted gene correction with 5' acridine-oligonucleotide conjugates

Single-stranded oligonucleotides (SSOs) mediate gene repair of punctual chromosomal mutations at a low frequency. We hypothesized that enhancement of DNA binding affinity of SSOs by intercalating agents may increase the number of corrected cells. Several biochemical modifications of SSOs were tested for their capability to correct a chromosomally integrated and mutated GFP reporter gene in human 293 cells. SSOs of 25 nucleotide length conjugated with acridine at their 5' end increased the efficiency of gene correction up to 10-fold compared to nonmodified SSOs. Acridine and psoralen conjugates were both evaluated, and acridine-modified SSOs were the most effective. Conjugation with acridine at the 3' end of the SSO inhibited gene correction, whereas flanking the SSO by acridine on both sides provided an intermediate level of correction. These results suggest that increasing the stability of hybridization between SSO and its target without hampering a 3' extension improves gene targeting, in agreement with the "annealing-integration" model of DNA repair.

Oligonucleotide transformation of yeast reveals mismatch repair complexes to be differentially active on DNA replication strands

Transformation of both prokaryotes and eukaryotes with single-stranded oligonucleotides can transfer sequence information from the oligonucleotide to the chromosome. We have studied this process using oligonucleotides that correct a -1 frameshift mutation in the LYS2 gene of Saccharomyces cerevisiae. We demonstrate that transformation by oligonucleotides occurs preferentially on the lagging strand of replication and is strongly inhibited by the mismatch-repair system. These results are consistent with a mechanism in which oligonucleotides anneal to single-stranded regions of DNA at a replication fork and serve as primers for DNA synthesis. Because the mispairs the primers create are efficiently removed by the mismatch-repair system, single-stranded oligonucleotides can be used to probe mismatch-repair function in a chromosomal context. Removal of mispairs created by annealing of the single-stranded oligonucleotides to the chromosomal DNA is as expected, with 7-nt loops being recognized solely by MutS beta and 1-nt loops being recognized by both MutS alpha and MutS beta. We also find evidence for Mlh1-independent repair of 7-nt, but not 1-nt, loops. Unexpectedly, we find a strand asymmetry of mismatch-repair function; transformation is blocked more efficiently by MutS alpha on the lagging strand of replication, whereas MutS beta does not show a significant strand bias. These results suggest an inherent strand-related difference in how the yeast MutS alpha and MutS beta complexes access and/or repair mismatches that arise in the context of DNA replication.

Manipulation of cell cycle progression can counteract the apparent loss of correction frequency following oligonucleotide-directed gene repair

Background

Single-stranded oligonucleotides (ssODN) are used routinely to direct specific base alterations within mammalian genomes that result in the restoration of a functional gene. Despite success with the technique, recent studies have revealed that following repair events, correction frequencies decrease as a function of time, possibly due to a sustained activation of damage response signals in corrected cells that lead to a selective stalling. In this study, we use thymidine to slow down the replication rate to enhance repair frequency and to maintain substantial levels of correction over time.

Results

First, we utilized thymidine to arrest cells in G1 and released the cells into S phase, at which point specific ssODNs direct the highest level of correction. Next, we devised a protocol in which cells are maintained in thymidine following the repair reaction, in which the replication is slowed in both corrected and non-corrected cells and the initial correction frequency is retained. We also present evidence that cells enter a senescence state upon prolonged treatment with thymidine but this passage can be avoided by removing thymidine at 48 hours.

Conclusion

Taken together, we believe that thymidine may be used in a therapeutic fashion to enable the maintenance of high levels of treated cells bearing repaired genes.

Active transcription promotes single-stranded oligonucleotide mediated gene repair

The lambda-Red-mediated recombination has been exploited as an efficient means for DNA manipulation. We previously reported that replication plays a pivotal role during this process. Replication direction dictates strand bias, such that single stranded oligonucleotide (SSO) with sequence corresponding to the nascent lagging strand directs higher levels of recombinant formation compared to its complementary SSO. In addition, the Escherichia coli methyl-directed mismatch repair system impedes efficient SSO-mediated site-specific gene repair. However, the role of transcription in determining strand bias and recombination efficiency is unclear. To address the potential role of transcriptional processes, we constructed plasmid substrates that harbor a mutant antibiotic reporter under the control of an inducible promoter. We found that transcription activation can promote recombinant formation to more than 10-folds whilst it has negligible effect on strand bias. Our findings provide evidence for a role of transcription in SSO-mediated gene repair process.

The involvement of replication in single stranded oligonucleotide-mediated gene repair

Targeted gene repair mediated by single-stranded oligonucleotides (SSOs) has great potential for use in functional genomic studies and gene therapy. Genetic changes have been created using this approach in a number of prokaryotic and eukaryotic systems, including mouse embryonic stem cells. However, the underlying mechanisms remain to be fully established. In one of the current models, the ‘annealing-integration’ model, the SSO anneals to its target locus at the replication fork, serving as a primer for subsequent DNA synthesis mediated by the host replication machinery. Using a λ-Red recombination-based system in the bacterium Escherichia coli, we systematically examined several fundamental premises that form the mechanistic basis of this model. Our results provide direct evidence strongly suggesting that SSO-mediated gene repair is mechanistically linked to the process of DNA replication, and most likely involves a replication intermediate. These findings will help guide future experiments involving SSO-mediated gene repair in mammalian and prokaryotic cells, and suggest several mechanisms by which the efficiencies may be reliably and substantially increased.

The potential of oligonucleotides for therapeutic applications

Viral-derived particles have been widely used and described in gene therapy clinical trials. Although substantial results have been achieved, major safety issues have also arisen. For more than a decade, oligonucleotides have been seen as an alternative to gene complementation by viral vectors or DNA plasmids, either to correct the genetic defect or to silence gene expression. The development of RNA interference has strengthened the potential of this approach. Recent clinical trials have also tested the ability of aptamer molecules and decoy oligonucleotides to sequestrate pathogenic proteins. Here, we review the potential of oligonucleotides in gene therapy, outline what has already been accomplished, and consider what remains to be done.

Delivery and mechanistic considerations for the production of knock-in mice by single-stranded oligonucleotide gene targeting

Single-stranded oligodeoxynucleotide (ssODN) gene targeting may facilitate animal model creation and gene repair therapy. Lipofection of ssODN can introduce point mutations into target genes. However, typical efficiencies in mouse embryonic stem cells (ESC) are <10(-4), leaving corrections too rare to effectively identify. We developed ESC lines with an integrated mutant neomycin resistance gene (Tyr22Ter). After targeting with ssODN, repaired cells survive selection in G418. Correction efficiencies varied with different lipofection procedures, clonal lines, and ssODN designs, ranging from 1 to 100 corrections per million cells plated. Uptake studies using cell sorting of Cy5-labelled ssODN showed 40% of the corrections concentrated in the best transfected 22% of cells. Four different basepair mismatches were tested and results show that the base-specificity of the mismatch is critical. Dual mismatch ssODN also showed mismatch preferences. These ESC lines may facilitate development of improved ssODN targeting technologies for either animal production or ex vivo gene therapy.

Single-stranded oligonucleotide-mediated gene repair in mammalian cells has a mechanism distinct from homologous recombination repair

Single-stranded DNA oligonucleotide (SSO)-mediated gene repair has great potentials for gene therapy and functional genomic studies. However, its underlying mechanism remains unclear. Previous studies from other groups have suggested that DNA damage response via the ATM/ATR pathway may be involved in this process. In this study, we measured the effect of two ATM/ATR inhibitors caffeine and pentoxifylline on the correction efficiency in SSO-mediated gene repair. We also checked their effect on double-stranded break (DSB)-induced homologous recombination repair (HRR) as a control, which is well known to be dependent on the ATM/ATR pathway. We found these inhibitors could completely inhibit DSB-induced HRR, but could only partially inhibit SSO-mediated process, indicating SSO-mediated gene repair is not dependent on the ATM/ATR pathway. Furthermore, we found that thymidine treatment promotes SSO-mediated gene repair, but inhibits DSB-induced HRR. Collectively, our results demonstrate that SSO-mediated and DSB-induced gene repairs have distinct mechanisms.

Effective oligonucleotide-mediated gene disruption in ES cells lacking the mismatch repair protein MSH3

We have previously demonstrated that site-specific insertion, deletion or substitution of one or two nucleotides in mouse embryonic stem cells (ES cells) by single-stranded deoxyribo-oligonucleotides is several hundred-fold suppressed by DNA mismatch repair (MMR) activity. Here, we have investigated whether compound mismatches and larger insertions escape detection by the MMR machinery and can be effectively introduced in MMR-proficient cells. We identified several compound mismatches that escaped detection by the MMR machinery to some extent, but could not define general rules predicting the efficacy of complex base-pair substitutions. In contrast, we found that four-nucleotide insertions were largely subject to suppression by the MSH2/MSH3 branch of MMR and could be effectively introduced in Msh3-deficient cells. As these cells have no overt mutator phenotype and Msh3-deficient mice do not develop cancer, Msh3-deficient ES cells can be used for oligonucleotide-mediated gene disruption. As an example, we present disruption of the Fanconi anemia gene Fancf.

Targeted chromosomal gene modification in human cells by single-stranded oligodeoxynucleotides in the presence of a DNA double-strand break

A DNA double-strand break (DSB) cannot be tolerated by a cell and is dealt with by several pathways. Here, it was hypothesized that DSB induction close to a targeted mutation in the genome of a mammalian cell might attract oligodeoxynucleotide (ODN)-directed gene repair. A HEK-293-derived cell line had been engineered harboring a single target locus with open reading frames encoding the living-cell reporter proteins LacZ and EGFP, the latter translationally decoupled by a DNA spacer with a unique I-SceI recognition site for defined DSB induction. To enable expression of a fluorescent LacZ-EGFP fusion protein, single-stranded (ss) ODNs (80 or 96 nucleotides long) spanning the DSB were designed to fuse both reading frames by altering a few base-pair positions, deleting 59 bp or introducing a 10-bp fragment. The ssODNs alone generated few EGFP-positive cells. With I-SceI transiently expressed, more than 0.3% of cells revealed EGFP expression 7 days after transfection, with up to 96% of the loci faithfully corrected, depending on the ssODN used. During these correction events, the ssODN did not become physically incorporated into the chromosome, but served only as information template. Unwanted insertional mutagenesis also occurred. Both observations have important implications for gene therapy.

Genomic sequence correction by single-stranded DNA oligonucleotides: role of DNA synthesis and chemical modifications of the oligonucleotide ends

BACKGROUND

Single-stranded oligonucleotides (ssODN) can induce site-specific genetic alterations in selected mammalian cells, but the involved mechanisms are not known.

METHODS

We corroborate the potential of genomic sequence correction by ssODN using chromosomally integrated mutated enhanced green fluorescent protein (mEGFP) reporter genes in CHO cell lines. The role of integration site was studied in a panel of cell clones with randomly integrated reporters and in cell lines with site-specific single copy integration of the mEGFP reporter in opposite orientations. Involvement of end modification was examined on ssODN with unprotected or phosphorothioate (PS) protected ends. Also ssODN containing octyl or hexaethylene glycol (HEG) end blocking groups were tested. The significance of DNA synthesis was investigated by cell cycle analysis and by the DNA polymerases alpha, delta and epsilon inhibitor aphidicolin.

RESULTS

Correction rates of up to 5% were observed upon a single transfection of ssODN. Independent of the mEGFP chromosomal integration site and of its orientation towards the replication fork, antisense ssODN were more effective than sense ssODN. When ssODN ends were blocked by either octyl or HEG groups, correction rates were reduced. Finally, we demonstrate a dependence of the process on DNA synthesis.

CONCLUSIONS

We show that, on a chromosomal level, the orientation of the replication fork towards the targeted locus is not central in the strand bias of ssODN-based targeted sequence correction. We demonstrate the importance of accessible ssODN ends for sequence alteration. Finally, we provide evidence for the involvement of DNA synthesis in the process.

Physical incorporation of a single-stranded oligodeoxynucleotide during targeted repair of a human chromosomal locus

BACKGROUND

Targeted gene repair is an attractive method to correct point-mutated genes at their natural chromosomal sites, but it is still rather inefficient. As revealed by earlier studies, successful gene correction requires a productive interaction of the repair molecule with the target locus. The work here set out to investigate whether DNA repair, e.g., mismatch repair, or a direct incorporation of the correction molecule follows as the step upon the initial interaction.

METHODS

Single-stranded 21mer oligodeoxynucleotides (ODNs) of sense orientation were directed towards point-mutated enhanced green fluorescence protein transgene loci in HEK-293-derived cell clones. First gene repair assays compared ODNs carrying the canonical termini 5'-phosphate and 3'-OH with their respective variants harbouring non-canonical termini (5'-OH, 3'-H). Second, a protocol was established to allow efficient recovery of integrated short biotin-labelled ODNs from the genomes of gene-corrected cells using streptavidin-coated beads in order to test directly whether transfected ODNs become bona fide parts of the target locus DNA.

RESULTS

Oligodeoxynucleotides with canonical termini were about 34-fold more efficient than their counterparts carrying non-canonical termini in a phosphorothioate-modified backbone. Furthermore, biotinylated fragments were successfully recovered from genomic DNAs of gene-corrected cells.

CONCLUSIONS

The experiment showed that ODNs are incorporated into a mammalian genome. This unravels one early repair step and also sets an unexpected example of genome dynamics possibly relevant to other ODN-based cell techniques.

Targeted gene repair activates Chk1 and Chk2 and stalls replication in corrected cells

Oligonucleotides (ODNs) can direct the exchange of single nucleotides at specific sites in the mammalian genome. It is generally believed that the ODN aligns in homologous register with its complementary site in the target gene and provides a template for the endogenous repair machinery to alter the sequence of the gene. We have been studying the initial phase of the reaction with particular emphasis on the cellular events that occur when the oligonucleotide enters the cell. Our results show that, following introduction of the oligonucleotide, the DNA-damage response pathway is activated, evidenced by the presence of phosphorylated p53, Chk1 and Chk2, respectively. As a result, progression of some of these cells through the cell cycle is slowed and those bearing corrected genes all contain phosphorylated Chk1 and Chk2. In contrast, uncorrected cells contain much lower levels of these proteins in the activated state and pass through the cell cycle in a normal fashion. We suggest that gene repair directed by oligonucleotides activates a pathway that prevents corrected cells from proliferating in cell culture through the activation of Chk1 and Chk2. Our results impact the future use of gene repair for ex vivo gene therapy applications.

Oligonucleotide-mediated gene modification and its promise for animal agriculture

One of the great aspirations in modern biology is the ability to utilise the expanding knowledge of the genetic basis of phenotypic diversity through the purposeful tailoring of the mammalian genome. A number of technologies are emerging which have the capacity to modify genes in their chromosomal context. Not surprisingly, the major thrust in this area has come from the evaluation of gene therapy applications to correct mutations implicated in human genetic diseases. The recent development of somatic cell nuclear transfer (SCNT) provides access to these technologies for the purposeful modification of livestock animals. The enormous phenotypic variety existent in contemporary livestock animals has in many cases been linked to quantitative trait loci (QTL) and their underlying point mutations, often referred to as single-nucleotide polymorphisms (SNPs). Thus, the ability for the targeted genetic modification of livestock animals constitutes an attractive opportunity for future agricultural applications. In this review, we will summarize attempts and approaches for oligonucleotide-mediated gene modification (OGM) strategies for the site-specific modification of the genome, with an emphasis on chimeric RNA-DNA oligonucleotides (RDOs) and single-stranded oligonucletides (ssODNs). The potential of this approach for the directed genetic improvement of livestock animals is illustrated through examples, outlining the effects of point mutations on important traits, including meat and milk production, reproductive performance, disease resistance and superior models of human diseases. Current technological hurdles and potential strategies that might remove these barriers in the future are discussed.