Factors affecting double-strand break-induced homologous recombination in mammalian cells

Systematic discovery of recombinases for efficient integration of large DNA sequences into the human genome

Large serine recombinases (LSRs) are DNA integrases that facilitate the site-specific integration of mobile genetic elements into bacterial genomes. Only a few LSRs, such as Bxb1 and PhiC31, have been characterized to date, with limited efficiency as tools for DNA integration in human cells. In this study, we developed a computational approach to identify thousands of LSRs and their DNA attachment sites, expanding known LSR diversity by >100-fold and enabling the prediction of their insertion site specificities. We tested their recombination activity in human cells, classifying them as landing pad, genome-targeting or multi-targeting LSRs. Overall, we achieved up to seven-fold higher recombination than Bxb1 and genome integration efficiencies of 40-75% with cargo sizes over 7 kb. We also demonstrate virus-free, direct integration of plasmid or amplicon libraries for improved functional genomics applications. This systematic discovery of recombinases directly from microbial sequencing data provides a resource of over 60 LSRs experimentally characterized in human cells for large-payload genome insertion without exposed DNA double-stranded breaks.

Molecular insights into the mechanism of nonrecurrent F8 structural variants: Full breakpoint characterization and bioinformatics of DNA elements implicated in the upmost severe phenotype in hemophilia A

Hemophilia A (HA) provides excellent models to analyze genotype-phenotype relationships and mutational mechanisms. NhF8ld's breakpoints were characterized using case-specific DNA-tags, direct- or inverse-polymerase chain reaction amplification, and Sanger sequencing. DNA-break's stimulators (n = 46), interspersed repeats, non-B-DNA, and secondary structures were analyzed around breakpoints versus null hypotheses (E-values) based on computer simulations and base-frequency probabilities. Nine of 18 (50%) severe-HA patients with nhF8lds developed inhibitors, 1/8 affecting one exon and 8/10 (80%) affecting multi-exons. NhF8lds range: 2-165 kb. Five (45%) nhF8lds involve F8-extragenic regions including three affecting vicinal genes (SMIM9 and BRCC3) but none shows an extra-phenotype not related to severe-HA. The contingency analysis of recombinogenic motifs at nhF8ld breakpoints indicated a significant involvement of several DNA-break stimulator elements. Most nhF8ld's breakpoint junctions showed microhomologies (1-7 bp). Three (27%) nhF8lds show complexities at the breakpoints: an 8-bp inverted-insertion, and the remnant two, inverted- and direct-insertions (46-68 bp) supporting replicative models microhomology-mediated break-induced replication/Fork Stalling and Template Switching. The remnant eight (73%) nhF8lds may support nonhomologous end joining/microhomology-mediated end joining models. Our study suggests the involvement of the retroposition machinery (e.g., Jurka-targets, Alu-elements, long interspersed nuclear elements, long terminal repeats), microhomologies, and secondary structures at breakpoints playing significant roles in the origin of the upmost severe phenotype in HA.

CRISPR-Mediated Integration of Large Gene Cassettes Using AAV Donor Vectors

The CRISPR/Cas9 system has recently been shown to facilitate high levels of precise genome editing using adeno-associated viral (AAV) vectors to serve as donor template DNA during homologous recombination (HR). However, the maximum AAV packaging capacity of ∼4.5 kb limits the donor size. Here, we overcome this constraint by showing that two co-transduced AAV vectors can serve as donors during consecutive HR events for the integration of large transgenes. Importantly, the method involves a single-step procedure applicable to primary cells with relevance to therapeutic genome editing. We use the methodology in primary human T cells and CD34⁺ hematopoietic stem and progenitor cells to site-specifically integrate an expression cassette that, as a single donor vector, would otherwise amount to a total of 6.5 kb. This approach now provides an efficient way to integrate large transgene cassettes into the genomes of primary human cells using HR-mediated genome editing with AAV vectors.

Modern Genome Editing Technologies in Huntington's Disease Research

The development of new revolutionary technologies for directed gene editing has made it possible to thoroughly model and study NgAgo human diseases at the cellular and molecular levels. Gene editing tools like ZFN, TALEN, CRISPR-based systems, NgAgo and SGN can introduce different modifications. In gene sequences and regulate gene expression in different types of cells including induced pluripotent stem cells (iPSCs). These tools can be successfully used for Huntington's disease (HD) modeling, for example, to generate isogenic cell lines bearing different numbers of CAG repeats or to correct the mutation causing the disease. This review presents common genome editing technologies and summarizes the progress made in using them in HD and other hereditary diseases. Furthermore, we will discuss prospects and limitations of genome editing in understanding HD pathology.

Using the CRISPR/Cas9 system to understand neuropeptide biology and regulation

Neuropeptides and their receptors play a role in physiological responses such as appetite, stress and inflammatory pain. With neuropeptides having such diverse and important physiological roles, knocking-out the genes encoding them, their receptors, parts of their regulatory sequences, or reproducing disease associated polymorphic variants are important steps in studying neuropeptides and how they may contribute to disease. Previously, knock-outs were generated using methods such as targeted homologous recombination in embryonic stem cells but this method is costly and time-consuming. The CRISPR/Cas9 system has rapidly taken over the genome editing field and will advance our understanding of neuropeptide genes and their regulation. With CRISPR/Cas9 technology, the time and costs involved in producing transgenic animal models, is greatly reduced. In this review, we describe how the system can be used to manipulate genomic sequences by "knock-out" or "knock-in" mutations in cell lines or in animal models. We also discuss the specificity of the system and methods to limit off-target effects. When combined with the availability of genome sequences, CRISPR/Cas9 directed genome editing in vitro and in vivo, promises to provide a deeper understanding of the biology of the neuropeptides in health and disease than has ever been available before.

Tools and strategies for scarless allele replacement in Drosophila using CRISPR/Cas9

Genome editing via the CRISPR/Cas9 RNA-guided nuclease system has opened up exciting possibilities for genetic analysis. However, technical challenges associated with homology-directed repair have proven to be roadblocks for producing changes in the absence of unwanted, secondary mutations commonly known as "scars." To address these issues, we developed a 2-stage, marker-assisted strategy to facilitate precise, "scarless" edits in Drosophila with a minimal requirement for molecular screening. Using this method, we modified 2 base pairs in a gene of interest without altering the final sequence of the CRISPR cut sites. We executed this 2-stage allele swap using a novel transformation marker that drives expression in the pupal wings, which can be screened for in the presence of common eye-expressing reporters. The tools we developed can be used to make a single change or a series of allelic substitutions in a region of interest in any D. melanogaster genetic background as well as in other Drosophila species.

Dual-reporter surrogate systems for efficient enrichment of genetically modified cells

Isolation of genetically modified cells generated by designed nucleases are challenging, since they are often phenotypically indistinguishable from their parental cells. To efficiently enrich genetically modified cells, we developed two dual-reporter surrogate systems, namely NHEJ-RPG and SSA-RPG based on NHEJ and SSA repair mechanisms, respectively. Repair and enrichment efficiencies of these two systems were compared using different nucleases. In both CRISPR-Cas9- and ZFNs-induced DSB repair studies, we found that the efficiency and sensitivity of the SSA-RPG reporter with direct repeat length more than 200 bp were much higher than the NHEJ-RPG reporter. By utilizing the SSA-RPG reporter, we achieved the enrichment for indels in several endogenous loci with 6.3- to 34.8-fold of non-selected cells. Thus, the highly sensitive SSA-RPG reporter can be used for activity validation of designed nucleases and efficient enrichment of genetically modified cells. Besides, our systems offer alternative enrichment choices either by puromycin selection or FACS.

A suicidal zinc finger nuclease expression coupled with a surrogate reporter for efficient genome engineering

Genome editing with engineered nucleases, such as zinc-finger nucleases (ZFNs) and TALE nucleases, remains confronted with a high risk of cellular toxicity induced by off-targeting. Here we describe the construction of a suicidal nuclease expression vector in which a pair of ZFNs genes were flanked of its target sites. To further enrich the targeted cells, the suicidal ZFN expression cassette was also inserted within an eGFP reporter, to disrupt the ORF of eGFP gene. ZFN-associated toxicity was reduced by ~40 % with this new system, and the activities of ZFNs were ~4.5 % lower. We conclude that using this new suicidal ZFN expression and surrogate reporter system represents an improvement for genomic editing by reducing toxicity and allowing easy detection of edited cells by eGFP analysis.

Genetic modification stimulated by the induction of a site-specific break distant from the locus of correction in haploid and diploid yeast Saccharomyces cerevisiae

Generation of a site-specific break at a genomic locus to stimulate homologous recombination (HR) is used in many organisms to efficiently target genes for various types of genetic modification. Additionally, a site-specific chromosomal break can be used to trigger HR at genomic regions distant from the break, thereby largely expanding the region available for introducing desired mutations. In contrast to the former approach, the latter presents an alternative way in which genes can be efficiently modified also when it is not possible or desirable to introduce a break in the vicinity of the targeting locus. This type of in vivo site-directed mutagenesis distant from a break can be accomplished in the yeast model organism Saccharomyces cerevisiae because the generation of a double-strand break (DSB) in yeast chromosomal DNA activates HR at long regions upstream and downstream from the break site. Here we provide a protocol for efficiently altering a yeast chromosomal locus following the induction of a DSB several kilobase pairs distant from the site of gene correction. The techniques described can be used in both diploid and haploid yeast strains, and we provide examples of the gene correction assays.

Targeted disruption of the sheep MSTN gene by engineered zinc-finger nucleases

Prior to the development of zinc-finger nuclease technology, genetic manipulation by gene targeting achieved limited success in mammals, with the exception of mice and rat. Although ZFNs demonstrated highly effective gene targeted disruption in various model organisms, the activity of ZFNs in large domestic animals may be very low, and the probability of identifying ZFN-mediated positive targeted disruption events is small. In this paper, we used the context-dependent assembly method to synthesize two pairs of ZFNs targeted to the sheep MSTN gene. We verified the activity of these ZFNs using an mRFP-MBS-eGFP dual-fluorescence reporter system in HEK293T cells and, according to the expression level of eGFP, we obtained a pair of ZFNs that can recognize and cut the targeted MSTN site in the reporter vector. The activity of ZFN was increased by cold stimulation at 30 °C and by mutant the wildtype FokI in ZFN with its counterpart Sharkeys. Finally, the ZF-Sharkeys and reporter vector were cotransfected into sheep fetal fibroblasts and two MSTN mutant cell lines, identified by flow cytometry and sequencing, were obtained.

The Rate and Tract Length of Gene Conversion between Duplicated Genes

Interlocus gene conversion occurs such that a certain length of DNA fragment is non-reciprocally transferred (copied and pasted) between paralogous regions. To understand the rate and tract length of gene conversion, there are two major approaches. One is based on mutation-accumulation experiments, and the other uses natural DNA sequence variation. In this review, we overview the two major approaches and discuss their advantages and disadvantages. In addition, to demonstrate the importance of statistical analysis of empirical and evolutionary data for estimating tract length, we apply a maximum likelihood method to several data sets.

Meganucleases and other tools for targeted genome engineering: perspectives and challenges for gene therapy

The importance of safer approaches for gene therapy has been underscored by a series of severe adverse events (SAEs) observed in patients involved in clinical trials for Severe Combined Immune Deficiency Disease (SCID) and Chromic Granulomatous Disease (CGD). While a new generation of viral vectors is in the process of replacing the classical gamma-retrovirus-based approach, a number of strategies have emerged based on non-viral vectorization and/or targeted insertion aimed at achieving safer gene transfer. Currently, these methods display lower efficacies than viral transduction although many of them can yield more than 1% of engineered cells in vitro. Nuclease-based approaches, wherein an endonuclease is used to trigger site-specific genome editing, can significantly increase the percentage of targeted cells. These methods therefore provide a real alternative to classical gene transfer as well as gene editing. However, the first endonuclease to be in clinic today is not used for gene transfer, but to inactivate a gene (CCR5) required for HIV infection. Here, we review these alternative approaches, with a special emphasis on meganucleases, a family of naturally occurring rare-cutting endonucleases, and speculate on their current and future potential.

The I-CreI meganuclease and its engineered derivatives: applications from cell modification to gene therapy

Meganucleases (MNs) are highly specific enzymes that can induce homologous recombination in different types of cells, including mammalian cells. Consequently, these enzymes are used as scaffolds for the development of custom gene-targeting tools for gene therapy or cell-line development. Over the past 15 years, the high resolution X-ray structures of several MNs from the LAGLIDADG family have improved our understanding of their protein-DNA interaction and mechanism of DNA cleavage. By developing and utilizing high-throughput screening methods to test a large number of variant-target combinations, we have been able to re-engineer scores of I-CreI derivatives into custom enzymes that target a specific DNA sequence of interest. Such customized MNs, along with wild-type ones, have allowed for exploring a large range of biotechnological applications, including protein-expression cell-line development, genetically modified plants and animals and therapeutic applications such as targeted gene therapy as well as a novel class of antivirals.

Microhomology-mediated and nonhomologous repair of a double-strand break in the chloroplast genome of Arabidopsis

Chloroplast DNA (cpDNA) is under great photooxidative stress, yet its evolution is very conservative compared with nuclear or mitochondrial genomes. It can be expected that DNA repair mechanisms play important roles in cpDNA survival and evolution, but they are poorly understood. To gain insight into how the most severe form of DNA damage, a double-strand break (DSB), is repaired, we have developed an inducible system in Arabidopsis that employs a psbA intron endonuclease from Chlamydomonas, I-CreII, that is targeted to the chloroplast using the rbcS1 transit peptide. In Chlamydomonas, an I-CreII-induced DSB in psbA was repaired, in the absence of the intron, by homologous recombination between repeated sequences (20-60 bp) abundant in that genome; Arabidopsis cpDNA is very repeat poor, however. Phenotypically strong and weak transgenic lines were examined and shown to correlate with I-CreII expression levels. Southern blot hybridizations indicated a substantial loss of DNA at the psbA locus, but not cpDNA as a whole, in the strongly expressing line. PCR analysis identified deletions nested around the I-CreII cleavage site indicative of DSB repair using microhomology (6-12 bp perfect repeats, or 10-16 bp with mismatches) and no homology. These results provide evidence of alternative DSB repair pathways in the Arabidopsis chloroplast that resemble the nuclear, microhomology-mediated and nonhomologous end joining pathways, in terms of the homology requirement. Moreover, when taken together with the results from Chlamydomonas, the data suggest an evolutionary relationship may exist between the repeat structure of the genome and the organelle's ability to repair broken chromosomes.

Targeted approaches for gene therapy and the emergence of engineered meganucleases

BACKGROUND

In spite of significant advances in gene transfer strategies in the field of gene therapy, there is a strong emphasis on the development of alternative methods, providing better control of transgene expression and insertion patterns.

OBJECTIVE

Several new approaches consist of targeting a desired transgene or gene modification in a well defined locus, and we collectively refer to them as 'targeted approaches'. The use of redesigned meganucleases is one of these emerging technologies. Here we try to define the potential of this method, in the larger scope of targeted strategies.

METHODS

We survey the different types of targeted strategies, presenting the achievements and the potential applications, with a special emphasis on the use of redesigned endonucleases.

CONCLUSION

redesigned endonucleases represent one of the most promising tools for targeted approaches, and the opening of a clinical trial for AIDS patients has recently shown the maturity of these strategies. However, there is still a 'quest' for the best reagents, that is the endonucleases providing the best efficacy:toxicity ratio. New advances in protein design have allowed the engineering of new scaffolds, such as meganucleases, and the landscape of existing methods is likely to change over the next few years.

Chlamydomonas chloroplasts can use short dispersed repeats and multiple pathways to repair a double-strand break in the genome

Certain group I introns insert into intronless DNA via an endonuclease that creates a double-strand break (DSB). There are two models for intron homing in phage: synthesis-dependent strand annealing (SDSA) and double-strand break repair (DSBR). The Cr.psbA4 intron homes efficiently from a plasmid into the chloroplast psbA gene in Chlamydomonas, but little is known about the mechanism. Analysis of co-transformants selected using a spectinomycin-resistant 16S gene (16S(spec)) provided evidence for both pathways. We also examined the consequences of the donor DNA having only one-sided or no homology with the psbA gene. When there was no homology with the donor DNA, deletions of up to 5 kb involving direct repeats that flank the psbA gene were obtained. Remarkably, repeats as short as 15 bp were used for this repair, which is consistent with the single-strand annealing (SSA) pathway. When the donor had one-sided homology, the DSB in most co-transformants was repaired using two DNAs, the donor and the 16S(spec) plasmid, which, coincidentally, contained a region that is repeated upstream of psbA. DSB repair using two separate DNAs provides further evidence for the SDSA pathway. These data show that the chloroplast can repair a DSB using short dispersed repeats located proximally, distally, or even on separate molecules relative to the DSB. They also provide a rationale for the extensive repertoire of repeated sequences in this genome.

Catalytic domain of restriction endonuclease BmrI as a cleavage module for engineering endonucleases with novel substrate specificities

Creating endonucleases with novel sequence specificities provides more possibilities to manipulate DNA. We have created a chimeric endonuclease (CH-endonuclease) consisting of the DNA cleavage domain of BmrI restriction endonuclease and C.BclI, a controller protein of the BclI restriction-modification system. The purified chimeric endonuclease, BmrI198-C.BclI, cleaves DNA at specific sites in the vicinity of the recognition sequence of C.BclI. Double-strand (ds) breaks were observed at two sites: 8 bp upstream and 18 bp within the C-box sequence. Using DNA substrates with deletions of C-box sequence, we show that the chimeric endonuclease requires the 5' half of the C box only for specific cleavage. A schematic model is proposed for the mode of protein-DNA binding and DNA cleavage. The present study demonstrates that the BmrI cleavage domain can be used to create combinatorial endonucleases that cleave DNA at specific sequences dictated by the DNA-binding partner. The resulting endonucleases will be useful in vitro and in vivo to create ds breaks at specific sites and generate deletions.

Methods to produce marker-free transgenic plants

Selectable marker genes (SMGs) have been extraordinarily useful in enabling plant transformation because of the low efficiency of transgene integration. The most used SMGs encode proteins resistant to antibiotics or herbicides and use negative selection, i.e., by killing nontransgenic tissue. However, there are perceived risks in wide-scale deployment of SMG-transgenic plants, and therefore research has recently been performed to develop marker-free systems. In this review, transformation using markers not based on antibiotic or herbicide resistance genes, as well as different systems of marker gene deletion, are discussed.

DNA annealing mediated by Rad52 and Rad59 proteins

In the budding yeast Saccharomyces cerevisiae, the RAD52 gene is essential for all homologous recombination events and its homologue, the RAD59 gene, is important for those that occur independently of RAD51. Both Rad52 and Rad59 proteins can anneal complementary single-stranded (ss) DNA. We quantitatively examined the ssDNA annealing activity of Rad52 and Rad59 proteins and found significant differences in their biochemical properties. First, and most importantly, they differ in their ability to anneal ssDNA that is complexed with replication protein A (RPA). Rad52 can anneal an RPA-ssDNA complex, but Rad59 cannot. Second, Rad59-promoted DNA annealing follows first-order reaction kinetics, whereas Rad52-promoted annealing follows second-order reaction kinetics. Last, Rad59 enhances Rad52-mediated DNA annealing at increased NaCl concentrations, both in the absence and presence of RPA. These results suggest that Rad59 performs different functions in the recombination process, and should be more accurately viewed as a Rad52 paralogue.

Efficientin toto targeted recombination in mouse liver by meganuclease-induced double-strand break

BACKGROUND

Sequence-specific endonucleases with large recognition sites can cleave DNA in living cells, and, as a consequence, stimulate homologous recombination (HR) up to 10 000-fold. The recent development of artificial meganucleases with chosen specificities has provided the potential to target any chromosomal locus. Thus, they may represent a universal genome engineering tool and seem to be very promising for acute gene therapy. However, in toto applications depend on the ability to target somatic tissues as well as the proficiency of somatic cells to perform double-strand break (DSB)-induced HR.

METHODS

In order to investigate DSB-induced HR in toto, we have designed transgenic mouse lines carrying a LagoZ gene interrupted by one I-SceI cleavage site surrounded by two direct repeats. The LagoZ gene can be rescued upon cleavage by I-SceI and HR between the two repeats in a process called single-strand annealing. beta-Galactosidase activity is monitored in liver after tail vein injection of adenovirus expressing the meganuclease I-SceI.

RESULTS

In toto staining revealed a strong dotted pattern in all animals injected with adenovirus expressing I-SceI. In contrast, no staining could be detected in the control. beta-Galactosidase activity in liver extract, tissue section staining, and PCR analysis confirmed the presence of the recombined LagoZ gene.

CONCLUSIONS

We demonstrate for the first time that meganucleases can be successfully delivered in animal and induce targeted genomic recombination in mice liver in toto. These results are an essential step towards the use of designed meganucleases and show the high potential of this technology in the field of gene therapy.

Engineering of large numbers of highly specific homing endonucleases that induce recombination on novel DNA targets

The last decade has seen the emergence of a universal method for precise and efficient genome engineering. This method relies on the use of sequence-specific endonucleases such as homing endonucleases. The structures of several of these proteins are known, allowing for site-directed mutagenesis of residues essential for DNA binding. Here, we show that a semi-rational approach can be used to derive hundreds of novel proteins from I-CreI, a homing endonuclease from the LAGLIDADG family. These novel endonucleases display a wide range of cleavage patterns in yeast and mammalian cells that in most cases are highly specific and distinct from I-CreI. Second, rules for protein/DNA interaction can be inferred from statistical analysis. Third, novel endonucleases can be combined to create heterodimeric protein species, thereby greatly enhancing the number of potential targets. These results describe a straightforward approach for engineering novel endonucleases with tailored specificities, while preserving the activity and specificity of natural homing endonucleases, and thereby deliver new tools for genome engineering.