Assessment of protein dynamics and DNA repair following generation of DNA double-strand breaks at defined genomic sites

The formation of protein aggregates (foci) at sites of DNA double-strand breaks (DSBs) is mainly studied by immunostaining and is hence limited by the low resolution of light microscopy and the availability of appropriate and selective antibodies. Here, we describe a system using enzymatic creation of site-specific DNA DSBs within the human genome combined with chromatin immunoprecipitation (ChIP) that enables molecular probing of a DSB. Following induction of the I-PpoI enzyme and generation of DSBs, cellular DNA and proteins are crosslinked and analyzed by ChIP for specific proteins at the site of the break. The system allows the direct detection of protein and chromatin dynamics at the site of the break with high resolution, as well as direct measurement of DNA repair defects in human cells. Starting with fragmented chromatin, results can be achieved in 2-3 d.     

Crystal Structure of the Homing Endonuclease I-CvuI Provides a New Template for Genome Modification

Homing endonucleases recognize and generate a DNA double-strand break, which has been used to promote gene targeting. These enzymes recognize long DNA stretches; they are highly sequence-specific enzymes and display a very low frequency of cleavage even in complete genomes. Although a large number of homing endonucleases have been identified, the landscape of possible target sequences is still very limited to cover the complexity of the whole eukaryotic genome. Therefore, the finding and molecular analysis of homing endonucleases identified but not yet characterized may widen the landscape of possible target sequences. The previous characterization of protein-DNA interaction before the engineering of new homing endonucleases is essential for further enzyme modification. Here we report the crystal structure of I-CvuI in complex with its target DNA and with the target DNA of I-CreI, a homologue enzyme widely used in genome engineering. To characterize the enzyme cleavage mechanism, we have solved the I-CvuI DNA structures in the presence of non-catalytic (Ca²⁺) and catalytic ions (Mg²⁺). We have also analyzed the metal dependence of DNA cleavage using Mg²⁺ ions at different concentrations ranging from non-cleavable to cleavable concentrations obtained from in vitro cleavage experiments. The structure of I-CvuI homing endonuclease expands the current repertoire for engineering custom specificities, both by itself as a new scaffold alone and in hybrid constructs with other related homing endonucleases or other DNA-binding protein templates.     

Isolation and characterization of new homing endonuclease specificities at individual target site positions

Homing endonucleases are highly specific DNA endonucleases, encoded within mobile introns or inteins, that induce targeted recombination, double-strand repair and gene conversion of their cognate target sites. Due to their biological function and high level of target specificity, these enzymes are under intense investigation as tools for gene targeting. These studies require that naturally occurring enzymes be redesigned to recognize novel target sites. Here, we report studies in which the homodimeric LAGLIDADG homing endonuclease I-CreI is altered at individual side-chains corresponding to contact points to distinct base-pairs in its target site. The resulting enzyme constructs drive specific elimination of selected DNA targets in vivo and display shifted specificities of DNA binding and cleavage in vitro. Crystal structures of two of these constructs demonstrate that substitution of individual side-chain/DNA contact patterns can occur with almost no structural deformation or rearrangement of the surrounding complex, facilitating an isolated, modular redesign strategy for homing endonuclease activity and specificity.     

Design,activity, and structure of a highly specific artificial endonuclease

We have generated an artificial highly specific endonuclease by fusing domains of homing endonucleases I-DmoI and I-CreI and creating a new 1400 A(2) protein interface between these domains. Protein engineering was accomplished by combining computational redesign and an in vivo protein-folding screen. The resulting enzyme, E-DreI (Engineered I-DmoI/I-CreI), binds a long chimeric DNA target site with nanomolar affinity, cleaving it precisely at a rate equivalent to its natural parents. The structure of an E-DreI/DNA complex demonstrates the accuracy of the protein interface redesign algorithm and reveals how catalytic function is maintained during the creation of the new endonuclease. These results indicate that it may be possible to generate novel highly specific DNA binding proteins from homing endonucleases.     

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.     

Divalent metal ion differentially regulates the sequential nicking reactions of the GIY-YIG homing endonuclease I-BmoI

Homing endonucleases are site-specific DNA endonucleases that function as mobile genetic elements by introducing double-strand breaks or nicks at defined locations. Of the major families of homing endonucleases, the modular GIY-YIG endonucleases are least understood in terms of mechanism. The GIY-YIG homing endonuclease I-BmoI generates a double-strand break by sequential nicking reactions during which the single active site of the GIY-YIG nuclease domain must undergo a substantial reorganization. Here, we show that divalent metal ion plays a significant role in regulating the two independent nicking reactions by I-BmoI. Rate constant determination for each nicking reaction revealed that limiting divalent metal ion has a greater impact on the second strand than the first strand nicking reaction. We also show that substrate mutations within the I-BmoI cleavage site can modulate the first strand nicking reaction over a 314-fold range. Additionally, in-gel DNA footprinting with mutant substrates and modeling of an I-BmoI-substrate complex suggest that amino acid contacts to a critical GC-2 base pair are required to induce a bottom-strand distortion that likely directs conformational changes for reaction progress. Collectively, our data implies mechanistic roles for divalent metal ion and substrate bases, suggesting that divalent metal ion facilitates the re-positioning of the GIY-YIG nuclease domain between sequential nicking reactions.     

Generation of single-chain LAGLIDADG homing endonucleases from native homodimeric precursor proteins

Homing endonucleases (HEs) cut long DNA target sites with high specificity to initiate and target the lateral transfer of mobile introns or inteins. This high site specificity of HEs makes them attractive reagents for gene targeting to promote DNA modification or repair. We have generated several hundred catalytically active, monomerized versions of the well-characterized homodimeric I-CreI and I-MsoI LAGLIDADG family homing endonuclease (LHE) proteins. Representative monomerized I-CreI and I-MsoI proteins (collectively termed mCreIs or mMsoIs) were characterized in detail by using a combination of biochemical, biophysical and structural approaches. We also demonstrated that both mCreI and mMsoI proteins can promote cleavage-dependent recombination in human cells. The use of single chain LHEs should simplify gene modification and targeting by requiring the expression of a single small protein in cells, rather than the coordinate expression of two separate protein coding genes as is required when using engineered heterodimeric zinc finger or homing endonuclease proteins.     

Characterization of homing endonucleases

Homing endonucleases are a class of site-specific DNA endonucleases encoded by open reading frames within introns and inteins. They initiate the mobility of their host element by recognizing intronless or inteinless alleles of their host gene and making a double-strand break. The homing endonucleases are notable for their long target sites and a tolerance for sequence polymorphisms in their substrates. The methods used to study homing endonucleases are similar to those used to study protein-DNA interactions in general. However, some variations and specialized techniques are useful in characterizing homing endonucleases and these methods are discussed.     

Double-strand DNA break formation mediated by flap endonuclease-1

Double-strand DNA breaks are the most lethal type of DNA damage induced by ionizing radiations. Previously, we reported that double-strand DNA breaks can be enzymatically produced from two DNA damages located on opposite DNA strands 18 or 30 base pairs apart in a cell-free double-strand DNA break formation assay (Vispé, S., and Satoh, M. S. (2000) J. Biol. Chem. 275, 27386-27392). In the assay that we developed, these two DNA damages are converted into single-strand interruptions by enzymes involved in base excision repair. We showed that these single-strand interruptions are converted into double-strand DNA breaks; however, it was not due to spontaneous denaturation of DNA. Thus, we proposed a model in which DNA polymerase delta/epsilon, by producing repair patches at single-strand interruptions, collide, resulting in double-strand DNA break formation. We tested the model and investigated whether other enzymes/factors are involved in double-strand DNA break formation. Here we report that, instead of DNA polymerase delta/epsilon, flap endonuclease-1 (FEN-1), an enzyme involved in base excision repair, is responsible for the formation of double-strand DNA break in the assay. Furthermore, by transfecting a flap endonuclease-1 expression construct into cells, thus altering their flap endonuclease-1 content, we found an increased number of double-strand DNA breaks after gamma-ray irradiation of these cells. These results suggest that flap endonuclease-1 acts as a double-strand DNA break formation factor. Because FEN-1 is an essential enzyme that plays its roles in DNA repair and DNA replication, DSBs may be produced in cells as by-products of the activity of FEN-1.     

Structural and functional characteristics of homing endonucleases

Mobile genetic elements constitute a remarkably diverse group of nonessential selfish genes that provide no apparent function to the host. These selfish genes have been implicated in host extinction, speciation and architecture of genetic systems. Homing endonucleases, encoded by the open reading frames embedded in introns or inteins of mobile genetic elements, possess double-stranded DNA-specific endonuclease activity. They inflict sequence-specific double-strand breaks at or near the homing site in intron- or intein-less allele. Subsequently, through nonreciprocal exchange the insertion sequence (intron or intein) is transferred from an intein- or intron-containing allele to an intein- or intron-less allele. The components of host double-strand break repair pathway are thought to finish the "homing" process. Several lines of evidence suggest that homing endonucleases are capable of promoting transposition into ectopic sites within or across genomes for their survival as well as dispersal in natural populations. The occurrence of inteins at high frequencies serves as instructive models for understanding the mechanistic aspects of the process of homing and its evolution. This review focuses on genetic, biochemical, structural, and phylogenetic aspects of homing endonucleases, and their comparison with restriction endonucleases.     

Structural and Functional Characteristics of Homing Endonucleases

Mobile genetic elements constitute a remarkably diverse group of nonessential selfish genes that provide no apparent function to the host. These selfish genes have been implicated in host extinction, speciation and architecture of genetic systems. Homing endonucleases, encoded by the open reading frames embedded in introns or inteins of mobile genetic elements, possess double-stranded DNA-specific endonuclease activity. They inflict sequence-specific double-strand breaks at or near the homing site in intron- or intein-less allele. Subsequently, through nonreciprocal exchange the insertion sequence (intron or intein) is transferred from an intein- or intron-containing allele to an intein- or intron-less allele. The components of host double-strand break repair pathway are thought to finish the “homing” process. Several lines of evidence suggest that homing endonucleases are capable of promoting transposition into ectopic sites within or across genomes for their survival as well as dispersal in natural populations. The occurrence of inteins at high frequencies serves as instructive models for understanding the mechanistic aspects of the process of homing and its evolution. This review focuses on genetic, biochemical, structural, and phylogenetic aspects of homing endonucleases, and their comparison with restriction endonucleases.     

Induction of homologous recombination in mammalian chromosomes by using the I-SceI system of Saccharomyces cerevisiae.

The mitochondrial intron-encoded endonuclease I-SceI of Saccharomyces cerevisiae has an 18-bp recognition sequence and, therefore, has a very low probability of cutting DNA, even within large genomes. We demonstrate that double-strand breaks can be initiated by the I-SceI endonuclease at a predetermined location in the mouse genome and that the breaks can be repaired with a donor molecule homologous regions flanking the breaks. This induced homologous recombination is approximately 2 orders of magnitude more frequent than spontaneous homologous recombination and at least 10 times more frequent than random integration near an active promoter. As a consequence of induced homologous recombination, a heterologous novel sequence can be inserted at the site of the break. This recombination can occur at a variety of chromosomal targets in differentiated and multipotential cells. These results demonstrate homologous recombination involving chromosomal DNA by the double-strand break repair mechanism in mammals and show the usefulness of very rare cutter endonucleases, such as I-SceI, for designing genome rearrangements.     

Highly efficient sex chromosome interchanges produced by I-CreI expression in Drosophila

The homing endonuclease I-CreI recognizes a site in the gene encoding the 23S rRNA of Chlamydomonas reinhardtii. A very similar sequence is present in the 28S rRNA genes that are located on the X and Y chromosomes of Drosophila melanogaster. In this work we show that I-CreI expression in Drosophila is capable of causing induced DNA damage and eliciting cell cycle arrest. Expression also caused recombination between the X and Y chromosomes in the heterochromatic regions where the rDNA is located, presumably as a result of a high frequency of double-strand breaks in these regions. Approximately 20% of the offspring of males expressing I-CreI showed exceptional inheritance of X- and Y-linked markers, consistent with chromosome exchange at rDNA loci. Cytogenetic analysis confirmed the structures of many of these products. Exchange between the X and Y chromosomes can be induced in males and females to produce derivative-altered Y chromosomes, attached-XY, and attached-X chromosomes. This method has advantages over the traditional use of X rays for generating X-Y interchanges because it is very frequent and it generates predictable products.     

A highly sensitive selection method for directed evolution of homing endonucleases

Homing endonucleases are enzymes that catalyze DNA sequence specific double-strand breaks and can significantly stimulate homologous recombination at these breaks. These enzymes have great potential for applications such as gene correction in gene therapy or gene alteration in systems biology and metabolic engineering. However, homing endonucleases have a limited natural repertoire of target sequences, which severely hamper their applications. Here we report the development of a highly sensitive selection method for the directed evolution of homing endonucleases that couples enzymatic DNA cleavage with the survival of host cells. Using I-SceI as a model homing endonuclease, we have demonstrated that cells with wild-type I-SceI showed a high cell survival rate of 80–100% in the presence of the original I-SceI recognition site, whereas cells without I-SceI showed a survival rate <0.003%. This system should also be readily applicable for directed evolution of other DNA cleavage enzymes.     

Comprehensive homing endonuclease target site specificity profiling reveals evolutionary constraints and enables genome engineering applications

Homing endonucleases (HEs) promote the evolutionary persistence of selfish DNA elements by catalyzing element lateral transfer into new host organisms. The high site specificity of this lateral transfer reaction, termed homing, reflects both the length (14–40 bp) and the limited tolerance of target or homing sites for base pair changes. In order to better understand molecular determinants of homing, we systematically determined the binding and cleavage properties of all single base pair variant target sites of the canonical LAGLIDADG homing endonucleases I-CreI and I-MsoI. These Chlorophyta algal HEs have very similar three-dimensional folds and recognize nearly identical 22 bp target sites, but use substantially different sets of DNA-protein contacts to mediate site-specific recognition and cleavage. The site specificity differences between I-CreI and I-MsoI suggest different evolutionary strategies for HE persistence. These differences also provide practical guidance in target site finding, and in the generation of HE variants with high site specificity and cleavage activity, to enable genome engineering applications.     

Partners and pathwaysrepairing a double-strand break

Double-strand chromosome breaks can arise in a number of ways, by ionizing radiation, by spontaneous chromosome breaks during DNA replication, or by the programmed action of endonucleases, such as in meiosis. Broken chromosomes can be repaired either by one of several homologous recombination mechanisms, or by a number of nonhomologous repair processes. Many of these pathways compete actively for the repair of a double-strand break. Which of these repair pathways is used appears to be regulated developmentally, genetically and during the cell cycle.     

Illegitimate recombination induced by DNA double-strand breaks in a mammalian chromosome.

We examined DNA double-strand-break-induced mutations in the endogenous adenine phosphoribosyl-transferase (APRT) gene in cultured Chinese hamster ovary cells after exposure to restriction endonucleases. PvuII, EcoRV, and StuI, all of which produce blunt-end DNA double-strand breaks, were electroporated into CHO-AT3-2 cells hemizygous at the APRT locus. Colonies of viable cells containing mutations at APRT were expanded, and the mutations that occurred during break repair were analyzed at the DNA sequence level. Restriction enzyme-induced mutations consisted of small deletions of 1 to 36 bp, insertions, and combinations of insertions and deletions at the cleavage sites. Most of the small deletions involved overlaps of one to four complementary bases at the recombination junctions. Southern blot analysis revealed more complex mutations, suggesting translocation, inversion, or insertion of larger chromosomal fragments. These results indicate that blunt-end DNA double-strand breaks can induce illegitimate (nonhomologous) recombination in mammalian chromosomes and that they play an important role in mutagenesis.     

Reduced repair of DNA double-strand breaks by homologous recombination in a DNA ligase I-deficient human cell line

Genetic and biochemical studies of mammalian DNA ligase I indicate that this multifunctional enzyme plays a key role in the completion of DNA replication and certain DNA excision repair pathways. However, the involvement of DNA ligase I in DNA double-strand break repair has not been examined. Here we have determined the effect of DNA ligase I-deficiency on the frequency of homologous recombination initiated by a site-specific DNA double-strand break. We found that expression of wild-type DNA ligase I in a human DNA ligase I mutant cell line significantly increased the frequency of homologous recombination. Notably, the ability of DNA ligase I to promote the recombinational repair of DNA double-strand breaks was dependent upon its interaction with proliferating cell nuclear antigen. Thus, our results demonstrate that DNA ligase I-deficiency reduces recombinational repair of DNA double-strand breaks.     

Homing endonuclease mediated gene targeting in Anopheles gambiae cells and embryos

Homing endonuclease genes (HEGs) are ‘selfish’ genetic elements that combine the capability to selectively disrupt specific gene sequences with the ability to rapidly spread from a few individuals to an entire population through homologous recombination repair events. Because of these properties, HEGs are regarded as promising candidates to transfer genetic modifications from engineered laboratory mosquitoes to wild-type populations including Anopheles gambiae the vector of human malaria. Here we show that I-SceI and I-PpoI homing endonucleases cleave their recognition sites with high efficiency in A. gambiae cells and embryos and we demonstrate HEG-induced homologous and non-homologous repair events in a variety of functional assays. We also propose a gene drive system for mosquitoes that is based on our finding that I-PpoI cuts genomic rDNA located on the X chromosome in A. gambiae, which could be used to selectively incapacitate X-carrying spermatozoa thereby imposing a severe male-biased sex ratio.