Multi-Invasion-Induced Rearrangements as a Pathway for Physiological and Pathological Recombination

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.

Delineation of two multi-invasion-induced rearrangement pathways that differently affect genome stability

Punctuated bursts of structural genomic variations (SVs) have been described in various organisms, but their etiology remains incompletely understood. Homologous recombination (HR) is a template-guided mechanism of repair of DNA double-strand breaks and stalled or collapsed replication forks. We recently identified a DNA break amplification and genome rearrangement pathway originating from the endonucleolytic processing of a multi-invasion (MI) DNA joint molecule formed during HR. Genome-wide approaches confirmed that multi-invasion-induced rearrangement (MIR) frequently leads to several repeat-mediated SVs and aneuploidies. Using molecular and genetic analysis and a novel, highly sensitive proximity ligation-based assay for chromosomal rearrangement quantification, we further delineate two MIR subpathways. MIR1 is a universal pathway occurring in any sequence context, which generates secondary breaks and frequently leads to additional SVs. MIR2 occurs only if recombining donors exhibit substantial homology and results in sequence insertion without additional breaks or SVs. The most detrimental MIR1 pathway occurs late on a subset of persisting DNA joint molecules in a PCNA/Polδ-independent manner, unlike recombinational DNA synthesis. This work provides a refined mechanistic understanding of these HR-based SV formation pathways and shows that complex repeat-mediated SVs can occur without displacement DNA synthesis. Sequence signatures for inferring MIR1 from long-read data are proposed.

Saccharomyces cerevisiae DNA polymerase IV overcomes Rad51 inhibition of DNA polymerase δ in Rad52-mediated direct-repeat recombination

Saccharomyces cerevisiae DNA polymerase IV (Pol4) like its homolog, human DNA polymerase lambda (Polλ), is involved in Non-Homologous End-Joining and Microhomology-Mediated Repair. Using genetic analysis, we identified an additional role of Pol4 also in homology-directed DNA repair, specifically in Rad52-dependent/Rad51-independent direct-repeat recombination. Our results reveal that the requirement for Pol4 in repeat recombination was suppressed by the absence of Rad51, suggesting that Pol4 counteracts the Rad51 inhibition of Rad52-mediated repeat recombination events. Using purified proteins and model substrates, we reconstituted in vitro reactions emulating DNA synthesis during direct-repeat recombination and show that Rad51 directly inhibits Polδ DNA synthesis. Interestingly, although Pol4 was not capable of performing extensive DNA synthesis by itself, it aided Polδ in overcoming the DNA synthesis inhibition by Rad51. In addition, Pol4 dependency and stimulation of Polδ DNA synthesis in the presence of Rad51 occurred in reactions containing Rad52 and RPA where DNA strand-annealing was necessary. Mechanistically, yeast Pol4 displaces Rad51 from ssDNA independent of DNA synthesis. Together our in vitro and in vivo data suggest that Rad51 suppresses Rad52-dependent/Rad51-independent direct-repeat recombination by binding to the primer-template and that Rad51 removal by Pol4 is critical for strand-annealing dependent DNA synthesis.

Delineation of two multi-invasion-induced rearrangement pathways that differently affect genome stability

Punctuated bursts of structural genomic variations (SVs) have been described in various organisms, but their etiology remains incompletely understood. Homologous recombination (HR) is a template-guided mechanism of repair of DNA double-strand breaks and stalled or collapsed replication forks. We recently identified a DNA break amplification and genome rearrangement pathway originating from the endonucleolytic processing of a multi-invasion (MI) DNA joint molecule formed during HR. Genome-wide sequencing approaches confirmed that multi-invasion-induced rearrangement (MIR) frequently leads to several repeat-mediated SVs and aneuploidies. Using molecular and genetic analysis, and a novel, highly sensitive proximity ligation-based assay for chromosomal rearrangement quantification, we further delineate two MIR sub-pathways. MIR1 is a universal pathway occurring in any sequence context, which generates secondary breaks and frequently leads to additional SVs. MIR2 occurs only if recombining donors exhibit substantial homology, and results in sequence insertion without additional break or SV. The most detrimental MIR1 pathway occurs late on a subset of persisting DNA joint molecules in a PCNA/Polδ-independent manner, unlike recombinational DNA synthesis. This work provides a refined mechanistic understanding of these HR-based SV formation pathways and shows that complex repeat-mediated SVs can occur without displacement DNA synthesis. Sequence signatures for inferring MIR1 from long-read data are proposed.

Fusing an exonuclease with Cas9 enhances homologous recombination in Pichia pastoris

BACKGROUND

The methylotrophic yeast Pichia pastoris is considered as an ideal host for the production of recombinant proteins and chemicals. However, low homologous recombination (HR) efficiency hinders its precise and extensive genetic manipulation. To enhance the homology-directed repair over non-homologous end joining (NHEJ), we expressed five exonucleases that were fused with the Cas9 for enhancing end resection of double strand breaks (DSBs) of DNA cuts.

RESULTS

The endogenous exonuclease Mre11 and Exo1 showed the highest positive rates in seamless deletion of FAA1, and fusing the MRE11 to the C-terminal of CAS9 had the highest positive rate and relatively high number of clones. We observed that expression of CAS9-MRE11 significantly improved positive rates when simultaneously seamless deletion of double genes (from 76.7 to 86.7%) and three genes (from 10.8 to 16.7%) when overexpressing RAD52. Furthermore, MRE11 overexpression significantly improved the genomic integration of multi-fragments with higher positive rate and clone number.

CONCLUSIONS

Fusion expression of the endogenous exonuclease Mre11 with Cas9 enhances homologous recombination efficiency in P. pastoris. The strategy described here should facilitate the metabolic engineering of P. pastoris toward high-level production of value-added compounds.

Diffusion and distal linkages govern interchromosomal dynamics during meiotic prophase

SignificanceEssential for sexual reproduction, meiosis is a specialized cell division required for the production of haploid gametes. Critical to this process are the pairing, recombination, and segregation of homologous chromosomes (homologs). While pairing and recombination are linked, it is not known how many linkages are sufficient to hold homologs in proximity. Here, we reveal that random diffusion and the placement of a small number of linkages are sufficient to establish the apparent "pairing" of homologs. We also show that colocalization between any two loci is more dynamic than anticipated. Our study provides observations of live interchromosomal dynamics during meiosis and illustrates the power of combining single-cell measurements with theoretical polymer modeling.

A Pan-Cancer Bioinformatic Analysis of RAD51 Regarding the Values for Diagnosis, Prognosis, and Therapeutic Prediction

Background

RAD51, a critical protein for DNA repairment, has been found to associate with multiple cancer types, but, so far, a systematic pan-cancer analysis of RAD51 has not been done yet.

Methods

Data were obtained from multiple open databases and genetic alteration, gene expression, survival association, functional enrichment, stemness, mutation association, immunity association, and drug therapy association of RAD51were analyzed. A prognostic model of RAD51 for overall glioma was constructed as an example application of RAD51 as a biomarker.

Results

RAD51 was overexpressed in 28 types of cancers and was associated with worse overall survival in 11 cancer types. RAD51 correlated genes were enriched in cell cycle terms. RAD51 was associated with cancer stemness, tumor mutational burden, and multiple immunomodulators in different cancer types. RAD51 expression was different across immune subtypes in 11 cancer types. RAD51 was closely associated with cancer immune microenvironments in some cancer types. Proliferating T cells was the cell type that expressed highest RAD51 across most of the cancer samples analyzed. RAD51 expression had an AUC of over 0.5 in 12 of the 23 ICB subcohorts. The Tumor Immune Dysfunction and Exclusion of 9 cancer types were different between RAD51 high and low groups. RAD51 expression showed negative correlations with the sensitivity of most drugs. A prognostic nomogram was constructed with a high confidence.

Conclusion

RAD51 is a clinical valuable biomarker for multiple cancer types, regarding its potential power for diagnosis, prognosis, and therapeutic prediction.

Recombination-mediated genome rearrangements

Homologous recombination (HR) is a universal DNA double-strand break (DSB) repair pathway that uses an intact DNA molecule as a template. Signature HR reactions are homology search and DNA strand invasion catalyzed by the prototypical RecA-ssDNA filament (Rad51 and Dmc1 in eukaryotes), which produces heteroduplex DNA-containing joint molecules (JMs). These reactions uniquely infringe on the DNA strands association established at replication, on the basis of substantial sequence similarity. For that reason, and despite the high fidelity of its templated nature, DSB repair by HR authorizes the alteration of genome structure, guided by repetitive DNA elements. The resulting structural variations (SVs) can involve vast genomic regions, potentially affecting multiple coding sequences and regulatory elements at once, with possible pathological consequences. Here, we discuss recent advances in our understanding of genetic and molecular vulnerabilities of HR leading to SVs, and of the various fidelity-enforcing factors acting across scales on the balancing act of this complex pathway. An emphasis is put on extra-chomosomal DNAs, both product of, and substrate for HR-mediated chromosomal rearrangements.

Double-strand breaks induce short-scale DNA replication and damage amplification in the fully grown mouse oocytes

Break-induced replication (BIR) is essential for the repair of DNA double-strand breaks (DSBs) with single ends. DSBs-induced microhomology-mediated BIR (mmBIR) and template-switching can increase the risk of complex genome rearrangement. In addition, DSBs can also induce the multi-invasion-mediated DSB amplification. The mmBIR-induced genomic rearrangement has been identified in cancer cells and patients with rare diseases. However, when and how mmBIR is initiated have not been fully and deeply studied. Furthermore, it is not well understood about the conditions for initiation of multi-invasion-mediated DSB amplification. In the G2 phase oocyte of mouse, we identified a type of short-scale BIR (ssBIR) using the DNA replication indicator 5-ethynyl-2'-deoxyuridine (EdU). These ssBIRs could only be induced in the fully grown oocytes but not the growing oocytes. If the DSB oocytes were treated with Rad51 or Chek1/2 inhibitors, both EdU signals and DSB marker γH2A.X foci would decrease. In addition, the DNA polymerase inhibitor Aphidicolin could inhibit the ssBIR and another inhibitor ddATP could reduce the number of γH2A.X foci in the DSB oocytes. In conclusion, our results showed that DNA DSBs in the fully grown oocytes can initiate ssBIR and be amplified by Rad51 or DNA replication.

Physical and Genetic Assays for the Study of DNA Joint Molecules Metabolism and Multi-invasion-Induced Rearrangements in S. cerevisiae

DNA double-strand breaks (DSBs) are genotoxic lesions that can be repaired in a templated fashion by homologous recombination (HR). HR is a complex pathway that involves the formation of DNA joint molecules (JMs) containing heteroduplex DNA. Various types of JMs are formed throughout the pathway, including displacement loops (D-loops), multi-invasions (MI), and double Holliday junction intermediates. Dysregulation of JM metabolism in various mutant contexts revealed the propensity of HR to generate repeat-mediated chromosomal rearrangements. Specifically, we recently identified MI-induced rearrangements (MIR), a tripartite recombination mechanism initiated by one end of a DSB that exploits repeated regions to generate rearrangements between intact chromosomal regions. MIR occurs upon MI-JM processing by endonucleases and is suppressed by JM disruption activities. Here, we detail two assays: a physical assay for JM detection in Saccharomyces cerevisiae cells and genetic assays to determine the frequency of MIR in various chromosomal contexts. These assays enable studying the regulation of the HR pathway and the consequences of their defects for genomic instability by MIR.

Origin, Regulation, and Fitness Effect of Chromosomal Rearrangements in the Yeast Saccharomyces cerevisiae

Chromosomal rearrangements comprise unbalanced structural variations resulting in gain or loss of DNA copy numbers, as well as balanced events including translocation and inversion that are copy number neutral, both of which contribute to phenotypic evolution in organisms. The exquisite genetic assay and gene editing tools available for the model organism Saccharomyces cerevisiae facilitate deep exploration of the mechanisms underlying chromosomal rearrangements. We discuss here the pathways and influential factors of chromosomal rearrangements in S. cerevisiae. Several methods have been developed to generate on-demand chromosomal rearrangements and map the breakpoints of rearrangement events. Finally, we highlight the contributions of chromosomal rearrangements to drive phenotypic evolution in various S. cerevisiae strains. Given the evolutionary conservation of DNA replication and recombination in organisms, the knowledge gathered in the small genome of yeast can be extended to the genomes of higher eukaryotes.

Implications of Metastable Nicks and Nicked Holliday Junctions in Processing Joint Molecules in Mitosis and Meiosis

Joint molecules (JMs) are intermediates of homologous recombination (HR). JMs rejoin sister or homolog chromosomes and must be removed timely to allow segregation in anaphase. Current models pinpoint Holliday junctions (HJs) as a central JM. The canonical HJ (cHJ) is a four-way DNA that needs specialized nucleases, a.k.a. resolvases, to resolve into two DNA molecules. Alternatively, a helicase–topoisomerase complex can deal with pairs of cHJs in the dissolution pathway. Aside from cHJs, HJs with a nick at the junction (nicked HJ; nHJ) can be found in vivo and are extremely good substrates for resolvases in vitro. Despite these findings, nHJs have been neglected as intermediates in HR models. Here, I present a conceptual study on the implications of nicks and nHJs in the final steps of HR. I address this from a biophysical, biochemical, topological, and genetic point of view. My conclusion is that they ease the elimination of JMs while giving genetic directionality to the final products. Additionally, I present an alternative view of the dissolution pathway since the nHJ that results from the second end capture predicts a cross-join isomerization. Finally, I propose that this isomerization nicely explains the strict crossover preference observed in synaptonemal-stabilized JMs in meiosis.

Rdh54/Tid1 inhibits Rad51-Rad54-mediated D-loop formation and limits D-loop length

Displacement loops (D-loops) are critical intermediates formed during homologous recombination. Rdh54 (a.k.a. Tid1), a Rad54 paralog in Saccharomyces cerevisiae, is well-known for its role with Dmc1 recombinase during meiotic recombination. Yet contrary to Dmc1, Rdh54/Tid1 is also present in somatic cells where its function is less understood. While Rdh54/Tid1 enhances the Rad51 DNA strand invasion activity in vitro, it is unclear how it interplays with Rad54. Here, we show that Rdh54/Tid1 inhibits D-loop formation by Rad51 and Rad54 in an ATPase-independent manner. Using a novel D-loop Mapping Assay, we further demonstrate that Rdh54/Tid1 uniquely restricts the length of Rad51-Rad54-mediated D-loops. The alterations in D-loop properties appear to be important for cell survival and mating-type switch in haploid yeast. We propose that Rdh54/Tid1 and Rad54 compete for potential binding sites within the Rad51 filament, where Rdh54/Tid1 acts as a physical roadblock to Rad54 translocation, limiting D-loop formation and D-loop length.

Bisulfite treatment and single-molecule real-time sequencing reveal D-loop length, position, and distribution

Displacement loops (D-loops) are signature intermediates formed during homologous recombination. Numerous factors regulate D-loop formation and disruption, thereby influencing crucial aspects of DNA repair, including donor choice and the possibility of crossover outcome. While D-loop detection methods exist, it is currently unfeasible to assess the relationship between D-loop editors and D-loop characteristics such as length and position. Here, we developed a novel in vitro assay to characterize the length and position of individual D-loops with near base-pair resolution and deep coverage, while also revealing their distribution in a population. Non-denaturing bisulfite treatment modifies the cytosines on the displaced strand of the D-loop to uracil, leaving a permanent signature for the displaced strand. Subsequent single-molecule real-time sequencing uncovers the cytosine conversion patch as a D-loop footprint. The D-loop Mapping Assay is widely applicable with different substrates and donor types and can be used to study factors that influence D-loop properties.

Centromere scission drives chromosome shuffling and reproductive isolation

A fundamental characteristic of eukaryotic organisms is the generation of genetic variation via sexual reproduction. Conversely, significant large-scale genome structure variations could hamper sexual reproduction, causing reproductive isolation and promoting speciation. The underlying processes behind large-scale genome rearrangements are not well understood and include chromosome translocations involving centromeres. Recent genomic studies in the Cryptococcus species complex revealed that chromosome translocations generated via centromere recombination have reshaped the genomes of different species. In this study, multiple DNA double-strand breaks (DSBs) were generated via the CRISPR/Cas9 system at centromere-specific retrotransposons in the human fungal pathogen Cryptococcus neoformans The resulting DSBs were repaired in a complex manner, leading to the formation of multiple interchromosomal rearrangements and new telomeres, similar to chromothripsis-like events. The newly generated strains harboring chromosome translocations exhibited normal vegetative growth but failed to undergo successful sexual reproduction with the parental wild-type strain. One of these strains failed to produce any spores, while another produced ∼3% viable progeny. The germinated progeny exhibited aneuploidy for multiple chromosomes and showed improved fertility with both parents. All chromosome translocation events were accompanied without any detectable change in gene sequences and thus suggest that chromosomal translocations alone may play an underappreciated role in the onset of reproductive isolation and speciation.

A mutant form of Dmc1 that bypasses the requirement for accessory protein Mei5-Sae3 reveals independent activities of Mei5-Sae3 and Rad51 in Dmc1 filament stability

During meiosis, homologous recombination repairs programmed DNA double-stranded breaks. Meiotic recombination physically links the homologous chromosomes (“homologs”), creating the tension between them that is required for their segregation. The central recombinase in this process is Dmc1. Dmc1’s activity is regulated by its accessory factors including the heterodimeric protein Mei5-Sae3 and Rad51. We use a gain-of-function dmc1 mutant, dmc1-E157D, that bypasses Mei5-Sae3 to gain insight into the role of this accessory factor and its relationship to mitotic recombinase Rad51, which also functions as a Dmc1 accessory protein during meiosis. We find that Mei5-Sae3 has a role in filament formation and stability, but not in the bias of recombination partner choice that favors homolog over sister chromatids. Analysis of meiotic recombination intermediates suggests that Mei5-Sae3 and Rad51 function independently in promoting filament stability. In spite of its ability to load onto single-stranded DNA and carry out recombination in the absence of Mei5-Sae3, recombination promoted by the Dmc1 mutant is abnormal in that it forms foci in the absence of DNA breaks, displays unusually high levels of multi-chromatid and intersister joint molecule intermediates, as well as high levels of ectopic recombination products. We use super-resolution microscopy to show that the mutant protein forms longer foci than those formed by wild-type Dmc1. Our data support a model in which longer filaments are more prone to engage in aberrant recombination events, suggesting that filament lengths are normally limited by a regulatory mechanism that functions to prevent recombination-mediated genome rearrangements.

During meiosis, two rounds of division follow a single round of DNA replication to create the gametes for biparental reproduction. The first round of division requires that the homologous chromosomes become physically linked to one another to create the tension that is necessary for their segregation. This linkage is achieved through DNA recombination between the two homologous chromosomes, followed by resolution of the recombination intermediate into a crossover. Central to this process is the meiosis-specific recombinase Dmc1, and its accessory factors, which provide important regulatory functions to ensure that recombination is accurate, efficient, and occurs predominantly between homologous chromosomes, and not sister chromatids. To gain insight into the regulation of Dmc1 by its accessory factors, we mutated Dmc1 such that it was no longer dependent on its accessory factor Mei5-Sae3. Our analysis reveals that Dmc1 accessory factors Mei5-Sae3 and Rad51 have independent roles in stabilizing Dmc1 filaments. Furthermore, we find that although Rad51 is required for promoting recombination between homologous chromosomes, Mei5-Sae3 is not. Lastly, we show that our Dmc1 mutant forms abnormally long filaments, and high levels of aberrant recombination intermediates and products. These findings suggest that filaments are actively maintained at short lengths to prevent deleterious genome rearrangements.

The canine RAD51 mutation leads to the attenuation of interaction with PALB2

RAD51 forms a complex with BRCA2 and plays a central role in the DNA damage response pathway that is associated with homologous recombination. The structures of RAD51 and its homologues are highly conserved from prokaryotes to higher eukaryotes. Although a large number of BRCA2 mutations have been reported, there are only a few reports on the mutations of RAD51, which have been shown in humans and dogs. However, several mutations of canine RAD51 were identified from mammary gland tumour tissues in a recent study. Some of these mutations seem to have an influence on the homo-oligomerization or interaction with "Partner and localizer of BRCA2" (PALB2). In this study, we cloned the canine PALB2 homologue and investigated the effect on its interaction with the RAD51 mutants to evaluate the alteration in the function of RAD51 mutants. The A209S and T225S mutants of RAD51 show an attenuation of the interaction between RAD51 and PALB2. These results indicate that the canine RAD51 mutations can potentially alter the homologous recombination pathways in response to DNA damage in dogs.

Moving forward one step back at a time: reversibility during homologous recombination

DNA double-strand breaks are genotoxic lesions whose repair can be templated off an intact DNA duplex through the conserved homologous recombination (HR) pathway. Because it mainly consists of a succession of non-covalent associations of molecules, HR is intrinsically reversible. Reversibility serves as an integral property of HR, exploited and tuned at various stages throughout the pathway with anti- and pro-recombinogenic consequences. Here, we focus on the reversibility of displacement loops (D-loops), a central DNA joint molecule intermediate whose dynamics and regulation have recently been physically probed in somatic S. cerevisiae cells. From homology search to repair completion, we discuss putative roles of D-loop reversibility in repair fidelity and outcome.

Dynamic Processing of Displacement Loops during Recombinational DNA Repair

Displacement loops (D-loops) are pivotal intermediates of homologous recombination (HR), a universal DNA double strand break (DSB) repair pathway. We developed a versatile assay for the physical detection of D-loops in vivo, which enabled studying the kinetics of their formation and defining the activities controlling their metabolism. Nascent D-loops are detected within 2 h of DSB formation and extended in a delayed fashion in a genetic system designed to preclude downstream repair steps. The majority of nascent D-loops are disrupted by two pathways: one supported by the Srs2 helicase and the other by the Mph1 helicase and the Sgs1-Top3-Rmi1 helicase-topoisomerase complex. Both pathways operate without significant overlap and are delineated by the Rad54 paralog Rdh54 in an ATPase-independent fashion. This study uncovers a layer of quality control of HR relying on nascent D-loop dynamics.

Homologous Recombination and the Formation of Complex Genomic Rearrangements

The maintenance of genome integrity involves multiple independent DNA damage avoidance and repair mechanisms. However, the origin and pathways of the focal chromosomal reshuffling phenomena collectively referred to as chromothripsis remain mechanistically obscure. We discuss here the role, mechanisms, and regulation of homologous recombination (HR) in the formation of simple and complex chromosomal rearrangements. We emphasize features of the recently characterized multi-invasion (MI)-induced rearrangement (MIR) pathway which uniquely amplifies the initial DNA damage. HR intermediates and cellular contexts that endanger genomic stability are discussed as well as the emerging roles of various classes of nucleases in the formation of genome rearrangements. Long-read sequencing and improved mapping of repeats should enable better appreciation of the significance of recombination in generating genomic rearrangements.

Variation in RAD51 details a hub of functions: opportunities to advance cancer diagnosis and therapy

Loss of genome stability is one of the hallmarks of the enabling characteristics of cancer development. Homologous recombination is a DNA repair process that often breaks down as a prelude to developing cancer. Conversely, homologous recombination can be the Achilles’ heel in common anti-cancer therapies, which are effective by inducing irreparable DNA damage. Here, we review recent structural and functional studies of RAD51, the protein that catalyzes the defining step of homologous recombination: homology recognition and DNA strand exchange. Specific mutations can be linked to structural changes and known essential functions. Additional RAD51 interactions and functions may be revealed. The identification of viable mutations in this essential protein may help define the range of activity and interactions needed. All of this information provides opportunities to fine-tune existing therapies based on homologous recombination status, guide diagnosis, and hopefully develop new clinical tools.

Variation in RAD51 details a hub of functions: opportunities to advance cancer diagnosis and therapy

Loss of genome stability is one of the hallmarks of the enabling characteristics of cancer development. Homologous recombination is a DNA repair process that often breaks down as a prelude to developing cancer. Conversely, homologous recombination can be the Achilles' heel in common anti-cancer therapies, which are effective by inducing irreparable DNA damage. Here, we review recent structural and functional studies of RAD51, the protein that catalyzes the defining step of homologous recombination: homology recognition and DNA strand exchange. Specific mutations can be linked to structural changes and known essential functions. Additional RAD51 interactions and functions may be revealed. The identification of viable mutations in this essential protein may help define the range of activity and interactions needed. All of this information provides opportunities to fine-tune existing therapies based on homologous recombination status, guide diagnosis, and hopefully develop new clinical tools.