DNA double-strand break repair-pathway choice in somatic mammalian cells.

The major pathways of DNA double-strand break (DSB) repair are crucial for maintaining genomic stability. However, if deployed in an inappropriate cellular context, these same repair functions can mediate chromosome rearrangements that underlie various human diseases, ranging from developmental disorders to cancer. The two major mechanisms of DSB repair in mammalian cells are non-homologous end joining (NHEJ) and homologous recombination. In this Review, we consider DSB repair-pathway choice in somatic mammalian cells as a series of 'decision trees', and explore how defective pathway choice can lead to genomic instability. Stalled, collapsed or broken DNA replication forks present a distinctive challenge to the DSB repair system. Emerging evidence suggests that the 'rules' governing repair-pathway choice at stalled replication forks differ from those at replication-independent DSBs.

FANCM regulates repair pathway choice at stalled replication forks.

Repair pathway "choice" at stalled mammalian replication forks is an important determinant of genome stability; however, the underlying mechanisms are poorly understood. FANCM encodes a multi-domain scaffolding and motor protein that interacts with several distinct repair protein complexes at stalled forks. Here, we use defined mutations engineered within endogenous Fancm in mouse embryonic stem cells to study how Fancm regulates stalled fork repair. We find that distinct FANCM repair functions are enacted by molecularly separable scaffolding domains. These findings define FANCM as a key mediator of repair pathway choice at stalled replication forks and reveal its molecular mechanism. Notably, mutations that inactivate FANCM ATPase function disable all its repair functions and "trap" FANCM at stalled forks. We find that Brca1 hypomorphic mutants are synthetic lethal with Fancm null or Fancm ATPase-defective mutants. The ATPase function of FANCM may therefore represent a promising "druggable" target for therapy of BRCA1-linked cancer.

The Protexin complex counters resection on stalled forks to promote homologous recombination and crosslink repair.

Protection of stalled replication forks is critical to genomic stability. Using genetic and proteomic analyses, we discovered the Protexin complex containing the ssDNA binding protein SCAI and the DNA polymerase REV3. Protexin is required specifically for protecting forks stalled by nucleotide depletion, fork barriers, fragile sites, and DNA inter-strand crosslinks (ICLs), where it promotes homologous recombination and repair. Protexin loss leads to ssDNA accumulation and profound genomic instability in response to ICLs. Protexin interacts with RNA POL2, and both oppose EXO1's resection of DNA on forks remodeled by the FANCM translocase activity. This pathway acts independently of BRCA/RAD51-mediated fork stabilization, and cells with BRCA2 mutations were dependent on SCAI for survival. These data suggest that Protexin and its associated factors establish a new fork protection pathway that counteracts fork resection in part through a REV3 polymerase-dependent resynthesis mechanism of excised DNA, particularly at ICL stalled forks.

DNA Polymerase θ: Duct Tape and Zip Ties for a Fragile Genome.

Using a combination of genetics and cellular DNA rejoining assays, in this issue of Molecular Cell, Wyatt et al. (2016) demonstrate a critical role for mammalian DNA polymerase θ in the rejoining of DNA ends that are poor substrates for classical non-homologous end joining.

SIRT1 redistribution on chromatin promotes genomic stability but alters gene expression during aging.

Genomic instability and alterations in gene expression are hallmarks of eukaryotic aging. The yeast histone deacetylase Sir2 silences transcription and stabilizes repetitive DNA, but during aging or in response to a DNA break, the Sir complex relocalizes to sites of genomic instability, resulting in the desilencing of genes that cause sterility, a characteristic of yeast aging. Using embryonic stem cells, we show that mammalian Sir2, SIRT1, represses repetitive DNA and a functionally diverse set of genes across the mouse genome. In response to DNA damage, SIRT1 dissociates from these loci and relocalizes to DNA breaks to promote repair, resulting in transcriptional changes that parallel those in the aging mouse brain. Increased SIRT1 expression promotes survival in a mouse model of genomic instability and suppresses age-dependent transcriptional changes. Thus, DNA damage-induced redistribution of SIRT1 and other chromatin-modifying proteins may be a conserved mechanism of aging in eukaryotes.

Mechanism of tandem duplication formation in BRCA1-mutant cells.

Small, approximately 10-kilobase microhomology-mediated tandem duplications are abundant in the genomes of BRCA1-linked but not BRCA2-linked breast cancer. Here we define the mechanism underlying this rearrangement signature. We show that, in primary mammalian cells, BRCA1, but not BRCA2, suppresses the formation of tandem duplications at a site-specific chromosomal replication fork barrier imposed by the binding of Tus proteins to an array of Ter sites. BRCA1 has no equivalent role at chromosomal double-stranded DNA breaks, indicating that tandem duplications form specifically at stalled forks. Tandem duplications in BRCA1 mutant cells arise by a replication restart-bypass mechanism terminated by end joining or by microhomology-mediated template switching, the latter forming complex tandem duplication breakpoints. Solitary DNA ends form directly at Tus-Ter, implicating misrepair of these lesions in tandem duplication formation. Furthermore, BRCA1 inactivation is strongly associated with ~10 kilobase tandem duplications in ovarian cancer. This tandem duplicator phenotype may be a general signature of BRCA1-deficient cancer.

RFWD3-Dependent Ubiquitination of RPA Regulates Repair at Stalled Replication Forks.

We have used quantitative proteomics to profile ubiquitination in the DNA damage response (DDR). We demonstrate that RPA, which functions as a protein scaffold in the replication stress response, is multiply ubiquitinated upon replication fork stalling. Ubiquitination of RPA occurs on chromatin, involves sites outside its DNA binding channel, does not cause proteasomal degradation, and increases under conditions of fork collapse, suggesting a role in repair at stalled forks. We demonstrate that the E3 ligase RFWD3 mediates RPA ubiquitination. RFWD3 is necessary for replication fork restart, normal repair kinetics during replication stress, and homologous recombination (HR) at stalled replication forks. Mutational analysis suggests that multisite ubiquitination of the entire RPA complex is responsible for repair at stalled forks. Multisite protein group sumoylation is known to promote HR in yeast. Our findings reveal a similar requirement for multisite protein group ubiquitination during HR at stalled forks in mammalian cells.

Akt-mediated phosphorylation of XLF impairs non-homologous end-joining DNA repair.

Deficiency in repair of damaged DNA leads to genomic instability and is closely associated with tumorigenesis. Most DNA double-strand-breaks (DSBs) are repaired by two major mechanisms, homologous-recombination (HR) and non-homologous-end-joining (NHEJ). Although Akt has been reported to suppress HR, its role in NHEJ remains elusive. Here, we report that Akt phosphorylates XLF at Thr181 to trigger its dissociation from the DNA ligase IV/XRCC4 complex, and promotes its interaction with 14-3-3ß leading to XLF cytoplasmic retention, where cytosolic XLF is subsequently degraded by SCF(ß-TRCP) in a CKI-dependent manner. Physiologically, upon DNA damage, XLF-T181E expressing cells display impaired NHEJ and elevated cell death. Whereas a cancer-patient-derived XLF-R178Q mutant, deficient in XLF-T181 phosphorylation, exhibits an elevated tolerance of DNA damage. Together, our results reveal a pivotal role for Akt in suppressing NHEJ and highlight the tight connection between aberrant Akt hyper-activation and deficiency in timely DSB repair, leading to genomic instability and tumorigenesis.

Rad51 recruitment and exclusion of non-homologous end joining during homologous recombination at a Tus/Ter mammalian replication fork barrier.

Classical non-homologous end joining (C-NHEJ) and homologous recombination (HR) compete to repair mammalian chromosomal double strand breaks (DSBs). However, C-NHEJ has no impact on HR induced by DNA nicking enzymes. In this case, the replication fork is thought to convert the DNA nick into a one-ended DSB, which lacks a readily available partner for C-NHEJ. Whether C-NHEJ competes with HR at a non-enzymatic mammalian replication fork barrier (RFB) remains unknown. We previously showed that conservative "short tract" gene conversion (STGC) induced by a chromosomal Tus/Ter RFB is a product of bidirectional replication fork stalling. This finding raises the possibility that Tus/Ter-induced STGC proceeds via a two-ended DSB intermediate. If so, Tus/Ter-induced STGC might be subject to competition by C-NHEJ. However, in contrast to the DSB response, where genetic ablation of C-NHEJ stimulates HR, we report here that Tus/Ter-induced HR is unaffected by deletion of either of two C-NHEJ genes, Xrcc4 or Ku70. These results show that Tus/Ter-induced HR does not entail the formation of a two-ended DSB to which C-NHEJ has competitive access. We found no evidence that the alternative end-joining factor, DNA polymerase θ, competes with Tus/Ter-induced HR. We used chromatin-immunoprecipitation to compare Rad51 recruitment to a Tus/Ter RFB and to a neighboring site-specific DSB. Rad51 accumulation at Tus/Ter was more intense and more sustained than at a DSB. In contrast to the DSB response, Rad51 accumulation at Tus/Ter was restricted to within a few hundred base pairs of the RFB. Taken together, these findings suggest that the major DNA structures that bind Rad51 at a Tus/Ter RFB are not conventional DSBs. We propose that Rad51 acts as an "early responder" at stalled forks, binding single stranded daughter strand gaps on the arrested lagging strand, and that Rad51-mediated fork remodeling generates HR intermediates that are incapable of Ku binding and therefore invisible to the C-NHEJ machinery.

BRCA1 controls homologous recombination at Tus/Ter-stalled mammalian replication forks.

Replication fork stalling can promote genomic instability, predisposing to cancer and other diseases. Stalled replication forks may be processed by sister chromatid recombination (SCR), generating error-free or error-prone homologous recombination (HR) outcomes. In mammalian cells, a long-standing hypothesis proposes that the major hereditary breast/ovarian cancer predisposition gene products, BRCA1 and BRCA2, control HR/SCR at stalled replication forks. Although BRCA1 and BRCA2 affect replication fork processing, direct evidence that BRCA gene products regulate homologous recombination at stalled chromosomal replication forks is lacking, due to a dearth of tools for studying this process. Here we report that the Escherichia coli Tus/Ter complex can be engineered to induce site-specific replication fork stalling and chromosomal HR/SCR in mouse cells. Tus/Ter-induced homologous recombination entails processing of bidirectionally arrested forks. We find that the Brca1 carboxy (C)-terminal tandem BRCT repeat and regions of Brca1 encoded by exon 11-two Brca1 elements implicated in tumour suppression-control Tus/Ter-induced homologous recombination. Inactivation of either Brca1 or Brca2 increases the absolute frequency of 'long-tract' gene conversions at Tus/Ter-stalled forks, an outcome not observed in response to a site-specific endonuclease-mediated chromosomal double-strand break. Therefore, homologous recombination at stalled forks is regulated differently from homologous recombination at double-strand breaks arising independently of a replication fork. We propose that aberrant long-tract homologous recombination at stalled replication forks contributes to genomic instability and breast/ovarian cancer predisposition in BRCA mutant cells.

BRCA1 is required for postreplication repair after UV-induced DNA damage.

BRCA1 contributes to the response to UV irradiation. Utilizing its BRCT motifs, it is recruited during S/G2 to UV-damaged sites in a DNA replication-dependent but nucleotide excision repair (NER)-independent manner. More specifically, at UV-stalled replication forks, it promotes photoproduct excision, suppression of translesion synthesis, and the localization and activation of replication factor C complex (RFC) subunits. The last function, in turn, triggers post-UV checkpoint activation and postreplicative repair. These BRCA1 functions differ from those required for DSBR.

FANCJ helicase controls the balance between short- and long-tract gene conversions between sister chromatids.

The FANCJ DNA helicase is linked to hereditary breast and ovarian cancers as well as bone marrow failure disorder Fanconi anemia (FA). Although FANCJ has been implicated in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR), the molecular mechanism underlying the tumor suppressor functions of FANCJ remains obscure. Here, we demonstrate that FANCJ deficient human and hamster cells exhibit reduction in the overall gene conversions in response to a site-specific chromosomal DSB induced by I-SceI endonuclease. Strikingly, the gene conversion events were biased in favour of long-tract gene conversions in FANCJ depleted cells. The fine regulation of short- (STGC) and long-tract gene conversions (LTGC) by FANCJ was dependent on its interaction with BRCA1 tumor suppressor. Notably, helicase activity of FANCJ was essential for controlling the overall HR and in terminating the extended repair synthesis during sister chromatid recombination (SCR). Moreover, cells expressing FANCJ pathological mutants exhibited defective SCR with an increased frequency of LTGC. These data unravel the novel function of FANCJ helicase in regulating SCR and SCR associated gene amplification/duplications and imply that these functions of FANCJ are crucial for the genome maintenance and tumor suppression.

Correction: Complex Breakpoints and Template Switching Associated with Non-canonical Termination of Homologous Recombination in Mammalian Cells.

[This corrects the article DOI: 10.1371/journal.pgen.1006410.].

Complex Breakpoints and Template Switching Associated with Non-canonical Termination of Homologous Recombination in Mammalian Cells.

A proportion of homologous recombination (HR) events in mammalian cells resolve by "long tract" gene conversion, reflecting copying of several kilobases from the donor sister chromatid prior to termination. Cells lacking the major hereditary breast/ovarian cancer predisposition genes, BRCA1 or BRCA2, or certain other HR-defective cells, reveal a bias in favor of long tract gene conversion, suggesting that this aberrant HR outcome might be connected with genomic instability. If termination of gene conversion occurs in regions lacking homology with the second end of the break, the normal mechanism of HR termination by annealing (i.e., homologous pairing) is not available and termination must occur by as yet poorly defined non-canonical mechanisms. Here we use a previously described HR reporter to analyze mechanisms of non-canonical termination of long tract gene conversion in mammalian cells. We find that non-canonical HR termination can occur in the absence of the classical non-homologous end joining gene XRCC4. We observe obligatory use of microhomology (MH)-mediated end joining and/or nucleotide addition during rejoining with the second end of the break. Notably, non-canonical HR termination is associated with complex breakpoints. We identify roles for homology-mediated template switching and, potentially, MH-mediated template switching/microhomology-mediated break-induced replication, in the formation of complex breakpoints at sites of non-canonical HR termination. This work identifies non-canonical HR termination as a potential contributor to genomic instability and to the formation of complex breakpoints in cancer.

53BP1 Protects against CtIP-Dependent Capture of Ectopic Chromosomal Sequences at the Junction of Distant Double-Strand Breaks.

DNA double-strand breaks (DSB) are very harmful lesions that can generate genome rearrangements. In this study, we used intrachromosomal reporters to compare both the efficiency and accuracy of end-joining occurring with close (34 bp apart) vs. distant DSBs (3200 bp apart) in human fibroblasts. We showed that a few kb between two intrachromosomal I-SceI-induced DSBs are sufficient to foster deletions and capture/insertions at the junction scar. Captured sequences are mostly coupled to deletions and can be partial duplications of the reporter (i.e., sequences adjacent to the DSB) or insertions of ectopic chromosomal sequences (ECS). Interestingly, silencing 53BP1 stimulates capture/insertions with distant but not with close double-strand ends (DSEs), although deletions were stimulated in both case. This shows that 53BP1 protects both close and distant DSEs from degradation and that the association of unprotection with distance between DSEs favors ECS capture. Reciprocally, silencing CtIP lessens ECS capture both in control and 53BP1-depleted cells. We propose that close ends are immediately/rapidly tethered and ligated, whereas distant ends first require synapsis of the distant DSEs prior to ligation. This "spatio-temporal" gap gives time and space for CtIP to initiate DNA resection, suggesting an involvement of single-stranded DNA tails for ECS capture. We therefore speculate that the resulting single-stranded DNA copies ECS through microhomology-mediated template switching.

The tandem duplicator phenotype as a distinct genomic configuration in cancer.

Next-generation sequencing studies have revealed genome-wide structural variation patterns in cancer, such as chromothripsis and chromoplexy, that do not engage a single discernable driver mutation, and whose clinical relevance is unclear. We devised a robust genomic metric able to identify cancers with a chromotype called tandem duplicator phenotype (TDP) characterized by frequent and distributed tandem duplications (TDs). Enriched only in triple-negative breast cancer (TNBC) and in ovarian, endometrial, and liver cancers, TDP tumors conjointly exhibit tumor protein p53 (TP53) mutations, disruption of breast cancer 1 (BRCA1), and increased expression of DNA replication genes pointing at rereplication in a defective checkpoint environment as a plausible causal mechanism. The resultant TDs in TDP augment global oncogene expression and disrupt tumor suppressor genes. Importantly, the TDP strongly correlates with cisplatin sensitivity in both TNBC cell lines and primary patient-derived xenografts. We conclude that the TDP is a common cancer chromotype that coordinately alters oncogene/tumor suppressor expression with potential as a marker for chemotherapeutic response.

Phosphoinositide 3-kinase inhibitors induce DNA damage through nucleoside depletion.

We previously reported that combining a phosphoinositide 3-kinase (PI3K) inhibitor with a poly-ADP Rib polymerase (PARP)-inhibitor enhanced DNA damage and cell death in breast cancers that have genetic aberrations in BRCA1 and TP53. Here, we show that enhanced DNA damage induced by PI3K inhibitors in this mutational background is a consequence of impaired production of nucleotides needed for DNA synthesis and DNA repair. Inhibition of PI3K causes a reduction in all four nucleotide triphosphates, whereas inhibition of the protein kinase AKT is less effective than inhibition of PI3K in suppressing nucleotide synthesis and inducing DNA damage. Carbon flux studies reveal that PI3K inhibition disproportionately affects the nonoxidative pentose phosphate pathway that delivers Rib-5-phosphate required for base ribosylation. In vivo in a mouse model of BRCA1-linked triple-negative breast cancer (K14-Cre BRCA1(f/f)p53(f/f)), the PI3K inhibitor BKM120 led to a precipitous drop in DNA synthesis within 8 h of drug treatment, whereas DNA synthesis in normal tissues was less affected. In this mouse model, combined PI3K and PARP inhibition was superior to either agent alone to induce durable remissions of established tumors.

RAP80-directed tuning of BRCA1 homologous recombination function at ionizing radiation-induced nuclear foci.

In response to DNA double-strand breaks (DSBs), BRCA1 forms biochemically distinct complexes with certain other DNA damage response proteins. These structures, some of which are required for homologous recombination (HR)-type DSB repair, concentrate at distinct nuclear foci that demarcate sites of genome breakage. Polyubiquitin binding by one of these structures, the RAP80/BRCA1 complex, is required for efficient BRCA1 focal recruitment, but the relationship of this process to the execution of HR has been unclear. We found that this complex actively suppresses otherwise exaggerated, BRCA1-driven HR. By controlling the kinetics by which other BRCA1-interacting proteins that promote HR concentrate together with BRCA1 in nuclear foci, RAP80/BRCA1 complexes suppress excessive DSB end processing, HR-type DSB repair, and overt chromosomal instability. Since chromosomal instability emerges when BRCA1 HR function is either unbridled or absent, active tuning of BRCA1 activity, executed in nuclear foci, is important to genome integrity maintenance.