Ectopic expression of RNF168 and 53BP1 increases mutagenic but not physiological non-homologous end joining

E3 ligases: a ubiquitous link between DNA repair, DNA replication and human disease

Maintenance of genome stability is of paramount importance for the survival of an organism. However, genomic integrity is constantly being challenged by various endogenous and exogenous processes that damage DNA. Therefore, cells are heavily reliant on DNA repair pathways that have evolved to deal with every type of genotoxic insult that threatens to compromise genome stability. Notably, inherited mutations in genes encoding proteins involved in these protective pathways trigger the onset of disease that is driven by chromosome instability e.g. neurodevelopmental abnormalities, neurodegeneration, premature ageing, immunodeficiency and cancer development. The ability of cells to regulate the recruitment of specific DNA repair proteins to sites of DNA damage is extremely complex but is primarily mediated by protein post-translational modifications (PTMs). Ubiquitylation is one such PTM, which controls genome stability by regulating protein localisation, protein turnover, protein-protein interactions and intra-cellular signalling. Over the past two decades, numerous ubiquitin (Ub) E3 ligases have been identified to play a crucial role not only in the initiation of DNA replication and DNA damage repair but also in the efficient termination of these processes. In this review, we discuss our current understanding of how different Ub E3 ligases (RNF168, TRAIP, HUWE1, TRIP12, FANCL, BRCA1, RFWD3) function to regulate DNA repair and replication and the pathological consequences arising from inheriting deleterious mutations that compromise the Ub-dependent DNA damage response.

UBE2D3 facilitates NHEJ by orchestrating ATM signalling through multi-level control of RNF168

Maintenance of genome integrity requires tight control of DNA damage response (DDR) signalling and repair, with phosphorylation and ubiquitination representing key elements. How these events are coordinated to achieve productive DNA repair remains elusive. Here we identify the ubiquitin-conjugating enzyme UBE2D3 as a regulator of ATM kinase-induced DDR that promotes non-homologous end-joining (NHEJ) at telomeres. UBE2D3 contributes to DDR-induced chromatin ubiquitination and recruitment of the NHEJ-promoting factor 53BP1, both mediated by RNF168 upon ATM activation. Additionally, UBE2D3 promotes NHEJ by limiting RNF168 accumulation and facilitating ATM-mediated phosphorylation of KAP1-S824. Mechanistically, defective KAP1-S824 phosphorylation and telomeric NHEJ upon UBE2D3-deficiency are linked to RNF168 hyperaccumulation and aberrant PP2A phosphatase activity. Together, our results identify UBE2D3 as a multi-level regulator of NHEJ that orchestrates ATM and RNF168 activities. Moreover, they reveal a negative regulatory circuit in the DDR that is constrained by UBE2D3 and consists of RNF168- and phosphatase-mediated restriction of KAP1 phosphorylation.

Comprehensive mapping of cell fates in microsatellite unstable cancer cells supports dual targeting of WRN and ATR

Addiction to the WRN helicase is a unique vulnerability of human cancers with high levels of microsatellite instability (MSI-H). However, while prolonged loss of WRN ultimately leads to cell death, little is known about how MSI-H cancers initially respond to acute loss of WRN-knowledge that would be helpful for informing clinical development of WRN targeting therapy, predicting possible resistance mechanisms, and identifying useful biomarkers of successful WRN inhibition. Here, we report the construction of an inducible ligand-mediated degradation system in which the stability of endogenous WRN protein can be rapidly and specifically tuned, enabling us to track the complete sequence of cellular events elicited by acute loss of WRN function. We found that WRN degradation leads to immediate accrual of DNA damage in a replication-dependent manner that curiously did not robustly engage checkpoint mechanisms to halt DNA synthesis. As a result, WRN-degraded MSI-H cancer cells accumulate DNA damage across multiple replicative cycles and undergo successive rounds of increasingly aberrant mitoses, ultimately triggering cell death. Of potential therapeutic importance, we found no evidence of any generalized mechanism by which MSI-H cancers could adapt to near-complete loss of WRN. However, under conditions of partial WRN degradation, addition of low-dose ATR inhibitor significantly increased their combined efficacy to levels approaching full inactivation of WRN. Overall, our results provide the first comprehensive view of molecular events linking upstream inhibition of WRN to subsequent cell death and suggest that dual targeting of WRN and ATR might be a useful strategy for treating MSI-H cancers.

Comprehensive mapping of cell fates in microsatellite unstable cancer cells support dual targe6ng of WRN and ATR

Addiction to the WRN helicase is a unique vulnerability of human cancers with high levels of microsatellite instability (MSI-H). However, while prolonged loss of WRN ultimately leads to cell death, little is known about how MSI-H cancers initially respond to acute loss of WRN, knowledge that would be helpful for informing clinical development of WRN-targeting therapy, predicting possible resistance mechanisms, and identifying useful biomarkers of successful WRN inhibition. Here, we report the construction of an inducible ligand-mediated degradation system wherein the stability of endogenous WRN protein can be rapidly and specifically tuned, enabling us to track the complete sequence of cellular events elicited by acute loss of WRN function. We find that WRN degradation leads to immediate accrual of DNA damage in a replication-dependent manner that curiously did not robustly engage checkpoint mechanisms to halt DNA synthesis. As a result, WRN-degraded MSI-H cancer cells accumulate DNA damage across multiple replicative cycles and undergo successive rounds of increasingly aberrant mitoses, ultimately triggering cell death. Of potential therapeutic importance, we find no evidence of any generalized mechanism by which MSI-H cancers could adapt to near-complete loss of WRN. However, under conditions of partial WRN degradation, addition of low dose ATR inhibitor significantly increased their combined efficacy to levels approaching full inactivation of WRN. Overall, our results provided the first comprehensive view of molecular events linking upstream inhibition of WRN to subsequent cell death and suggested a potential therapeutical rationale for dual targeting of WRN and ATR.

Immediate-Early, Early, and Late Responses to DNA Double Stranded Breaks

Loss or rearrangement of genetic information can result from incorrect responses to DNA double strand breaks (DSBs). The cellular responses to DSBs encompass a range of highly coordinated events designed to detect and respond appropriately to the damage, thereby preserving genomic integrity. In analogy with events occurring during viral infection, we appropriate the terms Immediate-Early, Early, and Late to describe the pre-repair responses to DSBs. A distinguishing feature of the Immediate-Early response is that the large protein condensates that form during the Early and Late response and are resolved upon repair, termed foci, are not visible. The Immediate-Early response encompasses initial lesion sensing, involving poly (ADP-ribose) polymerases (PARPs), KU70/80, and MRN, as well as rapid repair by so-called ‘fast-kinetic’ canonical non-homologous end joining (cNHEJ). Initial binding of PARPs and the KU70/80 complex to breaks appears to be mutually exclusive at easily ligatable DSBs that are repaired efficiently by fast-kinetic cNHEJ; a process that is PARP-, ATM-, 53BP1-, Artemis-, and resection-independent. However, at more complex breaks requiring processing, the Immediate-Early response involving PARPs and the ensuing highly dynamic PARylation (polyADP ribosylation) of many substrates may aid recruitment of both KU70/80 and MRN to DSBs. Complex DSBs rely upon the Early response, largely defined by ATM-dependent focal recruitment of many signalling molecules into large condensates, and regulated by complex chromatin dynamics. Finally, the Late response integrates information from cell cycle phase, chromatin context, and type of DSB to determine appropriate pathway choice. Critical to pathway choice is the recruitment of p53 binding protein 1 (53BP1) and breast cancer associated 1 (BRCA1). However, additional factors recruited throughout the DSB response also impact upon pathway choice, although these remain to be fully characterised. The Late response somehow channels DSBs into the appropriate high-fidelity repair pathway, typically either ‘slow-kinetic’ cNHEJ or homologous recombination (HR). Loss of specific components of the DSB repair machinery results in cells utilising remaining factors to effect repair, but often at the cost of increased mutagenesis. Here we discuss the complex regulation of the Immediate-Early, Early, and Late responses to DSBs proceeding repair itself.

SPT6-driven error-free DNA repair safeguards genomic stability of glioblastoma cancer stem-like cells

Glioblastoma cancer-stem like cells (GSCs) display marked resistance to ionizing radiation (IR), a standard of care for glioblastoma patients. Mechanisms underpinning radio-resistance of GSCs remain largely unknown. Chromatin state and the accessibility of DNA lesions to DNA repair machineries are crucial for the maintenance of genomic stability. Understanding the functional impact of chromatin remodeling on DNA repair in GSCs may lay the foundation for advancing the efficacy of radio-sensitizing therapies. Here, we present the results of a high-content siRNA microscopy screen, revealing the transcriptional elongation factor SPT6 to be critical for the genomic stability and self-renewal of GSCs. Mechanistically, SPT6 transcriptionally up-regulates BRCA1 and thereby drives an error-free DNA repair in GSCs. SPT6 loss impairs the self-renewal, genomic stability and tumor initiating capacity of GSCs. Collectively, our results provide mechanistic insights into how SPT6 regulates DNA repair and identify SPT6 as a putative therapeutic target in glioblastoma.

Cancer stem cells can evade treatment. Here, the authors perform an in vitro screen to identify proteins that are involved in protecting glioma cancer stem cells from therapy and find that SPT6 increases BRCA1 expression and drives error-free DNA repair, thereby ensuring the survival of the cells.

Regulation of Histone Ubiquitination in Response to DNA Double Strand Breaks

Eukaryotic cells are constantly exposed to both endogenous and exogenous stressors that promote the induction of DNA damage. Of this damage, double strand breaks (DSBs) are the most lethal and must be efficiently repaired in order to maintain genomic integrity. Repair of DSBs occurs primarily through one of two major pathways: non-homologous end joining (NHEJ) or homologous recombination (HR). The choice between these pathways is in part regulated by histone post-translational modifications (PTMs) including ubiquitination. Ubiquitinated histones not only influence transcription and chromatin architecture at sites neighboring DSBs but serve as critical recruitment platforms for repair machinery as well. The reversal of these modifications by deubiquitinating enzymes (DUBs) is increasingly being recognized in a number of cellular processes including DSB repair. In this context, DUBs ensure proper levels of ubiquitin, regulate recruitment of downstream effectors, dictate repair pathway choice, and facilitate appropriate termination of the repair response. This review outlines the current understanding of histone ubiquitination in response to DSBs, followed by a comprehensive overview of the DUBs that catalyze the removal of these marks.

Ectopic RNF168 expression promotes break-induced replication-like DNA synthesis at stalled replication forks

The RNF168 E3 ubiquitin ligase is activated in response to double stranded DNA breaks (DSBs) where it mono-ubiquitinates γH2AX (ub-H2AX). RNF168 protein expression and ubiquitin signaling are finely regulated during the sensing, repair and resolution of DNA damage in order to avoid excessive spreading of ubiquitinated chromatin. Supra-physiological RNF168 protein expression levels have been shown to block DNA end resection at DSBs and increase PARP inhibitor (PARPi) sensitivity. In this study, we examined the impact of ectopic RNF168 overexpression on hydroxyurea (HU)-induced stalled replication forks in the setting of BRCA1 deficiency. Surprisingly, RNF168 overexpression resulted in the extension of DNA fibers, despite the presence of HU, in BRCA1 deficient cells. Mechanistically, RNF168 overexpression recruited RAD18 to ub-H2AX at HU-induced DNA breaks. Subsequently, a RAD18-SLF1 axis was responsible for initiating DNA synthesis in a manner that also required the break-induced replication (BIR) factors RAD52 and POLD3. Strikingly, the presence of wild-type BRCA1 blocked RNF168-induced DNA synthesis. Notably, BIR-like repair has previously been linked with tandem duplication events found in BRCA1-mutated genomes. Thus, in the absence of BRCA1, excessive RNF168 expression may drive BIR, and contribute to the mutational signatures observed in BRCA1-mutated cancers.

The nuclear structural protein NuMA is a negative regulator of 53BP1 in DNA double-strand break repair

P53-binding protein 1 (53BP1) mediates DNA repair pathway choice and promotes checkpoint activation. Chromatin marks induced by DNA double-strand breaks and recognized by 53BP1 enable focal accumulation of this multifunctional repair factor at damaged chromatin. Here, we unveil an additional level of regulation of 53BP1 outside repair foci. 53BP1 movements are constrained throughout the nucleoplasm and increase in response to DNA damage. 53BP1 interacts with the structural protein NuMA, which controls 53BP1 diffusion. This interaction, and colocalization between the two proteins in vitro and in breast tissues, is reduced after DNA damage. In cell lines and breast carcinoma NuMA prevents 53BP1 accumulation at DNA breaks, and high NuMA expression predicts better patient outcomes. Manipulating NuMA expression alters PARP inhibitor sensitivity of BRCA1-null cells, end-joining activity, and immunoglobulin class switching that rely on 53BP1. We propose a mechanism involving the sequestration of 53BP1 by NuMA in the absence of DNA damage. Such a mechanism may have evolved to disable repair functions and may be a decisive factor for tumor responses to genotoxic treatments.

53BP1 Enforces Distinct Pre- and Post-resection Blocks on Homologous Recombination

53BP1 activity drives genome instability and lethality in BRCA1-deficient mice by inhibiting homologous recombination (HR). The anti-recombinogenic functions of 53BP1 require phosphorylation-dependent interactions with PTIP and RIF1/shieldin effector complexes. While RIF1/shieldin blocks 5'-3' nucleolytic processing of DNA ends, it remains unclear how PTIP antagonizes HR. Here, we show that mutation of the PTIP interaction site in 53BP1 (S25A) allows sufficient DNA2-dependent end resection to rescue the lethality of BRCA1^Δ11 mice, despite increasing RIF1 "end-blocking" at DNA damage sites. However, double-mutant cells fail to complete HR, as excessive shieldin activity also inhibits RNF168-mediated loading of PALB2/RAD51. As a result, BRCA1^Δ1153BP1^S25A mice exhibit hallmark features of HR insufficiency, including premature aging and hypersensitivity to PARPi. Disruption of shieldin or forced targeting of PALB2 to ssDNA in BRCA1^D1153BP1^S25A cells restores RNF168 recruitment, RAD51 nucleofilament formation, and PARPi resistance. Our study therefore reveals a critical function of shieldin post-resection that limits the loading of RAD51.

Human papillomavirus E7 oncoprotein targets RNF168 to hijack the host DNA damage response

High-risk human papillomaviruses (HR-HPVs) promote cervical cancer as well as a subset of anogenital and head and neck cancers. Due to their limited coding capacity, HPVs hijack the host cell's DNA replication and repair machineries to replicate their own genomes. How this host-pathogen interaction contributes to genomic instability is unknown. Here, we report that HPV-infected cancer cells express high levels of RNF168, an E3 ubiquitin ligase that is critical for proper DNA repair following DNA double-strand breaks, and accumulate high numbers of 53BP1 nuclear bodies, a marker of genomic instability induced by replication stress. We describe a mechanism by which HPV E7 subverts the function of RNF168 at DNA double-strand breaks, providing a rationale for increased homology-directed recombination in E6/E7-expressing cervical cancer cells. By targeting a new regulatory domain of RNF168, E7 binds directly to the E3 ligase without affecting its enzymatic activity. As RNF168 knockdown impairs viral genome amplification in differentiated keratinocytes, we propose that E7 hijacks the E3 ligase to promote the viral replicative cycle. This study reveals a mechanism by which tumor viruses reshape the cellular response to DNA damage by manipulating RNF168-dependent ubiquitin signaling. Importantly, our findings reveal a pathway by which HPV may promote the genomic instability that drives oncogenesis.

BRCA1 Haploinsufficiency Is Masked by RNF168-Mediated Chromatin Ubiquitylation

BRCA1 functions at two distinct steps during homologous recombination (HR). Initially, it promotes DNA end resection, and subsequently it recruits the PALB2 and BRCA2 mediator complex, which stabilizes RAD51-DNA nucleoprotein filaments. Loss of 53BP1 rescues the HR defect in BRCA1-deficient cells by increasing resection, suggesting that BRCA1's downstream role in RAD51 loading is dispensable when 53BP1 is absent. Here we show that the E3 ubiquitin ligase RNF168, in addition to its canonical role in inhibiting end resection, acts in a redundant manner with BRCA1 to load PALB2 onto damaged DNA. Loss of RNF168 negates the synthetic rescue of BRCA1 deficiency by 53BP1 deletion, and it predisposes BRCA1 heterozygous mice to cancer. BRCA1^+/-RNF168^-/- cells lack RAD51 foci and are hypersensitive to PARP inhibitor, whereas forced targeting of PALB2 to DNA breaks in mutant cells circumvents BRCA1 haploinsufficiency. Inhibiting the chromatin ubiquitin pathway may, therefore, be a synthetic lethality strategy for BRCA1-deficient cancers.

Mechanism of 53BP1 activity regulation by RNA-binding TIRR and a designer protein

Dynamic protein interaction networks such as DNA double-strand break (DSB) signaling are modulated by post-translational modifications. The DNA repair factor 53BP1 is a rare example of a protein whose post-translational modification-binding function can be switched on and off. 53BP1 is recruited to DSBs by recognizing histone lysine methylation within chromatin, an activity directly inhibited by the 53BP1-binding protein TIRR. X-ray crystal structures of TIRR and a designer protein bound to 53BP1 now reveal a unique regulatory mechanism in which an intricate binding area centered on an essential TIRR arginine residue blocks the methylated-chromatin-binding surface of 53BP1. A 53BP1 separation-of-function mutation that abolishes TIRR-mediated regulation in cells renders 53BP1 hyperactive in response to DSBs, highlighting the key inhibitory function of TIRR. This 53BP1 inhibition is relieved by TIRR-interacting RNA molecules, providing proof-of-principle of RNA-triggered 53BP1 recruitment to DSBs.

Retroviral insertional mutagenesis implicates E3 ubiquitin ligase RNF168 in the control of cell proliferation and survival

The E3 ubiquitin ligase RNF168 is a ring finger protein that has previously been identified to play an important regulatory role in the repair of double-strand DNA breaks.  In the present study, an unbiased forward genetics functional screen in mouse granulocyte/ macrophage progenitor cell line FDCP1 has identified E3 ubiquitin ligase RNF168 as a key regulator of cell survival and proliferation. Our data indicate that RNF168 is an important component of the mechanisms controlling cell fate, not only in human and mouse haematopoietic growth factor-dependent cells, but also in the human breast epithelial cell line MCF-7. These observations therefore suggest that RNF168 provides a connection to key pathways controlling cell fate, potentially through interaction with PML nuclear bodies and/or epigenetic control of gene expression. Our study is the first to demonstrate a critical role for RNF168 in the in the mechanisms regulating cell proliferation and survival, in addition to its well-established role in DNA repair.

TIRR regulates 53BP1 by masking its histone methyl-lysine binding function

P53-binding protein 1 (53BP1) is a multi-functional double-strand break repair protein that is essential for class switch recombination in B lymphocytes and for sensitizing BRCA1-deficient tumours to poly-ADP-ribose polymerase-1 (PARP) inhibitors. Central to all 53BP1 activities is its recruitment to double-strand breaks via the interaction of the tandem Tudor domain with dimethylated lysine 20 of histone H4 (H4K20me2). Here we identify an uncharacterized protein, Tudor interacting repair regulator (TIRR), that directly binds the tandem Tudor domain and masks its H4K20me2 binding motif. Upon DNA damage, the protein kinase ataxia-telangiectasia mutated (ATM) phosphorylates 53BP1 and recruits RAP1-interacting factor 1 (RIF1) to dissociate the 53BP1-TIRR complex. However, overexpression of TIRR impedes 53BP1 function by blocking its localization to double-strand breaks. Depletion of TIRR destabilizes 53BP1 in the nuclear-soluble fraction and alters the double-strand break-induced protein complex centring 53BP1. These findings identify TIRR as a new factor that influences double-strand break repair using a unique mechanism of masking the histone methyl-lysine binding function of 53BP1.

Regulatory cross-talk determines the cellular levels of 53BP1 protein, a critical factor in DNA repair

DNA double strand breaks (DSBs) severely disrupt DNA integrity. 53BP1 plays critical roles in determining DSB repair. Whereas the recruitment of 53BP1 to the DSB site is key for its function, recent evidence suggests that 53BP1's abundance also plays an important role in DSB repair because recruitment to damage sites will be influenced by protein availability. Initial evidence has pointed to three proteins, the ubiquitin-conjugating enzyme UbcH7, the cysteine protease cathepsin L (CTSL), and the nuclear structure protein lamin A/C, that may impact 53BP1 levels, but the roles of each protein and any interplay between them were unclear. Here we report that UbcH7-dependent degradation plays a major role in controlling 53BP1 levels both under normal growth conditions and during DNA damage. CTSL influenced 53BP1 degradation during DNA damage while having little effect under normal growth conditions. Interestingly, both the protein and the mRNA levels of CTSL were reduced in UbcH7-depleted cells. Lamin A/C interacted with 53BP1 under normal conditions. DNA damage disrupted the lamin A/C-53BP1 interaction, which preceded the degradation of 53BP1 in soluble, but not chromatin-enriched, cellular fractions. Inhibition of 53BP1 degradation by a proteasome inhibitor or by UbcH7 depletion restored the 53BP1-lamin A/C interaction. Depletion of lamin A/C, but not CTSL, caused a similar enhancement in cell sensitivity to DNA damage as UbcH7 depletion. These data suggest that multiple pathways collectively fine-tune the cellular levels of 53BP1 protein to ensure proper DSB repair and cell survival.

Writers, Readers, and Erasers of Histone Ubiquitylation in DNA Double-Strand Break Repair

DNA double-strand breaks (DSBs) are highly cytotoxic DNA lesions, whose faulty repair may alter the content and organization of cellular genomes. To counteract this threat, numerous signaling and repair proteins are recruited hierarchically to the chromatin areas surrounding DSBs to facilitate accurate lesion repair and restoration of genome integrity. In vertebrate cells, ubiquitin-dependent modifications of histones adjacent to DSBs by RNF8, RNF168, and other ubiquitin ligases have a key role in promoting the assembly of repair protein complexes, serving as direct recruitment platforms for a range of genome caretaker proteins and their associated factors. These DNA damage-induced chromatin ubiquitylation marks provide an essential component of a histone code for DSB repair that is controlled by multifaceted regulatory circuits, underscoring its importance for genome stability maintenance. In this review, we provide a comprehensive account of how DSB-induced histone ubiquitylation is sensed, decoded and modulated by an elaborate array of repair factors and regulators. We discuss how these mechanisms impact DSB repair pathway choice and functionality for optimal protection of genome integrity, as well as cell and organismal fitness.

53BP1 fosters fidelity of homology-directed DNA repair

Repair of DNA double-strand breaks (DSBs) in mammals is coordinated by the ubiquitin-dependent accumulation of 53BP1 at DSB-flanking chromatin. Owing to its ability to limit DNA-end processing, 53BP1 is thought to promote nonhomologous end-joining (NHEJ) and to suppress homology-directed repair (HDR). Here, we show that silencing 53BP1 or exhausting its capacity to bind damaged chromatin changes limited DSB resection to hyper-resection and results in a switch from error-free gene conversion by RAD51 to mutagenic single-strand annealing by RAD52. Thus, rather than suppressing HDR, 53BP1 fosters its fidelity. These findings illuminate causes and consequences of synthetic viability acquired through 53BP1 silencing in cells lacking the BRCA1 tumor suppressor. We show that such cells survive DSB assaults at the cost of increasing reliance on RAD52-mediated HDR, which may fuel genome instability. However, our findings suggest that when challenged by DSBs, BRCA1- and 53BP1-deficient cells may become hypersensitive to, and be eliminated by, RAD52 inhibition.

Role of Deubiquitinating Enzymes in DNA Repair

Both proteolytic and nonproteolytic functions of ubiquitination are essential regulatory mechanisms for promoting DNA repair and the DNA damage response in mammalian cells. Deubiquitinating enzymes (DUBs) have emerged as key players in the maintenance of genome stability. In this minireview, we discuss the recent findings on human DUBs that participate in genome maintenance, with a focus on the role of DUBs in the modulation of DNA repair and DNA damage signaling.

Repair Pathway Choices and Consequences at the Double-Strand Break

DNA double-strand breaks (DSBs) are cytotoxic lesions that threaten genomic integrity. Failure to repair a DSB has deleterious consequences, including genomic instability and cell death. Indeed, misrepair of DSBs can lead to inappropriate end-joining events, which commonly underlie oncogenic transformation due to chromosomal translocations. Typically, cells employ two main mechanisms to repair DSBs: homologous recombination (HR) and classical nonhomologous end joining (C-NHEJ). In addition, alternative error-prone DSB repair pathways, namely alternative end joining (alt-EJ) and single-strand annealing (SSA), have been recently shown to operate in many different conditions and to contribute to genome rearrangements and oncogenic transformation. Here, we review the mechanisms regulating DSB repair pathway choice, together with the potential interconnections between HR and the annealing-dependent error-prone DSB repair pathways.