BRCA1 Haploinsufficiency Is Masked by RNF168-Mediated Chromatin Ubiquitylation

DNA repair and the contribution to chemotherapy resistance

The DNA damage response comprises a set of imperfect pathways that maintain cell survival following exposure to DNA damaging agents. Cancers frequently exhibit DNA repair pathway alterations that contribute to their intrinsic genome instability. This, in part, facilitates a therapeutic window for many chemotherapeutic agents whose mechanisms of action often converge at the generation of a double-strand DNA break. The development of therapy resistance occurs through countless molecular mechanisms that promote tolerance to DNA damage, often by preventing break formation or increasing repair capacity. This review broadly discusses the DNA damaging mechanisms of action for different classes of chemotherapeutics, how avoidance and repair of double-strand breaks can promote resistance, and strategic directions for counteracting therapy resistance.

Cell-cycle dependent inhibition of BRCA1 signaling by the lysine methyltransferase SET8

The cell-cycle regulated methyltransferase SET8 is the sole enzyme responsible for the mono-methylation of histone H4 at lysine 20 (H4K20) that is the substrate for di- and trimethylation mainly by SUV4-20Hs enzymes. Both SET8 and SUV4-20Hs have been implicated in regulating DNA repair pathway choice through the inverse affinities of BRCA1-BARD1 and 53BP1 complexes for disparate methylation states of H4K20. However, the precise and respective functions of each H4K20 methyltransferase in DNA repair pathways remain to be clarified. Here, we show that SET8 acts as a potent chromatin inhibitor of homologous recombination and that its timely degradation during DNA replication is essential for the spontaneous nuclear focal accumulation of BRCA1 and RAD51 complexes during the S phase. Strikingly, the anti-recombinogenic function of SET8 is independent of SUV4-20 h activity but requires the subsequent recruitment of the ubiquitin ligase RNF168. Moreover, we show that SET8-induced BRCA1 inhibition is not necessarily related to the loss of BARD1 binding to unmethylated histone H4K20. Instead, it is largely caused by the accumulation of 53BP1 in a manner depending on the concerted activities of SET8 and RNF168 on chromatin. Conversely, the lack of SET8 and H4K20 mono-methylation on newly assembly chromatin after DNA replication led to the untimely accumulation of BRCA1 on chromatin at the subsequent G1 phase. Altogether, these results establish the de novo activity of SET8 on chromatin as a primordial epigenetic lock of the BRCA1-mediated HR pathway during the cell.

TIPIN is essential for chromosome stability and cell viability in BRCA1-deficient cells

The mutations of breast cancer type 1 susceptibility gene (BRCA1) cause hereditary breast cancer. One of the medical revolutions of cancer therapy for BRCA1-mutated breast cancer is the drug approval of Poly (ADP-ribose) polymerase (PARP) inhibitors because of the synthetic lethal interaction between BRCA1 mutation and PARP inhibition. Here, we report another synthetic lethal interaction between BRCA1 and TIMELESS interacting protein (TIPIN), the latter of which encodes a protein involved in DNA replication, DNA damage checkpoint and sister chromatid cohesion. Cells deficient for both BRCA1 and TIPIN die due to elevated chromosomal aberrations including chromosomal breaks and radial chromosomes. The synthetic lethality of TIPIN/BRCA1-deficient cells is restored by the depletion of Tumor protein p53 binding protein 1 (53BP1), which prevents homologous recombination (HR) by its restricting DNA processing. Thus, spontaneous DNA lesions in TIPIN deficient cells could be preferentially repaired by BRCA1-mediated HR pathway. The viability of TIPIN/53BP1/BRCA1 triple mutant is lost by the depletion of Ring finger protein 8 (RNF8) E3-ubiquitin ligase, implicating that RNF8-mediated sub-HR pathway may work in a complementary manner of BRCA1 and 53BP1 pathway.

Regulation of pathway choice in DNA repair after double-strand breaks

DNA damage signaling is a highly coordinated cellular process which is required for the removal of DNA lesions. Amongst the different types of DNA damage, double-strand breaks (DSBs) are the most harmful type of lesion that attenuates cellular proliferation. DSBs are repaired by two major pathways-homologous recombination (HR), and non-homologous end-joining (NHEJ) and in some cases by microhomology-mediated end-joining (MMEJ). Preference of the pathway depends on multiple parameters including site of the DNA damage, the cell cycle phase and topology of the DNA lesion. Deregulated repair response contributes to genomic instability resulting in a plethora of diseases including cancer. This review discusses the different molecular players of HR, NHEJ, and MMEJ pathways that control the switch among the different DSB repair pathways. We also highlight the various functions of chromatin modifications in modulating repair response and how deregulated DNA damage repair response may promote oncogenic transformation.

BRCA1 and BRCA2: from cancer susceptibility to synthetic lethality

The discovery of BRCA1 and BRCA2 as tumor susceptibility genes and their role in genome maintenance has transformed our understanding of hereditary breast and ovarian cancer. This review traces the evolution of BRCA1/2 research over the past 30 years, highlighting key discoveries in the field and their contributions to tumor development. Additionally, we discuss current preventive measures for BRCA1/2 mutation carriers and targeted treatment options based on the concept of synthetic lethality. Finally, we explore the challenges of acquired therapy resistance and discuss potential alternative avenues for targeting BRCA1/2 mutant tumors.

One-ended and two-ended breaks at nickase-broken replication forks

Replisome collision with a nicked parental DNA template can lead to the formation of a replication-associated double strand break (DSB). How this break is repaired has implications for cancer initiation, cancer therapy and therapeutic gene editing. Recent work shows that collision of a replisome with a nicked DNA template can give rise to either a single-ended (se) or a double-ended (de)DSB, with potentially divergent effects on repair pathway choice and genomic instability. Emerging evidence suggests that the biochemical environment of the broken mammalian replication fork may be specialized in such a way as to skew repair in favor of homologous recombination at the expense of non-homologous end joining.

VGLL3 modulates chemosensitivity through promoting DNA double-strand break repair

Transcription cofactor vestigial-like 3 (VGLL3), as a master regulator of female-biased autoimmunity, also functions in tumor development, while the underlying mechanisms remain largely elusive. Here, we report that VGLL3 plays an important role in DNA damage response (DDR). VGLL3 can be recruited to damage sites in a PARylation-dependent manner. VGLL3 depletion impairs the accumulation of RNF8 and RAD51 at sites of DNA damage, leading to reduced homologous recombination efficiency and increased cellular sensitivity to chemotherapeutic drugs. Mechanistically, VGLL3 can prevent CtIP from KLHL15-mediated ubiquitination and degradation through competitive binding with KLHL15 and, meanwhile, stabilize MDC1 by limiting TRIP12-MDC1 but promoting USP7-MDC1 associations for optimal RNF8 signaling initiation. Consistently, VGLL3 depletion delays tumor development and sensitizes the xenografts to etoposide treatment. Overall, our results reveal an unexpected role of VGLL3 in DDR, which is distinct from its transcriptional cofactor function and not conserved among VGLL family members.

NEAT1 modulates the TIRR/53BP1 complex to maintain genome integrity

Tudor Interacting Repair Regulator (TIRR) is an RNA-binding protein (RBP) that interacts directly with 53BP1, restricting its access to DNA double-strand breaks (DSBs) and its association with p53. We utilized iCLIP to identify RNAs that directly bind to TIRR within cells, identifying the long non-coding RNA NEAT1 as the primary RNA partner. The high affinity of TIRR for NEAT1 is due to prevalent G-rich motifs in the short isoform (NEAT1_1) region of NEAT1. This interaction destabilizes the TIRR/53BP1 complex, promoting 53BP1's function. NEAT1_1 is enriched during the G1 phase of the cell cycle, thereby ensuring that TIRR-dependent inhibition of 53BP1's function is cell cycle-dependent. TDP-43, an RBP that is implicated in neurodegenerative diseases, modulates the TIRR/53BP1 complex by promoting the production of the NEAT1 short isoform, NEAT1_1. Together, we infer that NEAT1_1, and factors regulating NEAT1_1, may impact 53BP1-dependent DNA repair processes, with implications for a spectrum of diseases.

DNA damage response in breast cancer and its significant role in guiding novel precise therapies

DNA damage response (DDR) deficiency has been one of the emerging targets in treating breast cancer in recent years. On the one hand, DDR coordinates cell cycle and signal transduction, whose dysfunction may lead to cell apoptosis, genomic instability, and tumor development. Conversely, DDR deficiency is an intrinsic feature of tumors that underlies their response to treatments that inflict DNA damage. In this review, we systematically explore various mechanisms of DDR, the rationale and research advances in DDR-targeted drugs in breast cancer, and discuss the challenges in its clinical applications. Notably, poly (ADP-ribose) polymerase (PARP) inhibitors have demonstrated favorable efficacy and safety in breast cancer with high homogenous recombination deficiency (HRD) status in a series of clinical trials. Moreover, several studies on novel DDR-related molecules are actively exploring to target tumors that become resistant to PARP inhibition. Before further clinical application of new regimens or drugs, novel and standardized biomarkers are needed to develop for accurately characterizing the benefit population and predicting efficacy. Despite the promising efficacy of DDR-related treatments, challenges of off-target toxicity and drug resistance need to be addressed. Strategies to overcome drug resistance await further exploration on DDR mechanisms, and combined targeted drugs or immunotherapy will hopefully provide more precise or combined strategies and expand potential responsive populations.

A RAD18-UBC13-PALB2-RNF168 axis mediates replication fork recovery in BRCA1-deficient cancer cells

BRCA1/2 proteins function in genome stability by promoting repair of double-stranded DNA breaks through homologous recombination and by protecting stalled replication forks from nucleolytic degradation. In BRCA1/2-deficient cancer cells, extensively degraded replication forks can be rescued through distinct fork recovery mechanisms that also promote cell survival. Here, we identified a novel pathway mediated by the E3 ubiquitin ligase RAD18, the E2-conjugating enzyme UBC13, the recombination factor PALB2, the E3 ubiquitin ligase RNF168 and PCNA ubiquitination that promotes fork recovery in BRCA1- but not BRCA2-deficient cells. We show that this pathway does not promote fork recovery by preventing replication fork reversal and degradation in BRCA1-deficient cells. We propose a mechanism whereby the RAD18-UBC13-PALB2-RNF168 axis facilitates resumption of DNA synthesis by promoting re-annealing of the complementary single-stranded template strands of the extensively degraded forks, thereby allowing re-establishment of a functional replication fork. We also provide preliminary evidence for the potential clinical relevance of this novel fork recovery pathway in BRCA1-mutated cancers, as RAD18 is over-expressed in BRCA1-deficient cancers, and RAD18 loss compromises cell viability in BRCA1-deficient cancer cells.

Structure and repair of replication-coupled DNA breaks

Using CRISPR-Cas9 nicking enzymes, we examined the interaction between the replication machinery and single-strand breaks, one of the most common forms of endogenous DNA damage. We show that replication fork collapse at leading-strand nicks generates resected single-ended double-strand breaks (seDSBs) that are repaired by homologous recombination (HR). If these seDSBs are not promptly repaired, arrival of adjacent forks creates double-ended DSBs (deDSBs), which could drive genomic scarring in HR-deficient cancers. deDSBs can also be generated directly when the replication fork bypasses lagging-strand nicks. Unlike deDSBs produced independently of replication, end resection at nick-induced seDSBs and deDSBs is BRCA1-independent. Nevertheless, BRCA1 antagonizes 53BP1 suppression of RAD51 filament formation. These results highlight distinctive mechanisms that maintain replication fork stability.

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.

Functional Characterization of the Human BRCA1 ∆11 Splicing Isoforms in Yeast

BRCA1, a crucial tumor suppressor gene, has several splicing isoforms, including Δ9-11, Δ11, and Δ11q, which lack exon 11, coding for significant portions of the protein. These isoforms are naturally present in both normal and cancerous cells, exhibiting altered activity compared to the full-length BRCA1. Despite this, the impact on cancer risk of the germline intronic variants promoting the exclusive expression of these Δ11 isoforms remains uncertain. Consequently, they are classified as variants of uncertain significance (VUS), posing challenges for traditional genetic classification methods due to their rarity and complexity. Our research utilizes a yeast-based functional assay, previously validated for assessing missense BRCA1 variants, to compare the activity of the Δ11 splicing isoforms with known pathogenic missense variants. This approach allows us to elucidate the functional implications of these isoforms and determine whether their exclusive expression could contribute to increased cancer risk. By doing so, we aim to provide insights into the pathogenic potential of intronic VUS-generating BRCA1 splicing isoforms and improve the classification of BRCA1 variants.

SOD1 is a synthetic-lethal target in PPM1D-mutant leukemia cells

The DNA damage response is critical for maintaining genome integrity and is commonly disrupted in the development of cancer. PPM1D (protein phosphatase Mg2+/Mn2+-dependent 1D) is a master negative regulator of the response; gain-of-function mutations and amplifications of PPM1D are found across several human cancers making it a relevant pharmacological target. Here, we used CRISPR/Cas9 screening to identify synthetic-lethal dependencies of PPM1D, uncovering superoxide dismutase-1 (SOD1) as a potential target for PPM1D-mutant cells. We revealed a dysregulated redox landscape characterized by elevated levels of reactive oxygen species and a compromised response to oxidative stress in PPM1D-mutant cells. Altogether, our results demonstrate a role for SOD1 in the survival of PPM1D-mutant leukemia cells and highlight a new potential therapeutic strategy against PPM1D-mutant cancers.

Evolved histone tail regulates 53BP1 recruitment at damaged chromatin

The master DNA damage repair histone protein, H2AX, is essential for orchestrating the recruitment of downstream mediator and effector proteins at damaged chromatin. The phosphorylation of H2AX at S139, γH2AX, is well-studied for its DNA repair function. However, the extended C-terminal tail is not characterized. Here, we define the minimal motif on H2AX for the canonical function in activating the MDC1-RNF8-RNF168 phosphorylation-ubiquitination pathway that is important for recruiting repair proteins, such as 53BP1 and BRCA1. Interestingly, H2AX recruits 53BP1 independently from the MDC1-RNF8-RNF168 pathway through its evolved C-terminal linker region with S139 phosphorylation. Mechanistically, 53BP1 recruitment to damaged chromatin is mediated by the interaction between the H2AX C-terminal tail and the 53BP1 Oligomerization-Tudor domains. Moreover, γH2AX-linker mediated 53BP1 recruitment leads to camptothecin resistance in H2AX knockout cells. Overall, our study uncovers an evolved mechanism within the H2AX C-terminal tail for regulating DNA repair proteins at damaged chromatin.

BRCA1/BARD1 ubiquitinates PCNA in unperturbed conditions to promote continuous DNA synthesis

Deficiencies in the BRCA1 tumor suppressor gene are the main cause of hereditary breast and ovarian cancer. BRCA1 is involved in the Homologous Recombination DNA repair pathway and, together with BARD1, forms a heterodimer with ubiquitin E3 activity. The relevance of the BRCA1/BARD1 ubiquitin E3 activity for tumor suppression and DNA repair remains controversial. Here, we observe that the BRCA1/BARD1 ubiquitin E3 activity is not required for Homologous Recombination or resistance to Olaparib. Using TULIP2 methodology, which enables the direct identification of E3-specific ubiquitination substrates, we identify substrates for BRCA1/BARD1. We find that PCNA is ubiquitinated by BRCA1/BARD1 in unperturbed conditions independently of RAD18. PCNA ubiquitination by BRCA1/BARD1 avoids the formation of ssDNA gaps during DNA replication and promotes continuous DNA synthesis. These results provide additional insight about the importance of BRCA1/BARD1 E3 activity in Homologous Recombination.

TRIP12 governs DNA Polymerase β involvement in DNA damage response and repair

The multitude of DNA lesion types, and the nuclear dynamic context in which they occur, present a challenge for genome integrity maintenance as this requires the engagement of different DNA repair pathways. Specific 'repair controllers' that facilitate DNA repair pathway crosstalk between double strand break (DSB) repair and base excision repair (BER), and regulate BER protein trafficking at lesion sites, have yet to be identified. We find that DNA polymerase β (Polβ), crucial for BER, is ubiquitylated in a BER complex-dependent manner by TRIP12, an E3 ligase that partners with UBR5 and restrains DSB repair signaling. Here we find that, TRIP12, but not UBR5, controls cellular levels and chromatin loading of Polβ. Required for Polβ foci formation, TRIP12 regulates Polβ involvement after DNA damage. Notably, excessive TRIP12-mediated shuttling of Polβ affects DSB formation and radiation sensitivity, underscoring its precedence for BER. We conclude that the herein discovered trafficking function at the nexus of DNA repair signaling pathways, towards Polβ-directed BER, optimizes DNA repair pathway choice at complex lesion sites.

Mechanisms of RNF168 nucleosome recognition and ubiquitylation

RNF168 plays a central role in the DNA damage response (DDR) by ubiquitylating histone H2A at K13 and K15. These modifications direct BRCA1-BARD1 and 53BP1 foci formation in chromatin, essential for cell-cycle-dependent DNA double-strand break (DSB) repair pathway selection. The mechanism by which RNF168 catalyzes the targeted accumulation of H2A ubiquitin conjugates to form repair foci around DSBs remains unclear. Here, using cryoelectron microscopy (cryo-EM), nuclear magnetic resonance (NMR) spectroscopy, and functional assays, we provide a molecular description of the reaction cycle and dynamics of RNF168 as it modifies the nucleosome and recognizes its ubiquitylation products. We demonstrate an interaction of a canonical ubiquitin-binding domain within full-length RNF168, which not only engages ubiquitin but also the nucleosome surface, clarifying how such site-specific ubiquitin recognition propels a signal amplification loop. Beyond offering mechanistic insights into a key DDR protein, our study aids in understanding site specificity in both generating and interpreting chromatin ubiquitylation.

Meiotic protein SYCP2 confers resistance to DNA-damaging agents through R-loop-mediated DNA repair

Drugs targeting the DNA damage response (DDR) are widely used in cancer therapy, but resistance to these drugs remains a major clinical challenge. Here, we show that SYCP2, a meiotic protein in the synaptonemal complex, is aberrantly and commonly expressed in breast and ovarian cancers and associated with broad resistance to DDR drugs. Mechanistically, SYCP2 enhances the repair of DNA double-strand breaks (DSBs) through transcription-coupled homologous recombination (TC-HR). SYCP2 promotes R-loop formation at DSBs and facilitates RAD51 recruitment independently of BRCA1. SYCP2 loss impairs RAD51 localization, reduces TC-HR, and renders tumors sensitive to PARP and topoisomerase I (TOP1) inhibitors. Furthermore, our studies of two clinical cohorts find that SYCP2 overexpression correlates with breast cancer resistance to antibody-conjugated TOP1 inhibitor and ovarian cancer resistance to platinum treatment. Collectively, our data suggest that SYCP2 confers cancer cell resistance to DNA-damaging agents by stimulating R-loop-mediated DSB repair, offering opportunities to improve DDR therapy.

SOD1 is a synthetic lethal target in PPM1D-mutant leukemia cells

The DNA damage response is critical for maintaining genome integrity and is commonly disrupted in the development of cancer. PPM1D (protein phosphatase, Mg2+/Mn2+ dependent 1D) is a master negative regulator of the response; gain-of-function mutations and amplifications of PPM1D are found across several human cancers making it a relevant pharmacologic target. Here, we used CRISPR/Cas9 screening to identify synthetic-lethal dependencies of PPM1D, uncovering superoxide dismutase-1 (SOD1) as a potential target for PPM1D-mutant cells. We revealed a dysregulated redox landscape characterized by elevated levels of reactive oxygen species and a compromised response to oxidative stress in PPM1D-mutant cells. Altogether, our results demonstrate the protective role of SOD1 against oxidative stress in PPM1D-mutant leukemia cells and highlight a new potential therapeutic strategy against PPM1D-mutant cancers.

BRCA1/2 Haploinsufficiency: Exploring the Impact of Losing one Allele

Since their discovery in the late 20th century, significant progress has been made in elucidating the functions of the tumor suppressor proteins BRCA1 and BRCA2. These proteins play vital roles in maintaining genome integrity, including DNA repair, replication fork protection, and chromosome maintenance. It is well-established that germline mutations in BRCA1 and BRCA2 increase the risk of breast and ovarian cancer; however, the precise mechanism underlying tumor formation in this context is not fully understood. Contrary to the long-standing belief that the loss of the second wild-type allele is necessary for tumor development, a growing body of evidence suggests that tumorigenesis can occur despite the presence of a single functional allele. This entails that heterozygosity in BRCA1/2 confers haploinsufficiency, where a single copy of the gene is not sufficient to fully suppress tumor formation. Here we provide an overview of the findings and the ongoing debate regarding BRCA haploinsufficiency. We further put out the challenges in studying this topic and discuss its potential relevance in the prevention and treatment of BRCA-related cancers.

Mechanisms of synthetic lethality between BRCA1/2 and 53BP1 deficiencies and DNA polymerase theta targeting

A synthetic lethal relationship exists between disruption of polymerase theta (Polθ), and loss of either 53BP1 or homologous recombination (HR) proteins, including BRCA1; however, the mechanistic basis of these observations are unclear. Here we reveal two distinct mechanisms of Polθ synthetic lethality, identifying dual influences of 1) whether Polθ is lost or inhibited, and 2) the underlying susceptible genotype. Firstly, we find that the sensitivity of BRCA1/2- and 53BP1-deficient cells to Polθ loss, and 53BP1-deficient cells to Polθ inhibition (ART558) requires RAD52, and appropriate reduction of RAD52 can ameliorate these phenotypes. We show that in the absence of Polθ, RAD52 accumulations suppress ssDNA gap-filling in G2/M and encourage MRE11 nuclease accumulation. In contrast, the survival of BRCA1-deficient cells treated with Polθ inhibitor are not restored by RAD52 suppression, and ssDNA gap-filling is prevented by the chemically inhibited polymerase itself. These data define an additional role for Polθ, reveal the mechanism underlying synthetic lethality between 53BP1, BRCA1/2 and Polθ loss, and indicate genotype-dependent Polθ inhibitor mechanisms.

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.

RNF126, 168 and CUL1: The Potential Utilization of Multi-Functional E3 Ubiquitin Ligases in Genome Maintenance for Cancer Therapy

Ubiquitination is a post-translational modification (PTM) that is involved in proteolysis, protein-protein interaction, and signal transduction. Accumulation of mutations and genomic instability are characteristic of cancer cells, and dysfunction of the ubiquitin pathway can contribute to abnormal cell physiology. Because mutations can be critical for cells, DNA damage repair, cell cycle regulation, and apoptosis are pathways that are in close communication to maintain genomic integrity. Uncontrolled cell proliferation due to abnormal processes is a hallmark of cancer, and mutations, changes in expression levels, and other alterations of ubiquitination factors are often involved. Here, three E3 ubiquitin ligases will be reviewed in detail. RNF126, RNF168 and CUL1 are involved in DNA damage response (DDR), DNA double-strand break (DSB) repair, cell cycle regulation, and ultimately, cancer cell proliferation control. Their involvement in multiple cellular pathways makes them an attractive candidate for cancer-targeting therapy. Functional studies of these E3 ligases have increased over the years, and their significance in cancer is well reported. There are continuous efforts to develop drugs targeting the ubiquitin pathway for anticancer therapy, which opens up the possibility for these E3 ligases to be evaluated for their potential as a target protein for anticancer therapy.

FBL promotes cancer cell resistance to DNA damage and BRCA1 transcription via YBX1

Fibrillarin (FBL) is a highly conserved nucleolar methyltransferase responsible for methylation of ribosomal RNA and proteins. Here, we reveal a role for FBL in DNA damage response and its impact on cancer proliferation and sensitivity to DNA-damaging agents. FBL is highly expressed in various cancers and correlates with poor survival outcomes in cancer patients. Knockdown of FBL sensitizes tumor cells and xenografts to DNA crosslinking agents, and leads to homologous recombination-mediated DNA repair defects. We identify Y-box-binding protein-1 (YBX1) as a key interacting partner of FBL, and FBL increases the nuclear accumulation of YBX1 in response to DNA damage. We show that FBL promotes the expression of BRCA1 by increasing the binding of YBX1 to the BRCA1 promoter. Our study sheds light on the regulatory mechanism of FBL in tumorigenesis and DNA damage response, providing potential therapeutic targets to overcome chemoresistance in cancer.

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.

ATR protects ongoing and newly assembled DNA replication forks through distinct mechanisms

The ATR kinase safeguards genomic integrity during S phase, but how ATR protects DNA replication forks remains incompletely understood. Here, we combine four distinct assays to analyze ATR functions at ongoing and newly assembled replication forks upon replication inhibition by hydroxyurea. At ongoing forks, ATR inhibitor (ATRi) increases MRE11- and EXO1-mediated nascent DNA degradation from PrimPol-generated, single-stranded DNA (ssDNA) gaps. ATRi also exposes template ssDNA through fork uncoupling and nascent DNA degradation. Electron microscopy reveals that ATRi reduces reversed forks by increasing gap-dependent nascent DNA degradation. At new forks, ATRi triggers MRE11- and CtIP-initiated template DNA degradation by EXO1, exposing nascent ssDNA. Upon PARP inhibition, ATRi preferentially exacerbates gap-dependent nascent DNA degradation at ongoing forks in BRCA1/2-deficient cells and disrupts the restored gap protection in BRCA1-deficient, PARP-inhibitor-resistant cells. Thus, ATR protects ongoing and new forks through distinct mechanisms, providing an extended view of ATR's functions in stabilizing replication forks.

BRCA Mutations-The Achilles Heel of Breast, Ovarian and Other Epithelial Cancers

Two related tumor suppressor genes, BRCA1 and BRCA2, attract a lot of attention from both fundamental and clinical points of view. Oncogenic hereditary mutations in these genes are firmly linked to the early onset of breast and ovarian cancers. However, the molecular mechanisms that drive extensive mutagenesis in these genes are not known. In this review, we hypothesize that one of the potential mechanisms behind this phenomenon can be mediated by Alu mobile genomic elements. Linking mutations in the BRCA1 and BRCA2 genes to the general mechanisms of genome stability and DNA repair is critical to ensure the rationalized choice of anti-cancer therapy. Accordingly, we review the literature available on the mechanisms of DNA damage repair where these proteins are involved, and how the inactivating mutations in these genes (BRCAness) can be exploited in anti-cancer therapy. We also discuss a hypothesis explaining why breast and ovarian epithelial tissues are preferentially susceptible to mutations in BRCA genes. Finally, we discuss prospective novel therapeutic approaches for treating BRCAness cancers.

Emerging Roles of RNF168 in Tumor Progression

RING finger protein 168 (RNF168) is an E3 ubiquitin ligase with the RING finger domain. It is an important protein contributing to the DNA double-strand damage repair pathway. Recent studies have found that RNF168 is significantly implicated in the occurrence and development of various cancers. Additionally, RNF168 contributes to the drug resistance of tumor cells by enhancing their DNA repair ability or regulating the degradation of target proteins. This paper summarizes and prospects the research progress of the structure and main functions of RNF168, especially its roles and the underlying mechanisms in tumorigenesis.

Leveraging the replication stress response to optimize cancer therapy

High-fidelity DNA replication is critical for the faithful transmission of genetic information to daughter cells. Following genotoxic stress, specialized DNA damage tolerance pathways are activated to ensure replication fork progression. These pathways include translesion DNA synthesis, template switching and repriming. In this Review, we describe how DNA damage tolerance pathways impact genome stability, their connection with tumorigenesis and their effects on cancer therapy response. We discuss recent findings that single-strand DNA gap accumulation impacts chemoresponse and explore a growing body of evidence that suggests that different DNA damage tolerance factors, including translesion synthesis polymerases, template switching proteins and enzymes affecting single-stranded DNA gaps, represent useful cancer targets. We further outline how the consequences of DNA damage tolerance mechanisms could inform the discovery of new biomarkers to refine cancer therapies.

Mechanisms of PARP Inhibitor Resistance

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) represent the first medicines based on the targeting of the DNA damage response (DDR). PARPi have become standard of care for first-line maintenance treatment in ovarian cancer and have also been approved in other cancer indications including breast, pancreatic and prostate. Despite their efficacy, resistance to PARPi has been reported clinically and represents a growing patient population with unmet clinical need. Here, we describe the various mechanisms of PARPi resistance that have been identified in pre-clinical models and in the clinic.

53BP1: Keeping It under Control, Even at a Distance from DNA Damage

Double-strand breaks (DSBs) are toxic lesions that can be generated by exposure to genotoxic agents or during physiological processes, such as during V(D)J recombination. The repair of these DSBs is crucial to prevent genomic instability and to maintain cellular homeostasis. Two main pathways participate in repairing DSBs, namely, non-homologous end joining (NHEJ) and homologous recombination (HR). The P53-binding protein 1 (53BP1) plays a pivotal role in the choice of DSB repair mechanism, promotes checkpoint activation and preserves genome stability upon DSBs. By preventing DSB end resection, 53BP1 promotes NHEJ over HR. Nonetheless, the balance between DSB repair pathways remains crucial, as unscheduled NHEJ or HR events at different phases of the cell cycle may lead to genomic instability. Therefore, the recruitment of 53BP1 to chromatin is tightly regulated and has been widely studied. However, less is known about the mechanism regulating 53BP1 recruitment at a distance from the DNA damage. The present review focuses on the mechanism of 53BP1 recruitment to damage and on recent studies describing novel mechanisms keeping 53BP1 at a distance from DSBs.

Unpaved roads: How the DNA damage response navigates endogenous genotoxins

Accurate DNA repair is essential for cellular and organismal homeostasis, and DNA repair defects result in genetic diseases and cancer predisposition. Several environmental factors, such as ultraviolet light, damage DNA, but many other molecules with DNA damaging potential are byproducts of normal cellular processes. In this review, we highlight some of the prominent sources of endogenous DNA damage as well as their mechanisms of repair, with a special focus on repair by the homologous recombination and Fanconi anemia pathways. We also discuss how modulating DNA damage caused by endogenous factors may augment current approaches used to treat BRCA-deficient cancers. Finally, we describe how synthetic lethal interactions may be exploited to exacerbate DNA repair deficiencies and cause selective toxicity in additional types of cancers.

BRCA1-Dependent and Independent Recruitment of PALB2-BRCA2-RAD51 in the DNA Damage Response and Cancer

The BRCA1-PALB2-BRCA2 axis plays essential roles in the cellular response to DNA double-strand breaks (DSB), maintenance of genome integrity, and suppression of cancer development. Upon DNA damage, BRCA1 is recruited to DSBs, where it facilitates end resection and recruits PALB2 and its associated BRCA2 to load the central recombination enzyme RAD51 to initiate homologous recombination (HR) repair. In recent years, several BRCA1-independent mechanisms of PALB2 recruitment have also been reported. Collectively, these available data illustrate a series of hierarchical, context-dependent, and cooperating mechanisms of PALB2 recruitment that is critical for HR and therapy response either in the presence or absence of BRCA1. Here, we review these BRCA1-dependent and independent mechanisms and their importance in DSB repair, cancer development, and therapy. As BRCA1-mutant cancer cells regain HR function, for which PALB2 is generally required, and become resistant to targeted therapies, such as PARP inhibitors, targeting BRCA1-independent mechanisms of PALB2 recruitment represents a potential new avenue to improve treatment of BRCA1-mutant tumors.

DNA Damage Response Regulation by Histone Ubiquitination

Cells are constantly exposed to numerous genotoxic stresses that induce DNA damage. DNA double-strand breaks (DSBs) are among the most serious damages and should be systematically repaired to preserve genomic integrity. The efficiency of repair is closely associated with chromatin structure, which is regulated by posttranslational modifications of histones, including ubiquitination. Recent evidence shows crosstalk between histone ubiquitination and DNA damage responses, suggesting an integrated model for the systematic regulation of DNA repair. There are two major pathways for DSB repair, viz., nonhomologous end joining and homologous recombination, and the choice of the pathway is partially controlled by posttranslational modifications of histones, including ubiquitination. Histone ubiquitination changes chromatin structure in the vicinity of DSBs and serves as a platform to select and recruit repair proteins; the removal of these modifications by deubiquitinating enzymes suppresses the recruitment of repair proteins and promotes the convergence of repair reactions. This article provides a comprehensive overview of the DNA damage response regulated by histone ubiquitination in response to DSBs.

Clinical Landscape of PARP Inhibitors in Ovarian Cancer: Molecular Mechanisms and Clues to Overcome Resistance

The survival of patients with advanced or recurrent ovarian cancer has improved tremendously in the past decade, mainly due to the establishment of maintenance therapy with poly (ADP-ribose) polymerase (PARP) inhibitors (PARPis) after conservative chemotherapies. Despite their superior efficacy, resistance to PARPis has been reported, and patients with resistance have a much worse prognosis. Therefore, the development of novel treatment strategies to overcome PARPi resistance is urgently needed. The present review article focuses on the molecular mechanisms of how PARPis exert cytotoxic effects on cancer cells through DNA repair processes, especially the genetic background and tumor microenvironment favored by PARPis. Furthermore, currently available information on PARPi resistance mechanisms is introduced and discussed to develop a novel therapeutic approach against them.

New answers to the old RIDDLE: RNF168 and the DNA damage response pathway

The chromatin-based DNA damage response pathway is tightly orchestrated by histone post-translational modifications, including histone H2A ubiquitination. Ubiquitination plays an integral role in regulating cellular processes including DNA damage signaling and repair. The ubiquitin E3 ligase RNF168 is essential in assembling a cohort of DNA repair proteins at the damaged chromatin via its enzymatic activity. RNF168 ubiquitinates histone H2A(X) at the N terminus and generates a specific docking scaffold for ubiquitin-binding motif-containing proteins. The regulation of RNF168 at damaged chromatin and the mechanistic implication in the recruitment of DNA repair proteins to the damaged sites remain an area of active investigation. Here, we review the function and regulation of RNF168 in the context of ubiquitin-mediated DNA damage signaling and repair. We will also discuss the unanswered questions that require further investigation and how understanding RNF168 targeting specificity could benefit the therapeutic development for cancer treatment.

BRCA2-DSS1 interaction is dispensable for RAD51 recruitment at replication-induced and meiotic DNA double strand breaks

The interaction between tumor suppressor BRCA2 and DSS1 is essential for RAD51 recruitment and repair of DNA double stand breaks (DSBs) by homologous recombination (HR). We have generated mice with a leucine to proline substitution at position 2431 of BRCA2, which disrupts this interaction. Although a significant number of mutant mice die during embryogenesis, some homozygous and hemizygous mutant mice undergo normal postnatal development. Despite lack of radiation induced RAD51 foci formation and a severe HR defect in somatic cells, mutant mice are fertile and exhibit normal RAD51 recruitment during meiosis. We hypothesize that the presence of homologous chromosomes in close proximity during early prophase I may compensate for the defect in BRCA2-DSS1 interaction. We show the restoration of RAD51 foci in mutant cells when Topoisomerase I inhibitor-induced single strand breaks are converted into DSBs during DNA replication. We also partially rescue the HR defect by tethering the donor DNA to the site of DSBs using streptavidin-fused Cas9. Our findings demonstrate that the BRCA2-DSS1 complex is dispensable for RAD51 loading when the homologous DNA is close to the DSB.

Mishra et al. have generated mice with a single amino acid substitution in BRCA2, which disrupts its interaction with DSS1 resulting in a severe HR defect. They show the interaction to be dispensable for HR at replication induced and meiotic DSBs.

PARP Inhibitors Resistance: Mechanisms and Perspectives

PolyADP-ribose polymerase (PARP) inhibitors (PARPis) represent the first clinically approved drugs able to provoke "synthetic lethality" in patients with homologous recombination-deficient (HRD) tumors. Four PARPis have just received approval for the treatment of several types of cancer. Besides, another three additional PARPis underlying the same mechanism of action are currently under investigation. Despite the success of these targeted agents, the increasing use of PARPis in clinical practice for the treatment of different tumors raised the issue of PARPis resistance, and the consequent disease relapse and dismal prognosis for patients. Several mechanisms of resistance have been investigated, and ongoing studies are currently focusing on strategies to address this challenge and overcome PARPis resistance. This review aims to analyze the mechanisms underlying PARPis resistance known today and discuss potential therapeutic strategies to overcome these processes of resistance in the future.

Preventing and Overcoming Resistance to PARP Inhibitors: A Focus on the Clinical Landscape

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are now a first-line maintenance treatment in ovarian cancer and have been approved in other cancer types, including breast, pancreatic and prostate. Despite their efficacy, and as is the case for other targeted therapies, resistance to PARPi has been reported clinically and is generating a growing patient population of unmet clinical need. Here, we discuss the mechanisms of resistance that have been described in pre-clinical models and focus on those that have been already identified in the clinic, highlighting the key challenges to fully characterise the clinical landscape of PARPi resistance and proposing ways of preventing and overcoming it.

Structural Insight into the Mechanism of PALB2 Interaction with MRG15

The tumor suppressor protein partner and localizer of BRCA2 (PALB2) orchestrates the interactions between breast cancer susceptibility proteins 1 and 2 (BRCA1, -2) that are critical for genome stability, homologous recombination (HR) and DNA repair. PALB2 mutations predispose patients to a spectrum of cancers, including breast and ovarian cancers. PALB2 localizes HR machinery to chromatin and links it with transcription through multiple DNA and protein interactions. This includes its interaction with MRG15 (Morf-related gene on chromosome 15), which is part of many transcription complexes, including the HAT-associated and the HDAC-associated complexes. This interaction is critical for PALB2 localization in actively transcribed genes, where transcription/replication conflicts lead to frequent replication stress and DNA breaks. We solved the crystal structure of the MRG15 MRG domain bound to the PALB2 peptide and investigated the effect of several PALB2 mutations, including patient-derived variants. PALB2 interacts with an extended surface of the MRG that is known to interact with other proteins. This, together with a nanomolar affinity, suggests that the binding of MRG15 partners, including PALB2, to this region is mutually exclusive. Breast cancer-related mutations of PALB2 cause only minor attenuation of the binding affinity. New data reveal the mechanism of PALB2-MRG15 binding, advancing our understanding of PALB2 function in chromosome maintenance and tumorigenesis.

Tools for Decoding Ubiquitin Signaling in DNA Repair

The maintenance of genome stability requires dedicated DNA repair processes and pathways that are essential for the faithful duplication and propagation of chromosomes. These DNA repair mechanisms counteract the potentially deleterious impact of the frequent genotoxic challenges faced by cells from both exogenous and endogenous agents. Intrinsic to these mechanisms, cells have an arsenal of protein factors that can be utilised to promote repair processes in response to DNA lesions. Orchestration of the protein factors within the various cellular DNA repair pathways is performed, in part, by post-translational modifications, such as phosphorylation, ubiquitin, SUMO and other ubiquitin-like modifiers (UBLs). In this review, we firstly explore recent advances in the tools for identifying factors involved in both DNA repair and ubiquitin signaling pathways. We then expand on this by evaluating the growing repertoire of proteomic, biochemical and structural techniques available to further understand the mechanistic basis by which these complex modifications regulate DNA repair. Together, we provide a snapshot of the range of methods now available to investigate and decode how ubiquitin signaling can promote DNA repair and maintain genome stability in mammalian cells.

Understanding and overcoming resistance to PARP inhibitors in cancer therapy

Developing novel targeted anticancer therapies is a major goal of current research. The use of poly(ADP-ribose) polymerase (PARP) inhibitors in patients with homologous recombination-deficient tumours provides one of the best examples of a targeted therapy that has been successfully translated into the clinic. The success of this approach has so far led to the approval of four different PARP inhibitors for the treatment of several types of cancers and a total of seven different compounds are currently under clinical investigation for various indications. Clinical trials have demonstrated promising response rates among patients receiving PARP inhibitors, although the majority will inevitably develop resistance. Preclinical and clinical data have revealed multiple mechanisms of resistance and current efforts are focused on developing strategies to address this challenge. In this Review, we summarize the diverse processes underlying resistance to PARP inhibitors and discuss the potential strategies that might overcome these mechanisms such as combinations with chemotherapies, targeting the acquired vulnerabilities associated with resistance to PARP inhibitors or suppressing genomic instability.

RNF168 E3 ligase participates in ubiquitin signaling and recruitment of SLX4 during DNA crosslink repair

SLX4/FANCP is a key Fanconi anemia (FA) protein and a DNA repair scaffold for incision around a DNA interstrand crosslink (ICL) by its partner XPF nuclease. The tandem UBZ4 ubiquitin-binding domains of SLX4 are critical for the recruitment of SLX4 to damage sites, likely by binding to K63-linked polyubiquitin chains. However, the identity of the ubiquitin E3 ligase that mediates SLX4 recruitment remains unknown. Using small interfering RNA (siRNA) screening with a GFP-tagged N-terminal half of SLX4 (termed SLX4-N), we identify the RNF168 E3 ligase as a critical factor for mitomycin C (MMC)-induced SLX4 foci formation. RNF168 and GFP-SLX4-N colocalize in MMC-induced ubiquitin foci. Accumulation of SLX4-N at psoralen-laser ICL tracks or of endogenous SLX4 at Digoxigenin-psoralen/UVA ICL is dependent on RNF168. Finally, we find that RNF168 is epistatic with SLX4 in promoting MMC tolerance. We conclude that RNF168 is a critical component of the signal transduction that recruits SLX4 to ICL damage.

DNA Double Strand Break Repair Pathways in Response to Different Types of Ionizing Radiation

The superior dose distribution of particle radiation compared to photon radiation makes it a promising therapy for the treatment of tumors. However, the cellular responses to particle therapy and especially the DNA damage response (DDR) is not well characterized. Compared to photons, particles are thought to induce more closely spaced DNA lesions instead of isolated lesions. How this different spatial configuration of the DNA damage directs DNA repair pathway usage, is subject of current investigations. In this review, we describe recent insights into induction of DNA damage by particle radiation and how this shapes DNA end processing and subsequent DNA repair mechanisms. Additionally, we give an overview of promising DDR targets to improve particle therapy.

PARP inhibitor resistance in ovarian cancer: Underlying mechanisms and therapeutic approaches targeting the ATR/CHK1 pathway

Ovarian cancer (OC) constitutes the most common cause of gynecologic cancer-related death in women worldwide. Despite consistent developments in treatment strategies for OC, the management of advanced-stage disease remains a significant challenge. Recent improvements in targeted treatments based on poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) have provided invaluable benefits to patients with OC. Unfortunately, numerous patients do not respond to PARPi due to intrinsic resistance or acquisition of resistance. Here, we discuss mechanisms of resistance to PARPi that have specifically emerged in OC including increased drug efflux, restoration of HR repair, re-establishment of replication fork stability, reduced PARP1 trapping, abnormalities in PARP signaling, and less common pathways associated with alternative DNA sensing and repair pathways. Elucidation of the precise mechanisms is essential for the development of novel strategies to re-sensitize OC cells to PARPi agents. Additionally, novel potential concepts for preventing and combating resistance to PARPi under development and relevant clinical reports on treatment strategies have been reviewed, with emphasis on the exploitation of the ATR/CHK1 kinase pathway in sensitization to PARPi to overcome resistance-induced vulnerability in ovarian cancer.

Role of 53BP1 in end protection and DNA synthesis at DNA breaks

Double-strand break (DSB) repair choice is greatly influenced by the initial processing of DNA ends. 53BP1 limits the formation of recombinogenic single-strand DNA (ssDNA) in BRCA1-deficient cells, leading to defects in homologous recombination (HR). However, the exact mechanisms by which 53BP1 inhibits DSB resection remain unclear. Previous studies have identified two potential pathways: protection against DNA2/EXO1 exonucleases presumably through the Shieldin (SHLD) complex binding to ssDNA, and localized DNA synthesis through the CTC1-STN1-TEN1 (CST) and DNA polymerase α (Polα) to counteract resection. Using a combinatorial approach of END-seq, SAR-seq, and RPA ChIP-seq, we directly assessed the extent of resection, DNA synthesis, and ssDNA, respectively, at restriction enzyme-induced DSBs. We show that, in the presence of 53BP1, Polα-dependent DNA synthesis reduces the fraction of resected DSBs and the resection lengths in G0/G1, supporting a previous model that fill-in synthesis can limit the extent of resection. However, in the absence of 53BP1, Polα activity is sustained on ssDNA yet does not substantially counter resection. In contrast, EXO1 nuclease activity is essential for hyperresection in the absence of 53BP1. Thus, Polα-mediated fill-in partially limits resection in the presence of 53BP1 but cannot counter extensive hyperresection due to the loss of 53BP1 exonuclease blockade. These data provide the first nucleotide mapping of DNA synthesis at resected DSBs and provide insight into the relationship between fill-in polymerases and resection exonucleases.

LIN37-DREAM prevents DNA end resection and homologous recombination at DNA double-strand breaks in quiescent cells

DNA double-strand break (DSB) repair by homologous recombination (HR) is thought to be restricted to the S- and G₂- phases of the cell cycle in part due to 53BP1 antagonizing DNA end resection in G₁-phase and non-cycling quiescent (G₀) cells. Here, we show that LIN37, a component of the DREAM transcriptional repressor, functions in a 53BP1-independent manner to prevent DNA end resection and HR in G₀ cells. Loss of LIN37 leads to the expression of HR proteins, including BRCA1, BRCA2, PALB2, and RAD51, and promotes DNA end resection in G₀ cells even in the presence of 53BP1. In contrast to 53BP1-deficiency, DNA end resection in LIN37-deficient G₀ cells depends on BRCA1 and leads to RAD51 filament formation and HR. LIN37 is not required to protect DNA ends in cycling cells at G₁-phase. Thus, LIN37 regulates a novel 53BP1-independent cell phase-specific DNA end protection pathway that functions uniquely in quiescent cells.

RNF168-mediated localization of BARD1 recruits the BRCA1-PALB2 complex to DNA damage

DNA damage prompts a diverse range of alterations to the chromatin landscape. The RNF168 E3 ubiquitin ligase catalyzes the mono-ubiquitination of histone H2A at lysine (K)13/15 (mUb-H2A), forming a binding module for DNA repair proteins. BRCA1 promotes homologous recombination (HR), in part, through its interaction with PALB2, and the formation of a larger BRCA1-PALB2-BRCA2-RAD51 (BRCA1-P) complex. The mechanism by which BRCA1-P is recruited to chromatin surrounding DNA breaks is unclear. In this study, we reveal that an RNF168-governed signaling pathway is responsible for localizing the BRCA1-P complex to DNA damage. Using mice harboring a Brca1^CC (coiled coil) mutation that blocks the Brca1-Palb2 interaction, we uncovered an epistatic relationship between Rnf168⁻ and Brca1^CC alleles, which disrupted development, and reduced the efficiency of Palb2-Rad51 localization. Mechanistically, we show that RNF168-generated mUb-H2A recruits BARD1 through a BRCT domain ubiquitin-dependent recruitment motif (BUDR). Subsequently, BARD1-BRCA1 accumulate PALB2-RAD51 at DNA breaks via the CC domain-mediated BRCA1-PALB2 interaction. Together, these findings establish a series of molecular interactions that connect the DNA damage signaling and HR repair machinery.

The BRCA1-PALB2-BRCA2-RAD51 (BRCA1-P) complex is well known to play a fundamental role in DNA repair, but how the complex recruitment is regulated is still a matter of interest. Here the authors reveal mechanistic insights into RNF168 activity being responsible for PALB2 recruitment, through BARD1-BRCA1 during homologous recombination repair.

Recent Advances in Enhancing the Therapeutic Index of PARP Inhibitors in Breast Cancer

Simple Summary

Two to three percent of breast cancer patients harbor germline mutation of either BRCA1 or BRCA2 genes. Their tumor cells are deficient in homologous recombination, a BRCA-dependent DNA repair machinery. These deficient cells survive thanks to the PARP-mediated alternative pathway. Therefore, PARP inhibitors have already shown some level of efficiency in the treatment of metastatic breast cancer patients. Unfortunately, some tumor cells inevitably resist PARP inhibitors by different mechanisms. In this review, we (i) present the notion of homologous recombination deficiency and its evaluation methods, (ii) detail the PARP inhibitor clinical trials in breast cancer, (iii) briefly describe the mechanisms to PARP inhibitors resistance, and (iv) discuss some strategies currently under evaluation to enhance the therapeutic index of PARP inhibitors in breast cancer.

Abstract

As poly-(ADP)-ribose polymerase (PARP) inhibition is synthetic lethal with the deficiency of DNA double-strand (DSB) break repair by homologous recombination (HR), PARP inhibitors (PARPi) are currently used to treat breast cancers with mutated BRCA1/2 HR factors. Unfortunately, the increasingly high rate of PARPi resistance in clinical practice has dented initial hopes. Multiple resistance mechanisms and acquired vulnerabilities revealed in vitro might explain this setback. We describe the mechanisms and vulnerabilities involved, including newly identified modes of regulation of DSB repair that are now being tested in large cohorts of patients and discuss how they could lead to novel treatment strategies to improve the therapeutic index of PARPi.