RNF168-Mediated Ubiquitin Signaling Inhibits the Viability of BRCA1-Null Cancers

RAD18 directs DNA double-strand break repair by homologous recombination to post-replicative chromatin

RAD18 is an E3 ubiquitin ligase that prevents replication fork collapse by promoting DNA translesion synthesis and template switching. Besides this classical role, RAD18 has been implicated in homologous recombination; however, this function is incompletely understood. Here, we show that RAD18 is recruited to DNA lesions by monoubiquitination of histone H2A at K15 and counteracts accumulation of 53BP1. Super-resolution microscopy revealed that RAD18 localizes to the proximity of DNA double strand breaks and limits the distribution of 53BP1 to the peripheral chromatin nanodomains. Whereas auto-ubiquitination of RAD18 mediated by RAD6 inhibits its recruitment to DNA breaks, interaction with SLF1 promotes RAD18 accumulation at DNA breaks in the post-replicative chromatin by recognition of histone H4K20me0. Surprisingly, suppression of 53BP1 function by RAD18 is not involved in homologous recombination and rather leads to reduction of non-homologous end joining. Instead, we provide evidence that RAD18 promotes HR repair by recruiting the SMC5/6 complex to DNA breaks. Finally, we identified several new loss-of-function mutations in RAD18 in cancer patients suggesting that RAD18 could be involved in cancer development.

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.

Discovery of a 53BP1 Small Molecule Antagonist Using a Focused DNA-Encoded Library Screen

Methyl-lysine reader p53 binding protein 1 (53BP1) is a central mediator of DNA break repair and is associated with various human diseases, including cancer. Thus, high-quality 53BP1 chemical probes can aid in further understanding the role of 53BP1 in genome repair pathways. Herein, we utilized focused DNA-encoded library screening to identify the novel hit compound UNC8531, which binds the 53BP1 tandem Tudor domain (TTD) with an IC50 of 0.47 ± 0.09 μM in a TR-FRET assay and Kd values of 0.85 ± 0.17 and 0.79 ± 0.52 μM in ITC and SPR, respectively. UNC8531 was cocrystallized with the 53BP1 TTD to guide further optimization efforts, leading to UNC9512. NanoBRET and 53BP1-dependent foci formation experiments confirmed cellular target engagement. These results show that UNC9512 is a best-in-class small molecule 53BP1 antagonist that can aid further studies investigating the role of 53BP1 in DNA repair, gene editing, and oncogenesis.

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.

RNF125 attenuates hepatocellular carcinoma progression by downregulating SRSF1-ERK pathway

Hepatocellular carcinoma (HCC) is one of the most deadly malignant cancers worldwide. Research into the crucial genes responsible for maintaining the aggressive behaviour of cancer cells is important for the clinical treatment of HCC. The purpose of this study was to determine whether the E3 ubiquitin ligase Ring Finger Protein 125 (RNF125) plays a role in the proliferation and metastasis of HCC. RNF125 expression in human HCC samples and cell lines was investigated using TCGA dataset mining, qRT‒PCR, western blot, and immunohistochemistry assays. In addition, 80 patients with HCC were studied for the clinical value of RNF125. Furthermore, the molecular mechanism by which RNF125 contributes to hepatocellular carcinoma progression was determined with mass spectrometry (MS), coimmunoprecipitation (Co-IP), dual-luciferase reporter assays, and ubiquitin ladder assays. We found that RNF125 was markedly downregulated in HCC tumour tissues, which was associated with a poor prognosis for patients with HCC. Moreover, the overexpression of RNF125 inhibited HCC proliferation and metastasis both in vitro and in vivo, whereas the knockdown of RNF125 exerted antithetical effects. Mechanistically, mass spectrometry analysis revealed a protein interaction between RNF125 and SRSF1, and RNF125 accelerated the proteasome-mediated degradation of SRSF1, which impeded HCC progression by inhibiting the ERK signalling pathway. Furthermore, RNF125 was detected to be the downstream target of miR-103a-3p. In this study, we identified that RNF125 is a tumour suppressor in HCC and inhibits HCC progression by inhibiting the SRSF1/ERK pathway. These findings provide a promising treatment target for HCC.

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.

COVID-19 mRNA Vaccine Effectiveness against Elderly Frail People

The frail, elderly population is often characterized by poor immunogenicity post COVID-19 mRNA vaccination. "Inflame-ageing" and "immune-senescence" are pathogenetic mechanisms that might explain this phenomenon. Complex interplay with cytokines and microbiota is also implicated in this inflammatory cascade. The abovementioned population, although very important from immunologic perspective, has barely been included in the mRNA vaccination clinical trials.

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.

Deubiquitinase OTUD6A promotes breast cancer progression by increasing TopBP1 stability and rendering tumor cells resistant to DNA-damaging therapy

The DNA damage response (DDR) is critical for maintaining cellular homeostasis and genome integrity. Mounting evidence has shown that posttranslational protein modifications play vital roles in the DDR. In this study, we showed that deubiquitinase OTUD6A is involved in the DDR and is important for maintaining genomic stability. Mechanistically, in response to DNA damage, the abundance of OTUD6A was increased; meanwhile, PP2A interacted with OTUD6A and dephosphorylated OTUD6A at sites S70/71/74, which promoted nuclear localization of OTUD6A. Subsequently, OTUD6A was recruited to the damage site, where it interacted with TopBP1 and blocked the interaction between TopBP1 and its ubiquitin E3 ligase UBR5, decreasing K48-linked polyubiquitination and increasing the stability of TopBP1. OTUD6A depletion impaired CHK1 S345 phosphorylation and blocked cell cycle progression under DNA replication stress. Consistently, knockout of OTUD6A rendered mice hypersensitive to irradiation, shortened survival, and inhibited tumor growth by regulating TopBP1 in xenografted nude mice. Moreover, OTUD6A is expressed at high levels in breast cancer, and OTUD6A overexpression promotes cell proliferation, migration and invasion, indicating that dysregulation of OTUD6A expression contributes to genomic instability and is associated with tumor development. In summary, this study demonstrates that OTUD6A plays a critical role in promoting tumor cell resistance to chemoradiotherapy by deubiquitinating and stabilizing TopBP1.

Multifaceted regulation and functions of 53BP1 in NHEJ‑mediated DSB repair (Review)

The repair of DNA double-strand breaks (DSBs) is crucial for the preservation of genomic integrity and the maintenance of cellular homeostasis. Non-homologous DNA end joining (NHEJ) is the predominant repair mechanism for any type of DNA DSB during the majority of the cell cycle. NHEJ defects regulate tumor sensitivity to ionizing radiation and anti-neoplastic agents, resulting in immunodeficiencies and developmental abnormalities in malignant cells. p53-binding protein 1 (53BP1) is a key mediator involved in DSB repair, which functions to maintain a balance in the repair pathway choices and in preserving genomic stability. 53BP1 promotes DSB repair via NHEJ and antagonizes DNA end overhang resection. At present, novel lines of evidence have revealed the molecular mechanisms underlying the recruitment of 53BP1 and DNA break-responsive effectors to DSB sites, and the promotion of NHEJ-mediated DSB repair via 53BP1, while preventing homologous recombination. In the present review article, recent advances made in the elucidation of the structural and functional characteristics of 53BP1, the mechanisms of 53BP1 recruitment and interaction with the reshaping of the chromatin architecture around DSB sites, the post-transcriptional modifications of 53BP1, and the up- and downstream pathways of 53BP1 are discussed. The present review article also focuses on the application perspectives, current challenges and future directions of 53BP1 research.

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.

RNF168 suppresses the cancer stem cell-like traits of nonsmall cell lung cancer cells by mediating RhoC ubiquitination

The critical roles of E3 ubiquitin ligase RNF168 have been widely revealed in various tumors, however, its roles in lung cancer progression are still confusing. Here, we found that RNF168 expression is positively correlated with the overall survival, first-progression survival, and postprogression survival of lung adenocarcinoma, but not correlated with these survivals of squamous cell carcinoma of lung. Furthermore, it was shown that RNF168 mRNA expression is lowly expressed in lung adenocarcinoma tissues, but highly expressed in squamous cell carcinoma of lung. Functional experiments indicated that RNF168 overexpression significantly suppressed the cancer stem cell (CSC)-like traits of nonsmall cell lung cancer (NSCLC) cells, as characterized by the attenuation of sphere-formation ability, ALDH activity, and the expression of lung CSC markers. Mechanistic studies demonstrated that RNF168 facilitated the ubiquitination of RhoC, which had been considered as a fascinating target for CSCs, and thus promoted RhoC protein degradation. Notably, RNF168 failed to affect the mRNA expression of RhoC and overexpression of RhoC rescued the inhibitory effects of RNF168 overexpression on the CSC-like traits of NSCLC cells. Therefore, this study revealed RNF168 as a novel regulator of RhoC protein in NSCLC cells, this RNF168/RhoC regulatory axis might be a potential target for NSCLC treatment.

MEOX1 suppresses the progression of lung cancer cells by inhibiting the cell-cycle checkpoint gene CCNB1

The previous study has shown that transcriptional factor MEOX1 could promote proliferation and sphere formation ability of non-small cell lung cancer (NSCLC) cells, however, we found that MEOX1 mRNA was lowly expressed in lung cancer tissues compared to that in normal adjacent tissues, and MEOX1 mRNA expression was positively correlated with the survival of lung cancer patients, especially in lung adenocarcinoma patients. Functional experiments using in vitro and in vivo experiments revealed that stable overexpression of MEOX1 significantly suppressed the proliferation ability, promoted cell cycle arrest in G2 phase, and apoptotic ability of NSCLC cells. Additionally, it was identified that MEOX1 and CCNB1 mRNA expression exhibited a negative correlation in different lung cancer tissues. Mechanistically, we indicated that MEOX1 bound to the transcriptional initiation site of CCNB1 and thus suppressed CCNB1 expression. Notably, CCNB1 overexpression rescued the inhibition of MEOX1 overexpression on NSCLC progression. This study deciphers a novel MEOX1/CCNB1 axis suppressing NSCLC progression.

Ofeleein i mi Vlaptin-Volume II: Immunity Following Infection or mRNA Vaccination, Drug Therapies and Non-Pharmacological Management at Post-Two Years SARS-CoV-2 Pandemic

The persistence of the coronavirus disease 2019 (COVID-19) pandemic has triggered research into limiting transmission, morbidity and mortality, thus warranting a comprehensive approach to guide balanced healthcare policies with respect to people's physical and mental health. The mainstay priority during COVID-19 is to achieve widespread immunity, which could be established through natural contact or vaccination. Deep knowledge of the immune response combined with recent specific data indicates the potential inferiority of induced immunity against infection. Moreover, the prevention of transmission has been founded on general non-pharmacological measures of protection, albeit debate exists considering their efficacy and, among other issues, their socio-psychological burden. The second line of defense is engaged after infection and is supported by a plethora of studied agents, such as antibiotics, steroids and non-steroid anti-inflammatory drugs, antiviral medications and other biological agents that have been proposed, though variability in terms of benefits and adverse events has not allowed distinct solutions, albeit certain treatments might have a role in prevention and/or treatment of the disease. This narrative review summarizes the existing literature on the advantages and weaknesses of current COVID-19 management measures, thus underlining the necessity of acting based on the classical principle of "ofeleein i mi vlaptin", that is, to help or not to harm.

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.

RNF168 promotes RHOC degradation by ubiquitination to restrain gastric cancer progression via decreasing HDAC1 expression

Gastric cancer (GC) is the most common cancer worldwide. Although advances in the treatments, the oncogenic mechanisms are still largely unknown. RNF168 (ring-finger nuclear factor 168) is an important regulator of DNA double-strand break (DSB) repair, and its defects have been involved in the pathogenesis of a number of human diseases including cancer. However, its effects on GC are still unclear. In the study, we demonstrated that RNF168 expression was remarkably down-regulated in human GC tissues, and its low expression showed worse overall survival rate in GC patients. Importantly, we here reported that RNF168 directly interacted with Ras homolog gene family member C (RHOC) and induced its ubiquitination to promote RHOC degradation. RHOC exhibited higher expression in human GC tissues, and its knockdown significantly restrained cell proliferation, migration and invasion in GC cell lines. Moreover, RHOC knockdown led to a significant reduction in GC tumor growth in a xenograft mouse model. Additionally, histone deacetylase 1 (HDAC1) was found to be markedly decreased in GC cells with RHOC knockdown. Intriguingly, RHOC suppression-ameliorated proliferative and migratory ability in GC cells were significantly diminished by HDAC1 over-expression. Our in vivo studies finally confirmed that RHOC inhibition dramatically reduced the lung metastasis in nude mice. Collectively, all our results demonstrated that RNF168 directly interacted with RHOC to induce its degradation via promoting its ubiquitination, contributing to the inhibition of cell proliferation and metastasis in GC through decreasing HDAC1. Thus, targeting RNF168/RHOC/HDAC1 axis might be promising to develop effective therapies for GC treatment.

Exploiting synthetic lethality to target BRCA1/2-deficient tumors: where we stand

The principle of synthetic lethality, which refers to the loss of viability resulting from the disruption of two genes, which, individually, do not cause lethality, has become an attractive target approach due to the development and clinical success of Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi). In this review, we present the most recent findings on the use of PARPi in the clinic, which are currently approved for second-line therapy for advanced ovarian and breast cancer associated with mutations of BRCA1 or BRCA2 (BRCA1/2) genes. PARPi efficacy, however, appears to be limited by acquired and inherent resistance, highlighting the need for alternative and synergistic targets to eliminate these tumors. Here, we explore other identified synthetic lethal interactors of BRCA1/2, including DNA polymerase theta (POLQ), Fanconi anemia complementation group D2 (FANDC2), radiation sensitive 52 (RAD52), Flap structure-specific endonuclease 1 (FEN1), and apurinic/apyrimidinic endodeoxyribonuclease 2 (APE2), as well as other protein and nonprotein targets, for BRCA1/2-mutated cancers and their implications for future therapies. A wealth of information now exists for phenotypic and functional characterization of these novel synthetic lethal interactors of BRCA1/2, and leveraging these findings can pave the way for the development of new targeted therapies for patients suffering from these cancers.

Research on the Expression and Regulatory Mechanism of Breast Cancer Susceptibility Gene-1 on Cell of Skin Cancer in Different Classification

The expression condition of breast cancer susceptibility gene-1 (BRCA1) in tissue of skin cancer in different classification was assessed in our study with analysis of the correlation between BRCA1 and classification of skin cancer. The influence of BRCA1 on the tyrosine kinase receptors (c-kit) was detected to discuss the possible-existing regulatory mechanism. The skin cancer was classified according to the clinical symptoms and was divided into zero to the fifth phase. Then the expression of BRCA1 and c-kit in tissue of skin cancer in different classification was detected by RT-PCR. The cell line of skin cancer HS-4 cultured in vitro was transfected with high-BRCA1-expressed vector. There was negative correlation between the expression of BRCA1 and the classification of skin cancer. There was no significant difference among the zero phase, the first phase and the second phase when the value of P was less than 0.05. There was no significant difference between the third phase and the fourth phase when the value of P was larger than 0.05. But there was significant difference between the latter two groups and the former three groups when the value of P was less than 0.05. The expression of c-kit was increased along with the elevation of classification of skin cancer with significant difference among groups. The reducing of expression of BRCA1 could affect the expression of c-kit from the cell experiment. There was negative correlation between the expression of BRCA1 and the expression of c-kit. There was correlation between the expression of BRCA1 and c-kit and the development of skin cancer. There was negative correlation between BRCA1 expression and c-kit level. It was illustrated that the BRCA1 could develop certain regulatory effect on skin cancer by influencing the expression level of c-kit.

DSB repair pathway choice is regulated by recruitment of 53BP1 through cell cycle-dependent regulation of Sp1

Although many of the factors, epigenetic changes, and cell cycle stages that distinguish repair of double-strand breaks (DSBs) by homologous recombination (HR) from non-homologous end joining (NHEJ) are known, the underlying mechanisms that determine pathway choice are incompletely understood. Previously, we found that the transcription factor Sp1 is recruited to DSBs and is necessary for repair. Here, we demonstrate that Sp1 localizes to DSBs in G1 and is necessary for recruitment of the NHEJ repair factor, 53BP1. Phosphorylation of Sp1-S59 in early S phase evicts Sp1 and 53BP1 from the break site; inhibition of that phosphorylation results in 53BP1 and Sp1 remaining at DSBs in S phase cells, precluding BRCA1 binding and suppressing HR. Expression of Sp1-S59A increases sensitivity of BRCA1^+/+ cells to poly (ADP-ribose) polymerase (PARP) inhibition similar to BRCA1 deficiency. These data demonstrate how Sp1 integrates the cell cycle and DSB repair pathway choice to favor NHEJ.

RNF168 regulates R-loop resolution and genomic stability in BRCA1/2-deficient tumors

Germline mutations in BRCA1 and BRCA2 (BRCA1/2) genes considerably increase breast and ovarian cancer risk. Given that tumors with these mutations have elevated genomic instability, they exhibit relative vulnerability to certain chemotherapies and targeted treatments based on poly (ADP-ribose) polymerase (PARP) inhibition. However, the molecular mechanisms that influence cancer risk and therapeutic benefit or resistance remain only partially understood. BRCA1 and BRCA2 have also been implicated in the suppression of R-loops, triple-stranded nucleic acid structures composed of a DNA:RNA hybrid and a displaced ssDNA strand. Here, we report that loss of RNF168, an E3 ubiquitin ligase and DNA double-strand break (DSB) responder, remarkably protected Brca1-mutant mice against mammary tumorigenesis. We demonstrate that RNF168 deficiency resulted in accumulation of R-loops in BRCA1/2-mutant breast and ovarian cancer cells, leading to DSBs, senescence, and subsequent cell death. Using interactome assays, we identified RNF168 interaction with DHX9, a helicase involved in the resolution and removal of R-loops. Mechanistically, RNF168 directly ubiquitylated DHX9 to facilitate its recruitment to R-loop-prone genomic loci. Consequently, loss of RNF168 impaired DHX9 recruitment to R-loops, thereby abrogating its ability to resolve R-loops. The data presented in this study highlight a dependence of BRCA1/2-defective tumors on factors that suppress R-loops and reveal a fundamental RNF168-mediated molecular mechanism that governs cancer development and vulnerability.

Not Black or White but Shades of Gray: Homologous Recombination Deficiency as a Continuous Variable Modulated by RNF168

In this issue of Cancer Research, the study by Krais and colleagues underscores that DNA damage repair by homologous recombination (HR) is not an all-or-nothing phenomenon, but that HR competency comes on a spectrum, ranging from complete deficiency to proficiency. Residual low-level HR in BRCA1-mutant cancer cells turns out to be critically important for their survival and is afforded by low levels of Histone 2A (H2A) ubiquitination resulting from lowered RNF168 levels. The findings raise the possibility that, if ubiquitination of H2A could be enforced by inhibition of deubiquitinases, residual HR in BRCA1mt cells might be extinguished. Extinction of residual HR might improve the therapeutic efficacy of the emerging inhibitors of DNA damage repair. The development of methods to measure HR directly and quantitatively is crucial to develop this field.See related article by Krais et al., p. 2848.

BRCA1 Mutations in Cancer: Coordinating Deficiencies in Homologous Recombination with Tumorigenesis

Cancers that arise from BRCA1 germline mutations are deficient for homologous recombination (HR) DNA repair and are sensitive to DNA-damaging agents such as platinum and PARP inhibitors. In vertebrate organisms, knockout of critical HR genes including BRCA1 and BRCA2 is lethal because HR is required for genome replication. Thus, cancers must develop strategies to cope with loss of HR activity. Furthermore, as established tumors respond to chemotherapy selection pressure, additional genetic adaptations transition cancers to an HR-proficient state. In this review, we discuss biological mechanisms that influence the ability of BRCA1-mutant cancers to perform HR. Furthermore, we consider how the HR status fluctuates throughout the cancer life course, from tumor initiation to the development of therapy refractory disease.