A mechanism for the suppression of homologous recombination in G1 cells

53BP1: a DSB escort

In this review, Mirman et al. summarize the current understanding of the role of 53BP1 in DSB repair at deprotected telomeres, in class switch recombination in the immune system, and in the context of PARPi-treated BRCA1-deficient cells. They argue that the primary function of 53BP1 is not to regulate the choice between c-NHEJ and HDR, but to ensure the fidelity of DSB repair, a function that is corrupted in diseases where DNA repair is rewired.

53BP1 is an enigmatic DNA damage response factor that gained prominence because it determines the efficacy of PARP1 inhibitory drugs (PARPi) in BRCA1-deficient cancers. Recent studies have elevated 53BP1 from its modest status of (yet another) DNA damage factor to master regulator of double-strand break (DSB) repair pathway choice. Our review of the literature suggests an alternative view. We propose that 53BP1 has evolved to avoid mutagenic repair outcomes and does so by controlling the processing of DNA ends and the dynamics of DSBs. The consequences of 53BP1 deficiency, such as diminished PARPi efficacy in BRCA1-deficient cells and altered repair of damaged telomeres, can be explained from this viewpoint. We further propose that some of the fidelity functions of 53BP1 coevolved with class switch recombination (CSR) in the immune system. We speculate that, rather than being deterministic in DSB repair pathway choice, 53BP1 functions as a DSB escort that guards against illegitimate and potentially tumorigenic recombination.

Non-canonical regulation of homologous recombination DNA repair by the USP9X deubiquitylase

In order to prevent the deleterious effects of genotoxic agents, cells have developed complex surveillance mechanisms and DNA repair pathways that allow them to maintain genome integrity. The ubiquitin-specific protease 9X (USP9X) contributes to genome stability during DNA replication and chromosome segregation. Depletion of USP9X leads to DNA double-strand breaks, some of which are triggered by replication fork collapse. Here, we identify USP9X as a novel regulator of homologous recombination (HR) DNA repair in human cells. By performing cellular HR reporter, irradiation-induced focus formation and colony formation assays, we show that USP9X is required for efficient HR. Mechanistically, we show USP9X is important to sustain the expression levels of key HR factors, namely BRCA1 and RAD51 through a non-canonical regulation of their mRNA abundance. Intriguingly, we find that the contribution of USP9X to BRCA1 and RAD51 expression is independent of its known catalytic activity. Thus, this work identifies USP9X as a regulator of HR, demonstrates a novel mechanism by which USP9X can regulate protein levels, and provides insights in to the regulation of BRCA1 and RAD51 mRNA.This article has an associated First Person interview with the first author of the paper.

Ubiquitin and ubiquitin-like molecules in DNA double strand break repair

Both environmental and endogenous factors induce various forms of DNA damage. DNA double strand break (DSB) is the most deleterious DNA lesion. The swift initiation of a complexed network of interconnected pathways to repair the DNA lesion is essential for cell survival. In the past years, the roles of ubiquitin and ubiquitin-like proteins in DNA damage response and DNA repair has been explored. These findings help us better understand the complicated mechanism of DSB signaling pathways.

DNA polymerase theta (Polθ) - an error-prone polymerase necessary for genome stability

Mammalian cells have evolved multiple pathways to repair DNA double strand breaks (DSBs) and ensure genome stability. In addition to non-homologous end-joining (NHEJ) and homologous recombination (HR), cells evolved an error-prone repair pathway termed microhomology-mediated end joining (MMEJ). The mutagenic outcome of MMEJ derives from the activity of DNA polymerase theta (Polθ) - a multidomain enzyme that is minimally expressed in normal tissue but overexpressed in tumors. Polθ expression is particularly crucial for the proliferation of HR deficient cancer cells. As a result, this mutagenic repair emerged as an attractive target for cancer therapy, and inhibitors are currently in pre-clinical development. Here, we review the multifunctionality of this enigmatic polymerase, focusing on its role during DSB repair in mammalian cells and its impact on cancer genomes.

Sharpening the Molecular Scissors: Advances in Gene-Editing Technology

The ability to precisely modify human genes has been made possible by the development of tools such as meganucleases, zinc finger nucleases, TALENs, and CRISPR/Cas. These now make it possible to generate targeted deletions, insertions, gene knock outs, and point variants; to modulate gene expression by targeting transcription factors or epigenetic machineries to DNA; or to target and modify RNA. Endogenous repair mechanisms are used to make the modifications required in DNA; they include non-homologous end joining, homology-directed repair, homology-independent targeted integration, microhomology-mediated end joining, base-excision repair, and mismatch repair. Off-target effects can be monitored using in silico prediction and sequencing and minimized using Cas proteins with higher accuracy, such as high-fidelity Cas9, enhanced-specificity Cas9, and hyperaccurate Cas9. Alternatives to Cas9 have been identified, including Cpf1, Cas12a, Cas12b, and smaller Cas9 orthologs such as CjCas9. Delivery of gene-editing components is performed ex vivo using standard techniques or in vivo using AAV, lipid nanoparticles, or cell-penetrating peptides. Clinical development of gene-editing technology is progressing in several fields, including immunotherapy in cancer treatment, antiviral therapy for HIV infection, and treatment of genetic disorders such as β-thalassemia, sickle cell disease, lysosomal storage disorders, and retinal dystrophy. Here we review these technological advances and the challenges to their clinical implementation.

A transcomplementing gene drive provides a flexible platform for laboratory investigation and potential field deployment

CRISPR-based gene drives can spread through wild populations by biasing their own transmission above the 50% value predicted by Mendelian inheritance. These technologies offer population-engineering solutions for combating vector-borne diseases, managing crop pests, and supporting ecosystem conservation efforts. Current technologies raise safety concerns for unintended gene propagation. Herein, we address such concerns by splitting the drive components, Cas9 and gRNAs, into separate alleles to form a trans-complementing split-gene-drive (tGD) and demonstrate its ability to promote super-Mendelian inheritance of the separate transgenes. This dual-component configuration allows for combinatorial transgene optimization and increases safety by restricting escape concerns to experimentation windows. We employ the tGD and a small-molecule-controlled version to investigate the biology of component inheritance and resistant allele formation, and to study the effects of maternal inheritance and impaired homology on efficiency. Lastly, mathematical modeling of tGD spread within populations reveals potential advantages for improving current gene-drive technologies for field population modification.

DNA Repair and the Stability of the Plant Mitochondrial Genome

The mitochondrion stands at the center of cell energy metabolism. It contains its own genome, the mtDNA, that is a relic of its prokaryotic symbiotic ancestor. In plants, the mitochondrial genetic information influences important agronomic traits including fertility, plant vigor, chloroplast function, and cross-compatibility. Plant mtDNA has remarkable characteristics: It is much larger than the mtDNA of other eukaryotes and evolves very rapidly in structure. This is because of recombination activities that generate alternative mtDNA configurations, an important reservoir of genetic diversity that promotes rapid mtDNA evolution. On the other hand, the high incidence of ectopic recombination leads to mtDNA instability and the expression of gene chimeras, with potential deleterious effects. In contrast to the structural plasticity of the genome, in most plant species the mtDNA coding sequences evolve very slowly, even if the organization of the genome is highly variable. Repair mechanisms are probably responsible for such low mutation rates, in particular repair by homologous recombination. Herein we review some of the characteristics of plant organellar genomes and of the repair pathways found in plant mitochondria. We further discuss how homologous recombination is involved in the evolution of the plant mtDNA.

Ubiquitin-Regulated Cell Proliferation and Cancer

Ubiquitin ligases (E3) play a crucial role in the regulation of different cellular processes such as proliferation and differentiation via recognition, interaction, and ubiquitination of key cellular proteins in a spatial and temporal regulated manner. The type of ubiquitin chain formed determines the fate of the substrates. The ubiquitinated substrates can be degraded by the proteasome, display altered subcellular localization, or can suffer modifications on their interaction with functional protein complexes. Deregulation of E3 activities is frequently found in various human pathologies, including cancer. The illegitimated or accelerated degradation of oncosuppressive proteins or, inversely, the abnormally high accumulation of oncoproteins, contributes to cell proliferation and transformation. Anomalies in protein abundance may be related to mutations that alter the direct or indirect recognition of proteins by the E3 enzymes or alterations in the level of expression or activity of ubiquitin ligases. Through a few examples, we illustrate here the complexity and diversity of the molecular mechanisms related to protein ubiquitination involved in cell cycle regulation. We will discuss the role of ubiquitin-dependent degradation mediated by the proteasome, the role of non-proteolytic ubiquitination during cell cycle progression, and the consequences of this deregulation on cellular transformation. Finally, we will highlight the novel opportunities that arise from these studies for therapeutic intervention.

CRISPR/Cas9 Genome Editing Technology: A Valuable Tool for Understanding Plant Cell Wall Biosynthesis and Function

For the past 5 years, clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) technology has appeared in the molecular biology research spotlight. As a game-changing player in genome editing, CRISPR/Cas9 technology has revolutionized animal research, including medical research and human gene therapy as well as plant science research, particularly for crop improvement. One of the most common applications of CRISPR/Cas9 is to generate genetic knock-out mutants. Recently, several multiplex genome editing approaches utilizing CRISPR/Cas9 were developed and applied in various aspects of plant research. Here we summarize these approaches as they relate to plants, particularly with respect to understanding the biosynthesis and function of the plant cell wall. The plant cell wall is a polysaccharide-rich cell structure that is vital to plant cell formation, growth, and development. Humans are heavily dependent on the byproducts of the plant cell wall such as shelter, food, clothes, and fuel. Genes involved in the assembly of the plant cell wall are often highly redundant. To identify these redundant genes, higher-order knock-out mutants need to be generated, which is conventionally done by genetic crossing. Compared with genetic crossing, CRISPR/Cas9 multi-gene targeting can greatly shorten the process of higher-order mutant generation and screening, which is especially useful to characterize cell wall related genes in plant species that require longer growth time. Moreover, CRISPR/Cas9 makes it possible to knock out genes when null T-DNA mutants are not available or are genetically linked. Because of these advantages, CRISPR/Cas9 is becoming an ideal and indispensable tool to perform functional studies in plant cell wall research. In this review, we provide perspectives on how to design CRISPR/Cas9 to achieve efficient gene editing and multi-gene targeting in plants. We also discuss the recent development of the virus-based CRISPR/Cas9 system and the application of CRISPR/Cas9 to knock in genes. Lastly, we summarized current progress on using CRISPR/Cas9 for the characterization of plant cell wall-related genes.

De novo pathogenic germline variant in PALB2 in a patient with pancreatic cancer

De novo mutations in the major breast/ovarian cancer susceptibility genes BRCA1 and BRCA2 are rare. De novo mutations in the PALB2 gene have never been reported. Here we report a de novo PALB2 germ line mutation (c.3455delC (p.Pro1152Hisfs*11) in a patient with pancreatic cancer, where non-paternity and somatic parental mosaicism have to the extent possible been excluded as a mechanism for detecting the de novo mutation. The lack of previous reports on de novo PALB2 mutations maybe the limited number of PALB2germline mutations reported overall.

Beyond reversal: ubiquitin and ubiquitin-like proteases and the orchestration of the DNA double strand break repair response

The cellular response to genotoxic DNA double strand breaks (DSBs) uses a multitude of post-translational modifications to localise, modulate and ultimately clear DNA repair factors in a timely and accurate manner. Ubiquitination is well established as vital to the DSB response, with a carefully co-ordinated pathway of histone ubiquitination events being a central component of DSB signalling. Other ubiquitin-like modifiers (Ubl) including SUMO and NEDD8 have since been identified as playing important roles in DSB repair. In the last five years ∼20 additional Ub/Ubl proteases have been implicated in the DSB response. The number of proteases identified highlights the complexity of the Ub/Ubl signal present at DSBs. Ub/Ubl proteases regulate turnover, activity and protein-protein interactions of DSB repair factors both catalytically and non-catalytically. This not only ensures efficient repair of breaks but has a role in channelling repair into the correct DSB repair sub-pathways. Ultimately Ub/Ubl proteases have essential roles in maintaining genomic stability. Given that deficiencies in many Ub/Ubl proteases promotes sensitivity to DNA damaging chemotherapies, they could be attractive targets for cancer treatment.

Pharmacological methods to transcriptionally modulate double-strand break DNA repair

There is much interest in targeting DNA repair pathways for use in cancer therapy, as the effectiveness of many therapeutic agents relies on their ability to cause damage to DNA, and deficiencies in DSB repair pathways can make cells more sensitive to specific cancer therapies. For example, defects in the double-strand break (DSB) pathways, non-homologous end joining (NHEJ) and homology-directed repair (HDR), induce sensitivity to radiation therapy and poly(ADP)-ribose polymerase (PARP) inhibitors, respectively. However, traditional approaches to inhibit DNA repair through small molecule inhibitors have often been limited by toxicity and poor bioavailability. This review identifies several pharmacologic manipulations that modulate DSB repair by reducing expression of DNA repair factors. A number of pathways have been identified that modulate activity of NHEJ and HDR through this mechanism, including growth and hormonal receptor signaling pathways as well as epigenetic modifiers. We also discuss the effects of anti-angiogenic therapy on DSB repair. Preclinically, these pharmacological manipulations of DNA repair factor expression have been shown to increase sensitivity to specific cancer therapies, including ionizing radiation and PARP inhibitors. When applicable, relevant clinical trials are discussed and areas for future study are identified.

Beyond classic editing: innovative CRISPR approaches for functional studies of long non-coding RNA

Long non-coding RNAs (lncRNAs) makeup a considerable part of the non-coding human genome and had been well-established as crucial players in an array of biological processes. In spite of their abundance and versatile roles, their functional characteristics remain largely undiscovered mainly due to the lack of suitable genetic manipulation tools. The emerging CRISPR/Cas9 technology has been widely adapted in several studies that aim to screen and identify novel lncRNAs as well as interrogate the functional properties of specific lncRNAs. However, the complexity of lncRNAs genes and the regulatory mechanisms that govern their transcription, as well as their unique functionality pose several limitations the utilization of classic CRISPR methods in lncRNAs functional studies. Here, we overview the unique characteristics of lncRNAs transcription and function and the suitability of the CRISPR toolbox for applications in functional characterization of lncRNAs. We discuss some of the novel variations to the classic CRISPR/Cas9 system that have been tailored and applied previously to study several aspects of lncRNAs functionality. Finally, we share perspectives on the potential applications of various CRISPR systems, including RNA-targeting, in the direct editing and manipulation of lncRNAs.

Harnessing DNA Double-Strand Break Repair for Cancer Treatment

DNA double-strand breaks (DSBs) are highly deleterious, with a single unrepaired DSB being sufficient to trigger cell death. Compared to healthy cells, cancer cells have a higher DSB burden due to oncogene-induced replication stress and acquired defects in DNA damage response (DDR) mechanisms. Consequently, hyperproliferating cancer cells rely on efficient DSB repair for their survival. Moreover, augmented DSB repair capacity is a major cause of radio- and chemoresistance and, ultimately, cancer recurrence. Although inherited DDR defects can predispose individuals to develop certain cancers, the very same vulnerability may be therapeutically exploited to preferentially kill tumor cells. A paradigm for DNA repair targeted therapy has emerged in cancers that exhibit mutations in BRCA1 or BRCA2 tumor suppressor genes, conferring a strong defect in homologous recombination, a major and error-free DSB repair pathway. Clinical validation of such approaches, commonly described as synthetic lethality (SL), has been provided by the regulatory approval of poly(ADP-ribose) polymerase 1 inhibitors (PARPi) as monotherapy for BRCA1/2-mutated breast and ovarian tumors. In this review, we will describe the different DSB repair mechanisms and discuss how their specific features could be exploited for cancer therapy. A major emphasis is put on advances in combinatorial treatment modalities and SL approaches arising from DSB repair pathway interdependencies.

USP11 acts as a histone deubiquitinase functioning in chromatin reorganization during DNA repair

How chromatin dynamics is regulated to ensure efficient DNA repair remains to be understood. Here, we report that the ubiquitin-specific protease USP11 acts as a histone deubiquitinase to catalyze H2AK119 and H2BK120 deubiquitination. We showed that USP11 is physically associated with the chromatin remodeling NuRD complex and functionally involved in DNA repair process. We demonstrated that USP11-mediated histone deubiquitination and NuRD-associated histone deacetylation coordinate to allow timely termination of DNA repair and reorganization of the chromatin structure. As such, USP11 is involved in chromatin condensation, genomic stability, and cell survival. Together, these observations indicate that USP11 is a chromatin modifier critically involved in DNA damage response and the maintenance of genomic stability.

Advances in genome editing through control of DNA repair pathways

Eukaryotic cells deploy overlapping repair pathways to resolve DNA damage. Advancements in genome editing take advantage of these pathways to produce permanent genetic changes. Despite recent improvements, genome editing can produce diverse outcomes that can introduce risks in clinical applications. Although homology-directed repair is attractive for its ability to encode precise edits, it is particularly difficult in human cells. Here we discuss the DNA repair pathways that underlie genome editing and strategies to favour various outcomes.

HMEJ-mediated efficient site-specific gene integration in chicken cells

BACKGROUND

The production of transgenic chicken cells holds great promise for several diverse areas, including developmental biology and biomedical research. To this end, site-specific gene integration has been an attractive strategy for generating transgenic chicken cell lines and has been successfully adopted for inserting desired genes and regulating specific gene expression patterns. However, optimization of this method is essential for improving the efficiency of genome modification in this species.

RESULTS

Here we compare gene knock-in methods based on homology-independent targeted integration (HITI), homology-directed repair (HDR) and homology mediated end joining (HMEJ) coupled with a clustered regularly interspaced short palindromic repeat associated protein 9 (CRISPR/Cas9) gene editing system in chicken DF-1 cells and primordial germ cells (PGCs). HMEJ was found to be a robust and efficient method for gene knock-in in chicken PGCs. Using this method, we successfully labeled the germ cell specific gene DAZL and the pluripotency-related gene Pou5f3 in chicken PGCs through the insertion of a fluorescent protein in the frame at the 3' end of the gene, allowing us to track cell migration in the embryonic gonad. HMEJ strategy was also successfully used in Ovalbumin, which accounts for more than 60% of proteins in chicken eggs, suggested its good promise for the mass production of protein with pharmaceutical importance using the chicken oviduct system.

CONCLUSIONS

Taken together, these results demonstrate that HMEJ efficiently mediates site-specific gene integration in chicken PGCs, which holds great potential for the biopharmaceutical engineering of chicken cells.

Nestin regulates cellular redox homeostasis in lung cancer through the Keap1-Nrf2 feedback loop

Abnormal cancer antioxidant capacity is considered as a potential mechanism of tumor malignancy. Modulation of oxidative stress status is emerging as an anti-cancer treatment. Our previous studies have found that Nestin-knockdown cells were more sensitive to oxidative stress in non-small cell lung cancer (NSCLC). However, the molecular mechanism by which Nestin protects cells from oxidative damage remains unclear. Here, we identify a feedback loop between Nestin and Nrf2 maintaining the redox homeostasis. Mechanistically, the ESGE motif of Nestin interacts with the Kelch domain of Keap1 and competes with Nrf2 for Keap1 binding, leading to Nrf2 escaping from Keap1-mediated degradation, subsequently promoting antioxidant enzyme generation. Interestingly, we also map that the antioxidant response elements (AREs) in the Nestin promoter are responsible for its induction via Nrf2. Taken together, our results indicate that the Nestin-Keap1-Nrf2 axis regulates cellular redox homeostasis and confers oxidative stress resistance in NSCLC.

USP22 Interacts with PALB2 and Promotes Chemotherapy Resistance via Homologous Recombination of DNA Double-Strand Breaks

Homologous recombination (HR) is a highly conserved pathway that can facilitate the repair of DNA double-strand breaks (DSB). Several Deubiquitinases (DUB) have been implicated as key players in DNA damage repair (DDR) through HR. Here, we report USP22, a DUB that is highly overexpressed in multiple cancer types, is necessary for HR through a direct interaction with PALB2 through its C-terminal WD40 domain. This interaction stimulates USP22 catalytic activity in vitro. Furthermore, we show USP22 is necessary for BRCA2, PALB2, and Rad51 recruitment to DSBs and this is, in part, through USP22 stabilizing BRCA2 and PALB2 levels. Taken together, our results describe a role for USP22 in DNA repair. IMPLICATIONS: This research provides new and exciting mechanistic insights into how USP22 overexpression promotes chemoresistance in lung cancer. We believe this study, and others, will help aid in developing targeted drugs toward USP22 and known binding partners for lung cancer treatment.

PML isoform expression and DNA break location relative to PML nuclear bodies impacts the efficiency of homologous recombination

Promyelocytic leukemia nuclear bodies (PML NBs) are nuclear subdomains that respond to genotoxic stress by increasing in number via changes in chromatin structure. However, the role of the PML protein and PML NBs in specific mechanisms of DNA repair has not been fully characterized. Here, we have directly examined the role of PML in homologous recombination (HR) using I-SceI extrachromosomal and chromosome-based homology-directed repair (HDR) assays, and in HDR by CRISPR/Cas9-mediated gene editing. We determined that PML loss can inhibit HR in an extrachromosomal HDR assay but had less of an effect on CRISPR/Cas9-mediated chromosomal HDR. Overexpression of PML also inhibited both CRISPR HDR and I-SceI-induced HDR using a chromosomal reporter, and in an isoform-specific manner. However, the impact of PML overexpression on the chromosomal HDR reporter was dependent on the intranuclear chromosomal positioning of the reporter. Specifically, HDR at the TAP1 gene locus, which is associated with PML NBs, was reduced compared with a locus not associated with a PML NB; yet, HDR could be reduced at the non-PML NB-associated locus by PML overexpression. Thus, both loss and overexpression of PML isoforms can inhibit HDR, and proximity of a chromosomal break to a PML NB can impact HDR efficiency.

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.

Cullin Ring Ubiquitin Ligases (CRLs) in Cancer: Responses to Ionizing Radiation (IR) Treatment

Treatment with ionizing radiation (IR) remains the cornerstone of therapy for multiple cancer types, including disseminated and aggressive diseases in the palliative setting. Radiotherapy efficacy could be improved in combination with drugs that regulate the ubiquitin-proteasome system (UPS), many of which are currently being tested in clinical trials. The UPS operates through the covalent attachment of ATP-activated ubiquitin molecules onto substrates following the transfer of ubiquitin from an E1, to an E2, and then to the substrate via an E3 enzyme. The specificity of ubiquitin ligation is dictated by E3 ligases, which select substrates to be ubiquitylated. Among the E3s, cullin ring ubiquitin ligases (CRLs) represent prototypical multi-subunit E3s, which use the cullin subunit as a central assembling scaffold. CRLs have crucial roles in controlling the cell cycle, hypoxia signaling, reactive oxygen species clearance and DNA repair; pivotal factors regulating the cancer and normal tissue response to IR. Here, we summarize the findings on the involvement of CRLs in the response of cancer cells to IR, and we discuss the therapeutic approaches to target the CRLs which could be exploited in the clinic.

Precise in vivo genome editing via single homology arm donor mediated intron-targeting gene integration for genetic disease correction

In vivo genome editing represents a powerful strategy for both understanding basic biology and treating inherited diseases. However, it remains a challenge to develop universal and efficient in vivo genome-editing tools for tissues that comprise diverse cell types in either a dividing or non-dividing state. Here, we describe a versatile in vivo gene knock-in methodology that enables the targeting of a broad range of mutations and cell types through the insertion of a minigene at an intron of the target gene locus using an intracellularly linearized single homology arm donor. As a proof-of-concept, we focused on a mouse model of premature-aging caused by a dominant point mutation, which is difficult to repair using existing in vivo genome-editing tools. Systemic treatment using our new method ameliorated aging-associated phenotypes and extended animal lifespan, thus highlighting the potential of this methodology for a broad range of in vivo genome-editing applications.

RBX1 prompts degradation of EXO1 to limit the homologous recombination pathway of DNA double-strand break repair in G1 phase

End resection of DNA double-strand breaks (DSBs) to form 3′ single-strand DNA (ssDNA) is critical to initiate the homologous recombination (HR) pathway of DSB repair. HR pathway is strictly limited in the G1-phase cells because of lack of homologous DNA as the templates. Exonuclease 1 (EXO1) is the key molecule responsible for 3′ ssDNA formation of DSB end resection. We revealed that EXO1 is inactivated in G1-phase cells via ubiquitination-mediated degradation, resulting from an elevated expression level of RING-box protein 1 (RBX1) in G1 phase. The increased RBX1 significantly prompted the neddylation of Cullin1 and contributed to the G1 phase-specific degradation of EXO1. Knockdown of RBX1 remarkedly attenuated the degradation of EXO1 and increased the end resection and HR activity in γ-irradiated G1-phase cells, as demonstrated by the increased formation of RPA32, BrdU, and RAD51 foci. And EXO1 depletion mitigated DNA repair defects due to RBX1 reduction. Moreover, increased autophosphorylation of DNA-PKcs at S2056 was found to be responsible for the higher expression level of the RBX1 in the G1 phase. Inactivation of DNA-PKcs decreased RBX1 expression, and simultaneously increased EXO1 expression and DSB end resection in G1-phase cells. This study demonstrates a new mechanism for restraining the HR pathway of DNA DSB repair in G1 phase via RBX1-prompted inactivation of EXO1.

Moving Mountains-The BRCA1 Promotion of DNA Resection

DNA double-strand breaks (DSBs) occur in our cells in the context of chromatin. This type of lesion is toxic, entirely preventing genome continuity and causing cell death or terminal arrest. Several repair mechanisms can act on DNA surrounding a DSB, only some of which carry a low risk of mutation, so that which repair process is utilized is critical to the stability of genetic material of cells. A key component of repair outcome is the degree of DNA resection directed to either side of the break site. This in turn determines the subsequent forms of repair in which DNA homology plays a part. Here we will focus on chromatin and chromatin-bound complexes which constitute the "mountains" that block resection, with a particular focus on how the breast and ovarian cancer predisposition protein-1 (BRCA1) contributes to repair outcomes through overcoming these blocks.

CENPO expression regulates gastric cancer cell proliferation and is associated with poor patient prognosis

Gastric cancer (GC) is one of the most common malignancies worldwide; however, understanding of its development and carcinogenesis is currently limited. Centromere protein O (CENPO), is a newly discovered constitutive centromeric protein, associated with cell death. The expression of CENPO in human cancers, including GC, is currently unknown. The aim of the present study was to investigate the clinical association between CENPO and GC, and to elucidate the potential mechanisms of CENPO in the process of GC progression. The results demonstrated that CENPO was expressed at high levels in GC and was correlated with p-TNM stage. In addition, CENPO was observed to be a marker of poor prognosis in patients with GC. Knockdown of CENPO contributed to GC cell growth inhibition and apoptosis induction. In addition, downregulation of CENPO expression suppressed GC cell growth in vivo. Furthermore, CENPO knockdown decreased ataxia telangiectasia mutated (ATM), cyclin D1 and c-Jun expression, indicating that the ATM signaling pathway may be involved in CENPO-mediated regulation of GC cell growth. In conclusion, increased CENPO expression may be associated with the aggressive tumor biology of GC and CENPO may present a novel therapeutic target and prognostic biomarker for patients with GC.

PARP Inhibitor Resistance: A Tug-of-War in BRCA-Mutated Cells

Poly-(ADP)-ribose polymerase (PARP) inhibition is synthetic lethal with deficiency for homologous recombination (HR), a pathway essential for DNA double-strand break repair. PARP inhibitors (PARPi) therefore hold great promise for the treatment of tumors with disruptive mutations in BRCA1/2 or other HR factors. Unfortunately, PARPi resistance has proved to be a major problem in the clinic. Knowledge about PARPi resistance is expanding quickly, revealing four main mechanisms that alter drug availability, affect (de)PARylation enzymes, restore HR, or restore replication fork stability. We discuss how studies on resistance mechanisms have yielded important insights into the regulation of DNA double-strand break (DSB) repair and replication fork protection, and how these studies could pave the way for novel treatment options to target resistance mechanisms or acquired vulnerabilities.

Regulation of Wound Healing by the NRF2 Transcription Factor-More Than Cytoprotection

The nuclear factor-erythroid 2-related factor 2 (NRF2) transcription factor plays a central role in mediating the cellular stress response. Due to their antioxidant properties, compounds activating NRF2 have received much attention as potential medications for disease prevention, or even for therapy. Accumulating evidence suggests that activation of the NRF2 pathway also has a major impact on wound healing and may be beneficial in the treatment of chronic wounds, which remain a considerable health and economic burden. While NRF2 activation indeed shows promise, important considerations need to be made in light of corresponding evidence that also points towards pro-tumorigenic effects of NRF2. In this review, we discuss the evidence to date, highlighting recent advances using gain- and loss-of-function animal models and how these data fit with observations in humans.

Pold3 is required for genomic stability and telomere integrity in embryonic stem cells and meiosis

Embryonic stem cells (ESCs) and meiosis are featured by relatively higher frequent homologous recombination associated with DNA double strand breaks (DSB) repair. Here, we show that Pold3 plays important roles in DSB repair, telomere maintenance and genomic stability of both ESCs and spermatocytes in mice. By attempting to generate Pold3 deficient mice using CRISPR/Cas9 or transcription activator-like effector nucleases, we show that complete loss of Pold3 (Pold3^−/−) resulted in early embryonic lethality at E6.5. Rapid DNA damage response and massive apoptosis occurred in both outgrowths of Pold3-null (Pold3^−/−) blastocysts and Pold3 inducible knockout (iKO) ESCs. While Pold3^−/− ESCs were not achievable, Pold3 iKO led to increased DNA damage response, telomere loss and chromosome breaks accompanied by extended S phase. Meanwhile, loss of Pold3 resulted in replicative stress, micronucleation and aneuploidy. Also, DNA repair was impaired in Pold3^+/− or Pold3 knockdown ESCs. Moreover, Pold3 mediates DNA replication and repair by regulating 53BP1, RIF1, ATR and ATM pathways. Furthermore, spermatocytes of Pold3 haploinsufficient (Pold3^+/−) mice with increasing age displayed impaired DSB repair, telomere shortening and loss, and chromosome breaks, like Pold3 iKO ESCs. These data suggest that Pold3 maintains telomere integrity and genomic stability of both ESCs and meiosis by suppressing replicative stress.

The Problem of the Low Rates of CRISPR/Cas9-Mediated Knock-ins in Plants: Approaches and Solutions

The main number of genome editing events in plant objects obtained during the last decade with the help of specific nucleases zinc finger (ZFN), transcription activator-like effector nucleases (TALEN), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas are the microindels causing frameshift and subsequent gene knock-out. The knock-ins of genes or their parts, i.e., the insertion of them into a target genome region, are between one and two orders of magnitude less frequent. First and foremost, this is associated with the specific features of the repair systems of higher eukaryotes and the availability of the donor template in accessible proximity during double-strand break (DSB) repair. This review briefs the main repair pathways in plants according to the aspect of their involvement in genome editing. The main methods for increasing the frequency of knock-ins are summarized both along the homologous recombination pathway and non-homologous end joining, which can be used for plant objects.

RNA-guided piggyBac transposition in human cells

Safer and more efficient methods for directing therapeutic genes to specific sequences could increase the repertoire of treatable conditions. Many current approaches act passively, first initiating a double-stranded break, then relying on host repair to uptake donor DNA. Alternatively, we delivered an actively integrating transposase to the target sequence to initiate gene insertion. We fused the hyperactive piggyBac transposase to the highly specific, catalytically dead SpCas9-HF1 (dCas9) and designed guide RNAs (gRNAs) to the CCR5 safe harbor sequence. We introduced mutations to the native DNA-binding domain of piggyBac to reduce non-specific binding of the transposase and cause the fusion protein to favor binding by dCas9. This strategy enabled us, for the first time, to direct transposition to the genome using RNA. We showed that increasing the number of gRNAs improved targeting efficiency. Interestingly, over half of the recovered insertions were found at a single TTAA hotspot. We also found that the fusion increased the error rate at the genome-transposon junction. We isolated clonal cell lines containing a single insertion at CCR5 and demonstrated long-term expression from this locus. These vectors expand the utility of the piggyBac system for applications in targeted gene addition for biomedical research and gene therapy.

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

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

Inadequate DNA Damage Repair Promotes Mammary Transdifferentiation, Leading to BRCA1 Breast Cancer

Loss of BRCA1 p220 function often results in basal-like breast cancer (BLBC), but the underlying disease mechanism is largely opaque. In mammary epithelial cells (MECs), BRCA1 interacts with multiple proteins, including NUMB and HES1, to form complexes that participate in interstrand crosslink (ICL) DNA repair and MEC differentiation control. Unrepaired ICL damage results in aberrant transdifferentiation to a mesenchymal state of cultured, human basal-like MECs and to a basal/mesenchymal state in primary mouse luminal MECs. Loss of BRCA1, NUMB, or HES1 or chemically induced ICL damage in primary murine luminal MECs results in persistent DNA damage that triggers luminal to basal/mesenchymal transdifferentiation. In vivo single-cell analysis revealed a time-dependent evolution from normal luminal MECs to luminal progenitor-like tumor cells with basal/mesenchymal transdifferentiation during murine BRCA1 BLBC development. Growing DNA damage accompanied this malignant transformation.

Nrf2 Activation Promotes Lung Cancer Metastasis by Inhibiting the Degradation of Bach1

Approximately 30% of human lung cancers acquire mutations in either Keap1 or Nfe2l2, resulting in the stabilization of Nrf2, the Nfe2l2 gene product, which controls oxidative homeostasis. Here, we show that heme triggers the degradation of Bach1, a pro-metastatic transcription factor, by promoting its interaction with the ubiquitin ligase Fbxo22. Nrf2 accumulation in lung cancers causes the stabilization of Bach1 by inducing Ho1, the enzyme catabolizing heme. In mouse models of lung cancers, loss of Keap1 or Fbxo22 induces metastasis in a Bach1-dependent manner. Pharmacological inhibition of Ho1 suppresses metastasis in a Fbxo22-dependent manner. Human metastatic lung cancer display high levels of Ho1 and Bach1. Bach1 transcriptional signature is associated with poor survival and metastasis in lung cancer patients. We propose that Nrf2 activates a metastatic program by inhibiting the heme- and Fbxo22-mediated degradation of Bach1, and that Ho1 inhibitors represent an effective therapeutic strategy to prevent lung cancer metastasis.

Genome editing for blood disorders: state of the art and recent advances

In recent years, tremendous advances have been made in the use of gene editing to precisely engineer the genome. This technology relies on the activity of a wide range of nuclease platforms - such as zinc-finger nucleases, transcription activator-like effector nucleases, and the CRISPR-Cas system - that can cleave and repair specific DNA regions, providing a unique and flexible tool to study gene function and correct disease-causing mutations. Preclinical studies using gene editing to tackle genetic and infectious diseases have highlighted the therapeutic potential of this technology. This review summarizes the progresses made towards the development of gene editing tools for the treatment of haematological disorders and the hurdles that need to be overcome to achieve clinical success.

Evaluation of Pharmacodynamic Responses to Cancer Therapeutic Agents Using DNA Damage Markers

PURPOSE

We sought to examine the pharmacodynamic activation of the DNA damage response (DDR) pathway in tumors following anticancer treatment for confirmation of target engagement.

EXPERIMENTAL DESIGN

We evaluated the time course and spatial activation of 3 protein biomarkers of DNA damage recognition and repair (γH2AX, pS343-Nbs1, and Rad51) simultaneously in a quantitative multiplex immunofluorescence assay (IFA) to assess DDR pathway activation in tumor tissues following exposure to DNA-damaging agents.

RESULTS

Because of inherent biological variability, baseline DDR biomarker levels were evaluated in a colorectal cancer microarray to establish clinically relevant thresholds for pharmacodynamic activation. Xenograft-bearing mice and clinical colorectal tumor biopsies obtained from subjects exposed to DNA-damaging therapeutic regimens demonstrated marked intratumor heterogeneity in the timing and extent of DDR biomarker activation due, in part, to the cell-cycle dependency of DNA damage biomarker expression.

CONCLUSIONS

We have demonstrated the clinical utility of this DDR multiplex IFA in preclinical models and clinical specimens following exposure to multiple classes of cytotoxic agents, DNA repair protein inhibitors, and molecularly targeted agents, in both homologous recombination-proficient and -deficient contexts. Levels exceeding 4% nuclear area positive (NAP) γH2AX, 4% NAP pS343-Nbs1, and 5% cells with ≥5 Rad51 nuclear foci indicate a DDR activation response to treatment in human colorectal cancer tissue. Determination of effect-level cutoffs allows for robust interpretation of biomarkers with significant interpatient and intratumor heterogeneity; simultaneous assessment of biomarkers induced at different phases of the DDR guards against the risk of false negatives due to an ill-timed biopsy.

Evolution of Resistance in Cancer: A Cell Cycle Perspective

Resistance of neoplastic cells to therapy is considered a key challenge in the treatment of cancer. Emergence of resistance is commonly attributed to the gradual mutational evolution of neoplastic cells. However, accumulating evidence suggests that exogenous stressors could significantly accelerate the emergence of resistant clones during the course of treatment. Herein, we review molecular mechanisms that regulate the evolution of resistance in a tumor with particular emphasis on the role of cell cycle.

CRISPR Cpf1 proteins: structure, function and implications for genome editing

CRISPR and CRISPR-associated (Cas) protein, as components of microbial adaptive immune system, allows biologists to edit genomic DNA in a precise and specific way. CRISPR-Cas systems are classified into two main classes and six types. Cpf1 is a putative type V (class II) CRISPR effector, which can be programmed with a CRISPR RNA to bind and cleave complementary DNA targets. Cpf1 has recently emerged as an alternative for Cas9, due to its distinct features such as the ability to target T-rich motifs, no need for trans-activating crRNA, inducing a staggered double-strand break and potential for both RNA processing and DNA nuclease activity. In this review, we attempt to discuss the evolutionary origins, basic architectures, and molecular mechanisms of Cpf1 family proteins, as well as crRNA designing and delivery strategies. We will also describe the novel Cpf1 variants, which have broadened the versatility and feasibility of this system in genome editing, transcription regulation, epigenetic modulation, and base editing. Finally, we will be reviewing the recent studies on utilization of Cpf1as a molecular tool for genome editing.

MDC1 Interacts with TOPBP1 to Maintain Chromosomal Stability during Mitosis

In mitosis, cells inactivate DNA double-strand break (DSB) repair pathways to preserve genome stability. However, some early signaling events still occur, such as recruitment of the scaffold protein MDC1 to phosphorylated histone H2AX at DSBs. Yet, it remains unclear whether these events are important for maintaining genome stability during mitosis. Here, we identify a highly conserved protein-interaction surface in MDC1 that is phosphorylated by CK2 and recognized by the DNA-damage response mediator protein TOPBP1. Disruption of MDC1-TOPBP1 binding causes a specific loss of TOPBP1 recruitment to DSBs in mitotic but not interphase cells, accompanied by mitotic radiosensitivity, increased micronuclei, and chromosomal instability. Mechanistically, we find that TOPBP1 forms filamentous structures capable of bridging MDC1 foci in mitosis, indicating that MDC1-TOPBP1 complexes tether DSBs until repair is reactivated in the following G1 phase. Thus, we reveal an important, hitherto-unnoticed cooperation between MDC1 and TOPBP1 in maintaining genome stability during cell division.

Enhancement of Precise Gene Editing by the Association of Cas9 With Homologous Recombination Factors

The CRISPR-Cas9 system is used for genome editing in mammalian cells by introducing double-strand breaks (DSBs) which are predominantly repaired via non-homologous end joining (NHEJ) or to lesser extent by homology-directed repair (HDR). To enhance HDR for improving the introduction of precise genetic modifications, we tested fusion proteins of Cas9 nuclease with HDR effectors to enforce their localization at DSBs. Using a traffic-light DSB repair reporter (TLR) system for the quantitative detection of HDR and NHEJ events in human HEK cells we found that Cas9 fusions with CtIP, Rad52, and Mre11, but not Rad51C promote HDR up to twofold in human cells and significantly reduce NHEJ events. We further compared, as an alternative to the direct fusion with Cas9, two components configurations that associate CtIP fusion proteins with a Cas9-SunTag fusion or with guide RNA that includes MS2 binding loops. We found that the Cas9-CtIP fusion and the MS2-CtIP system, but not the SunTag approach increase the ratio of HDR/NHEJ 4.5-6-fold. Optimal results are obtained by the combined use of Cas9-CtIP and MS2-CtIP, shifting the HDR/NHEJ ratio by a factor of 14.9. Thus, our findings provide a simple and effective tool to promote precise gene modifications in mammalian cells.

Novel RNA and DNA strand exchange activity of the PALB2 DNA binding domain and its critical role for DNA repair in cells

BReast Cancer Associated proteins 1 and 2 (BRCA1, -2) and Partner and Localizer of BRCA2 (PALB2) protein are tumour suppressors linked to a spectrum of malignancies, including breast cancer and Fanconi anemia. PALB2 coordinates functions of BRCA1 and BRCA2 during homology-directed repair (HDR) and interacts with several chromatin proteins. In addition to protein scaffold function, PALB2 binds DNA. The functional role of this interaction is poorly understood. We identified a major DNA-binding site of PALB2, mutations in which reduce RAD51 foci formation and the overall HDR efficiency in cells by 50%. PALB2 N-terminal DNA-binding domain (N-DBD) stimulates the function of RAD51 recombinase. Surprisingly, it possesses the strand exchange activity without RAD51. Moreover, N-DBD stimulates the inverse strand exchange and can use DNA and RNA substrates. Our data reveal a versatile DNA interaction property of PALB2 and demonstrate a critical role of PALB2 DNA binding for chromosome repair in cells.

Recent advances of therapeutic targets based on the molecular signature in breast cancer: genetic mutations and implications for current treatment paradigms

Breast cancer is the most common malignancy in women all over the world. Genetic background of women contributes to her risk of having breast cancer. Certain inherited DNA mutations can dramatically increase the risk of developing certain cancers and are responsible for many of the cancers that run in some families. Regarding the widespread multigene panels, whole exome sequencing is capable of providing the evaluation of genetic function mutations for development novel strategy in clinical trials. Targeting the mutant proteins involved in breast cancer can be an effective therapeutic approach for developing novel drugs. This systematic review discusses gene mutations linked to breast cancer, focusing on signaling pathways that are being targeted with investigational therapeutic strategies, where clinical trials could be potentially initiated in the future are being highlighted.

Disruptive Technology: CRISPR/Cas-Based Tools and Approaches

Designer nucleases are versatile tools for genome modification and therapy development and have gained widespread accessibility with the advent of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) technology. Prokaryotic RNA-guided nucleases of CRISPR/Cas type, since first being adopted as editing tools in eukaryotic cells, have experienced rapid uptake and development. Diverse modes of delivery by viral and non-viral vectors and ongoing discovery and engineering of new CRISPR/Cas-type tools with alternative target site requirements, cleavage patterns and DNA- or RNA-specific action continue to expand the versatility of this family of nucleases. CRISPR/Cas-based molecules may also act without double-strand breaks as DNA base editors or even without single-stranded cleavage, be it as epigenetic regulators, transcription factors or RNA base editors, with further scope for discovery and development. For many potential therapeutic applications of CRISPR/Cas-type molecules and their derivatives, efficiencies still need to be improved and safety issues addressed, including those of preexisting immunity against Cas molecules, off-target activity and recombination and sequence alterations relating to double-strand-break events. This review gives a concise overview of current CRISPR/Cas tools, applications, concerns and trends.

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.

Recruitment of BRCA1 limits MYCN-driven accumulation of stalled RNA polymerase

MYC is an oncogenic transcription factor that binds globally to active promoters and promotes transcriptional elongation by RNA polymerase II (RNAPII)^1,2. Deregulated expression of the paralogous protein MYCN drives the development of neuronal and neuroendocrine tumours and is often associated with a particularly poor prognosis³. Here we show that, similar to MYC, activation of MYCN in human neuroblastoma cells induces escape of RNAPII from promoters. If the release of RNAPII from transcriptional pause sites (pause release) fails, MYCN recruits BRCA1 to promoter-proximal regions. Recruitment of BRCA1 prevents MYCN-dependent accumulation of stalled RNAPII and enhances transcriptional activation by MYCN. Mechanistically, BRCA1 stabilizes mRNA decapping complexes and enables MYCN to suppress R-loop formation in promoter-proximal regions. Recruitment of BRCA1 requires the ubiquitin-specific protease USP11, which binds specifically to MYCN when MYCN is dephosphorylated at Thr58. USP11, BRCA1 and MYCN stabilize each other on chromatin, preventing proteasomal turnover of MYCN. Because BRCA1 is highly expressed in neuronal progenitor cells during early development⁴ and MYC is less efficient than MYCN in recruiting BRCA1, our findings indicate that a cell-lineage-specific stress response enables MYCN-driven tumours to cope with deregulated RNAPII function.

Recruitment of BRCA1 limits MYCN-driven accumulation of stalled RNA polymerase

MYC is an oncogenic transcription factor that binds globally to active promoters and promotes transcriptional elongation by RNA polymerase II (RNAPII)^1,2. Deregulated expression of the paralogous protein MYCN drives the development of neuronal and neuroendocrine tumours and is often associated with a particularly poor prognosis³. Here we show that, similar to MYC, activation of MYCN in human neuroblastoma cells induces escape of RNAPII from promoters. If the release of RNAPII from transcriptional pause sites (pause release) fails, MYCN recruits BRCA1 to promoter-proximal regions. Recruitment of BRCA1 prevents MYCN-dependent accumulation of stalled RNAPII and enhances transcriptional activation by MYCN. Mechanistically, BRCA1 stabilizes mRNA decapping complexes and enables MYCN to suppress R-loop formation in promoter-proximal regions. Recruitment of BRCA1 requires the ubiquitin-specific protease USP11, which binds specifically to MYCN when MYCN is dephosphorylated at Thr58. USP11, BRCA1 and MYCN stabilize each other on chromatin, preventing proteasomal turnover of MYCN. Because BRCA1 is highly expressed in neuronal progenitor cells during early development⁴ and MYC is less efficient than MYCN in recruiting BRCA1, our findings indicate that a cell-lineage-specific stress response enables MYCN-driven tumours to cope with deregulated RNAPII function.

The deubiquitylating enzyme USP15 regulates homologous recombination repair and cancer cell response to PARP inhibitors

Poly-(ADP-ribose) polymerase inhibitors (PARPi) selectively kill breast and ovarian cancers with defects in homologous recombination (HR) caused by BRCA1/2 mutations. There is also clinical evidence for the utility of PARPi in breast and ovarian cancers without BRCA mutations, but the underlying mechanism is not clear. Here, we report that the deubiquitylating enzyme USP15 affects cancer cell response to PARPi by regulating HR. Mechanistically, USP15 is recruited to DNA double-strand breaks (DSBs) by MDC1, which requires the FHA domain of MDC1 and phosphorylated Ser678 of USP15. Subsequently, USP15 deubiquitinates BARD1 BRCT domain, and promotes BARD1-HP1γ interaction, resulting in BRCA1/BARD1 retention at DSBs. USP15 knockout mice exhibit genomic instability in vivo. Furthermore, cancer-associated USP15 mutations, with decreased USP15-BARD1 interaction, increases PARP inhibitor sensitivity in cancer cells. Thus, our results identify a novel regulator of HR, which is a potential biomarker for therapeutic treatment using PARP inhibitors in cancers.

Deubiquitinases have been shown to be involved in double strand break repair pathways. Here the authors reveal that USP15 deybiquitinase plays a role in homologues recombination repair by targeting BARD1 and affecting cells response to PARP inhibitors.

H4K20me0 recognition by BRCA1-BARD1 directs homologous recombination to sister chromatids

Genotoxic DNA double-strand breaks (DSBs) can be repaired by error-free homologous recombination (HR) or mutagenic non-homologous end-joining1. HR supresses tumorigenesis1, but is restricted to the S and G2 phases of the cell cycle when a sister chromatid is present2. Breast cancer type 1 susceptibility protein (BRCA1) promotes HR by antagonizing the anti-resection factor TP53-binding protein 1(53BP1) (refs. 2-5), but it remains unknown how BRCA1 function is limited to the S and G2 phases. We show that BRCA1 recruitment requires recognition of histone H4 unmethylated at lysine 20 (H4K20me0), linking DSB repair pathway choice directly to sister chromatid availability. We identify the ankyrin repeat domain of BRCA1-associated RING domain protein 1 (BARD1)-the obligate BRCA1 binding partner3-as a reader of H4K20me0 present on new histones in post-replicative chromatin6. BARD1 ankyrin repeat domain mutations disabling H4K20me0 recognition abrogate accumulation of BRCA1 at DSBs, causing aberrant build-up of 53BP1, and allowing anti-resection activity to prevail in S and G2. Consequently, BARD1 recognition of H4K20me0 is required for HR and resistance to poly (ADP-ribose) polymerase inhibitors. Collectively, this reveals that BRCA1-BARD1 monitors the replicative state of the genome to oppose 53BP1 function, routing only DSBs within sister chromatids to HR.