DNA DNA Double-strand Breaks Repair Research Articles

RAD50 Deficiency and Its Effects on Zebrafish Embryonic Development and DNA Repair Mechanisms

The MRE11-RAD50-NBS1 (MRN) complex is essential in detecting, signaling, and repairing DNA double-strand breaks (DSBs), thus maintaining genomic integrity. Mutations in RAD50 are linked to severe conditions such as microcephaly, mental retardation, and growth retardation in humans. This study investigates the developmental impact of RAD50 protein disruption in zebrafish embryos. Zebrafish embryos were treated with MIRIN (35 µM) to inhibit RAD50 and subsequently exposed to gamma-ray irradiation (15 Gy) to analyze the role of RAD50 in managing DNA damage during embryogenesis. Time-point analysis indicated that inhibiting RAD50 and ATM proteins during early embryonic stages (at 1 hpf) leads to increased embryonic mortality and abnormalities. These adverse effects were exacerbated by irradiation, underscoring the critical role of RAD50 in DNA DSB repair. The study concludes that RAD50 deficiencies can lead to embryonic lethality and human deformities due to the inability of tissues to repair DNA DSBs effectively.

Impaired DNA Double-Strand Break Repair in Irradiated Sheep Lung Fibroblasts: Late Effects of Previous Irradiation of the Spinal Thecal Sac.

Children with cancer previously treated with radiotherapy face the likelihood of side effects that can be debilitating or fatal. This study aimed to assess the long-term effect of medulloblastoma radiotherapy on the DNA double-strand break (DSB) repair capability of primary fibroblasts derived from lung biopsies of previously irradiated young sheep. This study included biopsies from three control and five irradiated sheep. The treated sheep had previously received spinal radiotherapy at a total dose of 28 Gy, which is equivalent to pediatric medulloblastoma treatment. Lung biopsies were taken 4 years post-irradiation from high-dose (HD, >18 Gy) and low-dose (LD, <2 Gy) regions. Fifteen cell lines were extracted (six control, four LD and five HD). The cells were irradiated, and DNA DSB repair was analyzed by immunofluorescence. Clonogenic, trypan blue and micronuclei assays were performed. Both the HD and LD cell lines had a significantly higher number of residual γH2AX foci 24 h and a significant decrease in pATM activity post-irradiation compared to the control. There was no statistically significant difference in the clonogenic assay, trypan blue and micronuclei results. Our study showed that a previous irradiation can impair the DNA DSB repair mechanism of ovine lung fibroblasts.

The role of declining ataxia-telangiectasia-mutated (ATM) function in oocyte aging

Despite the advances in the understanding of reproductive physiology, the mechanisms underlying ovarian aging are still not deciphered. Recent research found an association between impaired ATM-mediated DNA double-strand break (DSB) repair mechanisms and oocyte aging. However, direct evidence connecting ATM-mediated pathway function decline and impaired oocyte quality is lacking. The objective of this study was to determine the role of ATM-mediated DNA DSB repair in the maintenance of oocyte quality in a mouse oocyte knockdown model. Gene interference, in vitro culture, parthenogenesis coupled with genotoxicity assay approaches, as well as molecular cytogenetic analyses based upon next-generation sequencing, were used to test the hypothesis that intact ATM function is critical in the maintenance of oocyte quality. We found that ATM knockdown impaired oocyte quality, resulting in poor embryo development. ATM knockdown significantly lowered or blocked the progression of meiosis in vitro, as well as retarding and reducing embryo cleavage after parthenogenesis. After ATM knockdown, all embryos were of poor quality, and none reached the blastocyst stage. ATM knockdown was also associated with an increased aneuploidy rate compared to controls. Finally, ATM knockdown increased the sensitivity of the oocytes to a genotoxic active metabolite of cyclophosphamide, with increased formation of DNA DSBs, reduced survival, and earlier apoptotic death compared to controls. These findings suggest a key role for ATM in maintaining oocyte quality and resistance to genotoxic stress, and that the previously observed age-induced decline in oocyte ATM function may be a prime factor contributing to oocyte aging.

DIPG-82. RADIOSENSITIZATION WITH RAD52 INHIBITION IN DIFFUSE MIDLINE GLIOMA

Abstract BACKGROUND Diffuse midline glioma (DMG) is one of the most devastating childhood cancers that limited response to radiation therapy (RT). There is a critical need for new therapeutics that enhance the radiation effect. We, therefore, tested the hypothesis that targeting RAD52 activity sensitizes the radiation response of DMG. METHODS Genome-wide CRISPR/Cas9 screening was performed to identify the potential therapeutic target. RAD52 inhibition was used by shRNA-mediated RAD52 depletion and treatment with RAD52 inhibitor, D-I03. The protein expression, cell proliferation, DNA damage marker expression, DNA repair pathway, and transcriptional alteration were analyzed by western blotting, MTS assay, colony formation assay, immunocytochemistry, DNA repair pathway assay, and RNA sequencing. Mice with DMG patient-derived xenograft (PDX) models were treated with RAD52 inhibitor alone or combination with RT. RESULTS Genome-wide CRISPR/Cas9 screening identified RAD52 as a potential therapeutic target in DMG cells. RAD52 inhibition suppressed DMG cell proliferation. Importantly, RAD52 inhibition in combination with RT further increased the radiosensitivity of DMG cells. Immunocytochemistry of DNA double-strand breaks (DSB) marker γH2AX and repair marker 53BP1 showed that RAD52 inhibition sustained DNA damage with high levels of γH2AX at 24 hours following radiation while the level of 53BPI was decreased, thereby inhibiting DNA DSB repair. Western blotting also revealed that RAD52 inhibition causes a sustained level of phosphorylated Rad50 and γH2AX in irradiated DIPG cells over 24 hours. DNA repair assay showed that RAD52 inhibition suppressed homologous recombination DNA repair pathway. RNA sequencing showed that RAD52 inhibition downregulated genes associated with the DNA repair pathway. Finally, the combination therapy of RAD52 inhibitor and RT further suppressed tumor growth and increased survival of mice bearing DMG PDXs, outperforming either monotherapy. CONCLUSIONS these results highlight RAD52 inhibition as a potential radiosensitization and provide a rationale for developing combination therapy with radiation in the treatment of DMG.

Abstract A019: Inhibition of nicotinamide adenine dinucleotide (NAD) production is a potent therapeutic strategy to inactivate homologous recombination in cancer cells

Abstract Homologous recombination (HR) is the most faithful DNA double-strand breaks (DSBs) repairing pathway. HR deficiency (HRD) can result from mutations in BRCA1, BRCA2, and PALB2, which are associated with genomic instability and cancer. HR deficient cells can survive through the use of alternative DNA repair pathways. Therefore, inhibiting these repair pathways in HRD cells trigger cancer cell death. This phenomenon is called synthetic lethality. The best-known example of synthetic lethality is that involving BRCA1/BRCA2/PALB2 deficiency with PARP-1 inhibition by Olaparib. However, ∼40% of patients will develop resistance to PARP inhibition because of several mechanisms, including a second mutation in BRCA1/2, restoring their full-length expression or RAD51 overexpression. RAD51 is one of the most important mediators of HR as it promotes the invasion of a sister chromatid allowing faithful DSB repair. There are many pieces of evidence which show that RAD51 overexpression is correlated with resistance and worse overall survival. Our hypothesis is that targeting RAD51 can trigger HR deficiency, particularly in cancer cells resistant to standard treatments. We thus sought to find inhibitors of the RAD51 recombinase. We have developed an in cellulo screening technique to test the direct effect of chemical compounds on RAD51 foci formation following irradiation. Using this technique, we identified , among 1381 chemical molecules, FK866 (Daporinad) which is a Nicotinamide Phosphoribosyltransferase Inhibitor (NAMPTi). NAMPT is involved in producing cellular Nicotinamide Adenine Dinucleotide (NAD+), which plays a crucial role in various metabolic processes within the cell, primarily in energy production and DNA repair. Remarkably, treatment of Daporinad reduced RAD51 foci formation and triggered a destabilization of the RAD51 protein by the proteasome. This destabilization seemed to be specific for RAD51, as NHEJ factors were not affected by NAMPTi. We will present data related to our objectives: i) Monitoring the effect of Daporinad on different cancer cell lines originated from cancer types which has shown resistance and worse overall survival because of RAD51 overexpression. Western blot and qPCR techniques will be used to investigate the effect of Daporinad on RAD51 protein and mRNA level and using Zeiss Celldiscoverer 7 system and Incucyte live cell imaging microscopy to observe the effect of Daporinad on cell survival; ii) To confirm the specific targeting of HR by Daporinad, we will monitor the effect of Daporinad on HR and other DNA DSB repair pathways using in cellulo reporter systems; iii) In the long term, we aim to evaluate the effect of Daporinad in vivo on tumor cell growth in pre-clinical models, such as mouse xenografts. In conclusion, small molecules affecting HR provide a source of new cancer therapies targeting RAD51 overexpression or using synthetic lethality. Citation Format: Sadaf Valeh Sheida, Thibaut Peterlini, Mélissa Thomas, Guy Poirier, Jean-Yves Masson. Inhibition of nicotinamide adenine dinucleotide (NAD) production is a potent therapeutic strategy to inactivate homologous recombination in cancer cells [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr A019.

Exploring factors influencing choice of DNA double-strand break repair pathways

DNA double-strand breaks (DSBs) represent one of the most severe threats to genomic integrity, demanding intricate repair mechanisms within eukaryotic cells. A diverse array of factors orchestrates the complex choreography of DSB signaling and repair, encompassing repair pathways, such as non-homologous end-joining, homologous recombination, and polymerase-θ-mediated end-joining. This review looks into the intricate decision-making processes guiding eukaryotic cells towards a particular repair pathway, particularly emphasizing the processing of two-ended DSBs. Furthermore, we elucidate the transformative role of Cas9, a site-specific endonuclease, in revolutionizing our comprehension of DNA DSB repair dynamics. Additionally, we explore the burgeoning potential of Cas9's remarkable ability to induce sequence-specific DSBs, offering a promising avenue for precise targeting of tumor cells. Through this comprehensive exploration, we unravel the intricate molecular mechanisms of cellular responses to DSBs, shedding light on both fundamental repair processes and cutting-edge therapeutic strategies.

H2AX promotes replication fork degradation and chemosensitivity in BRCA-deficient tumours

Histone H2AX plays a key role in DNA damage signalling in the surrounding regions of DNA double-strand breaks (DSBs). In response to DNA damage, H2AX becomes phosphorylated on serine residue 139 (known as γH2AX), resulting in the recruitment of the DNA repair effectors 53BP1 and BRCA1. Here, by studying resistance to poly(ADP-ribose) polymerase (PARP) inhibitors in BRCA1/2-deficient mammary tumours, we identify a function for γH2AX in orchestrating drug-induced replication fork degradation. Mechanistically, γH2AX-driven replication fork degradation is elicited by suppressing CtIP-mediated fork protection. As a result, H2AX loss restores replication fork stability and increases chemoresistance in BRCA1/2-deficient tumour cells without restoring homology-directed DNA repair, as highlighted by the lack of DNA damage-induced RAD51 foci. Furthermore, in the attempt to discover acquired genetic vulnerabilities, we find that ATM but not ATR inhibition overcomes PARP inhibitor (PARPi) resistance in H2AX-deficient tumours by interfering with CtIP-mediated fork protection. In summary, our results demonstrate a role for H2AX in replication fork biology in BRCA-deficient tumours and establish a function of H2AX separable from its classical role in DNA damage signalling and DSB repair.

DNA Double-Strand Break Repair Kinetics after Exposure to Photons and Ions: A Systematic Review.

This study offers a review of published data on DNA double strand break (DSB) repair kinetics after exposure to ionizing radiation. By compiling a database, which currently includes 285 DNA DSB repair experiments utilizing both photons and ions, we investigate the impact of distinct experimental parameters on the kinetics of DNA DSB repair. Methodological differences and inconsistencies in reporting make the comparison of data generated by different research groups challenging. Nevertheless, by implementing filtering criteria, we can compare repair kinetics obtained with normal and tumor cells derived from human or animal tissues, as well as cells exposed to photons or ions ranging from hydrogen to iron ions. In addition, several repair curves of repair deficient cell lines were included. The study aims to provide researchers with a comprehensive overview of experimental factors that may confound results and emphasize the importance of precise reporting of experimental parameters. Moreover, we identify gaps in the literature that require attention in future studies, aiming to address clinically relevant questions related to radiotherapy. The database can be freely accessed at: https://github.com/weradstake/DRDNA.

Abstract 1112: Modulated dNTP pools and their influence on DNA repair mechanisms and apoptosis in therapy resistant cancers

Abstract Glioblastoma (GBM) is a deadly primary tumor of the brain, accounting for almost half of all diagnosed malignant gliomas. Ionizing radiation (IR) and some chemotherapeutics strategically target cancer cells by causing DNA double-strand breaks (DSBs). However, cancers like GBM often develop resistance to DSB-inducing therapeutics due to dysregulated DNA repair. Homologous Recombination (HR) is an error-free DSB-repair process consisting of proteins that are often dysregulated in GBM, causing overactive DNA-repair, therapy resistance, and poor patient outcomes. Proteins involved in the HR-repair process have been extensively investigated; however, therapeutic resistance remains common. Here, we provide an alternative approach to target DSB-resistant GBM by modulating the dNTP pool instead of solely targeting DNA-repair proteins. Our goal is to understand the interplay between dNTP pools and DSB- repair. dNTPs are a necessary constituent to repair DSBs; however, the role of the dNTP pool in DNA damage repair is not fully understood. Previous research shows that the dNTP pool remains unchanged after inducing DSBs, highlighting the importance of a tightly regulated dNTP pool. Using a neutral comet assay, we quantified accumulated double-stranded breaks in different treatment groups. We then analyzed these groups using immunofluorescent microscopy to examine distinct proteins involved in HR-mediated repair. We carried out RNA sequencing analysis to determine differential gene expression. We quantified cell viability and apoptotic cell populations via flow cytometry and Alamar blue assays. We determined protein interactions using immunoblotting and immunoprecipitation assays. We found that increasing the dNTP pool prior to IR treatment led to impaired DNA DSB repair in GBM cells as determined by a neutral comet assay. When analyzing key proteins involved in HR via immunofluorescence, we observed a significant decrease in RPA70 localization to the DNA damage site, indicating a disruption in end resection, a necessary phase of HR. Our RNA sequencing results show that increasing the dNTP pool in LN229 GBM cells prior to IR treatment caused significant changes in gene expression when compared to IR treatment alone. The most impacted pathways most were apoptosis, p53 cell cycle arrest, and DNA repair. The pathway analysis has identified genes of interest in the synergistic dNTP + IR treatment model. Significant increases in apoptosis and decreases in viability were observed 48 hours after inducing DSBs in LN229 cells with elevated dNTP pools. Not only do these data suggest that HR efficiency is reduced due to end resection impairment, but they also show that as a result, cell viability is destabilized due to prolonged DNA damage, resulting in apoptosis. The identification of this impairment may be essential in finding novel therapeutic approaches to sensitize therapy resistant GBM. Citation Format: Dominique Monroe, Mercy Kehinde-Ige, Edidiong Usoro, Arilyn Williams, Emily Steinbeck, Shelby Aycox, Huidong Shi, Waaqo Daddacha. Modulated dNTP pools and their influence on DNA repair mechanisms and apoptosis in therapy resistant cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1112.

Abstract IA015: Using systems approaches to interrogate DNA double strand break signaling and repair for optimal tumor cell killing

Abstract Repair of DNA double strand breaks (DSBs) occurs through multiple distinct repair pathways including canonical and microhomology-mediated/Pol Q-dependent end joining (c-NHEJ and alt-NHEJ), homologous recombination (HR), and single strand annealing (SSA). Exactly how a particular DSB dictates which pathway is selected for repair, and the extent to which different DSB repair pathways cross-talk with each other is incompletely understood, but appears to be influenced by the specific type of DNA ends, the extent of DNA resection, the cell state and stage of the cell cycle, and the chromatin context around the break. To simultaneously interrogate multiple DNA DSB repair pathways in a systems-based manner at the single cell level, we built upon the pioneering DSB repair reporter systems created by the Jasin, Stark, Scharenberg and Certo labs to create several new CRISPR-based DNA DSB reporter systems that readout the repair of a single genomic DNA DSB by error-free c-NHEJ, mutagenic c- and alt-NHEJ, HR or SSA using multi-color fluorescence-based imaging and flow cytometry approaches. Using these DSB reporter systems we show that competition between short-range and long-range end resection influences the choice between repair by HR versus SSA, demonstrate accepted and novel functions for a variety of DNA damage sensors and repair proteins in NHEJ and HR, and illustrate the feasibility of using this system to interrogate the effects of patient-specific mutations in DNA repair proteins on DNA DSB repair pathway choice. Finally, we show that DNA DSBs created by topoisomerase inhibitor treatment in live but DNA-damaged tumor cells are capable of activating anti-tumor immune responses by signaling through both canonical and non-canonical DNA break sensing and response pathways. Citation Format: Alex Kruswick, Bert van de Kooij, Tiffany R. Emmons, Ganapathy Sriram, Emma D. Martin, Anjali Arvind, Haico van Attikum, Michael B. Yaffe. Using systems approaches to interrogate DNA double strand break signaling and repair for optimal tumor cell killing [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr IA015.

Effect of bisphosphonates and statins on the in vitro radiosensitivity of breast cancer cell lines.

Early-stage breast cancer is usually treated with breast-conserving surgery followed by adjuvant radiation therapy. Acute skin toxicity is a common radiation-induced side effect experienced by many patients. Recently, a combination of bisphosphonates (zoledronic acid) and statins (pravastatin), or ZOPRA, was shown to radio-protect normal tissues by enhancing DNA double-strand breaks (DSB) repair mechanism. However, there are no studies assessing the effect of ZOPRA on cancerous cells. The purpose of this study is to characterize the in vitro effect of the zoledronic acid (ZO), pravastatin (PRA), and ZOPRA treatment on the molecular and cellular radiosensitivity of breast cancer cell lines. Two breast cancer cell lines, MDA MB 231 and MCF-7, were tested. Cells were treated with different concentrations of pravastatin (PRA), zoledronate (ZO), as well as their ZOPRA combination, before irradiation. Anti-γH2AX and anti-pATM immunofluorescence were performed to study DNA DSB repair kinetics. MTT assay was performed to assess cell proliferation and viability, and flow cytometry was performed to analyze the effect of the drugs on the cell cycle distribution. The clonogenic assay was used to assess cell survival. ZO, PRA, and ZOPRA treatments were shown to increase the residual number of γH2AX foci for both cell lines. ZOPRA treatment was also shown to reduce the activity of the ATM kinase in MCF-7. ZOPRA induced a significant decrease in cell survival for both cell lines. Our findings show that pretreatment with ZOPRA can decrease the radioresistance of breast cancer cells at the molecular and cellular levels. The fact that ZOPRA was previously shown to radioprotect normal tissues, makes it a good candidate to become a therapeutic window-widening drug.

Boric Acid Alters the Expression ofDNA Double Break Repair Genes in MCF-7-Derived Breast Cancer Stem Cells.

Breast cancer pathology ranks second in mortality among women worldwide due to the resistance of cancer stem cells in tumor tissue to radiotherapy and chemotherapy and their effective DNA damage response system (DDR). Targeting the expression of DNA double-strand break (DSB) repair genes in breast cancer stem cells (BC-SCs) is essential for facilitating their elimination with conventional therapies. This study aims to investigate the effects of boric acid (BA) on the expression of DNA DSB repair genes in BC-SCs, which has not been studied in the literature before. BS-SCs were isolated by the MACS method and characterized by flow cytometry. The effects of BA on BC-SCs' DNA DSB repair genes were deciphered by cell viability assay, inverted microscopy, and RT-qPCR. While the expression of the BRCA1 and BRCA2 was upregulated, the expression of the ATM (p < 0.001), RAD51 (p < 0.001), and KU70 (p < 0.001) was downregulated in dose-treated BC-SCs (p < 0.001) to the qPCR results. Consequently, BA affects some of the DNA DSB repair genes of breast cancer stem cells. Findings from this study could provide new insights into the potential therapeutic application of BA in BC-SC elimination and cancer intervention.

Pediatric Burkitt Leukemia with Underlying MSH2, MUTYH and POT1 Germline Variants: The Complexities and Relevance of Understanding Cancer Predisposition

Monoallelic germline variants in POT1 can lead to POT1-tumor predisposition (POT1-TPD), an autosomal dominant disorder leading to a spectrum of neoplasia, predominantly, melanoma, glioma, angiosarcoma and chronic lymphocytic leukemia (CLL). Recently, germline POT1 variants have been described to be associated with familial clonal hematopoiesis syndrome.The POT1 protein forms a part of shelterin complex, which has been implicated in DNA damage protection, telomere length regulation and chromosomal stability. The proposed mechanism of oncogenesis by POT1 variants is presumed to be secondary to increased telomere length causing genomic instability and cellular immortality. While POT1-TPD can cause CLL in adulthood, childhood leukemias have not been commonly observed. Here, we present a case of a previously healthy 12-year-old male diagnosed with Burkitt leukemia. He presented with leukocytosis and tumor lysis syndrome. Bone marrow evaluation revealed hypercellular marrow involved by Burkitt lymphoma, comprising over 90% marrow cellularity. Flow cytometry showed an abnormal lambda monotypic B-lymphoid population positive for CD10, CD19, CD20, CD22 and CD45. FISH studies demonstrated low-level MYC rearrangement (9%); negative for BCL2 and BCL6 rearrangement. EBV staining was negative. Acute leukemia NGS panel (63 genes) revealed a Tier 1 somatic variant in PHF6. RNA sequencing did not detect additional fusions. His chemotherapy course was complicated by significant toxicities including prolonged count recovery, severe peripheral neuropathy, neurotoxicity, kidney injury and several infectious complications. Given extensive family history of cancers on the maternal side including colorectal, duodenal, brain and skin cancers, a germline cancer predisposition NGS panel was sent from cultured skin fibroblast and identified two heterozygous pathogenic variants in MSH2 (c.942+3A&amp;gt;T) and MUTYH (c.1187G&amp;gt;A) and a heterozygous likely pathogenic variant in POT1 (c.233T&amp;gt;C). To delineate potential roles of these germline variants in oncogenesis, additional testing was performed. Immunohistochemical staining of bone marrow revealed intact nuclear expression of MLH1, PMS2, MSH2,and MSH6,suggesting microsatellite stability. Extended tumor profiling through whole exome sequencing did not identify additional somatic variants or copy loss of heterozygosity in MSH2, MUTYH and POT1 but additional somatic loss of function variants were seen in CCND3, CASP8, ARID1, and PTPRD.To assess potential functional consequences of the POT1 germline variant on DNA double-strand break (DSB) damage repair, the patient lymphocytes were exposed to 2 Gy ionizing radiation and mobilization and kinetics of DNA DSB repair was assessed at 1h and 24h post-irradiation (IR) using phosphorylation (p) of ATM, SMC1 and H2AX (gamma), and cell death and apoptosis was also assessed at 24h post-irradiation by flow cytometry. There was normal mobilization and repair of DNA DSBs in T, B and NK cells although there was a subset of CD19dim B cells that showed constitutive DNA DSBs at1h with partial repair at 24h. It is not evident that this defect can be attributed to the germline defects as they are also observed in several other clinical contexts, including post-chemotherapy. Telomere length assay from peripheral blood demonstrated telomere length at 50 th percentile in both lymphocytes and granulocytes. Children in families with adult-onset cancer predisposition syndromes may present with cancers but the potential contribution of these germline variants to these childhood cancers is unknown. Our molecular and functional studies on both germline and tumor tissues did not suggest direct contribution of these germline variants on lymphomagenesis, however there might be other mechanism of inactivation not identified from this study. This case highlights the complexities of understanding genetic susceptibility in childhood cancers. We propose a multifactorial oligogenic inheritance of cancer susceptibility in this patient through several mechanisms of genomic instability.

RPS19 and RPL5 Contribute to DNA Double-Strand Break Repair

Diamond Blackfan anemia (DBA) is an inherited bone marrow failure syndrome most often due to germline pathogenic variants (PVs) in ribosomal protein (RP) genes, with RPS19 and RPL5 being the most frequently mutated. In addition to hypoplastic anemia, individuals have a moderately increased risk of myelodysplastic syndrome, acute myeloid leukemia, and solid cancers, most commonly colorectal cancer and osteosarcoma. A causal link between RP deficiency and cancer predisposition has yet to be firmly established. Our work aims to examine the impact of deficiencies in RPS19 and RPL5 on DNA double-strand break (DSB) repair with the overarching hypothesis that disruptions in DSB repair could increase mutagenesis and ultimately contribute to cancer development in DBA. We found that DBA patient-derived lymphoblastoid cells were hypersensitive to ionizing radiation (IR) and had delayed resolution of the DNA damage marker, γ-H2AX, at IR-induced foci. Additionally, we found that both RPS19- and RPL5-knocked down (KD) bone marrow-derived CD34+ cells had delayed resolution of DSBs after IR in neutral comet assays. These data suggest that RPS19 and RPL5 deficiencies lead to impaired DSB repair. We, therefore, assessed the effect of RPS19- and RPL5-KD on the four main DSB repair pathways using well-established U2OS (osteosarcoma) DSB reporter cell lines. We found decreased homologous recombination (HR) and single-strand annealing (SSA) upon RPS19-KD, whereas the efficiencies of nonhomologous end-joining (NHEJ) and alternative end-joining (alt-EJ) were unchanged. Consistent with an HR defect, RPS19-KD cells were hypersensitive to PARP inhibition. In contrast, RPL5-KD cells had increased NHEJ and alt-EJ, whereas the efficiencies of HR and SSA were unchanged. Together, these results suggest that shifts in DSB repair pathway choice, with less high-fidelity HR, or more error-prone NHEJ or mutagenic alt-EJ, could contribute to mutagenesis in the context of RPS19 or RPL5 PVs. To investigate the mechanism(s) underlying these effects, we examined cell cycle profiles and found decreased G1 and increased G2/M cells following both RPS19- and RPL5-KD, which suggests altered cell cycle profiles are not responsible for DSB repair effects since NHEJ occurs principally in G1 and HR is restricted to G2. Given the essential roles of RPS19 and RPL5 in translation, we also examined levels of crucial proteins in each DSB repair pathway. Surprisingly, we found increased levels of NHEJ proteins DNA ligase IV (LIG4) and XRCC4 in both RPS19 and RPL5-KD cells. However, restoring LIG4/XRCC4 to normal levels via siRNA KD did not normalize the DSB repair efficiencies. Since HR and SSA require extensive resection of the 5' DNA strand at DSBs, we examined whether RPS19-KD impaired this key step by assessing the accumulation of nuclear RPA2, which associates with end-resected DNA. We found nuclear RPA2 levels were not reduced but rather increased following IR compared to the control, indicating the reduction in HR and SSA efficiency in these cells was not due to impaired end resection and may, instead, involve a downstream step in HR that we are currently investigating. To assess if RPS19 and RPL5 might directly contribute to repair, we monitored the recruitment of fluorescently tagged RPS19 and RPL5 to DSBs. We found that these proteins were rapidly recruited to sites of laser microirradiation and sites of chromatin associated Fok1-nuclease (Figure 1), both of which induce DSBs. The recruitment of both proteins was hindered by PARP inhibition and enhanced for RPS19 with PARG inhibition, suggesting RPS19, in particular, may be parylated or interact with parylated proteins. Similar studies were performed in Saos2 cells, which, in contrast to U2OS cells, are p53 null. Whereas RPS19 was recruited to sites of laser microirradiation in Saos2 cells, RPL5 was not. Interestingly, a transcriptionally hyperactive p53 (∆CTD), but not a wild-type or a dominant-negative form of p53 (p.R248Q), restored RPL5 recruitment in Saos2 cells, suggesting a p53 downstream target is required for its recruitment. We are currently investigating proteins that mediate RPS19 and RPL5 DSB recruitment. Together, our results support a model whereby altered DNA DSB repair in RPS19- and RPL5-deficient cells may contribute to cancer development in DBA.

FAM21 interacts with Ku to promote the localization of WASH to DNA double strand break sites

Cytoplasmic FAM21 works as a guiding protein in Wiskott–Aldrich Syndrome Protein and SCAR Homolog (WASH) complex by linking WASH complex to endosomes through its interaction with retromer. Recently, we have reported that nuclear WASH localizes to DNA double strand break (DSB) sites to promote DNA repair through non-homologous end-joining (NHEJ). However, whether FAM21, the close partner of WASH, is involved in the nuclear WASH localization and DNA repair remains to be clarified. Here, we show that FAM21 interacts with Ku and the interaction between C-terminal FAM21 and Ku is essential for its recruitment to DSB sites. Moreover, FAM21 depletion led to decreases in WASH recruitment to damaged DNA and repair capacity upon DNA damage. Taken together, these results reveal that FAM21 promotes DNA repair by orchestrating the recruitment of WASH to DSB sites, providing a mechanistic insight into WASH-dependent DNA DSB repair.

DDDR-26. INHIBITION OF RAD52 ACTIVITY ENHANCES THE RADIOSENSITIVITY IN DIPG

Abstract Diffuse intrinsic pontine glioma (DIPG) is one of the most devastating childhood cancers that limited response to radiation therapy, resulting in median overall survival of approximately 10 months. There is a critical need for new therapeutics that enhance the radiation effect while enabling the use of lower doses of radiation to limit long-term side effects in young patients. We, therefore, tested the hypothesis that targeting RAD52 activity sensitizes the radiation response of DIPG. Genome-wide CRISPR/Cas9 screening identified RAD52 as potential therapeutic target in DIPG cells. RAD52 inhibition, using shRNA mediated RAD52 depletion and treatment with RAD52 inhibitor, D-I03, suppressed DIPG cell proliferation. Importantly, RAD52 inhibition in combination with radiation therapy further increased radiosensitivity of DIPG cells. Immunocytochemistry of DNA double-strand breaks (DSB) marker γH2AX and repair marker 53BP1 showed that RAD52 inhibition sustained DNA damage with high levels of γH2AX at 24 hours following radiation while the level of 53BPI was decreased, thereby inhibiting DNA DSB repair. Western blotting also revealed that RAD52 inhibition causes a sustained level of phosphorylated Rad50 and γH2AX in irradiated DIPG cells. DNA repair assay showed that RAD52 inhibition suppressed DNA DSB repair through a homologous recombination pathway. RNA sequencing showed that RAD52 inhibition downregulated genes associated with the DNA repair pathway. Finally, the combination therapy of D-I03 RAD52 inhibitor and radiation inhibited tumor growth and significantly increased survival of mice bearing DIPG patient-derived xenografts, outperforming either monotherapy. Together, these results highlight RAD52 inhibitor as a potential radiosensitizer and provide a rationale for developing combination therapy with radiation in the treatment of DIPG.

Novel insights into the ecDNA formation mechanism involving MSH3 in methotrexate‑resistant human colorectal cancer cells.

Extrachromosomal DNAs (ecDNAs), also known as double minutes (DMs), can induce a fast increase in gene copy numbers and promote the development of cancer, including drug resistance. MutS homolog 3 (MSH3), a key protein in mismatch repair, has been indicated to participate in the regulation of DNA double‑strand break (DSB) repair, which has been reported to be associated with the formation of ecDNAs. However, it remains unclear whether MSH3 can influence drug resistance via ecDNAs in cancer. In the present study, high MSH3 expression was observed in methotrexate (MTX)‑resistant HT29 cells [DM‑ and homogeneously staining region (HSR)‑containing cells] compared with parental HT29 cells. Additionally, decreased amounts of ecDNAs, HSRs and amplified genes locating on ecDNAs and HSRs were detected following depletion of MSH3 and this could be reversed by overexpressing MSH3 in DM‑containing cells. No corresponding changes were found in HSR‑containing cells. The present study further verified the involvement of MSH3‑regulated DNA DSB repair pathways in the formation of ecDNAs by detecting the expression of core proteins and pathway activity. Furthermore, expulsion of ecDNAs/HSRs was detected and increased frequencies of micronuclei/nuclear buds with dihydrofolate reductase (DHFR) signals were observed in MSH3‑depleted DM‑containing cells. Finally, changes in MSH3 expression could affect DHFR amplification‑derived DHFR expression and cell sensitivity to MTX, suggesting that MSH3 may influence cancer drug resistance by altering the amount of ecDNAs. In conclusion, the present study revealed a novel mechanism involving MSH3 in the regulation of ecDNAs by DSB repair, which will have clinical value in the treatment of ecDNA‑based drug resistance in cancer.

Xrcc5/KU80 is not required for the survival or activation of prophase-arrested oocytes in primordial follicles.

The non-growing, meiotically-arrested oocytes housed within primordial follicles are exquisitely sensitive to genotoxic insults from endogenous and exogenous sources. Even a single DNA double-strand break (DSB) can trigger oocyte apoptosis, which can lead to accelerated depletion of the ovarian reserve, early loss of fertility and menopause. Therefore, repair of DNA damage is important for preserving the quality of oocytes to sustain fertility across the reproductive lifespan. This study aimed to evaluate the role of KU80 (encoded by the XRCC5 gene) - an essential component of the non-homologous end joining (NHEJ) pathway - in the repair of oocyte DNA DSBs during reproductive ageing, and following insult caused by the DNA-damaging chemotherapies cyclophosphamide and cisplatin. To investigate the importance of KU80 following endogenous and exogenous DNA damage, ovaries from conditional oocyte-specific Xrcc5 knockout (Xrcc5 cKO) and wildtype (WT) mice that were aged or exposed to DNA damage-inducing chemotherapy were compared. Ovarian follicles and oocytes were quantified, morphologically assessed and analysed via immunohistochemistry for markers of DNA damage and apoptosis. In addition, chemotherapy exposed mice were superovulated, and the numbers and quality of mature metaphase- II (MII) oocytes were assessed. The number of healthy follicles, atretic (dying) follicles, and corpora lutea were similar in Xrcc5 cKO and WT mice at PN50, PN200 and PN300. Additionally, primordial follicle number and ovulation rates were similar in young adult Xrcc5 cKO and WT mice following treatment with cyclophosphamide (75mg/kg), cisplatin (4mg/kg), or vehicle control (saline). Furthermore, KU80 was not essential for the repair of exogenously induced DNA damage in primordial follicle oocytes. These data indicate that KU80 is not required for maintenance of the ovarian reserve, follicle development, or ovulation during maternal ageing. Similarly, this study also indicates that KU80 is not required for the repair of exogenously induced DSBs in the prophase-arrested oocytes of primordial follicles.

The fructose-bisphosphate, Aldolase A (ALDOA), facilitates DNA-PKcs and ATM kinase activity to regulate DNA double-strand break repair

Glucose metabolism and DNA repair are fundamental cellular processes frequently dysregulated in cancer. In this study, we define a direct role for the glycolytic Aldolase A (ALDOA) protein in DNA double-strand break (DSB) repair. ALDOA is a fructose biphosphate Aldolase that catalyses fructose-1,6-bisphosphate to glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), during glycolysis. Here, we show that upon DNA damage induced by ionising radiation (IR), ALDOA translocates from the cytoplasm into the nucleus, where it partially co-localises with the DNA DSB marker γ-H2AX. DNA damage was shown to be elevated in ALDOA-depleted cells prior to IR and following IR the damage was repaired more slowly. Consistent with this, cells depleted of ALDOA exhibited decreased DNA DSB repair via non-homologous end-joining and homologous recombination. In support of the defective repair observed in its absence, ALDOA was found to associate with the major DSB repair effector kinases, DNA-dependent Protein Kinase (DNA-PK) and Ataxia Telangiectasia Mutated (ATM) and their autophosphorylation was decreased when ALDOA was depleted. Together, these data establish a role for an essential metabolic protein, ALDOA in DNA DSB repair and suggests that targeting ALDOA may enable the concurrent targeting of cancer metabolism and DNA repair to induce tumour cell death.

Dose-Dependent Shift in Relative Contribution of Homologous Recombination to DNA Repair after Low-LET Ionizing Radiation Exposure: Empirical Evidence and Numerical Simulation.

Understanding the relative contributions of different repair pathways to radiation-induced DNA damage responses remains a challenging issue in terms of studying the radiation injury endpoints. The comparative manifestation of homologous recombination (HR) after irradiation with different doses greatly determines the overall effectiveness of recovery in a dividing cell after irradiation, since HR is an error-free mechanism intended to perform the repair of DNA double-strand breaks (DSB) during S/G2 phases of the cell cycle. In this article, we present experimentally observed evidence of dose-dependent shifts in the relative contributions of HR in human fibroblasts after X-ray exposure at doses in the range 20-1000 mGy, which is also supported by quantitative modeling of DNA DSB repair. Our findings indicate that the increase in the radiation dose leads to a dose-dependent decrease in the relative contribution of HR in the entire repair process.