DNA Double-strand Break Repair Research Articles

Viability and Radiosensitivity of Human Tumor Cells from Breast and Colon Are Influenced by Hypericum perforatum Extract HP01

To enhance the treatment of tumors that are resistant to radio- and chemotherapy while minimizing the side effects of radiochemotherapy, researchers are continuously seeking new active compounds for use in combination with radiotherapy. Therefore, the aim of our study was to examine the cytotoxic and radiosensitizing effects of an extract from St. John’s Wort (Hypericum perforatum), referred to as HP01, on human epithelial tumor cells in vitro. The growth of MCF-7 (breast carcinoma) and HT-29 (colon carcinoma) cells was examined under the influence of HP01. In combination with radiation, the effects of HP01 on cytotoxicity and long-term survival were assessed using a colony formation assay. The number of DNA double-strand breaks was analyzed using the γH2AX assay, while cell cycle distribution was examined via flow cytometry. A growth-inhibiting and cytotoxic effect was observed for both tumor cell lines starting at a concentration of 10 µg/mL HP01. Treatment with HP01 resulted in an inhibition of clonogenic survival of tumor cells after ionizing radiation (6 Gy). The number of DNA double-strand breaks (DSBs) in tumor cells increased with HP01 treatment, but the repair of radiation-induced DNA DSBs was not affected. Cell cycle analysis revealed that HP01, in addition to radiation, enhanced G2/M arrest in MCF-7 and HT-29 cells. Overall, HP01 not only showed a growth-inhibiting effect but also demonstrated a radiosensitizing effect on human tumor cells for the first time. We conclude that the HP01-induced G2/M accumulation of cells may be the main rationale for the drug-induced radiosensitivity. It is therefore a promising candidate for combined therapy in tumor diseases and warrants further investigation.

GDBr: genomic signature interpretation tool for DNA double-strand break repair mechanisms.

Large genetic variants can be generated via homologous recombination (HR), such as polymerase theta-mediated end joining (TMEJ) or single-strand annealing (SSA). Given that these HR-based mechanisms leave specific genomic signatures, we developed GDBr, a genomic signature interpretation tool for DNA double-strand break repair mechanisms using high-quality genome assemblies. We applied GDBr to a draft human pangenome reference. We found that 78.1% of non-repetitive insertions and deletions and 11.0% of non-repetitive complex substitutions contained specific signatures. Of these, we interpreted that 98.7% and 1.3% of the insertions and deletions were generated via TMEJ and SSA, respectively, and all complex substitutions via TMEJ. Since population-level pangenome datasets are being dramatically accumulated, GDBr can provide mechanistic insights into how variants are formed. GDBr is available on GitHub at https://github.com/Chemical118/GDBr.

Functions of the Bloom Syndrome Helicase N-terminal Intrinsically Disordered Region.

Bloom Syndrome helicase (Blm) is a RecQ family helicase involved in DNA repair, cell-cycle progression, and development. Pathogenic variants in human BLM cause the autosomal recessive disorder Bloom Syndrome, characterized by predisposition to numerous types of cancer. Prior studies of Drosophila Blm mutants lacking helicase activity or protein have shown sensitivity to DNA damaging agents, defects in repairing DNA double-strand breaks (DSBs), female sterility, and improper segregation of chromosomes in meiosis. Blm orthologs have a well conserved and highly structured RecQ helicase domain, but more than half of the protein, particularly in the N-terminus, is predicted to be intrinsically disordered. Because this region is poorly conserved across metazoa, we compared closely related species to identify regions of conservation that might be associated with important functions. We deleted two Drosophila-conserved regions in D. melanogaster using CRISPR/Cas9 gene editing and assessed the effects on several Blm functions. Each deletion had distinct effects. Deletion of either conserved region 1 (CR1) or conserved region 2 (CR2) compromised DSB repair through synthesis-dependent strand annealing and resulted in increased mitotic crossovers. In contrast, CR2 is critical for embryonic development but CR1 is less important. Loss of CR1 leads to defects in meiotic crossover designation and patterning but does not impact meiotic chromosome segregation, whereas deletion of CR2 does not result in significant meiotic defects. Thus, while the two regions have overlapping functions, there are distinct roles facilitated by each. These results provide novel insights into functions of the N-terminal region of Blm helicase.

Gestational exposure to arsenic reduces female offspring fertility by impairing the repair of DNA double-strand breaks and synapsis formation in oocytes

Arsenic is a pollutant that can cross the placenta; however, research on the effects of arsenic exposure during pregnancy on the fertility of female offspring is limited. To address this gap, we developed a mouse model to investigate the relationship between arsenic exposure during pregnancy and fertility in female offspring. Our fertility assessment revealed that gestational exposure to 1mg/kg arsenic or higher (10mg/kg) resulted in reduction in litter size, ovarian volume, and multistage-follicle number in female offspring. By assessing the in vitro developmental capacity of oocytes and zygotes, we confirmed that the reduced fertility was due not to impaired oocyte quality but rather to a reduction in oocyte quantity. Arsenic exposure impedes synapsis formation in MPI and compromises homologous recombination-mediated repair of double-strand breaks, resulting in fewer crossovers. This disruption activates the pachytene-checkpoint, hindering the progression of the MPI and resulting in the elimination of defective oocytes through p-Chk2 activation. Our study reveals for the first time the detrimental effects of arsenic exposure during pregnancy on the fertility of female offspring, underscoring the urgent need to prevent such exposure to safeguard reproductive health.

Single-Molecule Visualization of BLM-DNA2-Mediated DNA End Resection Using DNA Curtains.

Homologous recombination (HR) is the principal pathway undertaken by a cell for the error-free repair of DNA double-strand breaks that are frequently encountered by the cell. HR can be initiated at the sites of DNA double-strand breaks by generating long stretches of single-stranded 3' DNA overhang through a process called DNA end resection. In one DNA end resection pathway, this is achieved via the concerted effort of specialized machinery involving the RecQ family helicase BLM, the helicase/endonuclease DNA2, and a single-strand DNA binding protein complex RPA. BLM unwinds the DNA at the sites of DNA damage. The DNA unwound by BLM is cleaved in a 5' to 3' direction by DNA2 leaving behind a long single-stranded 3' DNA tail which is rapidly coated with RPA. This long single-stranded DNA provides the loading platform for recombinase proteins to form nucleoprotein filaments which initiate the homology search to undergo HR-based repair. Most of the insights into these processes have been gained using ensemble biochemical methods. However, these approaches have proven to be challenging for dissecting complex molecular mechanisms, especially because of the dynamic nature of these processes. Experiments involving single-molecule techniques have enabled us to counter these issues by allowing us to gather information on intermediates that are heterogeneous and transient in nature. We have developed a single-molecule technique called DNA curtains where we can study hundreds of protein-nucleic acid interaction events in real time. Here, we describe a method to prepare a single-tethered double-stranded DNA curtain to visualize, in combination with total internal reflection microscopy (TIRFM), the BLM- and DNA2-mediated DNA end resection in real time.

Functional conservation and divergence of arabidopsis VENOSA4 and human SAMHD1 in DNA repair.

The human deoxyribonucleoside triphosphatase (dNTPase) Sterile alpha motif and histidine-aspartate domain containing protein 1 (SAMHD1) has a dNTPase-independent role in repairing DNA double-strand breaks (DSBs) by homologous recombination (HR). Here, we show that VENOSA4 (VEN4), the probable Arabidopsis thaliana ortholog of SAMHD1, also functions in DSB repair by HR. The ven4 loss-of-function mutants showed increased DNA ploidy and deregulated DNA repair genes, suggesting DNA damage accumulation. Hydroxyurea, which blocks DNA replication and generates DSBs, induced VEN4 expression. The ven4 mutants were hypersensitive to hydroxyurea, with decreased DSB repair by HR. Metabolomic analysis of the strong ven4-0 mutant revealed depletion of metabolites associated with DNA damage responses. In contrast to SAMHD1, VEN4 showed no evident involvement in preventing R-loop accumulation. Our study thus reveals functional conservation in DNA repair by VEN4 and SAMHD1.

The Interplay Between Chromatin Remodeling and DNA Double-strand Break Repair: Implications for Cancer Biology and Therapeutics

The Interplay Between Chromatin Remodeling and DNA Double-strand Break Repair: Implications for Cancer Biology and Therapeutics

MeCP2 deficiency leads to the γH2AX nano foci expansion after ionizing radiation

DNA double-strand breaks (DSBs) trigger the recruitment of repair protein and promote signal transduction through posttranslational modifications such as phosphorylation. After DSB induction, ataxia telangiectasia mutated (ATM) phosphorylates H2AX on chromatin surrounds the mega-base pairs proximal to the DSBs. Advanced super-resolution microscopic technology has demonstrated the formation of γH2AX nano foci as a unit of nano domain comprised of multiple nucleosomes. The formation of γH2AX nano foci could be potentially affected by pre-existing chromatin structure prior to DSB induction; however, it remains unclear whether chromatin status around DSBs influences the formation of γH2AX nano foci. In this study, to investigate γH2AX nano foci formation in the context of chromatin relaxation, γH2AX nano foci were examined following the depletion of MeCP2, which is a factor promoting chromatin condensation. Remarkably, by using super resolution imaging analysis, we found that the volume of γH2AX nano foci cluster in MeCP2-depleted cells was significantly greater than that in control cells, both 5 and 30min after ionizing radiation (IR). Corresponding to the increased volume size, the number of γH2AX nano foci per cluster was greater than that in control cells, while the distance of each nano focus within foci clusters remained unchanged. These findings suggest that relaxed chromatin condition by MeCP2 depletion facilitates faster and more extensive γH2AX nano foci formation after IR. Collectively, our super-resolution analysis suggests that the chromatin status surrounding DSBs influences the expansion of γH2AX nano foci formation, thus, potentially influencing the DSB repair and signaling.

NGS Detects Extensive Genomic Alterations in Survivors of Irradiated Normal Human Fibroblast Cells.

It is thought that cells surviving ionizing radiation exposure repair DNA double-strand breaks (DSBs) and restore their genomes. However, the recent biochemical and genetic characterization of DSB repair pathways reveals that only homologous recombination (HR) can function in an error-free manner and that the non-homologous end joining (NHEJ) pathways canonical NHEJ (c-NHEJ), alternative end joining (alt-EJ), and single-strand annealing (SSA) are error-prone, and potentially leave behind genomic scars and altered genomes. The strong cell cycle restriction of HR to S/G2 phases and the unparalleled efficiency of c-NHEJ throughout the cell cycle, raise the intriguing question as to how far a surviving cell "reaches" after repairing the genome back to its pre-irradiation state. Indeed, there is evidence that the genomes of cells surviving radiation treatment harbor extensive genomic alterations. To directly investigate this possibility, we adopted next-generation sequencing (NGS) technologies and tested a normal human fibroblast cell line, 82-6 hTert, after exposure up to 6 Gy. Cells were irradiated and surviving colonies expanded and the cells froze. Sequencing analysis using the Illumina sequencing platform and comparison with the unirradiated genome detected frequent genomic alterations in the six investigated radiation survivor clones, including translocations and large deletions. Translocations detected by this analysis and predicted to generate visible cytogenetic alterations were frequently (three out of five) confirmed using mFISH cytogenetic analysis. PCR analysis of selected deletions also confirmed seven of the ten examined. We conclude that cells surviving radiation exposure tolerate and pass to their progeny a wide spectrum of genomic alterations. This recognition needs to be integrated into the interpretation of biological results at all endpoints, as well as in the formulation of mathematical models of radiation action. NGS analysis of irradiated genomes promises to enhance molecular cytogenetics by increasing the spectrum of detectable genomic alterations and advance our understanding of key molecular radiobiological effects and the logic underpinning DSB repair. However, further developments in the technology will be required to harness its full potential.

BCAT1 Associates with DNA Repair Proteins KU70 and KU80 and Contributes to Regulate DNA Repair in T-Cell Acute Lymphoblastic Leukemia (T-ALL).

Increased expression of branched-chain amino acid (BCAA) transaminase 1 (BCAT1) often correlates with tumor aggressiveness and drug resistance in cancer. We have recently reported that BCAT1 was overexpressed in a subgroup of T-cell acute lymphoblastic (T-ALL) samples, especially those with NOTCH1 activating mutations. Interestingly, BCAT1-depleted cells showed pronounced sensitivity to DNA-damaging agents such as etoposide; however, how BCAT1 regulates this sensitivity remains uncertain. Here, we provide further clues on its chemo-sensitizing effect. Indeed, BCAT1 protein regulates the non-homologous end joining (c-NHEJ) DNA repair pathway by physically associating with the KU70/KU80 heterodimer. BCAT1 inhibition during active repair of DNA double-strand breaks (DSBs) led to increased KU70/KU80 acetylation and impaired c-NHEJ repair, a dramatic increase in DSBs, and ultimately cell death. Our results suggest that, in T-ALL, BCAT1 possesses non-metabolic functions that confer a drug resistance mechanism and that targeting BCAT1 activity presents a novel strategy to improve chemotherapy response in T-ALL patients.

DNA double-strand break repair capacity and normal tissue toxicity induced by radiotherapy.

Radiation therapy is used in the treatment of various cancers, and advancements in irradiation techniques have further expanded its applicability. For radiation oncologists, predicting adverse events remains a critical challenge, even with these technological advancements. Although numerous studies have been conducted to predict individual radiosensitivity, no biomarkers have been clinically applied thus far. This review focuses on γ-H2AX foci and chromosomal aberrations, providing an overview of their association with normal tissue toxicities.

Lig3-dependent rescue of mouse viability and DNA double-strand break repair by catalytically inactive Lig4.

Recent studies have revealed a structural role for DNA ligase 4 (Lig4) in the maintenance of a repair complex during non-homologous end joining (NHEJ) of DNA double-strand breaks. In cultured cell lines, catalytically inactive Lig4 can partially alleviate the severe DNA repair phenotypes observed in cells lacking Lig4. To study the structural role of Lig4 in vivo, a mouse strain harboring a point mutation to Lig4's catalytic site was generated. In contrast to the ablation of Lig4, catalytically inactive Lig4 mice are born alive. These mice display marked growth retardation and have clear deficits in lymphocyte development. We considered that the milder phenotype results from inactive Lig4 help to recruit another ligase to the repair complex. We next generated a mouse strain deficient for nuclear Lig3. Nuclear Lig3-deficient mice are moderately smaller and have elevated incidences of cerebral ventricle dilation but otherwise appear normal. Strikingly, in experiments crossing these two strains, mice lacking nuclear Lig3 and expressing inactive Lig4 were not obtained. Timed mating revealed that fetuses harboring both mutations underwent resorption, establishing an embryonic lethal genetic interaction. These data suggest that Lig3 is recruited to NHEJ complexes to facilitate end joining in the presence (but not activity) of Lig4.

Structure guided functional analysis of the S. cerevisiae Mre11 complex.

The Mre11 complex comprises Mre11, Rad50 and Nbs1 (Xrs2 in S. cerevisiae ). The core components, Mre11 and Rad50 are highly conserved, with readily identifiable orthologs in all clades of life, whereas Nbs1/Xrs2 are present only in eukaryotes. In eukaryotes, the complex is integral to the DNA damage response, acting in DNA double strand break (DSB) detection and repair, and the activation of DNA damage signaling. We present here a 3.2 Å cryo-EM structure of the S. cerevisiae Mre11-Rad50 complex with bound dsDNA. The structure provided a foundation for detailed mutational analyses regarding homo and heterotypic protein interfaces, as well as DNA binding properties of Rad50. We define several conserved residues in Rad50 and Mre11 that are critical to complex assembly as well as for DNA binding. In addition, the data reveal that the Rad50 coiled coil domain influences ATP hydrolysis over long distances.

HDAC Inhibitors Can Enhance Radiosensitivity of Head and Neck Cancer Cells Through Suppressing DNA Repair.

Background/Objectives: The incidence of head and neck squamous cell carcinoma (HNSCC), currently ~800,000 cases per year worldwide, is rising. Radiotherapy remains a mainstay for the treatment of HNSCC, although inherent radioresistance, particularly in human papillomavirus (HPV)-negative disease subtypes, remains a significant barrier to effective treatment. Therefore, combinatorial strategies using drugs or inhibitors against specific cellular targets are necessary to enhance HNSCC radiosensitivity to lead to an improvement in patient outcomes. Given that radiotherapy acts through targeting and damaging DNA, a common strategy is to focus on enzymes within DNA-dependent cellular pathways, such as DNA damage repair. Methods: Here, we have employed a 3D spheroid model of HNSCC (FaDu) in combination with a targeted drug screen to identify novel radiosensitisers that suppress tumour growth. Results: We identified that histone deacetylases (HDACs) were prominent candidates, and subsequently identified that the HDAC inhibitors mocetinostat and pracinostat, as well as the combined HDAC-epidermal growth factor receptor inhibitor CUDC-101, were effective at radiosensitising cell models of HNSCC (FaDu, A253, UMSCC11b) through their impact on both spheroid growth and clonogenic survival assays. We also demonstrated that this combinatorial strategy leads to inhibition of the repair of DNA double-strand breaks through the neutral comet assay and γH2AX foci analysis using immunofluorescence microscopy, providing a mechanism of action through which HDAC inhibition functions in HNSCC radiosensitisation. Conclusions: We believe that this approach should be further investigated in preclinical models, in order to realise the full therapeutic potential of HDAC inhibition for the radiosensitisation of HNSCC, eventually leading to improved patient treatment efficacy and outcomes.

DTX3L-mediated TIRR nuclear export and degradation regulates DNA repair pathway choice and PARP inhibitor sensitivity

53BP1 plays an important role in DNA double-strand break (DSB) repair and this activity is negatively regulated by its interaction with Tudor interacting repair regulator (TIRR). However, how the TIRR-53BP1 repair axis is regulated in response to DNA damage remains elusive. Here, we demonstrate that TIRR is translocated to the cytoplasm and degraded upon DNA damage. Ubiquitination of TIRR at lysine 187 by DTX3L is a critical process that regulates NHEJ pathway activity and PARP inhibitor sensitivity by facilitating XPO1-mediated TIRR nuclear export and degradation after DNA damage. We show that DTX3L is overexpressed in prostate cancers in patients and that decreased expression of TIRR due to DTX3L overexpression impairs the negative regulatory effect of TIRR on 53BP1, which consequently induces HR deficiency and chromosomal instability and sensitizes prostate cancer cells to poly (ADP-ribose) polymerase (PARP) inhibitors. Our work reveals a dual action of DTX3L on TIRR degradation and nuclear exportation and identifies DTX3L as an upstream regulator of the TIRR-53BP1 axis that governs DNA repair pathway choice and PARP inhibitor sensitivity. These findings suggest that TIRR ubiquitination and DTX3L overexpression could be viable biomarkers predicting PARP inhibitor sensitivity in cancers.

Novel pof1 mutation suppresses the sensitivity to DNA replication inhibitor in fission yeast RecQ helicase mutant.

Homologous recombination is vital for DNA double-strand break repair. Dysfunction in homologous recombination can lead to cell death, mutations, and cancer. In fission yeast (Schizosaccharomyces pombe), RecQ helicase Rqh1 resolves recombination intermediates. We found that rqh1-hd strain impaired growth in media containing hydroxyurea and thiabendazole. Using this condition, we identified a novel pof1 mutation (pof1-A81T) that suppress the poor growth of the rqh1-hd strain on the plate containing hydroxyurea and thiabendazole. Compared to rqh1-hd, rqh1-hd pof1-A81T cells displayed reduced Replication Protein A foci on chromosome bridges after hydroxyurea treatment. This suggests that pof1-A81T mutation suppresses the accumulation of recombination intermediates in hydroxyurea-treated rqh1-hd cells. Additionally, pof1-A81T mutation rescued the segregation defect of nucleolar protein Gar2 observed in hydroxyurea-treated rqh1-hd cells, potentially by mitigating recombination intermediate accumulation in rDNA. These results suggest that the pof1-A81T mutation suppresses the accumulation of recombination intermediates, particularly in rDNA, and alleviates the rqh1 deficiency phenotype in S. pombe.

An organotypic model for investigating drug-radiation responses in the lung

Background: Established in vivo radiobiological models are commonly used to assess anti-tumor effects and normal tissue toxicity. However, these models have notable limitations, and additional models are necessary to gain a deeper insights into drug-radiation interactions. Objective: This study aimed to develop an organotypic ex vivo model by using precision-cut lung slices (PCLSs) to evaluate radiation-induced residual deoxyribonucleic acid (DNA) damage, both alone and in combination with a pharmacological inhibitor of DNA double-strand break (DSB) repair. Methods: Left lungs from female C57BL/6 mice were dissected, perfused with 4% low-gelling-temperature agarose, and sliced into 250 μm sections. Lung slices were then incubated ex vivo for up to 7 days. The slices were irradiated using 137Cs, either with or without a DNA-dependent protein kinase (DNA-PK) inhibitor (NU7441). Tissue sections were subsequently fixed and stained for γH2AX and 53BP1, which serve as histological markers of DNA DSBs. Results: The established conditions preserved tissue viability for up to 7 days and maintained structural integrity for 2 days. DNA damage, detected through γH2AX and 53BP1 staining, was consistent between lungs irradiated ex vivo and their counterparts irradiated in vivo. In the organotypic model, radiation alone in DNA-PK-deficient SCID mice and radiation combined with DNA-PK inhibition in C57BL/6 mice led to increased residual γH2AX and 53BP1 staining. Conclusion: This study demonstrates that residual DNA damage levels following ionizing radiation in lung tissue are comparable between in vivo and ex vivo tissue slices, suggesting that PCLSs serve as a valuable organotypic model for investigating the effects of drug-radiation combinations.

DNA Damage and Inflammatory Response of p53 Null H358 Non-Small Cell Lung Cancer Cells to X-Ray Exposure Under Chronic Hypoxia.

Hypoxia-induced radioresistance limits therapeutic success in cancer. In addition, p53 mutations are widespread in tumors including non-small cell lung carcinomas (NSCLCs), and they might modify the radiation response of hypoxic tumor cells. We therefore analyzed the DNA damage and inflammatory response in chronically hypoxic (1% O2, 48 h) p53 null H358 NSCLC cells after X-ray exposure. We used the colony-forming ability assay to determine cell survival, γH2AX immunofluorescence microscopy to quantify DNA double-strand breaks (DSBs), flow cytometry of DAPI-stained cells to measure cell cycle distribution, ELISAs to quantify IL-6 and IL-8 secretion in cell culture supernatants, and RNA sequencing to determine gene expression. Chronic hypoxia increased the colony-forming ability and radioresistance of H358 cells. It did not affect the formation or resolution of X-ray-induced DSBs. It reduced the fraction of cells undergoing G2 arrest after X-ray exposure and delayed the onset of G2 arrest. Hypoxia led to an earlier enhancement in cytokines secretion rate after X-irradiation compared to normoxic controls. Gene expression changes were most pronounced after the combined exposure to hypoxia and X-rays and pertained to senescence and different cell death pathways. In conclusion, hypoxia-induced radioresistance is present despite the absence of functional p53. This resistance is related to differences in clonogenicity, cell cycle regulation, cytokine secretion, and gene expression under chronic hypoxia, but not to differences in DNA DSB repair kinetics.

Gold Nanoparticles Cause Radiosensitization in 4T1 Cells by Inhibiting DNA Double Strand Break Repair: Single Cell Comparisons of DSB Formation and γH2AX Expression.

Metal nanoparticles sensitize cancers to radiotherapy however their mechanisms of action are complex. The conceptual inspiration arose from theories of physical dose deposition however various chemical and biological factors have also been identified. Interpretation of data has been limited by challenges in measuring true DNA damage compared to DNA damage repair factors. Here, we applied a new assay, STRIDE, for the first time to measure DNA double strand breaks (DSBs) in 4T1 cells as a model of triple negative breast cancer exposed to gold nanoparticles and radiation, and compared this to the common γH2AX assay for DSB repair. The STRIDE assay showed no increase in DSB detection 15 mins after irradiation for cells containing nanoparticles compared to cells without. Gold nanoparticles led to prolonged detection of DSBs after irradiation and delayed the DSB repair. The data show no evidence of increased radiation dose deposition with nanoparticles, but rather enhanced radiobiological effects resulting from nanoparticles which includes disruption of the recruitment of essential DDR machinery, thereby impairing DNA repair processes.

FET fusion oncoproteins disrupt physiologic DNA repair networks in cancer.

While oncogenes promote cancer cell growth, unrestrained proliferation represents a significant stressor to cellular homeostasis networks such as the DNA damage response (DDR). To enable oncogene tolerance, many cancers disable tumor suppressive DDR signaling through genetic loss of DDR pathways and downstream effectors (e.g., ATM or p53 tumor suppressor mutations). Whether and how oncogenes can help "self-tolerize" by creating analogous functional defects in physiologic DDR networks is not known. Here we focus on Ewing sarcoma, a FET fusion oncoprotein (EWSR1-FLI1) driven pediatric bone tumor, as a model for the class of FET rearranged cancers. Native FET family members are among the earliest factors recruited to DNA double-strand breaks (DSBs), though the function of both native FET proteins and FET fusion oncoproteins in DNA repair remains to be defined. We discover that the EWSR1-FLI1 fusion oncoprotein is recruited to DNA DSBs and interferes with native FET (EWSR1) protein function in activating the DNA damage sensor ATM. In multiple FET rearranged cancers, FET fusion oncoproteins induce functional ATM defects, rendering the compensatory ATR signaling axis as a collateral dependency and therapeutic target. More generally, we find that aberrant recruitment of a fusion oncoprotein to sites of DNA damage can disrupt physiologic DSB repair, revealing a mechanism for how growth-promoting oncogenes can also create functional defects within tumor suppressive DDR networks.