Cooperation of the ATM and Fanconi Anemia/BRCA Pathways in Double-Strand Break End Resection

Lenoir

Proc. Natl. Acad. Sci. U.S.A.

et al. 2020

Fanconi anemia (FA) is caused by defects in cellular responses to DNA crosslinking damage and replication stress. Given the constant occurrence of endogenous DNA damage and replication fork stress, it is unclear why complete deletion of FA genes does not have a major impact on cell proliferation and germ-line FA patients are able to progress through development well into their adulthood. To identify potential cellular mechanisms that compensate for the FA deficiency, we performed dropout screens in FA mutant cells with a whole genome guide RNA library. This uncovered a comprehensive genome-wide profile of FA pathway synthetic lethality, including POLI and CDK4. As little is known of the cellular function of DNA polymerase iota (Pol ι), we focused on its role in the loss-of-function FA knockout mutants. Loss of both FA pathway function and Pol ι leads to synthetic defects in cell proliferation and cell survival, and an increase in DNA damage accumulation. Furthermore, FA-deficient cells depend on the function of Pol ι to resume replication upon replication fork stalling. Our results reveal a critical role for Pol ι in DNA repair and replication fork restart and suggest Pol ι as a target for therapeutic intervention in malignancies carrying an FA gene mutation.

Section: Discussionsupporting

confidence: 71%

DNA polymerase ι compensates for Fanconi anemia pathway deficiency by countering DNA replication stress

Lenoir

Proc. Natl. Acad. Sci. U.S.A.

et al. 2020

“…Emerging data suggests that loss of Fanconi anemia genes results in hypersensitivity to ATM inhibition [54]. Whether tumors with Fanconi anemia pathway or other genetic alterations could provide additional tumor specificity with the ATM inhibition plus Bragg peak irradiation combination warrants additional investigation.…”

Section: Discussionmentioning

confidence: 99%

Inhibition of ATM Induces Hypersensitivity to Proton Irradiation by Upregulating Toxic End Joining

Zhou

Howard

et al. 2021

Cancer Research

Proton Bragg peak irradiation has a higher ionizing density than conventional photon irradiation or the entrance of the proton beam profile. Whether targeting the DNA damage response could enhance vulnerability to the distinct pattern of damage induced by proton Bragg peak irradiation is currently unknown. Here we performed genetic or pharmacologic manipulation of key DNA damage response elements and evaluated DNA damage signaling, DNA repair, and tumor control in cell lines and xenografts treated with the same physical dose across a radiotherapy linear energy transfer spectrum. Radiotherapy consisted of 6 MV photons and the entrance beam or Bragg peak of a 76.8 MeV spot scanning proton beam. More complex DNA double strand breaks induced by Bragg peak proton irradiation preferentially underwent resection and engaged homologous recombination (HR) machinery. Unexpectedly, the ATM inhibitor AZD0156 but not an inhibitor of ATR rendered cells hypersensitive to more densely ionizing proton Bragg peak irradiation. ATM inhibition blocked resection and shunted more double strand breaks to processing by toxic ligation through nonhomologous end-joining, whereas loss of DNA ligation via XRCC4 or Lig4 knockdown rescued resection and abolished the enhanced Bragg peak cell killing. Proton Bragg peak monotherapy selectively sensitized cell lines and tumorxenografts with inherent HR defects, and the repair defect induced by ATM inhibitor coadministration showed enhanced efficacy in HR proficient models. In summary, inherent defects in HR or administration of an ATM inhibitor in HR proficient tumors selectively enhance the relative biological effectiveness of proton Bragg peak irradiation.

“…By inhibiting ATM in BRCA1/p53 double-de cient tumor cells, activation of cell cycle checkpoint and DNA damage pathways could be abrogated leading to cell cycle progression, exacerbating the level of genetic lesions, and subsequently leading to apoptosis. Furthermore, synthetic lethality between genes of the Fanconi anemia/BRCA pathway and ATM inhibition were shown to induce toxic levels of NHEJ leading to genomic rearrangements (59), suggesting that toxic NHEJ levels might play a role in the synergistic cytotoxicity between ATM and EZH2 inhibition in BRCA1-de cient cells.…”

Section: Discussionmentioning

confidence: 99%

EZH2 Inhibition Sensitizes BRCA1-Deficient Breast Cancer to Synthetic Lethal Therapy with ATM Inhibitors

Ratz

Brambillasca

Bartke

et al. 2021

Preprint

Background: The majority of BRCA1-mutant breast cancers are characterized by a triple-negative phenotype (TNBC) and a basal-like molecular subtype, associated with aggressive clinical behavior. Current treatment options are limited, highlighting the need for the development of novel targeted therapies for this tumor subtype. Methods: Our group previously showed that EZH2 is functionally relevant in BRCA1-deficient breast tumors and blocking EZH2 enzymatic activity could be a potent treatment strategy. To validate the role of EZH2 as a therapeutic target and to identify new synergistic drug combinations, we performed a high-throughput drug combination screen in various cell lines derived from BRCA1-proficient and deficient - mouse mammary tumors. Results: We identified the combined inhibition of EZH2 and the proximal DNA damage response kinase ATM as a novel synthetic lethality-based therapy for the treatment of BRCA1-deficient breast tumors. We show that the combined treatment with the EZH2 inhibitor GSK126 and the ATM inhibitor AZD1390 led to reduced colony formation, increased genotoxic stress, and apoptosis-mediated cell death in BRCA1-deficient mammary tumor cells in vitro. These findings were corroborated by in vivo experiments showing that simultaneous inhibition of EZH2 and ATM significantly increased anti-tumor activity in mice bearing BRCA1-deficient mammary tumors. Conclusion: Taken together, we identified a synthetic lethal interaction between EZH2 and ATM and propose this synergistic interaction as a novel molecular combination for the treatment of BRCA1-mutant breast cancer.