Cambridge, UK—Mutations in the ATM gene cause the devastating neurodegenerative and cancer predisposition disease called ataxia-telangiectasia (A-T), and are also associated with various forms of sporadic cancer. Previous work has shown that the ATM protein, which is produced from the ATM gene, serves as a molecular guardian of the genome by detecting DNA damage and promoting its repair. Consequently, A-T patients and ATM-deficient cells are hyper-sensitive to various endogenous DNA lesions that can lead to neurodegeneration, as well as DNA-damaging agents used in cancer therapy such as PARP inhibitors.
As described by their recent publication in Nature Communications, researchers in the laboratory of Professor Steve Jackson at The Gurdon Institute, University of Cambridge, have collaborated with colleagues at AstraZeneca to identify mechanisms by which the drug sensitivities of ATM-deficient cells can be alleviated by changes in other genes. Thus, through using cutting-edge CRISPR-Cas9 genetic screens, the authors show that defects in the products of several genes also involved in DNA repair pathways, including components of the BRCA1-A complex and the non-homologous end joining factors LIG4, XRCC4 and XLF, can alleviate the hypersensitivity of ATM-deficient cells to PARP inhibitors and the chemotherapeutic drug topotecan.
The study notes that “in the absence of ATM, NHEJ [non-homologous end-joining]-mediated repair of seDSBs [single-ended double-strand breaks] induced by TOP1 or PARP1 inhibition results in aberrant chromatid fusions and cell death. Strikingly, both phenotypes can be rescued by impairing NHEJ, either via loss/mutation of LIG4 [DNA ligase IV], XLF, or XRCC4, or by promoting increased engagement of HRR via loss of specific components of the BRCA1-A complex. In addition to highlighting potential mechanisms for therapeutic resistance in ATM-deficient cancers, our results suggest that the prime mechanism by which ATM promotes cell survival in response to seDSB generation is not to remove Ku from such structures but to promote efficient DSB resection and thereby prevent seDSB repair by toxic NHEJ.”
Co-lead authors on the paper Dr. Gabriel Balmus from Dementia Research Institute at University of Cambridge and Domenic Pilger, Jackson Lab Cancer Research UK graduate student, noted that “We are excited by the publication of our research and by the possibility that it might improve cancer therapies and could lead to a therapeutic approach for the neurodegenerative disease A-T.”
In addition to this work providing new mechanistic insights into how cells respond to DNA damage, these findings also have potential medical relevance. First, they suggest how cancers with ATM mutations might evolve resistance in the clinic and how this may make these resistant cancers susceptible to other anti-cancer agents. Second, they suggest potential therapeutic targets for A-T.
“It has been wonderful collaborating with the group of Prof. Steve Jackson to carry out these exciting studies. My colleagues and I at AstraZeneca are now exploring how these findings might lead to the discovery of more effective cancer treatments,” said Dr. Josep Forment, Oncology Team Leader at AstraZeneca, who is co-lead and co-corresponding author of the study.
“Based on our findings, we suggest that exploring the genetic and transcriptional status of genes for BRCA1-A complex members and of the NHEJ components Ku70/80, XRCC4, XLF, and LIG4 in ATM-deficient tumors might help in predicting responses to seDSB-inducing agents such as TOP1 or PARP inhibitors,” the article points out.
“This study marks a major step forward in our understanding of how the ATM protein maintains genome stability and how ATM defects can cause cancer and neurodegeneration in human patients with A-T,” commented Prof. Steve Jackson. “My colleagues and I are very excited by the potential clinical applications for our findings, which we now plan to actively pursue in my laboratory and with our colleagues elsewhere.”
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