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Characterizing DNA Repair Processes at Transient and Long-lasting Double-strand DNA Breaks by Immunofluorescence Microscopy
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The BRCA1-A complex restricts replication fork reversal-dependent DNA repair in ATM deficient cells

Arindam Datta1, Jessica Jackson2, Yaroslav I Morozov1

  • 1Department of Cancer Biology, Penn Center for Genome Integrity, Basser Center for BRCA, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA.

Nature Communications
|July 4, 2026
PubMed

Insights

Ataxia Telangiectasia Mutated (ATM) kinase deficiency causes cancer and drug sensitivity. BRCA1-A complex loss in ATM-deficient cells restores drug resistance by enabling DNA end resection at damaged forks.

Area of Science:

  • DNA repair mechanisms
  • Cancer biology
  • Genetics

Background:

  • Ataxia Telangiectasia Mutated (ATM) kinase deficiency leads to cancer and drug hypersensitivity.
  • BRCA1-interacting protein A (BRCA1-A) complex or XRCC4/Ligase 4 deficiency confers resistance to Topoisomerase I or PARP1 inhibitors in ATM-deficient cells.
  • BRCA1-A complex may direct toxicity to DNA damaging agents in ATM-mutated cells via illegitimate end-joining.

Purpose of the Study:

  • To investigate the role of ATM inhibition in DNA damage response.
  • To elucidate the mechanism by which BRCA1-A complex influences drug sensitivity in ATM-deficient cells.
  • To identify key factors determining chemotherapy response in ATM-deficient cancers.

Main Methods:

  • Cellular assays to assess DNA damage recognition and repair.
  • SUMOylation and ubiquitination assays.
  • Chromatin accessibility and nuclease activity measurements.
  • Electron microscopy to visualize replication fork dynamics.
  • Drug sensitivity assays with Topoisomerase I inhibitors.

Main Results:

  • ATM inhibition triggers SUMO and ubiquitin-mediated BRCA1-A recognition of damaged forks, restricting end-resection and causing Topoisomerase I inhibitor hypersensitivity.
  • BRCA1-A deficient cells exhibit increased chromatin accessibility and nuclease activity at damaged forks, restoring DNA end resection and drug resistance.
  • ATM inhibition prevents replication fork reversal; loss of BRCA1-A restores fork reversal, generating substrates for end resection.

Conclusions:

  • BRCA1-A enforces a restrictive chromatin state to prevent the formation of DNA end resection substrates.
  • Replication fork reversal is a critical determinant of chemotherapy response in ATM-deficient cells.
  • Targeting BRCA1-A or modulating fork reversal may offer therapeutic strategies for ATM-deficient cancers.

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