Role for ATM in DNA damage-induced phosphorylation of BRCA1

The human genetic disorder ataxia-telangiectasia is characterized by immuno deficiency, progressive cerebellar ataxia, radiosensitivity, cell cycle che ckpoint defects, and cancer predisposition, The gene product [ataxia-telang iectasia mutation (ATM)] mutated in this syndrome is a component of the DNA damage detection pathway. Loss of ATM function in human and mouse cells ca uses defects in DNA repair and cell cycle checkpoint control and, not surpr isingly, humans and mice with compromised ATM function are prone to cancers . An excess of breast cancer in the relatives of atasia-telangiectasia pati ents has also been reported by epidemiological studies. Predisposition to b reast and ovarian cancers is also observed in women with germline mutations in BRCA1, a tumor suppressor gene. BRCA1 is a nuclear protein with a cell cycle-regulated expression pattern and is hyperphosphorylated in response t o DNA-damaging agents. Here we show that rapid ionizing radiation-induced i n vivo phosphorylation of BRCA1 requires the presence of functional ATM pro tein. Furthermore, we show that ATM interacts with BRCA1, and this associat ion is enhanced by radiation. We also demonstrate that BRCA1 is a substrate of ATR kinase in vitro and irt vivo. Using phospho-specific antibodies aga inst serines 1387, 1423, and 1457 of BRCA1, we demonstrate radiation-induce d, ATR I-dependent phosphorylation of BRCA1 at these sites. These findings show that BRCA1 is regulated by an ATM-dependent mechanism as a part of the cellular response to DNA damage. This interaction between ATM and BRCA1 ar gues in favor of the involvement of particular aspects of ATM function in b reast cancer predisposition.