Protein network in DNA damage response and breast carcinogenesis

We investigate the ATM (mutated in ataxia telangiectasia) signaling network, with focus on its interaction with DNA repair proteins. IR-induced phosphorylation of NBSl (Nijmegen breakage syndrome), a component of a recombination and repair complex requires ATM. Mutations of NBSl gene lead to developmental defects, chromosome breakage, and predisposition to lymphoma. Complexes containing ATM and NBSl exist in vivo in both untreated and cells treated with IR. We have identified residues of NBSl that are phosphorylated in vitro by ATM and whose modification in vivo is essential for the cellular response to DNA damage, including cell cycle checkpoint activation, formation of NBS1/Mre11/Rad50 (N/M/R) foci and rescue of hypersensitivity to IR. These works are among the first to provide mechanistic insights into the interaction between checkpoint kinases and DNA double strand break repair proteins. We demonstrate that optimal chromatin-association of Rad50/Mre11/NBSl proteins require both the conserved forkhead-associated (FHA) and breast cancer carboxyl-terminus (BRCT) domains of NBSl. Both FHA and BRCT domains are essential for IR-induced foci (IRIF) formation of N/M/R and S phase checkpoint activation, but only the BRCT domain is needed for cellular survival after IR. The A-T like syndrome is due to mutations of Mre11 nuclease that form the repair protein complex. We have determined the kinase/substrate relationship between ATM and Mre11. It has been shown recently that N/M/R complex is required for IR-induced activation of ATM. Chromatin SMCl protein is an effector of ATM and NBSl. Future studies are aimed toward understanding how N/M/R complexes and chromatin structure changes lead to ATM activation.