A DNA-damage selective role for BRCA1 E3 ligase in claspin ubiquitylation, CHK1 activation, and DNA repair.

The breast and ovarian cancer suppressor BRCA1 is essential for cellular responses to DNA damage. It heterodimerizes with BARD1 to acquire an E3 ubiquitin (Ub) ligase activity that is often compromised by cancer-associated mutations. Neither the significance of this activity to damage responses, nor a relevant in vivo substrate, is clear. We have separated DNA-damage responses requiring the BRCA1 E3 ligase from those independent of it, using a gene-targeted point mutation in vertebrate DT40 cells that abrogates BRCA1's catalytic activity without perturbing BARD1 binding. We show that BRCA1 ubiquitylates claspin, an essential coactivator of the CHK1 checkpoint kinase, after topoisomerase inhibition, but not DNA crosslinking by mitomycin C. BRCA1 E3 inactivation decreases chromatin-bound claspin levels and impairs homology-directed DNA repair by interrupting signal transduction from the damage-activated ATR kinase to its effector, CHK1. Our findings identify claspin as an in vivo substrate for the BRCA1 E3 ligase and suggest that its modification selectively triggers CHK1 activation for the homology-directed repair of a subset of genotoxic lesions. This mechanism unexpectedly defines an essential but selective function for BRCA1 E3 ligase activity in cellular responses to DNA damage.