A New Cell-Free System to Study BRCA1 Function
Abstract
DNA interstrand cross-links are extremely cytotoxic DNA lesions that are generated by chemotherapy agents and certain endogenous metabolites. Using this award, we showed that the tumor suppressor BRCA1-BARD1 plays a novel role in the repair of DNA interstrand cross-links (ICL). Normally, when replication forks collide with an ICL, leading strands stall 20nucleotides from the ICL due to steric hindrance by the CMG helicase, which unwinds DNA ahead of the polymerase. Subsequently, CMG dissociates and the leading strand is extended towards the ICL, followed by excision of the cross-link by nucleases that are controlled by the Fanconi anemia pathway. In BRCA1-depleted egg extracts, the leading strands remain arrested at the -20 position because the CMG helicase fails to be unloaded. As a result, all downstream repair events are inhibited. The action of BRCA1-BARD1 at ICLs is likely direct because the complex binds to ICLs coincident with CMG removal. We further show that CMG unloading involves ubiquitin signaling and the action of the AAA ATPase p97. BRCA1does not perform this novel function by acting through FANCJ, FANCM, or CTIP. Our data support a model in which stalled and converged CMG helicases undergo polyubiquitylation (possibly by BRCA1-BARD1, which has E3 ubiquitin ligase activity), followed by extraction of the ubiquitylated complex from chromatin by p97. Our results demonstrate that CMG unloading is the first active event in ICL repair, and that it is regulated by the BRCA1-BARD1 complex.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 01, 2016
- Accession Number
- AD1019343
Entities
People
- Johannes Walter
Organizations
- Harvard Medical School