Germline RBBP8 variants associated with early-onset breast cancer compromise replication fork stability

Abstract

Haploinsufficiency of factors governing genome stability underlies hereditary breast and ovarian cancer. Homologous recombination (HR) repair is a major pathway disabled in these cancers. With the aim of identifying new candidate genes, we examined early onset breast cancer patients negative for BRCA1 and BRCA2 pathogenic variants. Here, we focused on CtIP (RBBP8 gene) that mediates HR repair through the end-resection of DNA double-strand breaks (DSB). Notably, the patients exhibited a number of rare germline RBBP8 variants, and functional analysis revealed that these variants did not affect DNA DSB end-resection efficiency. However, expression of a subset of variants led to deleterious nucleolytic degradation of stalled DNA replication forks in a manner similar to cells lacking BRCA1 or BRCA2. In contrast to BRCA1 and BRCA2, CtIP deficiency promoted the helicase-driven destabilization of RAD51 nucleofilaments at damaged DNA replication forks. Taken together, our work identifies CtIP as a critical regulator of DNA replication fork integrity, which when compromised, may predispose to the development of early onset breast cancer.