BRCA1 suppresses microhomology-mediated tandem duplications at Tus/Ter-stalled replication forks

Small ~10 kb microhomology-mediated tandem duplications (“Group 1 TDs”) are abundant in BRCA1-linked but not BRCA2-linked breast cancer genomes. Here, we define the mechanism underlying this rearrangement signature. We show that BRCA1, but not BRCA2, suppresses TDs at a Tus/Ter site-specific chromosomal replication fork barrier in primary mammalian cells. BRCA1 has no equivalent role at chromosomal double strand breaks, indicating specificity for the stalled fork response. Two motor proteins—FANCM and the Bloom's syndrome helicase—suppress Tus/Ter-induced TDs in BRCA1 mutants, revealing the existence of a multi-gene TD suppressor network. TDs arise by a “replication restart-bypass” mechanism terminated by end joining or microhomology-mediated template switching, the latter forming complex TD breakpoints. We show that solitary DNA ends form directly at Tus/Ter, implicating misrepair of these lesions in TD formation. We find that BRCA1 inactivation is strongly associated with Group 1 TDs in ovarian cancer. The Group 1 TD phenotype may be a general signature of BRCA1-deficient cancer Overall design: LAM-HTGTS from a Cas9/gRNA bait DSB 30kb away from the 1xGFP Ter array cassette inserted in the ROSA26 locus