Defective BRCA1-mediated DNA end resection drives tandem duplication formation and FANCM synthetic lethality.

BRCA1 -linked cancer genomes contain abundant genome-wide ∼10 kb 'Group 1' tandem duplications (TDs) that are drivers of tumorigenesis. Group 1 TD formation is recapitulated at a chromosomal Tus/ Ter site-specific replication fork barrier in DNA end resection-defective mouse embryonic stem (mES) cells lacking Brca1 exon 11. To explore relationships between DNA end resection and Group 1 TD formation, we analyzed Brca1 coiled coil (CC) domain mutants-separation-of-function alleles that are impaired for homologous recombination but competent for DNA end resection. Notably, Brca1 CC mutants retain the ability to suppress Group 1 TDs in the Tus/ Ter system and in a mouse model of Brca1 -linked tumorigenesis. These data show that Brca1 CC domain mutant cancers follow a path of tumorigenesis distinct from that of other pathogenic Brca1 alleles. FANCM is a TD co-suppressor, the loss of which is synthetic lethal/sick in combination with Brca1 exon 11 mutation. In contrast, Fancm deletion is well-tolerated by Brca1 CC mutant mES cells. Thus, Group 1 TD formation and Fancm synthetic lethality are linked phenotypes related to defective BRCA1-mediated DNA end resection.