The BRCA1 coiled-coil domain is dispensable for suppression of tandem duplications and tolerance of FANCM loss

BRCA1-linked cancers contain abundant ~10 kb 'Group 1' tandem duplications (TDs). Group 1 TDs form at a Tus/Ter replication-fork barrier in DNA-end resection-defective mouse embryonic stem (mES) cells lacking Brca1 exon 11. To elucidate how BRCA1 suppresses Group 1 TDs, we analyzed Brca1 coiled-coil (CC)-domain mutants-separation-of-function alleles impaired for homologous recombination (HR) through loss of PALB2-binding and RAD51-loading functions 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 mammary tumorigenesis. These data suggest that Brca1 CC domain-mutant cancers follow a path of tumorigenesis distinct from that of other Brca1-linked cancers. FANCM is a TD co-suppressor, loss of which is synthetic lethal/sick in Brca1 exon 11-deleted cells. In contrast, Fancm deletion unexpectedly improves the growth of Brca1 CC mutant and Brca2 mutant mES cells. Thus, Group 1 tandem duplication formation and Fancm synthetic lethality are linked phenotypes, potentially related to defective BRCA1-mediated DNA end resection but genetically separable from BRCA1/PALB2-mediated RAD51 loading.