Defective BRCA1-mediated DNA end resection drives tandem duplication formation and <i>FANCM</i> synthetic lethality

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