A BRCA1 coiled-coil domain variant disrupting PALB2 interaction predisposes to mammary tumors with a targetable defect in homologous recombination repair [CNV-seq]

The BRCA1 tumor suppressor gene encodes a multi-domain protein for which several functions have been described. These include a key role in homologous recombination repair (HRR) of DNA double-strand breaks (DSBs), which is shared with two other high-risk hereditary breast cancer suppressors, BRCA2 and PALB2. Although both BRCA1 and BRCA2 interact with PALB2, BRCA1 missense variants affecting its PALB2-interacting coiled-coil domain are considered sequence variants of uncertain clinical significance (VUS). Using genetically engineered mice, we now show that a BRCA1 coiled-coil domain VUS, Brca1 p.L1363P, disrupting the interaction with PALB2 leads to embryonic lethality and loss of breast cancer suppression. Brca1 p.L1363P mammary tumors develop with a similar latency as Brca1-null tumors, but show different histopathological features and more stable DNA copy number profiles. Nevertheless, Brca1 p.L1363P mammary tumors are HRR-incompetent and responsive to cisplatin and PARP inhibition. Overall design: RNASeq and CNVSeq of Brca1 p.L1363P mammary tumors from different histology classes were compared to other KP, KB1P, and KB2P breast cancer mouse model mammary tumors