Rare germline variants in PALB2 and BRCA2 in familial and sporadic chordoma.

Chordoma is a rare bone tumor with genetic risk factors largely unknown. We conducted a whole-exome sequencing (WES) analysis of germline DNA from 19 familial chordoma cases in five pedigrees and 137 sporadic chordoma patients and identified 17 rare germline variants in PALB2 and BRCA2, whose products play essential roles in homologous recombination (HR) and tumor suppression. One PALB2 variant showed disease cosegregation in a family with four affected people or obligate gene carrier. Chordoma cases had a significantly increased burden of rare variants in both genes when compared to population-based controls. Four of the six PALB2 variants identified from chordoma patients modestly affected HR function and three of the 11 BRCA2 variants caused loss of function in experimental assays. These results, together with previous reports of abnormal morphology and Brachyury expression of the notochord in Palb2 knockout mouse embryos and genomic signatures associated with HR defect and HR gene mutations in advanced chordomas, suggest that germline mutations in PALB2 and BRCA2 may increase chordoma susceptibility. Our data shed light on the etiology of chordoma and support the previous finding that PARP-1 inhibitors may be a potential therapy for some chordoma patients.

ATR/ATM-Mediated Phosphorylation of BRCA1 T1394 Promotes Homologous Recombinational Repair and G2-M Checkpoint Maintenance.

BRCA1 maintains genome integrity and suppresses tumorigenesis by promoting homologous recombination (HR)-mediated repair of DNA double-strand breaks (DSB) and DNA damage-induced cell-cycle checkpoints. Phosphorylation of BRCA1 by ATM, ATR, CHK2, CDK, and PLK1 kinases has been reported to regulate its functions. Here we show that ATR and ATM-mediated phosphorylation of BRCA1 on T1394, a highly conserved but functionally uncharacterized site, is a key modification for its function in the DNA damage response (DDR). Following DNA damage, T1394 phosphorylation ensured faithful repair of DSBs by promoting HR and preventing single-strand annealing, a deletion-generating repair process. BRCA1 T1394 phosphorylation further safeguarded chromosomal integrity by maintaining the G2-M checkpoint. Moreover, multiple patient-derived BRCA1 variants of unknown significance were shown to affect T1394 phosphorylation. These results establish an important regulatory mechanism of BRCA1 function in the DDR and may have implications in the development or prognosis of BRCA1-associated cancers. SIGNIFICANCE: This study identifies a BRCA1 phosphorylation event critical for its DNA repair function and reveals the functional defects of several BRCA1 variants of unknown significance.