Prevalence of BRCA1/2 variants in an Ovarian Cancer Cohort: outcomes from a Nationwide Testing Program.

Poly(ADP-ribose) polymerase inhibitors (PARPi) have revolutionized the management of BRCA1- and BRCA2-associated ovarian cancer (OC). In 2020, Ireland implemented a nationwide, oncology-led pathway for mainstreamed BRCA1/2 testing in patients with OC. This study evaluated the pathway and characterised the BRCA1/2 landscape in an Irish cohort of patients with OC. Samples collected between January 2020 and December 2023 were tested in two national molecular diagnostic laboratories. Single-molecule molecular inversion probe sequencing was used to detect single nucleotide variants, while multiplex ligation-dependent probe amplification assessed large genomic rearrangements. Variants were classified according to the American College of Medical Genetics and Genomics and Association for Clinical Genomic Science 2020 guidelines and CanVIG-UK specifications. In total, 535 patients were included, with a median age of 65.5 years (range, 27-89 years). Of these, 455/535 (85.0%) underwent germline BRCA1/2 (gBRCA) testing using peripheral blood, and 360/535 (67.2%) underwent tumour BRCA1/2 (tBRCA) testing, resulting in 292/535 (54.6%) patients having paired testing. Among gBRCA-tested patients, 10.3% (47/455) had a clinically actionable variant (CAV), while 2.1% (10/455) had a variant of uncertain significance. Of the 262 patients who underwent paired testing and had negative gBRCA results, 9.5% (25/262) had a somatic BRCA CAV. Recurrent germline BRCA CAVs were identified in regional clusters, including BRCA1 c.5266dup, BRCA1 c.1175_1214del, and BRCA2 c.4398_4402del. This nationwide real-world study demonstrates that mainstreamed germline and somatic BRCA1/2 testing is feasible in Ireland and reports the prevalence of BRCA CAVs in a cohort of patients with OC. Although the observed prevalence of germline BRCA CAVs was lower than anticipated, it is consistent with UK real-world data. The identification of recurrent, regionally clustered germline variants further enhances the characterisation of the Irish genomic landscape.