Background and Objectives: Ovarian cancer (OC) is the seventh most common cancer among women worldwide, and at least 15% of ovarian cancer is considered to be hereditary. DNA repair genes play a crucial role in cancer susceptibility syndromes. The association between Hereditary Breast and Ovarian Cancer syndrome and deficient DNA damage response (DDR) is particularly well documented. Inactivating mutations in RAD51D, a key player in homologous recombination repair (HRR), has been implicated in the etiology of ovarian cancer. However, the link between rare non-truncating RAD51D variants and ovarian cancer is not yet confirmed.  Our aim is to fully characterize the previously described missense RAD51D variant (c.680C>T;(p.S227L), rs370228071) and its implications on a French Canadian kindred affected with high grade serous ovarian cancer (HGSC).

Methods: Sanger sequencing and whole exome sequencing have been applied to decipher the genetic cause of ovarian cancer in a woman and two sisters, all three having been diagnosed with HGSC of the ovary or endometrium. Sensitivity to PARP inhibitors was evaluated by treatment of RAD51D mutant cell lines with olaparib and BMN 673 in a cell survival assay.

Results: The RAD51D missense mutation c.680C>T;p.S227L was identified in the germline of a proband affected with HGSC of the ovary at the age of 77. Genetic testing reveals the presence of the mutation in her two affected sisters. Sanger-based loss of heterozygosity (LOH) study was inconclusive and the presence of second somatic mutation in RAD51D was excluded by whole exome sequencing. Subsequently, ExomeAI statistical approach applied to the WES dataset of the tumours paired to their normal tissue allowed us to confirm the presence of LOH for the entire RAD51D locus in all three carriers. Moreover allelic imbalances profiling of the tumours revealed a pattern of recurrent imbalances all over the genome, highlighting the full Chr17 imbalance in all three tumours. Given the known sensitivity of HRR-deficient cells to PARP inhibitors, RAD51D c.680C>T was stably expressed in RAD51D-null cells which were later treated with PARP inhibitors olaparib and BMN 673. RAD51D c.680C>T was found to confer sensitivity to olaparib and more strongly to BMN 673, in contrast to previously-characterized non-pathogenic c.47T>C-mutant and WT cell lines.

Conclusion: In this report, we present robust evidence for the pathogenicity of this rare missense variant in RAD51D (c.680C>T;p.S227L), characterizing it as a susceptibility allele for HGSC. Furthermore, PARP-inhibitor sensitivity conferred by this mutation confirms a targeted molecular therapeutic avenue for this disease as previously hypothesized.