A functionally impaired missense variant identified in French Canadian families implicates FANCI as a candidate ovarian cancer-predisposing gene

BACKGROUND: Familial ovarian cancer (OC) cases not harbouring pathogenic variants in either of the BRCA1 and BRCA2 OC-predisposing genes, which function in homologous recombination (HR) of DNA, could involve pathogenic variants in other DNA repair pathway genes. METHODS: Whole exome sequencing was used to identify rare variants in HR genes in a BRCA1 and BRCA2 pathogenic variant negative OC family of French Canadian (FC) ancestry, a population exhibiting genetic drift. OC cases and cancer-free individuals from FC and non-FC populations were investigated for carrier frequency of FANCI c.1813C>T; p.L605F, the top-ranking candidate. Gene and protein expression were investigated in cancer cell lines and tissue microarrays, respectively. RESULTS: In FC subjects, c.1813C>T was more common in familial (7.1%, 3/42) than sporadic (1.6%, 7/439) OC cases (P = 0.048). Carriers were detected in 2.5% (74/2950) of cancer-free females though female/male carriers were more likely to have a first-degree relative with OC (121/5249, 2.3%; Spearman correlation = 0.037; P = 0.011), suggesting a role in risk. Many of the cancer-free females had host factors known to reduce risk to OC which could influence cancer risk in this population. There was an increased carrier frequency of FANCI c.1813C>T in BRCA1 and BRCA2 pathogenic variant negative OC families, when including the discovery family, compared to cancer-free females (3/23, 13%; OR = 5.8; 95%CI = 1.7–19; P = 0.005). In non-FC subjects, 10 candidate FANCI variants were identified in 4.1% (21/516) of Australian OC cases negative for pathogenic variants in BRCA1 and BRCA2, including 10 carriers of FANCI c.1813C>T. Candidate variants were significantly more common in familial OC than in sporadic OC (P = 0.04). Localization of FANCD2, part of the FANCI-FANCD2 (ID2) binding complex in the Fanconi anaemia (FA) pathway, to sites of induced DNA damage was severely impeded in cells expressing the p.L605F isoform. This isoform was expressed at a reduced level, destabilized by DNA damaging agent treatment in both HeLa and OC cell lines, and exhibited sensitivity to cisplatin but not to a poly (ADP-ribose) polymerase inhibitor. By tissue microarray analyses, FANCI protein was consistently expressed in fallopian tube epithelial cells and only expressed at low-to-moderate levels in 88% (83/94) of OC samples. CONCLUSIONS: This is the first study to describe candidate OC variants in FANCI, a member of the ID2 complex of the FA DNA repair pathway. Our data suggest that pathogenic FANCI variants may modify OC risk in cancer families. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13073-021-00998-5.