Whole exome sequencing reveals a combination of rare high and low penetrance variants that correlates with familial breast cancer relative risk

Background: Genetic risk factors of breast cancer are very heterogeneous and complex. They vary according to the familial relative risk, the age of cancer diagnosis of the index case and the age of the affected relatives.Objectives: We aimed to investigate and identify simultaneously all rare pathogenic and common variants in unrelated BC cases with different relative risk ratios for breast cancer and evaluate the contribution of these variants in genetic susceptibility to breast cancer.Patients and Methods: All frequent mutations in BRCA genes previously identified in Tunisia have been excluded by Sanger sequencing in 42 women affected with high family risk having at least 3 cancer affected related individuals. Two unrelated cases having two different family histories (in terms of different numbers of affected first-degree relatives and young age onset) have been selected for whole exome sequencing. The first family is composed of three sisters F1.1, F1.2 and F1.3 affected at 46, 50, and 32 years old, respectively. The second has only two breast cancer cases, F2.2 and F2.4, affected at late age 61 and 70 years old, respectively, in addition to other 5 members affected by different kinds of cancer. Selected high risk variants were confirmed and segregation analysis was performed using Sanger sequencing. Results and discussion: For F1.1 case, we identified a pathogenic frame-shift loss of function variant in BRCA2 p.Val1283Lysfs. For F2.2 we identified a pathogenic rare variant in OGG1, p.Arg46Gln that co-segregates with a rare non sense variant in BRCA2 p.K3326X, only in the breast cancer affected cases. Moreover, F2.2 patient has 9 other common low penetrant variants in different loci known to represent independently minor, but cumulatively significant, increased risk for breast cancer.Conclusion: Family history and the young age at onset for patient F1.1 correlate with the presence of a rare high penetrant variant (p.Val1283Lysfs) in BRCA2 gene. However, the late age at onset and the less severe phenotype for patient F2.2 are probably the consequence of the presence of a pathogenic variant p.Arg46Gln in OGG1 gene that co-segregate with a low penetrant variant Lys3326X in BRCA2 only in breast cancer cases.