Evidence for pathogenicity of c.8351G>A p.(Arg2784Gln) and the challenges in classification of pathogenic variants with reduced penetrance.

[BACKGROUND] The c.8351G>A p.(Arg2784Gln) variant has long been classified as a variant of uncertain significance (VUS) due to conflicting evidence used in variant classification. This study aims to clarify its pathogenicity and associated risks for breast and ovarian cancer.

[METHODS] This study was conducted by the international Evidence-based Network for the Interpretation of Germline Mutant Alleles consortium. We collected data from 29 informative families with this variant. Co-segregation likelihood ratios (LRs) were calculated using the full-likelihood method to assess pathogenicity, and cancer risks were estimated with modified segregation analysis.

[RESULTS] Co-segregation analysis using a grid search across scaled penetrance levels for truncating variants yielded the strongest evidence in favour of pathogenicity, with LR maximised at approximately 20% of full penetrance (LR=11.026). Furthermore, estimated breast cancer risks were markedly higher for early onset breast cancer; women diagnosed at <50 years had a HR of 4.5, compared with a HR of 1.65 for women diagnosed at ≥50 years. The estimated lifetime risks were 25% for breast cancer and 6% for ovarian cancer.Evidence of pathogenicity was also supported by the presence of the variant allele in two patients with Fanconi anaemia.

[CONCLUSIONS] Our results indicate that c.8351G>A p.(Arg2784Gln) has a disease-causing effect, with reduced penetrance, similar to other pathogenic variants in moderate risk breast cancer genes such as and . We also provide risk-adapted recommendations for clinical management. Importantly, one should be aware of a reduced penetrance as the underlying reason for conflicting results among pieces of evidence used for variant classification.