Combined genetic and splicing analysis of BRCA1 c.[594-2A>C; 641A>G] highlights the relevance of naturally occurring in-frame transcripts for developing disease gene variant classification algorithms.

Abstract

A recent analysis using family history weighting and co-observation classification modeling indicated that BRCA1 c.594-2A > C (IVS9-2A > C), previously described to cause exon 10 skipping (a truncating alteration), displays characteristics inconsistent with those of a high risk pathogenic BRCA1 variant. We used large-scale genetic and clinical resources from the ENIGMA, CIMBA and BCAC consortia to assess pathogenicity of c.594-2A > C. The combined odds for causality considering case-control, segregation and breast tumor pathology information was 3.23 × 10-8 Our data indicate that c.594-2A > C is always in cis with c.641A > G. The spliceogenic effect of c.[594-2A > C;641A > G] was characterized using RNA analysis of human samples and splicing minigenes. As expected, c.[594-2A > C; 641A > G] caused exon 10 skipping, albeit not due to c.594-2A > C impairing the acceptor site but rather by c.641A > G modifying exon 10 splicing regulatory element(s). Multiple blood-based RNA assays indicated that the variant allele did not produce detectable levels of full-length transcripts, with a per allele BRCA1 expression profile composed of ≈70-80% truncating transcripts, and ≈20-30% of in-frame Δ9,10 transcripts predicted to encode a BRCA1 protein with tumor suppression function.We confirm that BRCA1c.[594-2A > C;641A > G] should not be considered a high-risk pathogenic variant. Importantly, results from our detailed mRNA analysis suggest that BRCA-associated cancer risk is likely not markedly increased for individuals who carry a truncating variant in BRCA1 exons 9 or 10, or any other BRCA1 allele that permits 20-30% of tumor suppressor function. More generally, our findings highlight the importance of assessing naturally occurring alternative splicing for clinical evaluation of variants in disease-causing genes.The research described was supported by Spanish Instituto de Salud Carlos III funding, an initiative of the Spanish Ministry of Economy and Innovation partially supported by European Regional Development FEDER Funds [PI12/00539 and PI15/00059 to M.d.H., PI13/02030 to A.V.]; the French Ministry of Higher Education and Research [to O.S.]; the University of Otago, Mackenzie Charitable Foundation, Maria Lupton, and .Health Research Council of New Zealand [to L.W.]; UK Higher Education Funding Council Senior Fellowship Scheme, the University of Southampton [to D.B.]; Cancer research UK [to D.B., M.R.]; FamilienHede Nielsen Foundation fund [to T.V.O.H.]; Cancer Research-UK Senior Cancer Research Fellowship [to A.C.A.]; National Institute of Health [CA128978 and CA11616 to F.J.C.]; an NIH specialized program of research excellence in breast cancer to the Mayo Clinic [P50 CA116201 to F.J.C.]; and the US Breast Cancer Research Foundation [to F.J.C.]; translational grant from the French National Cancer Institute and Direction Générale de l'Offre des Soins (INCa-DGOS AAP/CFB/CI) and a grant from the French North-West Canceropole (CNO) [to A.M.]; The Cancer Council Queensland [APP1086286 to A.B.S.]; the NHMRC Senior Research Fellowship Scheme [ID 1061779 to A.B.S.]; NHMRC Project grant scheme [ID 1010719 to A.B.S.]. EMBRACE is supported by Cancer Research UK Grants C1287/A10118 and C1287/A11990. The BBCS is funded by Cancer Research UK and Breakthrough Breast Cancer (recently merged with Breast Cancer Campaign forming Breast Cancer Now) and acknowledges NHS funding to the NIHR Biomedical Research Centre, and the National Cancer Research Network (NCRN). SEARCH was supported by grants CRUK A490/A11021, C490/A16561. CIMBA data management was supported by Cancer Research-UK grant C12292/A11174 and C1287/A10118. BCAC is funded by Cancer Research UK [C1287/A10118, C1287/A12014] and by the European Communitýs Seventh Framework Programme under grant agreement number 223175 (grant number HEALTH-F2-2009-223175) (COGS).This is the author accepted manuscript. The final version is available from Oxford University Press via http://dx.doi.org/10.1093/hmg/ddw09