Splicing Predictions, Splicing Assays, and Variant Classification Using ACMG/AMP Guidelines: Challenges Observed with BRCA1 and BRCA2 Variants.

Germline loss-of-function variants in BRCA1 and BRCA2 are established drivers of hereditary breast and ovarian cancer, often acting through aberrant splicing. However, not all spliceogenic changes are pathogenic, and many variants remain classified as uncertain due to insufficient experimental evidence and challenges in applying the ACMG/AMP variant interpretation framework to splicing alterations. In this study, we examined the splicing outcomes of 17 variants-10 in BRCA1 [c.135-2A>G; c.135-5T>C; c.5074+1G>C; c.5332+2_5332+4del; c.5333-8C>T; c.5335C>G p.(Gln1779Glu); c.302-24_302-22del; c.302-23A>G; c.547+57T>C; c.4096+34C>G] and 7 in BRCA2 [c.-39-5delT; c.67+3A>G; c.425G>A p.(Ser142Asn); c.425G>T p.(Ser142Ile); c.517-13_517-9del; c.681+5G>C; c.67+84_67+85del]-identified in families with suspected hereditary breast and/or ovarian cancer. Depending on sample availability, we assessed splicing either on carrier-derived mRNA or via splicing-reporter minigene assay. Eight variants triggered aberrant splicing, while 9 showed no spliceogenic effect. Our findings, combined in some cases with previously published data, allowed us to apply the PVS1_(RNA) criterion at full strength to some variants. For others, residual full-length transcripts or in-frame mis-spliced isoforms precluded full application of PVS1_(RNA). Following ClinGen ENIGMA BRCA1 and BRCA2 Variant Curation Expert Panel specifications based on ACMG/AMP guidelines, we classified 4 variants as pathogenic or likely pathogenic, 10 as benign or likely benign, and 3 as uncertain significance. This comprehensive analysis of splicing defects refines the clinical classification of BRCA1 and BRCA2 variants and highlights the value of combining experimental and computational evidence to enhance genetic risk assessment in hereditary cancer.