Generation of functional noncanonical donor splice sites by +2T variants in breast cancer susceptibility genes: impact on clinical interpretation.

Splicing dysregulation is a relevant mechanism of pathogenicity for variants in disease susceptibility genes. Variants affecting the critical intronic +1 and +2 GT nucleotides of the 5' splice sites (5'ss) are generally strong indicators of pathogenicity. However, some +2 T variants create functional noncanonical 5'ss that generate wildtype transcripts, hampering accurate variant interpretation and genetic counseling. We previously showed that variants PALB2 c.108+2T > C and ATM c.1898+2T > G generated significant levels of full-length (FL) transcripts by creating functional atypical GC and GG donor sites, respectively. In this study, we aimed to investigate the splicing impact of +2T variants in the breast cancer susceptibility genes ATM, BRCA1, and PALB2. For this purpose, five minigenes encompassing 29 exons of ATM, BRCA1, and PALB2 were employed. A total of 30 +2T > C/G/A variants were introduced into these constructs by site-directed mutagenesis and analyzed in MCF-7 cells. Four +2T > C variants (ATM c.6347+2T > C, BRCA1 c.5193+2T > C and c.5277+2T > C, and PALB2 c.2748+2T > C) and ATM variants c.6347+2T > A/G produced FL-transcripts (4%-81% of the overall expression). All +2T > C leaky variants conserved a central core of 6 nucleotides (AGgcaa). Variants were assessed according to the ClinGen specifications of the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) interpretation guidelines. Two variants (ATM c.6347+2T > C and BRCA1 c.5193+2T > C) were classified as likely benign, consistent with predictions based on their respective ACMG/AMP-based gene specifications. Conversely, two variants (ATM c.6347+2T > G and BRCA1 c.4675+2T > C), initially predicted as likely pathogenic, were reclassified as variant of uncertain significance (VUS). In conclusion, a significant proportion of +2T variants can create functional noncanonical 5'ss, resulting in the production of FL-transcripts that may preserve gene function. Variant-splicing assays provide essential data for accurate clinical classification and for the development of effective clinical management strategies for patients and their families. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.