Abstract B001: Does deep intronic splicing in breast cancer susceptibility genes contribute to breast cancer?
Authors: Akamandisa Mwangala P. , Wubbenhorst Bradley , D'Andrea Kurt , Symecko Heather , Gillette Rhonda , Shah Payal D. , Bradbury Angela R. , Maxwell Kara N. , Domchek Susan M. , Nathanson Katherine L. 
Published: February 2024

Abstract
Breast cancer (BC) is the most common cancer and leading cause of cancer mortality in women. Average lifetime BC risk for a woman in the United States is 13%. However, pathogenic variants in BC susceptibility genes such as BRCA1/2, ATM, and RAD51C, or high polygenic risk scores substantially increase BC risk. Women with a family history of BC and/or ovarian cancer, BC &lt; age 40, or BC and second primary tumor are considered high-risk with an increased likelihood of having a pathogenic variant in a BC susceptibility gene. Yet, more than 60% of women with high-risk BC test negative for coding variants. In these women, possible causal variation could be non-coding in deep intronic or regulatory regions, or cryptic structural variants (SV). To determine whether deep intronic splicing contributes to BC in high-risk women testing negative for pathogenic coding variants in BC genes, we analyzed the intronic sequences of 649 BRCA1/BRCA2 negative individuals with high-risk BC. The samples had undergone targeted capture sequencing of exons and introns of more than 100 target genes including ATM and RAD51C. From the 649 samples, we identified three intronic splicing variants in ATM and RAD51C in 10 samples (1.5%) that had SpliceAI delta scores &gt;0.5 suggesting changes to splicing. One splicing donor gain variant, RAD51C c.145+246A&gt;T, was found exclusively in women of self-reported African ancestry at a frequency of 0.047. In gnomAD, the variant was found exclusively in individuals of African ancestry at a frequency of 0.012. A 1.3-fold enrichment in variant frequency was observed in AFR women under 65 with breast cancer compared to age-matched cancer free controls in the All of Us dataset. Using lymphoblastoid-cell derived RNA from carriers of RAD51C c.145+246A&gt;T, we performed RT-PCR and cDNA amplification targeting the exons surrounding the variant and observed a lengthened exon. Targeted amplicon sequencing also confirmed the presence of intronic sequence in the variant cDNA. The introduced intronic sequence contains a UGA stop codon 12 bp from the end of the exon which could led to loss of RAD51C function. We have created minigene vectors in the RHCglo background and are performing minigene splicing assays to validate the splicing changes in ATM due to intronic variants. Gene functional assays and association studies will be conducted to determine the functional effects and risk conferred by these variants. Understanding the level of contribution of deep intronic splicing in known BC susceptibility genes is critically important, as results impact sequencing offered to patients, and if positive, their medical management.
Citation Format: Mwangala P. Akamandisa, Bradley Wubbenhorst, Kurt D'Andrea, Heather Symecko, Rhonda Gillette, Payal D. Shah, Angela R. Bradbury, Kara N. Maxwell, Susan M. Domchek, Katherine L. Nathanson. Does deep intronic splicing in breast cancer susceptibility genes contribute to breast cancer? [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr B001.