Solving Missing Heritability in Patients With Familial Adenomatous Polyposis With DNA-RNA Paired Testing.

PURPOSE:Patients with germline pathogenic variants (PVs) in APC develop tens (attenuated familial adenomatous polyposis [AFAP]) to innumerable (classic FAP) adenomatous polyps in their colon and are at significantly increased lifetime risk of colorectal cancer. Up to 10% of FAP and up to 50% of patients with AFAP who have undergone DNA-only multigene panel testing (MGPT) do not have an identified PV in APC. We seek to demonstrate how the addition of RNA sequencing run concurrently with DNA can improve detection of germline PVs in individuals with a clinical presentation of AFAP/FAP. METHODS:We performed a retrospective query of individuals tested with paired DNA-RNA MGPT from 2021 to 2022 at a single laboratory and included those with a novel APC PV located in intronic regions infrequently covered by MGPT, a personal history of polyposis, and family medical history provided. All clinical data were deidentified in this institutional review board-exempt study. RESULTS:Three novel APC variants were identified in six families and were shown to cause aberrant splicing because of the creation of a deep intronic cryptic splice site that leads to an RNA transcript subject nonsense-mediated decay. Several carriers had previously undergone DNA-only genetic testing and had received a negative result. CONCLUSION:Here, we describe how paired DNA-RNA MGPT can be used to solve missing heritability in FAP families, which can have important implications in family planning and treatment decisions for patients and their families.