Case series examining somatic test results for patients with hereditary cancer syndromes associated with gastrointestinal cancer risk.

680 Background: Somatic tumor testing may identify germline pathogenic variants (PV) associated with cancer predisposition syndromes. Labs differ whether they offer somatic only or paired germline analysis. Methods used by somatic testing labs, even those that include germline analysis, differ from designated germline labs that have optimized the identification of germline PV. Methods: Chart reviews were performed for patients who had testing through both somatic and designated germline laboratories. Cases with discrepant results in which germline PV were not detected by the somatic laboratory are summarized. Results: Nine cases with discrepant results. Five had paired germline testing and 4 somatic testing only. All 9 patients met the criteria to undergo designated germline testing, either for Lynch syndrome (3) or BRCA1/2 testing (6), based on personal and/or family history. Designated germline testing identified 4 MLH1, 1 BRCA1, 2 ATM, 1 MUTYH and 1 RAD50 PV not reported by the somatic labs’ tumor or germline analysis; 2 MLH1 PV were called variants of uncertain significance by somatic testing but classified as PV by ClinVar and designated germline labs. Three PV identified by designated germline labs are targets for PARP inhibitors and resulted in different treatment options. Three of the MLH1 PV were identified in patients meeting Lynch Syndrome test criteria while 1 was identified in a patient meeting BRCA1/2 criteria. Among the 5 other patients meeting BRCA1/2 test criteria, 3 had PV in breast cancer genes (2 ATM, 1 BRCA1) and 2 had PV in other cancer genes ( MUTYH and RAD50) not reported by the somatic labs, highlighting the importance of panel testing. Conclusions: Methods used by somatic labs, regardless of inclusion of germline analysis, are not equivalent to those of designated germline labs. Overlooked germline PV may miss identification of hereditary syndromes and targeted therapy opportunities (e.g. Anti-PD1 immunotherapy, PARP inhibitors). Patients meeting criteria for genetic evaluation should be referred for designated germline testing regardless of somatic testing outcomes.