Molecular Background of Colorectal Tumors From Patients With Lynch Syndrome Associated With Germline Variants in PMS2

BACKGROUND & AIMS: Germline variants in mismatch repair genes MLH1, MSH2 (EPCAM), MSH6, or PMS2 cause Lynch syndrome. Patients with these variants have an increased risk of developing colorectal cancers (CRCs) that differ from sporadic CRCs in genetic and histologic features. It has been a challenge to study CRCs associated with PMS2 variants (PMS2-associated CRCs) because these develop less frequently and in older patients than CRCs with variants in other mismatch repair genes. METHODS: We analyzed 20 CRCs associated with germline variants in PMS2, 22 sporadic CRCs, 18 CRCs with germline variants in MSH2, and 24 CRCs from patients with germline variants in MLH1. Tumor tissue blocks were collected from Dutch pathology departments in 2017. After extraction of tumor DNA, we used a platform designed to detect approximately 3,000 somatic hotspot variants in 55 genes (including KRAS, APC, CTNNB1, and TP53). Somatic variant frequencies were compared using the Fisher exact test. RESULTS: None of the PMS2-associated CRCs contained any somatic variants in the catenin-beta(1) gene (CTNNB1), which encodes beta-catenin, whereas 14 of 24 MLH1-associated CRCs (58%) contained variants in CTNNB1. Half the PMS2-associated CRCs contained KRAS variants, but only 20% of these were in hotspots that encoded G12D or G13D. These hotspot variants occurred more frequently in CRCs associated with variants in MLH1 (37.5%; P = .44) and MSH2 (71.4%; P = .035) than in those associated with variants in PMS2. CONCLUSIONS: In a genetic analysis of 84 colorectal tumors, we found tumors from patients with PMS2-associated Lynch syndrome to be distinct from colorectal tumors associated with defects in other mismatch repair genes. This might account for differences in development and less frequent occurrence.