Metachronous Colorectal Cancer in Icelandic MSH6 and PMS2 Lynch Syndrome Carriers in 1955-2017: A Population-based Study.

76 Background: The prevalence of Lynch syndrome in Iceland is the highest described so far or 1:226 with three founder mutations, one in MSH6 and two in PMS2, and was first reported in 2017. Lynch syndrome predisposes carriers to colorectal cancer and metachronous primaries. Most data on metachronous colorectal cancer reflects on MLH1 and MSH2 carriers and is not well described in MSH6 and PMS2 carriers. The purpose of this study was to examine the cumulative incidence of metachronous colorectal cancer in Icelandic Lynch syndrome carriers diagnosed with colorectal cancer from 1955-2017. Methods: This retrospective study included all individuals with pathogenic variants in mismatch repair genes identified in a nationwide genotype database and a colorectal cancer diagnosis in the Icelandic Cancer Registry from 1955-2017. DeCODE genetics has collected extensive genotypic information on the Icelandic population sequencing the whole genomes of 49,708 Icelanders and imputing genotypes into 116,573 Icelanders whose DNA had been genotyped with SNP chips. Cancer information was obtained from patient charts and cause of death obtained from the Cause of Death Register. All polyp pathology was accessed in the pathology registry and reassessed by two pathologists. Results: A total of 65 individuals developed colorectal cancer during the study period with 34 (52.3%) PMS2 carriers and 25 (38.5%) MSH6 carriers. The cumulative incidence of metachronous colorectal cancer was low or one case (1.5%) in an MSH2 carrier with a median follow-up time of 58 months (Q1 8.0, Q3 167.0, range 0-365 months) in the cohort. Only two (3.1%) individuals, one with MSH2 and one with MSH6 pathogenic variants, underwent a total colectomy. Median age at first colorectal cancer diagnosis was 63 years (Q1 54.0, Q3 72.5, range 28-94). Thirty-one (47.7%) individuals had polyps resected during the study period with adenomas (67.1%) being the most common. Significantly more MSH6 carriers had a history of advanced adenomas as compared to PMS2 carriers (36.0 % vs 11.8%, p = 0.026). Five-year colorectal cancer survival was 81.4% for PMS2 carriers and 71.5% for MSH6 carriers. Conclusions: The incidence of metachronous colorectal cancer was zero in Lynch syndrome carriers with MSH6 or PMS2 pathogenic variants from 1955-2017 despite these individuals not undergoing rigorous Lynch syndrome screening. This would support performing segmental colectomies and following European screening guidelines for MSH6 and PMS2 carriers in Iceland.