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Background & Aims: Lynch syndrome is caused by variants in DNA mismatch repair (MMR) genes and associated with an increased risk of colorectal cancer (CRC). In patients with Lynch syndrome, CRCs can develop via different pathways. We studied associations between Lynch syndrome-associated variants in MMR genes and risks of adenoma and CRC and somatic mutations in APC and CTNNB1 in tumors in an international cohort of patients., Methods: We combined clinical and molecular data from 3 studies. We obtained clinical data from 2747 patients with Lynch syndrome associated with variants in MLH1, MSH2, or MSH6 from Germany, the Netherlands, and Finland who received at least 2 surveillance colonoscopies and were followed for a median time of 7.8 years for development of adenomas or CRC. We performed DNA sequence analyses of 48 colorectal tumors (from 16 patients with mutations in MLH1, 29 patients with mutations in MSH2, and 3 with mutations in MSH6) for somatic mutations in APC and CTNNB1., Results: Risk of advanced adenoma in 10 years was 17.8% in patients with pathogenic variants in MSH2 vs 7.7% in MLH1 (P < .001). Higher proportions of patients with pathogenic variants in MLH1 or MSH2 developed CRC in 10 years (11.3% and 11.4%) than patients with pathogenic variants in MSH6 (4.7%) (P = .001 and P = .003 for MLH1 and MSH2 vs MSH6, respectively). Somatic mutations in APC were found in 75% of tumors from patients with pathogenic variants in MSH2 vs 11% in MLH1 (P = .015). Somatic mutations in CTNNB1 were found in 50% of tumors from patients with pathogenic variants in MLH1 vs 7% in MSH2 (P = .002). None of the 3 tumors with pathogenic variants in MSH6 had a mutation in CTNNB1, but all had mutations in APC., Conclusions: In an analysis of clinical and DNA sequence data from patients with Lynch syndrome from 3 countries, we associated pathogenic variants in MMR genes with risk of adenoma and CRC, and somatic mutations in APC and CTNNB1 in colorectal tumors. If these findings are confirmed, surveillance guidelines might be adjusted based on MMR gene variants., (Copyright © 2020 AGA Institute. Published by Elsevier Inc. All rights reserved.)

Background & Aims: Mutation carriers in Lynch syndrome families have a high risk for developing colorectal cancer during their lifetime. This study was designed to assess the cumulative risk for the development of colorectal adenoma or carcinoma in prospective colonoscopic surveillance., Methods: Data from the Finnish Hereditary Colorectal Cancer Registry electronic database on 420 Lynch syndrome mutation carriers without previous colorectal tumors were reviewed. Between March 1982 and May 2005 the mutation carriers underwent a total of 1252 colonoscopies. The total follow-up time was 3150 years (mean, 6.7 y/patient)., Results: The cumulative risk of adenoma by age 60 was estimated as 68% (95% confidence interval [CI], 50%-80%) in men and 48% (95% CI, 29%-62%) in women. The estimated cumulative risk up to age 60 years for the development of cancer found as a result of surveillance at an interval of 2-3 years was 35% (95% CI, 16%-49%) in men and 22% (95% CI, 7%-34%) in women. Half of the adenomas were located proximal to the splenic flexure. Extracolonic cancer was diagnosed in 73 patients (18%)., Conclusions: Adenoma would appear to be the most important lesion preceding cancer formation in Lynch syndrome and removal of adenomas decreases the risk for colorectal cancer (CRC). The Finnish surveillance protocol of colonoscopies at 2- to 3-year intervals facilitates patient adherence but includes an essential risk for CRC up to 60 years of age, but without CRC-related mortality when the surveillance instructions are followed.