Novel Findings in Swedish Patients With MYH-Associated Polyposis: Mutation Detection and Clinical Characterization

Background & Aims: Biallelic mutations in the base-excision repair gene MYH have recently been associated with recessive inheritance of multiple colorectal adenomas. An investigation and characterization of MYH mutations in Swedish patients were therefore carried out. Methods: A set of 15 unrelated adenomatous polyposis coli (APC)-mutation negative patients from the Swedish Polyposis Registry was screened for germline mutations in the MYH gene. The patients were clinically characterized and compared with 43 APC-mutation positive probands diagnosed during the same period. Results: Disease-causing biallelic MYH mutations were identified in 6 patients (40%). The mean age at diagnosis was 47.8 years versus 34.1 years in APC-mutation positive patients (P = .015). Colorectal cancer at diagnosis of polyposis was present in 67% (4/6) of the patients, and all were right-sided, compared with only 19% versus 12.5% right-sided cancer in APC-mutation positive patients. Upper gastrointestinal manifestations were diagnosed in 1 of 5 compared with 23 of 27 in APC-mutation positive patients (odds ratio, 23; 95% confidence interval, 2-263; P = .0086). One family exhibited apparent dominant inheritance of colorectal adenomatous polyposis. Two new pathogenic mutations, MYH p.G175E and p.P391L, were identified. The mutations are argued to introduce profound changes in substrate-recognizing domains of the protein. Conclusions: Biallelic MYH mutations, including 2 novel mutations, were found in a substantial number of the patients with multiple colorectal adenomas who were negative for APC-mutation. The examined MYH-mutation positive patients were found to have higher risks of colorectal cancer at diagnosis, right-sided location of cancers, and a significantly lower incidence of upper gastrointestinal manifestations, compared with APC-mutation positive patients.