Variable Phenotypic Expression of Identical MYH Germline Mutations in Siblings with Attenuated Familial Adenomatous Polyposis

Germline mutations in the Mutant-Y-homologue (MYH) gene have been linked to an attenuated form of familial adenomatous polyposis in patients who express a wild-type adenomatous polyposis coli gene. However, the diverse clinical manifestations of MYH mutations have not been fully elucidated. We report a case of siblings with identical germline mutations in the MYH gene, one of whom developed a locally advanced colon adenocarcinoma with few other adenomatous lesions, whereas the other had numerous benign colonic polyps. The variable genotype–phenotype manifestations of MYH mutations and the attenuated familial adenomatous polyposis syndrome are discussed.