A large family with MSH3-related polyposis.

Biallelic MSH3 germline variants are a rare cause of adenomatous polyposis as yet reported in two small families only. We describe the phenotype of a third family, the largest thus far, with adenomatous polyposis related to compound heterozygous MSH3 pathogenic variants. The index patient was a 55-years old male diagnosed with rectal cancer and adenomatous polyposis (cumulatively 52 polyps), with a family history of colorectal polyposis with unknown cause. Next-generation sequencing and copy number variation analysis of a panel of genes associated with colorectal cancer and polyposis revealed compound heterozygous germline pathogenic variants in the MSH3 gene. Nine out of 11 siblings were genotyped. Three siblings carried the same compound heterozygous MSH3 variants. Colonoscopy screening showed predominantly right-sided adenomatous polyposis in all compound heterozygous siblings, with a cumulative number of adenomas ranging from 18 to 54 in an average of four colonoscopies, and age at first adenoma detection ranging from 46 to 59. Microsatellite analysis demonstrated alterations at selected tetranucleotide repeats (EMAST) in DNA retrieved from the rectal adenocarcinoma, colorectal adenomas as well as of normal colonic mucosa. Gastro-duodenoscopy did not reveal adenomas in any of the four patients. Extra-intestinal findings included a ductal adenocarcinoma in ectopic breast tissue in one female sibling at the age of 46, and liver cysts in three affected siblings. None of the three heterozygous or wild type siblings who previously underwent colonoscopy had adenomatous polyposis. We conclude that biallelic variants in MSH3 are a rare cause of attenuated adenomatous polyposis with an onset in middle age.