Hereditary mixed polyposis syndrome

Hereditary mixed polyposis syndrome (HMPS) is an autosomal dominant inherited condition in which affected individuals develop colorectal polyps of multiple and mixed histological type including serrated lesions, Peutz-Jeghers polyps, juvenile polyps, conventional adenomas and colorectal carcinomas in the absence of any extra-colonic features. Most of the families described to date are Ashkenazi Jewish and have an ancestral founder mutation with a duplication of 40 kb upstream of the GREM1 gene. This leads to increased and ectopic expression of GREM1 in the colonic crypt. A smaller duplication of about 16 kb also upstream of GREM1 has been reported in a Swedish HMPS family. Presymptomatic genetic testing is now available in these families, and affected individuals require careful colonoscopic surveillance from an early age, with polypectomy to prevent the development of colorectal cancer.