Background: Germline mutation in a gene on chromosome 5 (the adenomatous polyposis coli gene) causes familial adenomatous polyposis of the colorectum. Phenotypic manifestations of this condition vary, but the exact relation of the phenotype to the mutation site along the gene has not been fully described.
Objective: To determine how the location of mutations along a gene that is associated with multiple colorectal polyps (the adenomatous polyposis coli gene) is related to the phenotypic expression of the syndrome in families.
Design: Prospective cohort study.
Setting: Polyposis registry.
Patients: 20 patients from 7 families that had mutations in the adenomatous polyposis coli gene that were located toward the 5′ end of codon 158 (proximal 5′ families), were compared with 52 patients from 7 families that had mutations downstream from codon 158, in codons 179 to 625 (distal 5′ families).
Measurements: Sex, age at diagnosis of familial adenomatous polyposis, number of polyps at first examination of the colon, distribution of polyps, age at diagnosis of colorectal cancer, and location of colorectal cancer.
Results: Mutations that were proximal to codon 158 were found in 7 of 112 families (6%). At the first examination of the colon, 8 of 17 (47%) patients in proximal 5′ families and 9 of 48 (19%) patients of similar ages in distal 5′ families were found to have fewer than 100 adenomas (P = 0.029). The distribution of polyps was frequently right-sided in patients in proximal 5′ families (P = 0.001). The cumulative probability of survival without colorectal cancer was greater for patients in proximal 5′ families (P = 0.041).
Conclusions: Families with adenomatous polyposis that have proximal 5′ mutations of the adenomatous polyposis coli gene are more likely to have a heterogeneous phenotype with delayed development of colonic polyposis and colorectal cancer than are families with distal 5′ mutations of the gene. Management should include genotyping of patients who are at risk, colonoscopic surveillance of genotypically positive persons, and prophylactic colectomy if several adenomas are found.