Different expressivity of two adjacent mutations of the APC gene

Aims and background: The phenotypic expression of different APC mutations i n familial adenomatous polyposis (FAP) is variable: two to three variants o f the disease have been defined based on the severity of colonic manifestat ions, Age of onset and number of polypectomies per person-year of postsurgi cal follow-up were compared in two FAP families with very close mutation si tes in the APC gene, in order to ascertain mutation-specific variation of e xpressivity. Families and APC mutations: Family A (5 patients) carried a newly character ized mutation, a four bp deletion at codon 843. Family B (5 patients) carri ed a previously identified mutation at codon 835, Results: Mean age of onset was 49.7 years in family A and 30.5 years in fam ily B; number of polypectomies per person-year of follow-up was 1.05 for fa mily A and 10.1 for family B (P<0.001). Conclusions: There is significant variation of expressivity (allelic hetero geneity) in FAP between two mutations separated by only eight codons, locat ed at the 5' extremity of APC gene exon 15.