Functional characterization of the novel APC N1026S variant associated with attenuated familial adenomatous polyposis.

We identified the APC N1026S variant of unknown malignant potential in the adenomatous polyposis coli (APC) gene in a Spanish attenuated familial adenomatous polyposis (AFAP) family. The variant was located in the first of the 4 highly conserved 15-amino acid (AA) repeats within the beta-catenin union domain. Our aim was to determine its functional relevance to establish its pathogenicity. N1026S variant was analyzed in 22 members of the AFAP family studied, in 236 sporadic colorectal cancer cases, 203 matched controls, and 205 unrelated familial colorectal cancer cases. To assess its effects on beta-catenin binding, beta-catenin/Tcf-4-mediated transcription and beta-catenin subcellular distribution we performed affinity chromatography experiments, BIAcore 1000 (BIAcore AB, Uppsala, Sweden) assays, luciferase reporter assays, assessment of c-myc messenger RNA levels, and cell fractionation. N1026S variant cosegregated with the disease in the AFAP family studied. None of the sporadic or familial cases as well as the controls analyzed was positive for the variant. N1026S variant completely precluded beta-catenin binding to the first 15-AA repeat and diminished it when all four 15-AA repeats were present. Expression of APC N1026S in SW480 and DLD-1 cells did not diminish beta-catenin/Tcf-4-mediated transcription as effectively as APC wild-type. N1026S did not decrease c-myc transcription in DLD1 cells and nuclear beta-catenin in SW480 cells as effectively as WT. These findings strongly support a pathogenic role of the APC N1026S variant in the AFAP phenotype, reinforcing the importance of functional characterization of APC variants for genetic counseling.