Definition of candidate low risk APC alleles in a Swedish population

Many families experience an apparently inherited increased risk of colorectal cancer (CRC) similar to the known syndromes familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC). Besides these high-risk syndromes, approximately 10% of all CRC cases come from families with 2 affected 1st-degree relatives, and even 1st-degree relatives to a single case of CRC are at increased risk. Risk subjects from these families frequently show polyps at colonoscopy, which suggests the APC gene as a good candidate susceptibility gene for these attenuated polypotic syndromes. We used the sensitive DHPLC technique to search for possible predisposing germline mutations in the entire APC gene in 91 risk subjects from these high- and low-risk syndromes with unknown predisposing genes. Most exons were also screened for mutations in 96 normal controls and 96 colorectal cancer cases. In our study we probably have identified the most common APC variants in a Swedish population. Among 30 germline variants identified, 1 clearly pathogenic nonsense mutation and 11 putative pathogenic variants (10 missense and one 3' UTR) were found in 20 index patients (22%). Twelve silent as well as 5 intronic variants were considered nonpathogenic. Two of the missense variants found here, E1317Q and D1822V, have previously been related to a difference in risk of colorectal cancer. One variant, 8636C>A, located within the 3' UTR region of the APC gene, was suggested to constitute an additional low risk allele with a similar relative risk as the Jewish I1307K mutation (OR = 1.8; 95% CI, 0.96-3.40). The question of whether all the other variants confer an increased colorectal cancer risk warrants future large association studies.