Germline mutations of EX01 gene in patients with hereditary nonpolyposis colorectal cancer (HNPCC) and atypical HNPCC forms

Background & Aims: Germline mutations in one of four mismatch repair genes have been found in the majority of families with hereditary nonpolyposis colorectal cancer (HNPCC), but only in a small part of families with atypical HNPCC. The recently cloned EX01 gene might be involved in the pathogenesis of HNPCC because the EX01 protein strongly interacts with the MSH2 protein. To determine its role in HNPCC, EXO1 was scanned for germline mutations. Methods: All 14 exons of EXO1 were scanned for mutations in index patients from 33 families with HNPCC fulfilling the Amsterdam criteria and in 225 index patients suspected of HNPCC. Results: Germline variants of EXO1 were detected in 14 patients, including one splice-site mutation in a family with HNPCC and 13 missense mutations in patients with atypical HNPCC. These variants did not occur in more than 200 control individuals. From 13 of these 14 patients, tumors were available for analysis of microsatellite instability and loss of heterozygosity. Six of the tumors showed microsatellite instability. Heterozygosity analysis showed one case without EXO1 allelic loss and 12 tumors with loss of the mutant allele and retention of the normal one. Conclusions: The results indicate a possible association of germline EXO1 variants with HNPCC and atypical HNPCC.