Germline characterization of early-aged onset of hereditary non-polyposis colorectal cancer

Objectives: Hereditary non-polyposis colorectal cancer (HNPCC) is character ized by the early onset of colorectal cancer (similar to 10 years). Adolesc ent colorectal cancer is unusual in HNPCC families. We speculated that some DNA mismatch repair germline mutations might be associated with early onse t of disease. Study design: Genomic DNA was extracted from members of a kindred with viru lent HNPCC fitting the Amsterdam Criteria for HNPCC and sequenced for 2 DNA mismatch repair genes, hMSH2 and hMLH1. A sigmoid adenocarcinoma from the 14-year-old proband was analyzed for high-frequency microsatellite instabil ity and immunostained for DNA mismatch repair gene expression. Results: A germline mutation was identified at nucleotide 676 (codon 226) o f the hIMLH1 gene. The C to T transition created a nonsense mutation, trunc ating the hMLH1 protein. This mutation also alters the splice donor sequenc e, because nucleotide 676 is 2 base pairs from the 3' end of the exon 8. Th e proband's tumor demonstrated high-frequency microsatellite instability an d displayed loss of hMLH1 expression, indicating bi-allelic inactivation of hMLH1. Conclusions: A complex mutation of hMLH1 at codon 226 is associated with ad olescent onset of colorectal cancer in an HNPCC family. Genetic screening o f other suspected HNPCC families with unusually young members with cancer m ight reveal certain genotypes with particularly virulent forms of this dise ase.