Tumorigenesis in colorectal tumors from patients with hereditary non-polyposis colorectal cancer

Tumorigenesis of colorectal cancer in patients with hereditary non-polyposis colorectal cancer (HNPCC) has been postulated to follow a different pathway from that of sporadic colorectal tumors. A characteristic of HNPCC-associated tumors is the replication error phenotype. We studied tumorigenesis in 8 fresh-frozen and 67 paraffin-embedded colorectal tumors derived from 29 families with HNPCC or a familial aggregation of colorectal cancer. By using intragenic markers, inactivation of the wild-type allele of hMLH1 was shown to occur through loss of heterozygosity and not through a somatic point mutation. Microsatellite instability is very common and occurs early in almost all colorectal tumors from HNPCC patients. Transforming growth factor β type II receptor (TβRII) mutations occur in these tumors at a high frequency. Of colorectal cancers from families with HNPCC, 63% have frameshift mutations in TβRII, compared with 10% of sporadic colorectal cancers. APC and K-RAS mutations appear to be as frequent in the HNPCC tumors as in the sporadic counterpart.