K-RAS MUTATIONS IN PUTATIVE PRENEOPLASTIC LESIONS IN HUMAN COLON

Background: A mutation in c-K-ras (KRAS2) has long been implicated as one of the important early events in the development of a large propor tion of human colon cancers. Aberrant crypt foci, putative preneoplast ic lesions identified microscopically in wholemounts of colons, have b een shown to occur with high frequency in the colons of animals treate d with colon carcinogens and in the grossly normal mucosas of patients with colon cancer. Purpose: In this study, we asked whether the mutat ional activation of K-ras occurs in the aberrant crypt foci of human c olon. Methods: Grossly normal colonic mucosas were obtained from seven patients during surgery and were provided to us by the Western Divisi on of the Cooperative Human Tissue Network located at Case Western Res erve University. A total of 42 samples, consisting of aberrant crypt f oci and similarly sized normal crypt areas, were microdissected from t he grossly normal colonic mucosas. The DNA region containing codon 12 of K-ras was amplified by polymerase chain reaction and analyzed for m utations by dot-blot hybridization with specific oligonucleotide probe s complementary to normal or mutant sequences. Results: Mutations in c odon 12 of K-ras were found in 11 (73%) of 15 aberrant crypt foci but not in any of 27 morphologically normal crypt areas from the same pati ents. Conclusions: The observed high frequency of K-ras mutations in t hese microscopically identifiable lesions makes mutation in K-ras the earliest identified gene-mutational event in human colon tumorigenesis , establishes that it often occurs prior to the development of polyps, and is consistent with the hypothesis that aberrant crypt foci are th e earliest identified precursors of human colon cancer. Implications: Further analysis of aberrant crypt foci may identify yet unknown early genetic events that precede human colon cancer.