DETECTION OF K-RAS MUTATION IN NORMAL AND MALIGNANT COLONIC TISSUES BY AN ENRICHED PCR METHOD

ras oncogene mutations appear in over 50% of colon tumors in humans. S tudies in animal systems have revealed that ras mutations are also pre sent in preneoplastic lesions, suggesting the possibility of early det ection of ras mutation in morphologically normal colon tissues for dia gnostic purposes. An Enriched PCR, developed by us, eliminates most of the normal ras alleles prior to amplification; subsequent analysis vi a RFLP enables the detection of one mutant allele within 10(4) normal alleles. Using the Enriched PCR, we have determined the frequency of m utant ras alleles in normal mucosae and in adenomatous polyps of patie nts with or without adenocarcinoma. Of the 42 patients who had colon t umors, 15 were found to harbor K-ras oncogene mutation (35%). In two o f the 14 cases with mutant K-ras in the tumor tissue we were able to i dentify mutations in tissues that had been obtained from a site at con siderable distance from the tumor (13%); Analysis of 7 adenomas identi fied one as a carrier of the mutant ras allele (14%). Of 11 normal col onic mucosa obtained from patients without neoplasia, one specimen con tained K-ras mutation. Thus, mutated alleles of K-ras may be present, at low frequency, throughout the 'normal appearing' tissue. Cells of n ormal appearance that harbor such mutation, have the potential to unde rgo further changes and to develop into the transformed phenotype. Ove rall, our findings suggest that mutant ras alleles can be detected in preneoplastic mucosa that is morphologically normal, and in adenomas, suggesting the occurrence of an initiation event, and possibly enablin g the identification of patients who may be at high risk for developin g malignant tumors.