Background: The majority of colorectal carcinomas, if not all, arise from a benign adenoma. The DNA of the carcinomatous cells frequently has mutations in several genes. However, it is not exactly clear when during the neoplastic process each mutation develops. An adenoma with an area of in situ carcinoma provides an opportunity to evaluate genetic changes within a single neoplasia whose separate areas are comprised of both the benign adenoma as well as the malignant carcinoma.
Methods: Thirty-seven neoplasms with areas of both benign adenoma and in situ carcinoma were studied. Both portions were evaluated for loss of heterozygosity (LOH) of the adenomatous polyposis coli (APC) gene and for mutations in codons 12/13 of the K-ras oncogene using the polymerase chain reaction technique.
Results: Twenty-eight neoplasms showed no LOH in either portion whereas both portions of 4 neoplasms revealed a loss of heterozygosity. In three lesions the APC gene was normal in the adenomatous portion but LOH was present in the carcinomatous portion. Two neoplasms were uninformative for LOH of the APC gene. Thirteen neoplasms showed the wild-type pattern for the K-ras oncogene whereas 15 contained the identical mutation in both portions. Of the remaining nine neoplasms, six had a K-ras mutation in the adenomatous portion only and three had one pattern in the adenomatous portion and a different pattern in the in situ carcinoma portion.
Conclusions: LOH of the APC gene is an early and persistent feature in the evolution of a benign colorectal adenoma into an in situ carcinoma. There is less consistency regarding K-ras mutations; one in five in situ carcinomas contains a K-ras mutation different from that observed in the adenomatous portion.