Genetic Heterogeneity of the c-K-ras Locus in Colorectal Adenomas but not in Adenocarcinomas

Abstract

Previous molecular genetics studies of colorectal cancer have identified multiple mutations in the c-K-ras gene (also known as KRAS2) in all phases of its development. Because of technical difficulty, prior studies rarely focused attention on the detailed distribution of c-K-ras mutations in multiple regions of the same primary tumor specimen. However, with recent development of the selective UV radiation fractionation method, characterization of c-K-ras mutations in multiple regions of the same primary tumor specimen can be performed. Our purpose was to describe how c-K-ras mutations were distributed among cells obtained from multiple regions of the same primary tumor in an attempt to describe differences between early and late colorectal carcinogenesis. Formalin-fixed, paraffin-embedded tissue blocks were obtained. Seven adenocarcinomas and seven adenomas were selected for the presence of mutant c-K-ras genes and histologic transitions between normal and neoplastic tissue. Tissue sections were prepared for analysis by the selective UV radiation fractionation method by placing thin, fixed tissue sections on a plastic slide with no coverslip. Under the microscope, small ink dots from a felt-tip pen were manually placed directly on relatively pure cell subpopulations. The slides were placed with the tissue side exposed to a UV transilluminator for 2–4 hours to inactivate the DNA present in the unprotected (“undotted”) cells. Individual dots were cut out of the plastic slide into 2 × 2-mm squares and placed into microfuge tubes. The DNA was extracted and supernatant used for polymerase chain reaction (PCR) analysis. Mutations at c-K-ras codons 12 and 13 were detected. The selective UV radiation fractionation method and PCR analysis revealed that c-K-ras mutations never extended into normal mucosa and were present in all neoplastic cells regardless of phenotypes in all seven adenocarcinomas and three of the seven adenomas. Further examination of two carcinomas for p53 (also known as TP53) mutations or loss of heterozygosity demonstrated that these additional mutations were also present in all tumor cells, suggesting that a single transformed clone was responsible for the majority of growth. However, in four other adenomas, tumor heterogeneity was demonstrated, since c-K-ras mutations were detected only in discrete portions. Adenoma formation may include a stage in which multiple and genetically distinct neoplastic clones are present, while most carcinomas appear to have a homogeneous composition that may result from the successful progression of one of these clones. [J Natl Cancer Inst 85:1058–1063, 1993]