Relative expression of wild-type and activated ki-ras2 oncogene in colorectal carcinomas

A quantitative, competitive RT-PCR-RFLP assay was developed to detect and discriminate the expression of mutant versus wild-type alleles of the Ki-ras oncogene. The aim was to establish whether these alleles are differentially expressed in human malignant neoplasma, since experiments in vitro have shown stoichiometric representation and expression of ras genes does not necessarily engender a cancer phenotype. Sixteen primary colorectal carcinomas and two colorectal carcinoma xenografts, passed in immune-suppressed mice, were studied. Previous sequence analysis had established that 9 of the primary tumours and both xenografts had codon 12 Ki-ras mutations, 4 tumours had codon 13 mutations and 3 were wild-type controls. Wild-type and mutant Ki-ras were co-expressed in all the primary tumours, but the assay showed that stoichiometrically equivalent amounts of the two mRNA species were present in only one-third: in the others, mutant Ki-ras was overexpressed by around 30-60% relative to wild-type. The xenografts showed a similar range of values, despite their near-total lack of stroma. Ki-ras activation by point mutation is known to be involved in the early, adenoma phase of evolution of colorectal tumorigenesis, but these results show that differential expression of the mutant allele is common in carcinomas and may be associated with persisting growth advantage.