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Allele‐specific detection of K‐ras oncogene expression in human non‐small‐cell lung carcinomas

Point mutations in codon 12 of the K‐ras oncogene are frequent in human lung adenocarcinomas. To study the expression of the K‐ras gene in these tumors we have developed a mRNA detection technique based on the polymerase chain reaction (PCR). By this technique, K‐ras expression can be detected semi‐quantitatively in samples of less than 100 ng total RNA. Hybridization of the amplified cDNA sequences with mutation‐specific oligonucleotides allows separate quantification of the expression of normal and point‐mutated alleles in a single sample. RNA samples from 24 human non‐small‐cell lung carcinomas (NSCLC), from 2 lung metastases of colonic adenocarcinomas, from 3 human lung adenocarcinoma cell lines, and from normal lung tissue were analyzed. In most tumors, expression of K‐ras was detected at levels equal to or several times higher than those found in normal lung tissue. A lung metastasis from a colon adenocarcinoma, known to contain an amplified K‐ras gene, highly over‐expressed the K‐ras gene. In those tumors in which the K‐ras oncogene was activated by a point mutation, both alleles of the gene were expressed. Our results show that a high over‐expression of K‐ras is a rare event in human lung carcinomas, but that a certain degree of over‐expression of the mutated allele can be demonstrated in tumors with an activated K‐ras gene. With the technique we describe here, adequate estimation of the expression of specific genes in minimal amounts of tumor cells becomes possible.