Codon 12 and codon 13 mutations in K-RAS differentially affect colorectal carcinoma cells

The RAS proteins are involved in one of the most conserved signal transduction pathways that regulates proliferation, differentiation and apoptosis in all animals, and their activating mutations have oncogenic effects. K-RAS mutations, usually point mutations in codons 12 or 13, are common in colorectal carcinomas. However, molecular epidemiological studies suggest that mutations in different codons or different mutations in the same codon of K-Ras may have diverse biological consequences. They may also lead to a different response to therapies based on Cetuximab, an anti-EGFR monoclonal antibody. K-RAS mutations are in fact considered to be a resistance factor to this drug, but it has been reported that tumors with K-RAS mutations in codon 13 might be less resistant to Cetuximab than tumors with codon 12 mutations. To shed more light on the molecular mechanisms responsible for the different effects of K-RAS mutations, we have established and analyzed HT-29 clones stably transfected with cDNAs codifying K-RASG12V (clone K12) and K-RASG13D (clone K13) under the control of a inducible promoter. The expression of the two mutated isoforms of K-RAS induces different effects on the growth rate and on the cell cycle, and also on the level of expression of several proteins controlling proliferation and cell survival. Thus, for example, K-RASG13D and K-RASG12V have a differential effect on the pro-apoptotic (MST2-RASSF1A-LATS1) and the anti-apoptotic (MST2-RAF-1) pathways and K-RASG13D, in our system, appears to have a significant cytotoxic effect. Finally, we observed that Cetuximab affects the cell cycle and proliferation of the K13 clone expressing K-RASG13D but not the HT29 cells and the K12 cells induced to express K-RASG12V. These results support the hypothesis that tumors with different K-RAS mutations could respond differently to therapies.