KRAS4A induces metastatic lung adenocarcinomas in vivo in the absence of the KRAS4B isoform

Significance KRAS encodes two protein isoforms which differ in intracellular trafficking and plasma membrane location. Moreover, KRAS mutations in cancer activate both isoforms. Thus, understanding their individual contributions to tumor development may facilitate the design of therapeutic strategies to selectively target each isoform. Here, we describe two strains of mice that fail to express KRAS4B without affecting expression of either wild-type KRAS4A or oncogenic KRAS4AG12V. Expression of KRAS4A in the absence of KRAS4B prevents postnatal development. More importantly, expression of the endogenous KRAS4AG12V isoform in the absence of KRAS4B is sufficient to induce lung adenocarcinomas that undergo proximal metastasis. Hence, development of therapeutic strategies against KRAS mutant tumors must take into account inhibition of both protein isoforms. In mammals, the KRAS locus encodes two protein isoforms, KRAS4A and KRAS4B, which differ only in their C terminus via alternative splicing of distinct fourth exons. Previous studies have shown that whereas KRAS expression is essential for mouse development, the KRAS4A isoform is expendable. Here, we have generated a mouse strain that carries a terminator codon in exon 4B that leads to the expression of an unstable KRAS4B154 truncated polypeptide, hence resulting in a bona fide Kras4B-null allele. In contrast, this terminator codon leaves expression of the KRAS4A isoform unaffected. Mice selectively lacking KRAS4B expression developed to term but died perinatally because of hypertrabeculation of the ventricular wall, a defect reminiscent of that observed in embryos lacking the Kras locus. Mouse embryonic fibroblasts (MEFs) obtained from Kras4B−/− embryos proliferated less than did wild-type MEFs, because of limited expression of KRAS4A, a defect that can be compensated for by ectopic expression of this isoform. Introduction of the same terminator codon into a KrasFSFG12V allele allowed expression of an endogenous KRAS4AG12V oncogenic isoform in the absence of KRAS4B. Exposure of Kras+/FSF4AG12V4B– mice to Adeno-FLPo particles induced lung tumors with complete penetrance, albeit with increased latencies as compared with control Kras+/FSFG12V animals. Moreover, a significant percentage of these mice developed proximal metastasis, a feature seldom observed in mice expressing both mutant isoforms. These results illustrate that expression of the KRAS4AG12V mutant isoform is sufficient to induce lung tumors, thus suggesting that selective targeting of the KRAS4BG12V oncoprotein may not have significant therapeutic consequences.