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

Abstract

In mammals, the <i>KRAS</i> 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 <i>Kras</i>4B-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 <i>Kras</i> locus. Mouse embryonic fibroblasts (MEFs) obtained from <i>Kras</i>4B^-/- 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 <i>Kras</i>^FSFG12V allele allowed expression of an endogenous KRAS4A^G12V oncogenic isoform in the absence of KRAS4B. Exposure of <i>Kras</i>^{+/FSF4AG12V4B-} mice to Adeno-FLPo particles induced lung tumors with complete penetrance, albeit with increased latencies as compared with control <i>Kras</i>^+/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 KRAS4A^G12V mutant isoform is sufficient to induce lung tumors, thus suggesting that selective targeting of the KRAS4B^G12V oncoprotein may not have significant therapeutic consequences.