The RAS genes HRAS, NRAS, and KRAS are mutated in a third of all human cancers to encode constitutively active, oncogenic proteins that drive the initiation and maintenance of cancer. While all three genes encode highly similar proteins, KRAS is the most commonly mutated isoform, suggesting some feature of the gene renders it more likely to promote tumorigenesis. In this regard, KRAS is highly enriched, compared to the other isoforms, in rare codons. This inherent rare codon bias within the coding sequence limits KRAS protein expression, but conversely, promotes Kras-driven tumor initiation in a urethane-driven murine model of de novo tumorigenesis. Thus, codon bias plays an integral role in Kras-driven tumor initiation, and may account for the higher frequency of mutations found in KRAS compared to NRAS and HRAS. Codon bias as a regulatory mechanism of RAS biology has never been taken into account with regards to cancer, and thus is it not clear how codon bias affects KRAS oncogenic potential. This proposal will address this deficiency through two independent aims. Completion of AIM 1 will elucidate precisely when and how codon bias influences Kras- driven tumor initiation through the use of novel mouse models that encode inducible oncogenic KrasG12D alleles that have been altered to contain either all rare or all common codons. While the rare codon bias in the Kras gene seems to be advantageous during tumor initiation, recent studies suggest that this bias limits progression once tumor cells bypass senescence. Despite this, increased RAS expression and signaling is associated with malignancy and cancer progression, suggesting that there is likely selection to overcome the poor expression of KRAS imposed by rare codons. Thus, AIM 2 will identify the mechanisms responsible for increasing oncogenic KRAS expression during cancer progression using a panel of KRAS mutant human cancer cell lines that exhibit enhanced translation of oncogenic KRAS transcripts. Results from these studies will shed light on the clinical mystery of why there are mutational biases among the RAS genes and, moreover, will reveal novel control points that drive oncogene addiction, thus opening up unexplored opportunities for therapeutic intervention in KRAS mutant cancers.
The RAS genes are mutated to encode constitutively active proteins that promote one-third of all human cancers. Thus, the proposed studies to elucidate how inherent differences in the sequence of these genes impact tumorigenesis has direct relevance to a broad spectrum of human cancers.