Evaluation of Altered Kras Codon Bias and NOS Inhibition During Lung Tumorigenesis

<p>The small GTPases <italic>HRAS, <italic>NRAS and <italic>KRAS are mutated in approximately one-third of all human cancers, rendering the proteins constitutively active and oncogenic. Lung cancer is the leading cause of cancer deaths worldwide, and more than 20% of human lung cancers harbor mutations in <italic>RAS, with 98% of those occurring in the <italic>KRAS isoform. While there have been many advances in the understanding of <italic>KRAS–driven lung tumorigenesis, it remains a therapeutic challenge. To further this understanding and assess novel approaches for treatment, I have investigated two aspects of <italic>Kras–driven tumorigenesis in the lung:</p><p>(<italic>I) Despite nearly identical protein sequences, the three <italic>RAS proto-oncogenes exhibit divergent codon usage. Of the three isoforms, <italic>KRAS contains the most rare codons resulting in lower levels of KRAS protein expression relative to <italic>HRAS and <italic>NRAS. To determine the consequences of rare codon bias during <italic>de <italic>novo tumorigenesis, we created a knock-in <italic>Kras<super>ex3op mouse in which synonymous mutations in exon 3 converted codons from rare to common. These mice had reduced tumor burden and fewer oncogenic mutations in the <italic>Kras<super>ex3op allele following carcinogen exposure. The reduction in tumorigenesis appeared to be a product of rare codons affecting both the oncogenic and non–oncogenic alleles. Converting rare codons to common codons yielded a more potent oncogenic allele that promoted growth arrest and enhanced tumor suppression by the non-oncogenic allele. Thus, rare codons play an integral role in <italic>Kras tumorigenesis.</p><p>(<italic>II) Lung cancer patients exhale higher levels of NO and <italic>iNOS<super>-/- mice are resistant to chemically induced lung tumorigenesis. I hypothesize that NO promotes <italic>Kras–driven lung adenocarcinoma, and NOS inhibition may decrease <italic>Kras–driven lung tumorigenesis. To test this hypothesis, I assessed efficacy of the NOS inhibitor L–NAME in a genetically engineered mouse model of <italic>Kras-driven lung adenocarcinoma. Adenoviral Cre recombinase was delivered into the lungs intranasally, resulting in expression of oncogenic <italic>Kras<super>G12D and dominant-negative <italic>Trp53<super>R172H in lung epithelial cells. L–NAME treatment was provided in the water and continued until survival endpoints. In this model, L–NAME treatment decreased tumor growth and prolonged survival. These data establish a potential clinical role for NOS inhibition in lung cancer treatment.</p>

Dissertation