Browsing by Subject "KRAS"

The small GTPases HRAS, NRAS and 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 RAS, with 98% of those occurring in the KRAS isoform. While there have been many advances in the understanding of 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 Kras–driven tumorigenesis in the lung:

(I) Despite nearly identical protein sequences, the three RAS proto-oncogenes exhibit divergent codon usage. Of the three isoforms, KRAS contains the most rare codons resulting in lower levels of KRAS protein expression relative to HRAS and NRAS. To determine the consequences of rare codon bias during de novo tumorigenesis, we created a knock-in Krasex3op 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 Krasex3op 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 Kras tumorigenesis.

(II) Lung cancer patients exhale higher levels of NO and iNOS-/- mice are resistant to chemically induced lung tumorigenesis. I hypothesize that NO promotes Kras–driven lung adenocarcinoma, and NOS inhibition may decrease Kras–driven lung tumorigenesis. To test this hypothesis, I assessed efficacy of the NOS inhibitor L–NAME in a genetically engineered mouse model of Kras-driven lung adenocarcinoma. Adenoviral Cre recombinase was delivered into the lungs intranasally, resulting in expression of oncogenic KrasG12D and dominant-negative Trp53R172H 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.

RAS is a small GTPase that helps to convert extracellular cues into intracellular actions. It is the most commonly mutated oncogene and is found in an active mutant form in 90% of pancreatic cancers. Therefore, study of how this protein is made and then how this protein signals in the cell could provide the foundation for novel approaches to treat RAS-driven malignancies.

First I demonstrate that the level of protein expressed from the gene KRAS, but not the highly homologous gene HRAS, is limited in mammalian cells by an abundance of underrepresented (rare) codons in the encoding mRNA. KRAS mRNA from both ectopic plasmids as well as from the endogenous cellular gene is subject to slowed translation due to these rare codons within its coding sequence. This has consequences for tumorigenesis, as replacement of the rare codons with commonly used codons accelerates RAS driven tumor growth. This may extend beyond HRAS and KRAS, as many other homologous gene pairs show a high divergence in codon usage and protein expression, suggesting that this could be a wider phenomenon used by mammalian cells to regulate protein levels.

Second, I demonstrate that RAS driven tumors partially depend on eNOS for growth. Using genetically engineered mouse models that recapitulate the spontaneous development of pancreatic cancer, I demonstrate that the protein eNOS is progressively upregulated as tumors develop. I then demonstrate that genetic ablation of eNOS partially blocks the development of preinvasive pancreatic lesions in these mice, and trends toward increasing survival in mice that develop lethal pancreatic adenocarcinoma. Furthermore, I then show that inhibition of eNOS using the clinically tested small molecule L-NAME can also slow the development of preinvasive neoplasia and nonsignificantly increase survival, although not to the level of eNOS genetic ablation. These findings are applicable to a clinical setting, as in conjunction with others I show that L-NAME treatment of human pancreatic cancer xenografts halves their growth, even when the main side effect of L-NAME, hypertension, is treated.

Together, these studies provide a better understanding of how RAS functions within the cell, and thus, ultimately, how RAS driven cancers may be treated.