Targeting cancer through tumor-selective mRNA stabilization

Abstract

The success of cancer gene therapy is hindered by various physiological barriers to therapeutic vector transport from the site of injection to the nucleus of all the tumor cells. The application of replicating viruses for the treatment of cancers can overcome this problem. But this approach is limited by normal tissue tolerance of toxicity determined by local concentration of transgene products and viral proteins. Major improvements in vector targeting technology are required before any clinical success. On this basis, this thesis tests the hypothesis to target transformed tumor cells by using a novel posttranslational mRNA stabilizing mechanism, which is occasionally deregulated in cancer. The overexpression of various proteins associated with rapid responses to inflammation and/or proliferation can be controlled at the level of mRNA stability. Since tumor cells continually recapitulate intracellular programs of proliferation, we hypothesize that we can use the tumor cell selective stabilization of mRNA as a novel means to target different malignant diseases. Cyclooxygenase (COX) is the key enzyme in the conversion of arachidonic acid to prostaglandin during inflammation and many studies have linked elevated expression of COX-2 to the pathology of breast, colorectal, head and neck and other types of cancer. It has been shown that the up-regulation of COX-2 is a downstream effect of RAS-mediated transformation. Although the COX-2 over-expression in cancer is associated with increased transcription of the COX-2 gene, a large component of RAS-induced upregulation is also mediated by a selective stabilization of the mRNA of the COX-2 gene in RAS-transformed cells. In this project, we show tumor selective mRNA stability via COX-2 3’ UTR by fusing it with the adenovirus early essential gene E1A, thereby obtaining a conditionally replicating adenovirus vector which will preferentially replicate in the RAS transformed cells. There are wide range of genes reported in the literature with 3'UTR, which confers destabilized activity on their cognate mRNA, but whose actions are reversed under certain physiological conditions. These include hypoxia responsive 3'UTR, radiation responsive elements and 3' UTR, which mediate increased mRNA stability in proliferating cells. Therefore, the linkage to 3’UTRs is a general strategy that could be used to confer tumor cell specificity to expression of therapeutic and/or replicative genes in a wide variety of vectors and to target specific physiological situations within tumors