Oncogenesis occurs when mechanisms of cell growth control are deregulated. This generally develops in a multistep process that involves mutations in oncogenes and tumor suppressor genes. The understanding of this process has been greatly enhanced through the study of tumor viruses. The piscine retroviruses and their associated neoplasias are new models of tumorigenesis. Walleye dermal sarcomas (WDS) are etiologically associated with the complex retrovirus, walleye dermal sarcoma virus (WDSV). These sarcomas develop and regress on a seasonal basis, providing a unique model to study molecular mechanisms of tumor development and regression in vertebrates. WDSV expression is highly regulated and the differential expression of viral transcripts correlates with the stage of disease development. Infectious virus is only produced during regression. Our working hypothesis is that the three WDSV accessory proteins, Orf A (rv-cyclin), Orf B, and Orf C, control viral gene expression and function in tumor growth and regression. WDSV rv-cyclin regulates transcription, inhibits virus expression, and enhances cell proliferation. Associations of rv-cyclin with cyclin dependent kinases 3 and 8 (cdk3 and cdk8), normally bound by host cyclin C, led to the hypothesis that rv-cyclin/cdk complexes alter viral and cellular gene expression and promote cell cycle progression. The long-term objective is to define novel mechanisms of tumor induction and control of gene expression. The proposed research has three specific aims: (1) Characterize rv-cyclin transregulation of transcription. The goals of this aim are to separate and define the mechanisms of rv-cyclin AD and cyclin box transcription control. These studies will focus on p53 and p53-responsive promoters that are regulated by both TAF9 and cyclin C/cdk8 and on NF-kB, which also contacts TAF9, and whose function is clearly inhibited by the rv-cyclin. (2) Define the functional outcome of rv-cyclin/cdk3 interaction. The identification of rv-cyclin-cdk3 interaction suggests a direct mechanism for cell cycle control by pRb phosphorylation. The kinase activity of rv-cyclin/cdk3 will be compared to cyclin C/cdk3, and the specific effects of the rv-cyclin on cell cycle regulation will be determined by flow cytometry. (3) Assess rv-cyclin function in control of virus expression and tumorigenesis. These studies will test wild type and mutated recombinant WDSV viruses with the rv-cyclin gene knocked out, with loss of function mutations in the cyclin box region and in the AD, and gain of function mutation of the AD. The replicative capacity of these infectious viruses will be tested in tissue culture cells and in fish, and the contribution of the rv-cyclin to tumor formation will be assessed.
The proposed research utilizes an efficient animal cancer model that combines tumor formation with natural and complete remission. The causative viral agent encodes a retroviral cyclin that controls cell growth and gene expression. These studies will identify targets for therapeutic intervention of cancer that are fundamental to all higher organisms.
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