The LTR and env gene of mink cell focus-forming viruses: Contributions to viral transcription and leukemogenicity.

摘要

Mink cell focus forming (MCF) viruses are recombinant murine leukemia viruses that arise de novo in mice of the inbred AKR strain. The leukemogenic phenotype of MCF viruses is determined by the viral long-terminal repeat (LTR) and env gene. The LTR determines the tissue-specificity of MCF viruses, and the proteins (TM and SU) encoded by the env gene bind to receptors on target cells. In this dissertation, I describe experiments designed to determine how sequences in the LTR contribute to MCF viral transcription and leukemogenicity. By analyzing the LTR sequences generated during the biological selection of leukemogenic MCF viruses, we identified a cluster of putative binding sites, for the transcription factors Nuclear Factor-1, Ets, Runt, Ikaros and Glucocorticoid Responsive Element, that, when duplicated, increased the leukemogenicity of an MCF virus. In experiments with transgenic mice carrying the LTR of a leukemogenic MCF virus, joined to either a gene encoding β- galactosidase or to the MCF env gene, I observed little-to-no transgene expression. The absence of transgene expression in these mice suggest that the MCF LTR may be silenced by a stem cell-specific mechanism that has been previously described in transgenic mice containing the LTR of the Moloney murine leukemia virus. Finally, in this dissertation I also describe experiments designed to determine whether expression of SU and/or TM initiates receptor-mediated events in leukemogenesis. For these experiments, I generated transgenic mice expressing TM and SU, under the control of regulatory sequences derived from the human CD2 gene, on the surface of their thymocytes. Analyses of transgene expression in these mice suggest that high expression of TM and SU may be negatively selected during thymocyte development. Taken together, the results of these experiments have further defined the region of the LTR that must be duplicated for increased leukemogenicity by MCF viruses, suggested that MCF viruses arise de novo because of transcriptional repression of endogenous murine leukemia viruses, and shed light on a mechanism by which expression of TM and SU could contribute to the transformation of infected thymocytes.