|M.Sc Student||Albert Elinor|
|Subject||Identification and Characterization of PML as a CML Tumor|
Suppressor Gene Using a Murine Model
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Ben-Zion Levi|
|Full Thesis text|
CML (Chronic Myelogenous Leukemia) is a myeloid progenitor cell disorder. The direct cause for CML is the product of the BCR-ABL fusion tyrosine kinase whose leukemogenic activity enhances survival of myeloid progenitor cells by activating anti-apoptotic genes. Recently, the role of the transcription factor Interferon Regulatory Factor-8 (IRF-8) in driving monocytopoiesis of bipotential progenitor cells was demonstrated. IRF-8 can also act as a tumor suppressor gene in myeloid leukemias in general and in CML in particular. Down regulation of IRF-8 expression levels was reported in CML patients that return to normal levels during remission following treatment. Our studies demonstrated that the Promyelocytic Leukemia (PML) gene is directly regulated by IRF-8. PML is a transcriptional regulator which is thought to act as a tumor suppressor, involved in genome stability, transcription, post-translational modifications, proteolysis, and DNA repair. Tumor progression is associated with loss of PML expression. Furthermore, a correlation between the low levels of IRF-8 and PML in CML patients was reported. The data pointed to a novel link between disregulated expression of PML and CML. Therefore, we hypothesized that some of the myeloleukemia suppressor activities of IRF-8 might be mediated through the regulation of its target gene PML. Our goal was to examine the role of PML in the pathogenesis of CML by using a murine model in which we expressed the BCR/ABL fusion gene in the murine myelocytic cell line, 32D. The BCR/ABL transformed 32D cells exhibited morphological changes, IL-3 independent proliferation and downregulation of genes participating in myelocyte differentiation. We transduced the BCR/ABL transformed cells with a retroviral vector harboring the murine PML-1 gene. The resultant cells were analyzed for various cells characteristics. In comparison to the BCR/ABL transformed cells, PML-1 expressing cells exhibited morphology that resembles 32D normal cells, reduction in colony forming potential in soft agar, increase in macrophage/granulocytes cell surface markers and elevated expression of granulocyte specific genes that were significantly repressed by BCR/ABL. We next investigated the tumor suppression activities of PML in vivo. BCR/ABL transformed 32D cells and the same cells expressing PML-1 were injected subcutaneously to syngeneic mice and the development of tumors was recorded. The PML expressing tumors exhibited a decreased growth rate and were much smaller comparing to the BCR/ABL tumors. Together, these results suggest that PML-1 was able to revert the transformed phenotype of BCR/ABL transformed cells and point to the important role of PML as a tumor suppressor of CML.