Impact of oncogenic retrovirus infection on hematopoiesis during the pre-malignant stage of disease
Description
The addition of a 21-bp triplication in the long terminal repeat (LTR) of feline leukemia virus (FeLV) affected disease outcome. The lymphoma from which the triplication-containing provirus, FeLV-945, was isolated was anatomically classified as multicentric. Subsequent studies showed that experimental FeLV-945 infection of cats resulted in multicentric lymphoma of B-cell origin. In this dissertation we wanted to explore the mechanism by which the FeLV-945 21-bp triplication affected disease outcome. The FeLV-945 21-bp triplication was demonstrated to contain 2 binding sites for the transcription factor, c-Myb. These binding sites span the junctions of the repeat indicating a selective pressure for the precise retention of the repeat. The 21-bp triplication renders the virus responsive to c-Myb in a dose-dependent manner. By direct interaction with c-Myb, the transcriptional co-factor CBP is recruited the LTR and increases transcriptional activity. Next the impact of the FeLV-945 LTR on bone marrow hematopoiesis was examined. A recombinant murine retrovirus, MoFe2-MuLV, was used which contains the U3 region of FeLV-945 in the backbone of Moloney murine leukemia virus (M-MuLV). MoFe2-MuLV and M-MuLV infection of laboratory mice resulted in decreased CFU-pre-B clonogenic potential during the preleukemic stage of disease. This is the first observation that M-MuLV affects B-lymphopoiesis. To explore the mechanism of decreased CFU-pre-B potential we used the murine multi-potential hematopoietic progenitor cell line, EML. This cell line gave us the opportunity to examine a possible block in B-lymphopoiesis following infection in vitro. A potential block in B-cell differentiation at the pro-B stage was observed in infected cells. A density-dependent reduction of proliferation during B-cell differentiation in infected cells was detected. Acute infection of EML cells with MoFe2-MuLV or M-MuLV resulted in a p53-mediated apoptotic crisis. Crisis lasted approximately 4-6 days, after which a chronically infected population arose. YB-1, a multifunctional protein which suppresses p53-mediated apoptosis, is upregulated in chronically infected EML cells. In vivo, a p53-dependent apoptotic crisis of hematopoietic progenitor cells, followed by increased YB-1 activity, represents a mechanism of rapid selection of a population of cells that are predisposed to malignant transformation