Pax5 negatively regulates B lineage cell proliferation through inhibition of D cyclin expression

Pax5 is a B cell specific transcription factor absolutely required for commitment and maintenance of the B lineage. It induces over 170 B lineage-specific genes and is expressed throughout B cell development. Pax5 is the most frequently mutated gene in acute lymphoblastic leukemia (ALL). Mutation to Pax5 diminishes its target gene activation contributing to the leukemogenesis in ALL. I over-expressed wild-type Pax5 in various human ALL and Burkitt's lymphoma (BL) cell lines and identified a remarkable reduction in cell proliferation. Furthermore, I identified a dramatic inhibition of cyclin D1 and D2 upon forced expression of Pax5 in Pax5 murine pro B cells (G5), which lead to the hypothesis that Pax5 acts as a negative regulator for B lineage cell proliferation through suppression of cyclin D1 and D2 expression. In mantle cell lymphoma (MCL) cells with abnormal cyclin D1 expression, over-expression of Pax5 failed to inhibit cell proliferation. However, simply over-expressing any of the D type cyclins could not rescue Pax5-mediated inhibition of cell proliferation in G5 cells. Further studies showed that forced expression of Pax5 in G5 cells induced apoptosis after 48 hours, accompanied by a shift in the balance of pro- versus anti-apoptotic BCL-2 family proteins. Mutation to Pax5 diminished this function and even created a growth advantage in vitro. Taken together, these results revealed a novel role for Pax5 as a negative regulator of B lineage cell proliferation and help explain why it's the most frequently mutated gene in ALL patients.;In my second project, I attempted to restore the B lineage phenotype in human Hodgkin's lymphoma (HL) cells. HL is a B-cell malignancy that has lost normal B lineage gene expression due to unknown causes. The absolute requirement of Pax5 for inducing B lineage-specific gene expression lead to the hypothesis that Pax5 over-expression could restore the B lineage phenotype in HL cells. Unfortunately, over-expression of Pax5 could not restore B lineage-specific gene expression in HL cells, while failing to inhibit cell proliferation as well. These results further support a non-B cell-specific phenotype in HL.