No abstract

We have identified a novel c-Myc-responsive gene, named JPO1, by representational difference analysis. JPO1 responds to two inducible c-Myc systems and behaves as a direct c-Myc target gene. JPO1 mRNA expression is readily detectable in the thymus, small intestine, and colon, whereas expression is relatively low in spleen, bone marrow, and peripheral leukocytes. We cloned a full-length JPO1 cDNA that encodes a 47-kDa nuclear protein.

The bmi-1 oncogene cooperates with c-myc in transgenic mice, resulting in accelerated lymphoma development. Altering the expression of Bmi-1 affects normal embryogenesis. The protein product of bmi-1 is homologous to certain Drosophila Polycomb group proteins that regulate homeotic gene expression through alteration of chromatin structure. Chimeric LexA-Bmi-1 protein has previously been shown to repress transcription. How Bmi-1 functions in embryogenesis and whether this relates to the ability of Bmi-1 to mediate cellular transformation is unknown. We demonstrate here that Bmi-1 is able to transform rodent fibroblasts in vitro, providing a system that has allowed us to correlate its molecular properties with its ability to transform cells. We map functional domains of Bmi-1 involved in transcriptional suppression by using the GAL4 chimeric transcriptional regulator system. Deletion analysis shows that the centrally located helix-turn-helix-turn-helixturn (HTHTHT) motif is necessary for transcriptional suppression whereas the N-terminal RING finger domain is not required. We demonstrate that nuclear localization requires KRMK (residues 230 to 233) and that the absence of nuclear entry ablates transformation. In addition, we find that the subnuclear localization of wild-type Bmi-1 to the rim of the nucleus requires the RING finger domain and correlates with its ability to transform. Our studies with Bmi-1 deletion mutants suggest that the ability of Bmi-1 to mediate cellular transformation correlates with its unique subnuclear localization but not its transcriptional suppression activity.The bmi-1 (B-cell-specific Moloney murine leukemia virus insertion site 1) oncogene was discovered by retroviral insertional mutagenesis when E-myc transgenic mice were infected with Moloney murine leukemia virus. These animals were noted to have a marked decrease in the latency period preceding the development of pre-B-cell lymphomas from approximately 150 to 50 days. Analysis of these tumors showed a variety of retroviral insertion sites with frequent integration (35 to 47%) near bmi-1, resulting in its overexpression (10, 39). E-bmi-1 transgenic mice develop lymphomas, with a majority being of the T-cell lineage. Consistent with the retroviral insertional mutagenesis studies, crossbreeding of E-bmi-1 and E-myc mice accelerates the onset of both B-and T-celllineage lymphomas (11).The significance of bmi-1 in lymphomagenesis is further underscored by its recurrent activation in naturally occurring feline lymphomas. LC-FeLV is a myc-containing strain of feline leukemia virus which has been shown to induce thymic lymphosarcoma in cats (23). Of these naturally occurring lymphosarcomas, 86% revealed preferential integration of LCFeLV at a locus termed flvi-2, which was found to be feline bmi-1. The involvement of bmi-1 in human cancers or lymphomas has not been established; however, its amino acid sequence shares 98% identity with murine Bmi-1 (23). Fluorescence in situ hybridization localizes the human bmi-1 gene to 10p13 (2), a reg...