ABSTRACT
Early B cell lymphopoiesis depends on E2A, Ebf1, Pax5 and Ikaros family members. In the present study, we used acute protein degradation in mice to identify direct target genes of these transcription factors in pro-B, small pre-B and immature B cells. E2A, Ebf1 and Pax5 predominantly function as transcriptional activators by inducing open chromatin at their target genes, have largely unique functions and are essential for early B cell maintenance. Ikaros and Aiolos act as dedicated repressors to cooperatively control early B cell development. The surrogate light-chain genes Igll1 and Vpreb1 are directly activated by Ebf1 and Pax5 in pro-B cells and directly repressed by Ikaros and Aiolos in small pre-B cells. Pax5 and E2A contribute to V(D)J recombination by activating Rag1, Rag2, Dntt, Irf4 and Irf8. Similar to Pax5, Ebf1 also represses the cohesin-release factor gene Wapl to mediate prolonged loop extrusion across the Igh locus. In summary, in vivo protein degradation has provided unprecedented insight into the control of early B cell lymphopoiesis by five transcription factors.
Subject(s)
B-Lymphocytes , Basic Helix-Loop-Helix Transcription Factors , Ikaros Transcription Factor , Lymphopoiesis , PAX5 Transcription Factor , Trans-Activators , Animals , Ikaros Transcription Factor/metabolism , Ikaros Transcription Factor/genetics , PAX5 Transcription Factor/metabolism , PAX5 Transcription Factor/genetics , Mice , Trans-Activators/metabolism , Trans-Activators/genetics , B-Lymphocytes/metabolism , B-Lymphocytes/immunology , Lymphopoiesis/genetics , Basic Helix-Loop-Helix Transcription Factors/metabolism , Basic Helix-Loop-Helix Transcription Factors/genetics , Proteolysis , Precursor Cells, B-Lymphoid/metabolism , Precursor Cells, B-Lymphoid/immunology , Mice, Inbred C57BL , Mice, Knockout , Cell Differentiation , Transcription Factor 7-Like 1 Protein/metabolism , Transcription Factor 7-Like 1 Protein/genetics , Transcription Factor 3/metabolism , Transcription Factor 3/genetics , V(D)J Recombination , Interferon Regulatory Factors/metabolism , Interferon Regulatory Factors/genetics , Immunoglobulin Light Chains, Surrogate/metabolism , Immunoglobulin Light Chains, Surrogate/genetics , Transcription, GeneticABSTRACT
While PAX5 is an important tumor suppressor gene in B-cell acute lymphoblastic leukemia (B-ALL), it is also involved in oncogenic translocations coding for diverse PAX5 fusion proteins. PAX5-JAK2 encodes a protein consisting of the PAX5 DNA-binding region fused to the constitutively active JAK2 kinase domain. Here, we studied the oncogenic function of the PAX5-JAK2 fusion protein in a mouse model expressing it from the endogenous Pax5 locus, resulting in inactivation of one of the two Pax5 alleles. Pax5Jak2/+ mice rapidly developed an aggressive B-ALL in the absence of another cooperating exogenous gene mutation. The DNA-binding function and kinase activity of Pax5-Jak2 as well as IL-7 signaling contributed to leukemia development. Interestingly, all Pax5Jak2/+ tumors lost the remaining wild-type Pax5 allele, allowing efficient DNA-binding of Pax5-Jak2. While we could not find evidence for a nuclear role of Pax5-Jak2 as an epigenetic regulator, high levels of active phosphorylated STAT5 and increased expression of STAT5 target genes were seen in Pax5Jak2/+ B-ALL tumors, implying that nuclear Pax5-Jak2 phosphorylates STAT5. Together, these data reveal Pax5-Jak2 as an important nuclear driver of leukemogenesis by maintaining phosphorylated STAT5 levels in the nucleus.