1009 – TRANSCRIPTIONAL CONTROL OF EARLY AND LATE B-LYMPHOPOIESIS ANALYZED BY IN VIVO ACUTE TRANSCRIPTION FACTOR DEGRADATION

Gene regulation by hematopoietic transcription factors (TFs) is largely studied by conditional mutagenesis combined with mRNA-seq and genome-wide TF binding. This approach does not reliably discriminate between direct target genes of TFs and secondary consequences of conditional mutagenesis. Moreover, the frequent loss of cell types upon elimination of important TFs and the paucity of stage-specific Cre lines prevent a systematic analysis of gene-regulatory networks of several TFs in different B cell types. Acute protein degradation bypasses these limitations by facilitating the identification of direct TF target genes. For this, we used the auxin-inducible degron (Aid) system to degrade Aid-tagged TFs in vivo in the mouse. As B cell development depends on E2A, Ebf1, Pax5, Ikaros and Aiolos, we have Aid-tagged these TFs to identify their target genes in pro-B, pre-B and immature B cells by degradation in vivo. This revealed that E2A, Ebf1 and Pax5 predominantly function as activators by inducing open chromatin at their target genes, while Ikaros and Aiolos act as dedicated repressors. Systematic analyses of their target genes provided unprecedented insight into the transcriptional regulation of early B-lymphopoiesis, including the control of pre-BCR transition, V(D)J recombination and chromosomal architecture. As plasma cells are not generated in the absence of Ikaros and Aiolos, we used acute TF degradation to investigate their role in mouse plasma cells, which also provided insight into the potential role of Ikaros and Aiolos in multiple myeloma.