EBF1 and PAX5 Control Pro-B Cell Expansion Via Opposing Regulation of the Myc Gene

Genes encoding B lineage-restricted transcription factors are frequently mutated in B-lymphoid leukemias, suggesting a close link between normal and malignant B-cell development. One of these transcription factors is early B-cell factor 1 (EBF1), a protein of critical importance for lineage specification and survival of B-lymphoid progenitors. Here, we report that impaired EBF1 function in mouse B-cell progenitors results in reduced expression of Myc. Ectopic expression of MYC partially rescued B-cell expansion in the absence of EBF1 both in vivo and in vitro. Using chromosome conformation analysis in combination with ATAC-sequencing, chromatin immunoprecipitation-sequencing, and reporter gene assays, six EBF1-responsive enhancer elements were identified within the Myc locus. CRISPR-Cas9-mediated targeting of EBF1-binding sites identified one element of key importance for Myc expression and pro-B cell expansion. These data provide evidence that Myc is a direct target of EBF1. Furthermore, chromatin immunoprecipitation-sequencing analysis revealed that several regulatory elements in the Myc locus are targets of PAX5. However, ectopic expression of PAX5 in EBF1-deficient cells inhibits the cell cycle and reduces Myc expression, suggesting that EBF1 and PAX5 act in an opposing manner to regulate Myc levels. This hypothesis is further substantiated by the finding that Pax5 inactivation reduces requirements for EBF1 in pro-B-cell expansion. The binding of EBF1 and PAX5 to regulatory elements in the human MYC gene in a B-cell acute lymphoblastic leukemia cell line indicates that the EBF1:PAX5:MYC regulatory loop is conserved and may control both normal and malignant B-cell development.