PU.1 and Spi-B Oppose Transformation Of Pre-B Cells Through Activation Of Key Genes Involved In B Cell Receptor Signalling

B cell development is controlled by stage-specific expression of transcription factors. Aberrant expression of such factors can lead to B cell acute lymphoblastic leukemia (B-ALL). Deletion of genes encoding the E26 transformation-specific (ETS) transcription factors, PU.1 and Spi-B, in B cells (CD19^+/CreSfpi1^lox/loxSpib^-/- mice, abbreviated to CD19-CreΔPB) leads to B-ALL at 100% incidence and with a median survival of 21 weeks. However, little is known about the target genes of PU.1 and Spi-B that explain leukemic transformation in these mice. In the current study, we investigated the developmental origins and mechanisms of leukemogenesis in CD19-CreΔPB mice. We found that B-ALL cells in CD19-CreΔPB mice had frequently rearranged both their heavy and light chain genes, but retained cell surface expression of interleukin-7 receptor (IL-7R), suggesting aberrant pre-B cell differentiation. Preleukemic CD19-CreΔPB mice had increased frequencies of pre-B cells compared to wild type mice. Pre-B cells, but not mature B cells, purified from the bone marrow of preleukemic CD19-CreΔPB mice could rapidly transfer disease to transplanted recipient mice. B-ALL cells from established tumors had uniform expression of markers indicating a pre-B cell phenotype and contained a high-frequency of leukemia-initiating cells as measured by transplantation assays. Genome-wide analysis of gene expression showed that B cell receptor signalling was the top impaired pathway in B-ALL cells from CD19-CreΔPB mice. Bone marrow cells from CD19-CreΔPB mice had increased responsiveness to IL-7R signalling and could be cultured as IL-7-dependent cell lines. Preleukemic or leukemic cells from CD19-CreΔPB mice expressed reduced levels of the gene encoding Bruton’s tyrosine kinase (Btk), which we show is a target gene of PU.1 and/or Spi-B that in combination with reduced BLNK is sufficient to explain increased IL-7R responsiveness. We conclude that mutation of PU.1 and Spi-B predispose developing B cells to leukemogenesis by impairing expression of key genes, such as Btk, that are required for BCR signalling and are involved in attenuation of IL-7 receptor signaling.