BCOR Is Involved in Myeloid Cell Growth Control by Regulating Hox Genes

Abstract 3445

BCOR interacts with Polycomb group (PcG) proteins to form a PRC1-like transcriptional repressive complex. BCOR is an essential developmental regulator: hemizygous mutation of BCOR in males causes gestational lethality, and heterozygous mutation in females causes the X-linked developmental disorder Oculofaciocardiodental syndrome (OFCD). Studies in humans, cell lines and mice indicate that BCOR plays key roles in the regulation of early embryonic development, mesenchymal stem cell function and hematopoiesis. Recently, recurrent somatic BCOR mutations have been reported in retinoblastoma, medulloblastoma and acute myelogenous leukaemia (AML) suggesting BCOR functions as a tumor suppressor protein. The goal of the current study was to begin elucidating the function of BCOR in normal hematopoiesis and in hematopoietic transformation. To examine this in the mouse we transduced bone marrow (BM) from mice harboring a conditional Bcor deletion allele (Bcor^Fl/Y) with lentivirus expressing CRE. FACS analysis showed that with CRE, 83% of Bcor^-/Y (Bcor KO) BM cells kept in IL3, IL6 and stem cell factor (SCF) were CD34+ compared to 95% of non-induced Bcor^Fl/Y (Bcor WT) BM cells. Moreover, following G-CSF treatment, Bcor KO BM cells showed a much higher incidence of cell differentiation (40.5%) compared to Bcor WT cells (5.4%) as shown by CD11b/Gr-1 FACS analysis. Surprisingly, Bcor KO BM cells also showed significantly higher proliferation rates in liquid culture and methycellulose colony assays. To identify BCOR downstream target genes, we compared the gene expression profiles between Bcor KO and WT BM cells. Real-time-PCR of individual Hox genes confirmed the upregulation of HoxA5, HoxA7 and HoxA9 in Bcor KO BM cells, which are known to increase self-renewal of immature hematopoietic cells. In addition, ChIP analysis showed specific binding of BCOR to HoxA7 promoter. In conclusion, our results from mouse experiments suggest that BCOR is involved in maintaining the CD34+ stem cell state and loss of BCOR leads to enhanced myeloid cell differentiation and proliferation possibly by regulating Hox genes. In addition, we examined BCOR expression levels in a large panel of human leukemic cell lines; high level BCOR mRNA and protein were detected in most cell lines. Notably, 2 cell lines, U937 and HL60, had low levels of BCOR mRNA and no detectable BCOR protein. Treatment of both cell lines with the differentiation inducers ATRA, Vitamin D3 or DMSO lead to a mark induction in BCOR expression both at the mRNA and protein level, suggesting that BCOR may be involved in myeloid cell differentiation. Further investigation should shed more light on the roles BCOR that plays in normal hematopoiesis and whether BCOR loss later in hematopoiesis could result in a differentiation block.