The vascular bone marrow niche influences outcome in chronic myeloid leukemia via the E-selectin - SCL/TAL1 - CD44 axis.

The endosteal bone marrow niche and vascular endothelial cells provide sanctuaries for leukemic cells. In murine chronic myeloid leukemia (CML) CD44 on leukemia cells and E-selectin on bone marrow endothelium are essential mediators for the engraftment of leukemic stem cells. We hypothesized that non-adhesion of CML-initiating cells to E-selectin on the bone marrow endothelium may lead to superior eradication of leukemic stem cells in CML after treatment with imatinib than imatinib alone. Indeed, here we show that treatment with the E-selectin inhibitor GMI-1271 in combination with imatinib prolongs survival of mice with CML via decreased contact time of leukemia cells with bone marrow endothelium. Non-adhesion of BCR-ABL1+ cells leads to an increase of cell cycle progression and an increase of expression of the hematopoietic transcription factor and proto-oncogene Scl/Tal1 in leukemia-initiating cells. We implicate SCL/TAL1 as an indirect phosphorylation target of BCR-ABL1 and as a negative transcriptional regulator of CD44 expression. We show that increased SCL/TAL1 expression is associated with improved outcome in human CML. These data demonstrate the BCR-ABL1-specific, cell-intrinsic pathways leading to altered interactions with the vascular niche via the modulation of adhesion molecules - which could be exploited therapeutically in the future.

Protein arginine methyltransferase 6 controls erythroid gene expression and differentiation of human CD34+ progenitor cells.

Hematopoietic differentiation is driven by transcription factors, which orchestrate a finely tuned transcriptional network. At bipotential branching points lineage decisions are made, where key transcription factors initiate cell type-specific gene expression programs. These programs are stabilized by the epigenetic activity of recruited chromatin-modifying cofactors. An example is the association of the transcription factor RUNX1 with protein arginine methyltransferase 6 (PRMT6) at the megakaryocytic/erythroid bifurcation. However, little is known about the specific influence of PRMT6 on this important branching point. Here, we show that PRMT6 inhibits erythroid gene expression during megakaryopoiesis of primary human CD34+ progenitor cells. PRMT6 is recruited to erythroid genes, such as glycophorin A Consequently, a repressive histone modification pattern with high H3R2me2a and low H3K4me3 is established. Importantly, inhibition of PRMT6 by shRNA or small molecule inhibitors leads to upregulation of erythroid genes and promotes erythropoiesis. Our data reveal that PRMT6 plays a role in the control of erythroid/megakaryocytic differentiation and open up the possibility that manipulation of PRMT6 activity could facilitate enhanced erythropoiesis for therapeutic use.

ABCB4 is frequently epigenetically silenced in human cancers and inhibits tumor growth.

Epigenetic silencing through promoter hypermethylation is an important hallmark for the inactivation of tumor-related genes in carcinogenesis. Here we identified the ATP-binding cassette sub-family B member 4 (ABCB4) as a novel epigenetically silenced target gene. We investigated the epigenetic regulation of ABCB4 in 26 human lung, breast, skin, liver, head and neck cancer cells lines and in primary cancers by methylation and expression analysis. Hypermethylation of the ABCB4 CpG island promoter occurred in 16 out of 26 (62%) human cancer cell lines. Aberrant methylation of ABCB4 was also revealed in 39% of primary lung cancer and in 20% of head and neck cancer tissues. In 37% of primary lung cancer samples, ABCB4 expression was absent. For breast cancer a significant hypermethylation occurred in tumor tissues (41%) compared to matching normal samples (0%, p = 0.002). Silencing of ABCB4 was reversed by 5-aza-2'-deoxycytidine and zebularine treatments leading to its reexpression in cancer cells. Overexpression of ABCB4 significantly suppressed colony formation and proliferation of lung cancer cells. Hypermethylation of Abcb4 occurred also in murine cancer, but was not found in normal tissues. Our findings suggest that ABCB4 is a frequently silenced gene in different cancers and it may act tumor suppressivly in lung cancer.