Targeting a Novel Epigenetic Silencing Mechanism to Efficiently Upregulate Fetal Globin Gene Expression

Abstract 352

L3MBTL1 is a Polycomb group protein, commonly deleted in patients with myeloid disorders associated with the 20q- chromosomal abnormality. After crystallizing the MBT repeat domain, we demonstrated that L3MBTL1 compacts chromatin by binding mono- and di-methylated lysine residues in histones H1 (H1K26) and H4 (H4K20), ultimately leading to gene repression. Despite its role in affecting the chromatin structure, the role of L3MBTL1 in hematopoiesis has remained largely unknown.

We recently demonstrated that lack of L3MBTL1 accelerates the erythroid differentiation of human hematopoietic stem cells and here we reveal that L3MBTL1 represses the expression of the fetal gamma globin gene. We lentivirally expressed shRNAs targeting L3MBTL1 in human cord blood (CB) CD34+ cells and in K562 erythroleukemia cells, and consistently observed upregulation of gamma globin gene expression, while beta globin gene expression decreased. Remarkably, we observed similar findings in human embryonic stem (hES) cells, where knock-down of L3MBTL1 triggered a BMP4-like spontaneous differentiation. Given the potential impact of therapeutically increasing fetal hemoglobin expression in patients with hemoglobinopathies, we targeted L3MBTL1 in induced pluripotent stem (iPS) cells derived from patients with β-thalassemia. The gene expression profile of L3MBTL1-KD normal and thalassemic iPS cells indicated clear activation of fetal hemoglobin (HbF) expression, activation of BMP4 signaling and upregulation of specific smad5 target genes (e.g. EKLF, HHEX, ID2/3). We generated and utilized a model of “stress erythropoiesis” in L3MBTL1 KO mice and observed in vivo BMP4-mediated expansion of spleen immature erythroid progenitors, as indicated by increased spleen weight and splenic BFU-E colonies in KO mice compared to controls.

We also examined K562 cells, human CB CD34+ cells and hES cells, using chromatin immunoprecipitation assays, and found that L3MBTL1 directly associates with the human β-globin locus, occupying discrete regions within the human β-globin cluster. Furthermore, L3MBTL1 colocalized with H4K20me within the Locus Control Region (LCR), a primary attachment site for chromatin modifiers. We observed clearance of L3MBTL1 and its associated histone marks (H4K20me1/2) from the LCR upon treatment with hemin, erythropoietin or TGFβ, three agents that potently induce erythroid differentiation. This suggests that this polycomb repressor complex responds to cytokine signaling.

In summary, we have identified a novel epigenetic regulatory mechanism to control fetal globin gene expression; the Polycomb protein L3MBTL1 regulates BMP4 signaling and the chromatin structure of globin genes. Targeting this regulatory system represents a means to efficiently increase HbF in a human model of β-thalassemia (i.e. with the use of patient-derived iPS cells) and to potentially ameliorate hematological and clinical symptoms of patients with red cell disorders.