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Myelodysplastic syndromes (MDSs) are clonal hematopoietic stem cell disorders characterized by ineffective hematopoiesis. Anemia is the defining cytopenia of MDS patients, yet the molecular mechanisms for dyserythropoiesis in MDSs remain to be fully defined. Recent studies have revealed that heterozygous loss-of-function mutation of DNA dioxygenase TET2 is 1 of the most common mutations in MDSs and that TET2 deficiency disturbs erythroid differentiation. However, mechanistic insights into the role of TET2 on disordered erythropoiesis are not fully defined. Here, we show that TET2 deficiency leads initially to stem cell factor (SCF)-dependent hyperproliferation and impaired differentiation of human colony-forming unit-erythroid (CFU-E) cells, which were reversed by a c-Kit inhibitor. We further show that this was due to increased phosphorylation of c-Kit accompanied by decreased expression of phosphatase SHP-1, a negative regulator of c-Kit. At later stages, TET2 deficiency led to an accumulation of a progenitor population, which expressed surface markers characteristic of normal CFU-E cells but were functionally different. In contrast to normal CFU-E cells that require only erythropoietin (EPO) for proliferation, these abnormal progenitors required SCF and EPO and exhibited impaired differentiation. We termed this population of progenitors "marker CFU-E" cells. We further show that AXL expression was increased in marker CFU-E cells and that the increased AXL expression led to increased activation of AKT and ERK. Moreover, the altered proliferation and differentiation of marker CFU-E cells were partially rescued by an AXL inhibitor. Our findings document an important role for TET2 in erythropoiesis and have uncovered previously unknown mechanisms by which deficiency of TET2 contributes to ineffective erythropoiesis. [ABSTRACT FROM AUTHOR]

The TET family of proteins plays important roles in a wide range of biological processes by oxidizing 5-methylcytosine (5mC) to 5-hydroxy-methylcytosine (5hmC). However, their function in erythropoiesis has remained unclear. We show here that TET2 and TET3, but not TET1 are expressed in human erythroid cells and we explore the role of these proteins in erythropoiesis. Knockdown experiments revealed that TET2 and TET3 have different functions. Suppression of TET3 expression in human CD34+ cells markedly impaired terminal erythroid differentiation, as reflected by increased apoptosis, generation of bi/multinucleated polychromatic/orthochromatic erythroblasts and impaired enucleation, though without effect on erythroid progenitors. In marked contrast, TET2 knockdown led to hyper-proliferation and impaired differentiation of erythroid progenitors. Surprisingly, knockdown of neither TET2 nor TET3 affected global levels of 5mC. Thus our findings have identified distinct roles for TET2 and TET3 in human erythropoiesis, and provide new insights into their role in regulating human erythroid differentiation at distinct stages of development. Moreover, as knockdown of TET2 recapitulates certain features of erythroid development defects characteristic of myelodysplastic syndromes (MDS), and TET2 gene mutation is one of the most common mutations in MDS, our findings may be relevant for improved understanding of dyserythropoiesis of MDS. [ABSTRACT FROM AUTHOR]