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SUMMARY Loss-of-function mutations in TET2 occur frequently in patients with clonal hematopoiesis, myelodysplastic syndrome (MDS), and acute myeloid leukemia (AML) and are associated with a DNA hypermethylation phenotype. To determine the role of TET2 deficiency in leukemia stem cell maintenance, we generated a reversible transgenic RNAi mouse to model restoration of endogenous Tet2 expression. Tet2 restoration reverses aberrant hematopoietic stem and progenitor cell (HSPC) self-renewal in vitro and in vivo. Treatment with vitamin C, a cofactor of Fe2+ and α-KG-dependent dioxygenases, mimics TET2 restoration by enhancing 5-hydroxymethylcytosine formation in Tet2-deficient mouse HSPCs and suppresses human leukemic colony formation and leukemia progression of primary human leukemia PDXs. Vitamin C also drives DNA hypomethylation and expression of a TET2-dependent gene signature in human leukemia cell lines. Furthermore, TET-mediated DNA oxidation induced by vitamin C treatment in leukemia cells enhances their sensitivity to PARP inhibition and could provide a safe and effective combination strategy to selectively target TET deficiency in cancer.

Summary Tet enzymes (Tet1/2/3) convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) in various embryonic and adult tissues. Mice mutant for either Tet1 or Tet2 are viable raising the question whether these enzymes have overlapping roles in development. Here, we have generated Tet1 and Tet2 double knockout (DKO) ESCs and mice. DKO ESCs remained pluripotent, but were depleted of 5hmC and caused developmental defects in chimeric embryos. While a fraction of double mutant embryos exhibited mid-gestation abnormalities with perinatal lethality, viable and overtly normal Tet1/Tet2 deficient mice were also obtained. DKO mice had reduced 5hmC and increased 5mC levels and abnormal methylation at various imprinted loci. Nevertheless, animals of both sexes were fertile with females having smaller ovaries and reduced fertility. Our data show that loss of both enzymes is compatible with development but promotes hypermethylation and compromises imprinting. It also suggests a significant contribution of Tet3 to hydroxylation of 5mC during development.

Molecular control of the pluripotent state is thought to reside in a core circuitry of master transcription factors including the homeodomain-containing protein Nanog1–2, which plays an essential role in establishing ground state pluripotency during somatic cell reprogramming3–4. While the genomic occupancy of Nanog has been extensively investigated, comparatively little is known about Nanog-associated proteins5 and their contribution to the Nanog-mediated reprogramming process. Using enhanced purification techniques and a stringent computational algorithm, we identified 27 high-confidence protein interaction partners of Nanog in mouse ES cells. These consist of 19 novel partners of Nanog that have not been reported before including the Ten eleven translocation (Tet) family methylcytosine hydroxylase Tet1. We confirmed physical association of Nanog with Tet1, and demonstrated that Tet1, in synergy with Nanog, enhances the efficiency of reprogramming. We also found physical association and reprogramming synergy of Tet2 with Nanog, and demonstrated that knockdown of Tet2 abolishes the reprogramming synergy of Nanog with a catalytically deficient mutant of Tet1 (Tet1Mut). These results indicate that the physical interaction between Nanog and Tet1/2 proteins facilitates reprogramming in a manner that is dependent on Tet1/2's catalytic activity. Tet1 and Nanog co-occupy genomic loci of genes associated with both maintenance of pluripotency and lineage commitment in ES cells, and Tet1 binding is reduced upon Nanog depletion. Co-expression of Nanog and Tet1 results in expression priming of and increased 5hmC levels at top ranked common targets Esrrb and Oct4 before reprogramming to naïve pluripotency. We propose that Tet1 is recruited by Nanog to enhance the expression of a subset of key reprogramming target genes. These results provide an insight into the reprogramming mechanism of Nanog and uncover a novel role for 5mC hydroxylases in the establishment of naïve pluripotency.

Identifying a bona fide population of cardiac stem cells (CSCs) is a critical step for developing cell-based therapies for heart failure patients. Previously, cardiac c-kit+ cells were reported to be CSCs with a potential to become myocardial, endothelial and smooth muscle cells in vitro and after cardiac injury. Here we provide further insights into the nature of cardiac c-kit+ cells. By targeting the c-kit locus with multiple reporter genes in mice, we find that c-kit expression rarely co-localizes with the expression of the cardiac progenitor and myogenic marker Nkx2.5, or that of the myocardial marker, cardiac troponin T (cTnT). Instead, c-kit predominantly labels a cardiac endothelial cell population in developing and adult hearts. After acute cardiac injury, c-kit+ cells retain their endothelial identity and do not become myogenic progenitors or cardiomyocytes. Thus, our work strongly suggests that c-kit+ cells in the murine heart are endothelial cells and not CSCs.

The ten-eleven translocation 1 (TET1) gene is the founding member of the TET family of enzymes (TET1/2/3) that convert 5-methylcytosine to 5-hydroxymethylcytosine. Although TET1 was first identified as a fusion partner of the mixed lineage leukemia (MLL) gene in acute myeloid leukemia carrying t(10,11), its definitive role in leukemia is unclear. In contrast to the frequent downregulation (or loss-of-function mutations) and critical tumor-suppressor roles of the three TET genes observed in various types of cancers, here we show that TET1 is a direct target of MLL-fusion proteins and is significantly up-regulated in MLL-rearranged leukemia, leading to a global increase of 5-hydroxymethylcytosine level. Furthermore, our both in vitro and in vivo functional studies demonstrate that Tet1 plays an indispensable oncogenic role in the development of MLL-rearranged leukemia, through coordination with MLL-fusion proteins in regulating their critical cotargets, including homeobox A9 (Hoxa9)/myeloid ecotropic viral integration 1 (Meis1)/pre-B-cell leukemia homeobox 3 (Pbx3) genes. Collectively, our data delineate an MLL-fusion/Tet1/Hoxa9/Meis1/ Pbx3 signaling axis in MLL-rearranged leukemia and highlight TET1 as a potential therapeutic target in treating this presently therapy-resistant disease.

Key Points Deletion/haploinsufficiency of Asxl1 causes MDS-like disease in mice. Asxl1 loss reduces the HSC pool and decreases HSC hematopoietic repopulating capacity in vivo.

In order to investigate the biologic processes underlying and resulting from the megakaryocytic hyperplasia that characterizes idiopathic myelofibrosis (IMF), peripheral blood CD34 ؉ cells isolated from patients with IMF, polycythemia vera (PV), and G-CSF-mobilized healthy volunteers were cultured in the presence of stem cell factor and thrombopoietin. IMF CD34 ؉ cells generated 24-fold greater numbers of megakaryocytes (MKs)