TET2 converts 5-methylcytosine to 5-hydroxymethylcytosine (5-hmC) in DNA and is frequently mutated in myeloid ma-lignancies, including myeloproliferative neoplasms. Here we show that the level of 5-hmC is decreased in granulocyte DNA from myeloproliferative neoplasm patients with TET2 mutations compared with granulocyte DNA from healthy patients. Inhibition of TET2 by RNA interference decreases 5-hmC levels in both human leukemia cell lines and cord blood CD34 cells. These results confirm the enzymatic function of TET2 in human hematopoietic cells. Knockdown of TET2 in cord blood CD34 cells skews progenitor differentiation toward the granulo-monocytic lineage at the expense of lym-phoid and erythroid lineages. In addition, by monitoring in vitro granulomonocytic development we found a decreased granulocytic differentiation and an increase in monocytic cells. Our results indicate that TET2 disruption affects 5-hmC levels in human myeloid cells and participates in the pathogenesis of my-eloid malignancies through the disturbance of myeloid differentiation. (Blood. 2011;118(9):2551-2555)

Human telomerase, a cellular reverse transcriptase (hTERT), is a nuclear ribonucleoprotein enzyme complex that catalyzes the synthesis and extension of telomeric DNA. This enzyme is specifically activated in most malignant tumors but is usually inactive in normal somatic cells, suggesting that telomerase plays an important role in cellular immortalization and tumorigenesis. Terminal maturation of tumor cells has been associated with the repression of telomerase activity. Using maturation-sensitive and -resistant NB4 cell lines, we analyzed the pattern of telomerase expression during the therapeutic treatment of acute promyelocytic leukemia (APL) by retinoids. Two pathways leading to the down-regulation of hTERT and telomerase activity were identified. The first pathway results in a rapid down-regulation of telomerase that is associated with retinoic acid receptor (RAR)-dependent maturation of NB4 cells. Furthermore, during NB4 cell maturation, obtained independently of RAR by retinoic X receptor (RXR)-specific agonists (rexinoids), no change in telomerase activity was observed, suggesting that hTERT regulation requires a specific signaling and occurs autonomously. A second pathway of hTERT regulation, identified in the RAR-responsive, maturation-resistant NB4-R1 cell line, results in a down-regulation of telomerase that develops slowly during two weeks of all-trans retinoic acid (ATRA) treatment. This pathway leads to telomere shortening, growth arrest, and cell death, all events that are overcome by ectopic expression of hTERT. These findings demonstrate a clear and full dissociation between the process of tumor cell maturation and the regulation of hTERT mRNA expression and telomerase activity by retinoids. We propose telomerase expression as an efficient and selective target of retinoids in the therapy of tumors.H uman telomerase, a ribonucleoprotein enzyme, extends chromosome ends with (TTAGGG)n telomeric sequences, and thus plays a key role in maintaining telomere length and in cellular replicative lifespan (1-3). Several observations indicate that telomeres, DNA-protein structures located at the ends of eukaryotic chromosomes, are important in the immortalization process (4). In most human normal cells, telomeric DNA is progressively lost with each round of cell division (5). Thus, telomeres shorten to a critical length and signal the onset of senescence (6). In contrast, telomere length is stable in immortalized cells, including tumor cells. This stabilization of telomere length seems to be achieved through the induction of a telomerase activity (7). The human telomerase is composed of template RNA components (hTR; ref. 8) and two proteins, telomerase-associated protein-1 (TP-1; ref. 9) and telomerase reverse transcriptase (hTERT), which is thought to be the enzyme's catalytic subunit (10)(11)(12)(13)(14). The level of expression of hTERT is the rate-limiting component of this complex: most normal human somatic cells do not have detectable telomerase activity and lack expression of hTERT, whereas most imm...

Protein N-e-acetylation is recognized as an important modification influencing many biological processes, and protein deacetylase inhibitors leading to N-e-hyperacetylation of histones are being clinically tested for their potential as anticancer drugs. In contrast to N-eacetyltransferases, the N-a-acetyltransferases transferring acetyl groups to the a-amino groups of protein N-termini have only been briefly described in mammalians. Human arrest defective 1 (hARD1), the only described human enzyme in this class, complexes with N-acetyltransferase human (NATH) and cotranslationally transfers acetyl groups to the N-termini of nascent polypeptides. Here, we demonstrate that knockdown of NATH and/or hARD1 triggers apoptosis in human cell lines. Knockdown of hARD1 also sensitized cells to daunorubicin-induced apoptosis, potentially pointing at the NATH-hARD1 acetyltransferase complex as a novel target for chemotherapy. Our results argue for an essential role of the NATH-hARD1 complex in cell survival and underscore the importance of protein N-aacetylation in mammalian cells.