Increased methylation of CpG islands and silencing of affected target genes is frequently found in human cancer; however, in vivo the question of causality has only been addressed by loss-of-function studies. To directly evaluate the role and mechanism of de novo methylation in tumor development, we overexpressed the de novo DNA methyltransferases Dnmt3a1 and Dnmt3b1 in Apc Min/+ mice. We found that Dnmt3b1 enhanced the number of colon tumors in Apc Min/+ mice approximately twofold and increased the average size of colonic microadenomas, whereas Dnmt3a1 had no effect. The overexpression of Dnmt3b1 caused loss of imprinting and increased expression of Igf2 as well as methylation and transcriptional silencing of the tumor suppressor genes Sfrp2, Sfrp4, and Sfrp5. Importantly, we found that Dnmt3b1 but not Dnmt3a1 efficiently methylates the same set of genes in tumors and in nontumor tissues, demonstrating that de novo methyltransferases can initiate methylation and silencing of specific genes in phenotypically normal cells. This suggests that DNA methylation patterns in cancer are the result of specific targeting of at least some tumor suppressor genes rather than of random, stochastic methylation followed by clonal selection due to a proliferative advantage caused by tumor suppressor gene silencing.[Keywords: DNA methylation; epigenetics; cancer; Dnmt3b; APC] Supplemental material is available at http://www.genesdev.org. Received July 17, 2007; revised version accepted October 11, 2007. Cancer cells show widespread epigenetic changes when compared with their normal parental tissue, including changes in DNA methylation and chromatin modification (Jones and Baylin 2007). The first epigenetic abnormality reported for human cancer was a global decrease in genomic cytosine methylation (Feinberg and Vogelstein 1983), most often seen in repetitive sequences and intergenic regions. It promotes genetic instability, increases the mobility of transposable elements (Walsh et al. 1998), and induces tumorigenesis in several different mouse models Gaudet et al. 2003;Yamada et al. 2005;Jones and Baylin 2007). Thus, hypomethylation predisposes to genetic damage and increases the risk of tumor development. Conversely, in some tissues global hypomethylation can also inhibit tumor outgrowth (Laird et al. 1995).In addition to global hypomethylation, it was also found that the cancer cell genome frequently contains regions with increased cytosine methylation (Baylin et al. 1986). This regional hypermethylation often affects CpG islands that are associated with promoter regions (Herman and Baylin 2003;Feinberg and Tycko 2004;Jones and Baylin 2007). Regional hypermethylation attracted attention when it was linked to transcriptional silencing of the RB tumor suppressor gene in patients with retinoblastoma tumors (Greger et al. 1989(Greger et al. , 1994. Multiple follow-up studies revealed that in cancer many tumor-relevant genes, in particular tumor suppressor genes, are transcriptionally silenced by hypermethylation. Aberrant DNA met...

Nicotinamide phosphoribosyltransferase (Nampt) inhibitors such as FK866 are potent inhibitors of NAD+ synthesis that show promise for the treatment of different forms of cancer. Based on Nampt upregulation in activated T lymphocytes and on preliminary reports of lymphopenia in FK866 treated patients, we have investigated FK866 for its capacity to interfere with T lymphocyte function and survival. Intracellular pyridine nucleotides, ATP, mitochondrial function, viability, proliferation, activation markers and cytokine secretion were assessed in resting and in activated human T lymphocytes. In addition, we used experimental autoimmune encephalomyelitis (EAE) as a model of T-cell mediated autoimmune disease to assess FK866 efficacy in vivo. We show that activated, but not resting, T lymphocytes undergo massive NAD+ depletion upon FK866-mediated Nampt inhibition. As a consequence, impaired proliferation, reduced IFN-γ and TNF-α production, and finally autophagic cell demise result. We demonstrate that upregulation of the NAD+-degrading enzyme poly-(ADP-ribose)-polymerase (PARP) by activated T cells enhances their susceptibility to NAD+ depletion. In addition, we relate defective IFN-γ and TNF-α production in response to FK866 to impaired Sirt6 activity. Finally, we show that FK866 strikingly reduces the neurological damage and the clinical manifestations of EAE. In conclusion, Nampt inhibitors (and possibly Sirt6 inhibitors) could be used to modulate T cell-mediated immune responses and thereby be beneficial in immune-mediated disorders.

Background and Aims-Epidemiologic studies have linked nutritional folate deficiency to an increased risk of cancer, but recent trials suggest that folate supplementation does not protect against tumor formation. Our aim was to analyze the genetic and epigenetic consequences of folate deficiency and to investigate whether impairment of the uracil base excision repair pathway can enhance its effects.

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Aberrant histone deacetylase (HDAC) activity is frequent in human leukemias. However, while classical, NAD+-independent HDACs are an established therapeutic target, the relevance of NAD+-dependent HDACs (sirtuins) in leukemia treatment remains unclear. Here, we assessed the antileukemic activity of sirtuin inhibitors and of the NAD+-lowering drug FK866, alone and in combination with traditional HDAC inhibitors. Primary leukemia cells, leukemia cell lines, healthy leukocytes and hematopoietic progenitors were treated with sirtuin inhibitors (sirtinol, cambinol, EX527) and with FK866, with or without addition of the HDAC inhibitors valproic acid, sodium butyrate, and vorinostat. Cell death was quantified by propidium iodide cell staining and subsequent flow-cytometry. Apoptosis induction was monitored by cell staining with FITC-Annexin-V/propidium iodide or with TMRE followed by flow-cytometric analysis, and by measuring caspase3/7 activity. Intracellular Bax was detected by flow-cytometry and western blotting. Cellular NAD+ levels were measured by enzymatic cycling assays. Bax was overexpressed by retroviral transduction. Bax and SIRT1 were silenced by RNA-interference. Sirtuin inhibitors and FK866 synergistically enhanced HDAC inhibitor activity in leukemia cells, but not in healthy leukocytes and hematopoietic progenitors. In leukemia cells, HDAC inhibitors were found to induce upregulation of Bax, a pro-apoptotic Bcl2 family-member whose translocation to mitochondria is normally prevented by SIRT1. As a result, leukemia cells become sensitized to sirtuin inhibitor-induced apoptosis. In conclusion, NAD+-independent HDACs and sirtuins cooperate in leukemia cells to avoid apoptosis. Combining sirtuin with HDAC inhibitors results in synergistic antileukemic activity that could be therapeutically exploited.

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4732 Nicotinamide phosphoribosyltransferase (Nampt) inhibitors such as FK866 are potent inhibitors of NAD+ synthesis that show promise for the treatment of different forms of cancer. Based on Nampt upregulation in activated T lymphocytes and on preliminary reports of lymphopenia in FK866 treated patients, we have investigated FK866 for its capacity to interfere with T lymphocyte function and survival. Intracellular pyridine nucleotides, ATP, mitochondrial function, viability, proliferation, activation markers and cytokine secretion were assessed in resting and in activated human T lymphocytes. In addition, we used experimental autoimmune encephalomyelitis (EAE) as a model of T-cell mediated autoimmune disease to assess FK866 efficacy in vivo. We show that activated, but not resting, T lymphocytes undergo massive NAD+ depletion upon FK866 mediated Nampt inhibition. As a consequence, impaired proliferation, reduced TNF-a production, ATP shortage, and, finally, autophagic cell demise result. We demonstrate that upregulation of the of the NAD+-degrading enzyme poly-(ADP-ribose)-polymerase (PARP) by activated T cells enhances their susceptibility to NAD+ depletion. We relate defective TNF-a production in response to FK866 to impaired SIRT6 activity. Finally, in line with its capacity to interfere with T lymphocyte function and survival, we show that FK866 strikingly reduces the neurological damage and the clinical manifestations of EAE. In conclusion, Nampt inhibitors (and possibly sirtuin inhibitors) could be used to modulate T cell-mediated immune responses and thereby be beneficial in immune-mediated disorders. Disclosures: No relevant conflicts of interest to declare.

No abstract