TET2 mutations are associated with specific 5-methylcytosine and 5-hydroxymethylcytosine profiles in patients with chronic myelomonocytic leukemia

Aberrant DNA methylation profile of chronic and transformed classic Philadelphia-negative myeloproliferative neoplasms

Most DNA methylation studies in classic Philadelphia-negative myeloproliferative neoplasms have been performed on a gene-by-gene basis. Therefore, a more comprehensive methylation profiling is needed to study the implications of this epigenetic marker in myeloproliferative neoplasms. Here, we have analyzed 71 chronic (24 polycythemia vera, 23 essential thrombocythemia and 24 primary myelofibrosis) and 13 transformed myeloproliferative neoplasms using genome-wide DNA methylation arrays. The three types of chronic Philadelphia-negative myeloproliferative neoplasms showed a similar aberrant DNA methylation pattern when compared to control samples. Differentially methylated regions were enriched in a gene network centered on the NF-κB pathway, indicating that they may be involved in the pathogenesis of these diseases. In the case of transformed myeloproliferative neoplasms, we detected an increased number of differentially methylated regions with respect to chronic myeloproliferative neoplasms. Interestingly, these genes were enriched in a list of differentially methylated regions in primary acute myeloid leukemia and in a gene network centered around the IFN pathway. Our results suggest that alterations in the DNA methylation landscape play an important role in the pathogenesis and leukemic transformation of myeloproliferative neoplasms. The therapeutic modulation of epigenetically-deregulated pathways may allow us to design targeted therapies for these patients.

5-hydroxymethylcytosine and its potential roles in development and cancer

Only a few years ago it was demonstrated that mammalian DNA contains oxidized forms of 5-methylcytosine (5mC). The base 5-hydroxymethylcytosine (5hmC) is the most abundant of these oxidation products and is referred to as the sixth DNA base. 5hmC is produced from 5mC in an enzymatic pathway involving three 5mC oxidases, Ten-eleven translocation (TET)1, TET2, and TET3. The biological role of 5hmC is still unclear. Current models propose that 5hmC is an intermediate base in an active or passive DNA demethylation process that operates during important reprogramming phases of mammalian development. Tumors originating in various human tissues have strongly depleted levels of 5hmC. Apparently, 5hmC cannot be maintained in proliferating cells. Furthermore, mutations in the TET2 gene are commonly observed in human myeloid malignancies. Since TET proteins and many lysine demethylases require 2-oxoglutarate as a cofactor, aberrations in cofactor biochemical pathways, including mutations in isocitrate dehydrogenase (IDH), may affect levels of 5hmC and 5mC in certain types of tumors, either directly or indirectly. We discuss current data and models of the function of 5hmC in general, with special emphasis on its role in mechanisms of development and cancer.

What do we know about IDH1/2 mutations so far, and how do we use it?

Whole genome analyses have facilitated the discovery of clinically relevant genetic alterations in a variety of diseases, most notably cancer. A prominent example of this was the discovery of mutations in isocitrate dehydrogenases 1 and 2 (IDH1/2) in a sizeable proportion of gliomas and some other neoplasms. Herein the normal functions of these enzymes, how the mutations alter their catalytic properties, the effects of their D-2-hydroxyglutarate metabolite, technical considerations in diagnostic neuropathology, implications about prognosis and therapeutic considerations, and practical applications and controversies regarding IDH1/2 mutation testing are discussed.

Chronic myelomonocytic leukemia: a review of the molecular biology, prognostic models and treatment

SUMMARY Chronic myelomonocytic leukemia (CMML) is a genetically heterogeneous hematologic neoplasm that manifests with features of both a myelodysplastic syndrome and a myeloproliferative neoplasm. Recent advances in the characterization of recurrent genetic markers have resulted in a better understanding of the leukemia-initiating events and have distinguished CMML from other clonal hematopoietic malignancies. Although these mutations may lead to CMML-specific therapies in the relatively near future, the current state of therapy for CMML is based on treatments designed for the myelodysplastic syndromes. Here we review the recurrent genetic mutations and, if known, their clinical significance. We also review the treatment and available CMML-specific prognostic models and novel therapies moving forward.

Tet1 is required for Rb phosphorylation during G1/S phase transition

DNA methylation plays an important role in many biological processes, including regulation of gene expression, maintenance of chromatin conformation and genomic stability. TET-family proteins convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), which indicates that these enzymes may participate in DNA demethylation. The function of TET1 has not yet been well characterized in somatic cells. Here, we show that depletion of Tet1 in NIH3T3 cells inhibits cell growth. Furthermore, Tet1 knockdown blocks cyclin D1 accumulation in G1 phase, inhibits Rb phosphorylation and consequently delays entrance to G1/S phase. Taken together, this study demonstrates that Tet1 is required for cell proliferation and that this process is mediated through the Rb pathway.

Oct4 and the small molecule inhibitor, SC1, regulates Tet2 expression in mouse embryonic stem cells

The ten eleven translocation (Tet) family of proteins includes three members (Tet1-3), all of which have the capacity to convert 5-methylcytosine to 5-hydroxymethylcytosine in a 2-oxoglutarate- and Fe(II)-dependent manner. Tet1 and Tet2 are highly expressed in undifferentiated embryonic stem cells (ESCs), and this expression decreases upon differentiation. Notably, the expression patterns of Tet1 and Tet2 in ESCs parallels that of pluripotency genes. To date, however, the mechanisms underlying the regulation of Tet gene expression in ESCs remain largely unexplored. Here we report that the pluripotency transcription factor, Oct4, directly regulates the expression of Tet2. Using RNAi, real time quantitative PCR, dual-luciferase reporter assays and electrophoretic mobility shift assays, we show that Oct4 promotes Tet2 transcription by binding to consensus sites in the proximal promoter region. Furthermore, we explored the role of the small molecule inhibitor, SC1 (pluripotin) on Tet gene expression. We show that SC1 promotes Tet3 expression, but represses Tet1 and Tet2 expression. Our findings indicate that Tet2 are crucial downstream targets of the pluripotency factor Oct4, and highlight a role for Oct4 in the regulation of DNA methylation in ESCs. In addition, these findings also provide a new insight into drug-mediated gene regulation.

Current insights in the cellular and molecular biology of chronic myelomonocytic leukemia

SUMMARY Chronic myelomonocytic leukemia is a rare clonal myeloid disorder most often seen in the elderly that remains a virtually incurable disease. Chronic myelomonocytic leukemia has long been considered as a myelodysplastic syndrome by diagnostic classifications, but recent insights in the cellular and molecular biology of the disease has refined its identity. The malignant clone was shown to generate myeloid-derived suppressive cells that may contribute to disease expansion, whereas the role of progenitor hypersensitivity to granulomonocyte colony-stimulating factor probably defines two distinct subgroups. At least one gene mutation can now be identified in almost all the patients. The most frequently mutated genes are TET2, SRSF2 and ASXL1, with a frequent combination of mutations in the first two genes, whereas ASXL1 mutations define a poor prognostic subgroup of patients. A number of additional mutations have been identified that confer to the disease its phenotype specificity; for example, mutations in RUNX1 induce thrombocytopenia, those in SF3B1 can be associated with anemia, and those in signaling molecules including NRAS, KRAS, CBL, JAK2 and FLT3, characterize the proliferative forms of the disease. Based on these recent observations, new working models on disease pathogenesis are proposed and may serve as a basis for the search for alternative and more efficient therapeutic approaches.

Treatment of chronic myelomonocytic leukemia with 5-Azacitidine: a case series and literature review

Chronic myelomonocytic leukemia (CMML) is a clinically heterogeneous disease, with no standard treatment. We present the outcome of ten patients diagnosed with CMML and treated with AZA in our institutions between 2005 and 2010. All patients were transfusion dependent at the time of initiation of therapy. The overall response rate was 60%. Responses were obtained in 2/3 of the patients with proliferative CMML. The median survival from start of therapy was 20 months. AZA treatment was well-tolerated and associated with a significant response rate in all forms of the disease.