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Abstract 4943 Myelodysplastic syndromes(MDS) are a heterogeneous group of myeloid neoplasms characterized by cytopenia, dysplasia in one or more cell lines, ineffective haematopoiesis, and increased risk of development of acute myeloid leukemias(AML). The precise mechanism of onset and evolution of MDS has not been clarified. Recently, the ten-eleven translocation 2 (TET2) gene has been found mutated in 15–26% of MDS and AML. The TET2 paralog TET1 catalyzes the conversion of 5-methylcytosine to 5-hydroxymethylcytosine; TET2 shares a homologous domain thought to catalyze this conversion, and is hypothesized to act as a tumor suppressor gene by regulating DNA methylation and epigenetic control of gene expression at critical loci important for myelopoiesis and leukemogenesis. Moreover, the hypomethylating agents 5-azacytidine and decitabine, have demonstrated good perspective in the treatment of high-risk MDS, but their effects might be limited to some patients. Given the effect of TET2 in epigenetic regulation, it is supposed that the status of TET2 gene mutation affect the therapeutic effect of hypomethylating agents related above. Hence, the aim of this study was to determine the clinical characteristics of Chinese patients with TET2 mutations and the vitro effect of 5-azacytidine and decitabine on CD34+Lin−bone marrow cells from patients with or without TET2 gene mutation in MDS and AML with previous history of MDS(AML-MDS). In our study, the entire coding sequence of the TET2 gene (exons 3 to 11) were sequenced. DNA for detection of mutations were from PB or BM samples of 17 MDS(RA 1, RARS 1, RCMD 4, RAEB-1 7 and RAEB-2 4) and 8 AML-MDS patients in our hospital. Among the patients, there were 19 males and 6 females. The median age was 59(46∼83) year old. Cytogenetic analysis was proceeded in 16 patients and abnormal karyotypes were detected in 7 patients. According to IPSS, cytogenetics was favorable in 9, intermediate in 3, unfavorable in 4. All patients were found with one or more TET2 mutation, including 20 frameshift and 57 point mutations. Except 2 mutations in exon 9 and 1 in exon7, others were all in exon 3 or 11. Only 9 mutations were homozygous and others were heterozygous. Both homozygous and heterozygous TET2 mutations were found, suggesting that the presence of wild type allele or residual activity is not protective. A large proportion of frameshift mutations caused stop codons resulting in loss of function while missense mutations may lead to decreased function. In addition, CD34+Lin-bone marrow cells from 4 AML-MDS patients were sorted with immune magnetic beads from MACS and cultured with StemSpan SFEM supplemented of StemSpan CC100. Decitabine and 5-azacytidine were respectively employed with concentration of 0. 1, 0. 25, 0. 50μM for 12h, 24h, 48h, and 72h. Cell proliferation was inhibited and apoptosis was induced by both drugs. Three AML-MDS patients had previously been treated with decitabine and LD AraC for 1 to 3 cycles. More observation is needed to evaluate if presence of TET2 mutation represent a negative predictor of response to demethylating agents. In conclusion, our preliminary data showed that high proportion of Chinese MDS and AML-MDS patients carried TET2 mutation, of which most showed heterozygous status and involved exon 3 or 11. Decitabine and 5-azacytidine might have effects on CD34+Lin− bone marrow cells from AML-MDS patients in vitro. Disclosures: No relevant conflicts of interest to declare.