Decitabine, a Potential Targeted Therapeutic for Juvenile Myelomonocytic Leukemia.

Juvenile myelomonocytic leukemia (JMML) is a mixed myelodysplastic /myeloproliferative disorder (MDS/MPD). It occurs in infancy and young children with a progressive course leading to death within one year after diagnosis. It is characterized by monocytosis, leukocytosis, elevated fetal hemoglobin, hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF), a low percentage of myeloblasts in the bone marrow, and absence of the Philadelphia chromosome or the BCR/ABL fusion gene. Mutations or other abnormalities in RAS, NF1, and PTPN11, have been linked to be responsible for the pathogenesis of JMML in up to 75% of cases. Treatment has been very difficult for JMML. Only allogeneic stem cell transplantation (SCT) can extend survival. However, the relapse rate from allogeneic SCT is inordinately high in JMML (28–55%), with 5-year disease-free survival rates of 25–40%. Decitabine, as one of the second generation of hypomethylating agents, has been demonstrated to produce encouraging responses in adult patients with chronic myelogenous leukemia or other hematopoietic disorders. Our recent studies have demonstrated that PTEN deficiency is detected in 67% of JMML patients. We hypothesize that hypermethylation of the PTEN promoter is one of the causes that lead to PTEN deficiency in JMML, and that a hypomethylating agent may improve PTEN expression in JMML cells, and thus inhibit hypersensitivity to GM-CSF. In order to test our hypothesis, we conducted an in vitro pilot study with JMML cells. Hypermethylation of the PTEN promoter was detected in 23/30 (77%) of JMML patients using Methylation-specific PCR. Sequencing confirmed that the CpG islands of the PTEN promoter were hypermethylated. A CFU-GM formation assay was used to evaluate the therapeutic sensitivity of Decitabine to JMML cells. Frozen mononuclear cells from peripheral blood samples of 5 JMML patients were plated in 0.3% agar medium with Decitabine ranging in concentration from 1nM to 1000nM. Significant inhibition of spontaneous CFU-GM growth was observed in all cultures in a dose-dependent fashion. The effective Decitabine concentrations in the cultures were lower or equivalent to the safe and tolerable plasma concentrations achievable in adult patients in clinical settings (30 mg/m²/day). Our data suggests that hypermethylation of the PTEN promoter is a common event in JMML, and Decitabine may be a potentially safe and effective reagent to treat JMML. Further pharmacokinetic studies should be conducted in the clinic to clarify the plasma concentration in pediatric patients, and the mechanism of Decitabine in inhibiting hypersensitivity of JMML cells to GM-CSF should be further explored since multiple genes are hypermethylated in cancers.