Evidence for PTEN Deficiency in Juvenile Myelomonocytic Leukemia.

Juvenile myelomonocytic leukemia (JMML) is a myelodysplastic/myeloproliferative disorder (MDS/MPD) of young children. It is characterized by monocytosis, leukocytosis, elevated fetal hemoglobin, hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF), low percentage of myeloblasts in bone marrow, and absence of the Philadelphia chromosome. The pathogenesis of JMML has been linked to dysregulated signal transduction through the NF1/RAS signaling pathway and PTPN11. This dysregulation results in JMML cells demonstrating selective hypersensitivity to GM-CSF in vitro dose-response assays. PTEN, a major negative regulator of the PI3-kinase pathway by virtue of its PIP3 phosphatase activity, was initially isolated as a tumor suppressor in a variety of malignancies. In order to evaluate the role of PTEN in the pathogenesis of JMML, we examined the status of PTEN in JMML patient samples. Peripheral blood or bone marrow was collected from 40 patients. Mononuclear cells (MNCs) were isolated and lysed in lysis buffer at a concentration of 10⁷/ml. Total RNA was extracted from MNCs of patients and 17 normal individuals. Protein and mRNA levels of PTEN were evaluated by Western-blot and relative-quantitative real-time RT-PCR, respectively. We found that PTEN protein was decreased in 18 of 30 (60%) JMML patients, and the patients had significantly lower RNA expression of PTEN than normal controls (p=0.015). With the available samples we also evaluated AKT activity and MAP kinase (MAPK) levels. We found that MAPK levels were correlated well with the status of the PTEN in 12 of 27(44%), and AKT activity in 13 of 25 patients (52%). Our data indicates that PTEN is significantly deficient in JMML patients, and the low PTEN protein level is related to its low transcription of RNA in JMML patients. The role of PTEN in regulation of MAPK and AKT activities in JMML is under further evaluation by studying the upstream status of the RAS pathway prior to PTEN. This is the first investigation of PTEN deficiency in JMML patients, and additional investigations may help to further understand the pathogenetic mechanisms in JMML, as well as to guide the development of targeted therapeutics for JMML.