The JAK2 V617F Mutation Is Uncommon in Patients with Juvenile Myelomonocytic Leukemia.

Juvenile myelomonocytic leukemia (JMML) is a rare, clonal myeloproliferative disorder afflicting young children and is characterized by specific hypersensitivity of JMML cells to GM-CSF in vitro. The pathogenesis of JMML seems to arise from dysregulation of GM-CSF signal transduction. Approximately, 75% of patients with JMML exhibit dysregulation due to mutations in RAS, NF1 or PTPN11, all of which are positioned in the GM-CSF pathway. Potential causative mutations in remaining patients are anticipated to be in signaling components downstream of the GM-CSF receptor. Recently, a somatic point mutation (JAK2 V617F) in the JAK2 gene, which plays a major role in the development of hematopoietic cells by transducing signals from growth factor receptors including GM-CSF receptor, was found commonly in polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis. All these myeloproliferative disorders are also characterized by hypersensitivity of hematopoietic progenitor cells to several growth factors and cytokines. Because the frequency of JAK2 V617F mutation in JMML remains unknown, we performed mutational analysis of JAK2 in 39 Japanese children (aged 1 to 69 months) with JMML to assess the pathogenetic relevance of this mutation in JMML, in addition to mutational analysis of PTPN11 and N-RAS. Genomic DNA were prepared from bone marrow or peripheral blood samples taken at initial diagnosis. Mutations of JAK2 (exon 12), PTPN11 (exon 3 and 13) and N-RAS (exon 1 and 2) were screened by direct sequencing. PTPN11 or N-RAS mutations were found in 19 (49%) or 3 (8%) patients respectively, and this is compatible with previous reports. In this study, no JAK2 V617F mutations were detected in any of the 39 JMML patients. In conclusion, we confirmed the presence of PTPN11 and N-RAS mutations in JMML. However, the JAK2 V617F mutation appears to be uncommon in patients with JMML, while it is extraordinarily common in classic MPD. Future investigation should focus on other signaling components in the same signaling pathway as JAK2, or on other parallel signal transduction cascades of the GM-CSF receptor.