Juvenile myelomonocytic leukemia (JMML) is a rare clonal myeloproliferative disorder that affects young children. It is characterized by specific hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF) in vitro. Mutations in RAS, NF1, or PTPN11 positioned in the GM-CSF signal pathway, are thought to be involved in the pathogenesis of JMML. However, no information is available on the relationship between these mutations and clinical features of JMML. The impacts of these mutations on clinical outcome also remain unclear. We tested 49 Japanese children with JMML for N-RAS, K-RAS, and PTPN11 mutations and evaluated their clinical significance. We also assessed correlations between mutational status and clinical and laboratory findings, including age at diagnosis, fetal hemoglobin (HbF), platelet count, and cytogenetic abnormality, all which have been proposed as prognostic factors for JMML. Of the 49 JMML patients, cytogenetic abnormalities were detected in 13, including 8 with monosomy 7. For 2 patients, a clinical diagnosis of neurofibromatosis type 1 (NF1) was confirmed. PTPN11 and N-/K-RAS mutations were found in 22 (45%) and 8 (16%) patients, respectively. Neither PTPN11 nor RAS mutations nor NF1 were present in 17 (35%) patients, and no simultaneous aberrations in these genes were found. In patients with the PTPN11 mutation, age at diagnosis was older (35 vs 11 months; P=0.001, or 12 months; P<0.01) and HbF level was higher (31 vs 10%; P=0.03, or 16%; P<0.01) than for patients with the RAS mutation or without any aberration, suggesting that the clinical outcome for patients with the PTPN11 mutation might be poorer, because a higher HbF level and older age have been reported to be poor prognostic factors. In fact, overall survival (OS) at 5 years was lower for patients with the PTPN11 mutation than for those without (20±9% vs 58±9%; P=0.02). In addition to PTPN11 mutation, age older than 24 months (P<0.01) and abnormal karyotype (P=0.02) were also associated with poor prognosis for OS. Of the 49 patients, 33 received stem cell transplantation (SCT). OS probabilities for patients with and without a mutation in PTPN11 at 5 years after SCT were 25±10% and 64±12%, respectively (P=0.04). More importantly, mutation in PTPN11 was the only unfavorable factor for relapse after SCT (P<0.01). Seven patients died owing to relapse and 12 from complications. All patients who died after relapse had a PTPN11 mutation. In summary, our results suggest that PTPN11-mutated JMML might be a distinct subgroup with specific clinical characteristics and a poor outcome.

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