Juvenile Myelomonocytic Leukemia (JMML)

Juvenile myelomonocytic leukemia (JMML) is a rare aggressive pediatric clonal hematologic malignancy classified under myelodysplastic-myeloproliferative neoplasms (MDS/MPN) in the WHO 2017/2022 classification, distinct from adult CMML and other pediatric leukemias. JMML accounts for 1-2% of pediatric leukemias with annual incidence of 1.2 per million children, predominantly affecting young children (median age 2 years, 95% under 6 years), with male predominance (2.5:1). Diagnostic criteria require: persistent peripheral monocytosis ≥1,000/uL, BCR-ABL1 negative, less than 20% blasts in blood and marrow, plus one or more of: somatic mutation in RAS pathway (PTPN11, KRAS, NRAS, CBL, NF1), monosomy 7, increased fetal hemoglobin (HbF for age), GM-CSF hypersensitivity in vitro (myeloid colony formation in low GM-CSF concentrations).

Pathogenesis: JMML is essentially a 'RASopathy' — characterized by constitutive activation of the RAS-MAPK signaling pathway leading to hyperactive proliferation of myeloid progenitors. Mutational landscape includes germline syndromes that predispose to JMML (representing 15-20% of cases): Noonan syndrome and related RASopathies with germline PTPN11 mutations (50% of inherited JMML), neurofibromatosis type 1 (NF1) with germline NF1 loss, CBL syndrome with germline CBL mutations (75% have spontaneous remission); somatic mutations as primary driver (75-85% of cases): PTPN11 (35%), NRAS (15%), KRAS (15%), NF1 (5%), CBL (10-15%, often with concurrent germline). Approximately 10-15% have monosomy 7. Recent genomic studies identify cooperating mutations in epigenetic regulators (ASXL1, EZH2, SETBP1, JAK3) that worsen prognosis.

Clinical presentation, diagnosis, and treatment: classical presentation includes hepatosplenomegaly (95% — often massive, dominant feature), lymphadenopathy (75%), pallor (anemia), fever, fatigue, recurrent respiratory infections, skin rash (eczema-like, café-au-lait macules, xanthomas, malar rash, leukemia cutis), bleeding/petechiae, growth failure, lymphadenitis, otitis media. Laboratory features: leukocytosis with monocytosis (median WBC 30-50,000/uL), anemia, thrombocytopenia, elevated fetal hemoglobin (HbF) for age (key feature), peripheral blood smear with myeloid precursors and blasts (<20%), elevated LDH. Bone marrow shows hypercellular marrow with myelomonocytic predominance, less than 20% blasts, dysplastic changes variable, monocytic/monocytoid cells, sometimes mild fibrosis. Differential diagnosis: viral infections (EBV, CMV, HHV-6, parvovirus — pseudo-JMML), other RAS-pathway disorders, atypical CML, leukemoid reactions. Treatment: allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative therapy with overall survival 50-70%, indicated for most patients except CBL syndrome (often spontaneous remission) and some Noonan syndrome cases (some self-limited courses); HLA-matched related or unrelated donor preferred, conditioning with busulfan-fludarabine-melphalan, treosulfan-fludarabine-thiotepa, or other intensive regimens; relapse occurs in 35-50% (highest among pediatric leukemias post-HSCT) often within first year, reduced immunosuppression and donor lymphocyte infusion may achieve remission. Pre-transplant chemotherapy: limited role, low-dose cytarabine, fludarabine, 6-mercaptopurine; azacitidine (hypomethylating agent) showing promise as bridge to transplant with response rates 30-50%, less toxicity than intensive chemotherapy. Targeted therapies (clinical trials): MEK inhibitors (trametinib, selumetinib) for RAS-pathway disease, JAK inhibitors, RAF inhibitors, combination strategies. CBL-associated JMML may have spontaneous resolution and observation in selected cases. Special considerations: family genetic counseling and testing for RAS pathway syndromes, surveillance of NF1 patients for malignancies, vaccination strategies post-HSCT, long-term follow-up for late effects of transplant including secondary malignancies, growth and endocrine consequences, fertility issues. Prognostic factors: age >2 years, platelet count <33,000/uL, HbF >10%, post-HSCT relapse risk, mutational profile (PTPN11 worst, CBL best). Long-term outcomes: 5-year overall survival 50-70% with HSCT; relapse remains main barrier to cure.