Effective Azacytidine Monotherapy in an Infant with Mosaic NRAS-Driven JMML
Authors: Grenier Jeremy , Tasian Sarah K. , Stieglitz Elliot , Abdullaev Zied , Wertheim Gerald , Li Marilyn M. , Phillips Charles A 
Published: December 2024

Juvenile myelomonocytic leukemia (JMML) is a rare RAS pathway-driven myelodysplastic neoplasm of infancy and early childhood with high rates of chemotherapy resistance and indication for allogeneic hematopoietic stem cell transplantation (HSCT) in most patients. However, a select subset of children have demonstrated excellent outcomes with minimal or no therapy. Herein, we report the case of a female infant diagnosed with somatic mosaic NRAS-driven JMML at 1 month of age after clinical presentation with a white blood cell (WBC) count of 73,000/µL with 38.6% peripheral blasts, platelets 45,000/µL, absolute monocyte count (AMC) 11,520/µL, and splenomegaly palpable 3 cm below the costal margin. Her hemoglobin F was not elevated for age and her bone marrow at diagnosis was hypercellular with trilineage hematopoiesis and 3-5% blasts. Cytogenetics were 46,XX with no monosomy 7 or BCR::ABL1 rearrangement detected. Clinical next-generation sequencing (NGS) of bone marrow cells identified an NRAS c.35G&gt;A (G12D) mutation with variant allele frequency (VAF) of 46% which was not detected in cultured skin fibroblasts. No secondary co-mutations were detected on our institutional comprehensive pediatric leukemia-focused NGS analyses (Chang J Mol Diagn Surrey Genome Med 2019). Research-level genome-wide methylation analysis revealed a low methylation status which has been associated with improved outcomes (Stieglitz Nat Commun 2017). These data stratified her as “low risk” in a new JMML risk stratification algorithm (Wintering Blood Adv 2021), suggesting that she might not require empiric hematopoietic stem cell transplant. She was treated with single-agent azacytidine (100 mg/m2 intravenously daily x 5 days/28 day cycle; dosing from Niswander Haematologica 2023) with resolution of splenomegaly, leukocytosis, thrombocytopenia, and monocytosis after two cycles. Recent evidence (Cseh Blood 2015) suggested additional cycles could eliminate the driver mutation. The patient thus continued azacytidine monotherapy for 12 cycles with serial monitoring of her bone marrow by flow cytometric measurable residual disease (MRD) analysis and quantitative NRAS mutation tracking. Despite resolution of splenomegaly and normalization of complete blood count parameters (WBC 12,400µL with AMC 1220/µL, hemoglobin 10.6 g/dL, platelets 183,000/µL), the NRAS c.35G&gt;A VAF remained near 50% after 12 cycles. Sanger sequencing of sorted bone marrow T cells, B cells, and myeloid cells at end of therapy confirmed the presence of the NRAS variant in all sorted populations despite undetectable immunophenotypic MRD. Importantly, the NRAS variant was not detected in her skin fibroblasts, and she had no clinical stigmata of a germline RASopathy, confirming a rare case of JMML driven by a mosaic NRAS variant which likely predisposed her to develop JMML. She continues to do well one year off therapy with no evidence of recurrent JMML. This case illustrates the importance of evaluating for RAS-pathway variant mosaicism in children with this genetically heterogeneous disease and offers additional evidence that a subset of patients with low-risk JMML may not require HSCT.