Activating FLT3 mutations are rare in children with juvenile myelomonocytic leukemia

Activating mutations of FLT3 have been identified in multiple myeloid malignancies. Two types of activating mutations have been described: (1) the internal tandem duplication (FLT3-ITD) and (2) point mutations within the activating loop (FLT3-ALM). Juvenile myelomonocytic leukemia (JMML) is a rare myelodysplastic/myeloproliferative disorder of early childhood. Mutations and other genetic abnormalities of RAS, NF1, and PTPN11 have been implicated as causative events in JMML, but approximately 25% of JMML patients harbor none of these abnormalities. We investigated whether FLT3 mutations might also contribute to JMML pathogenesis, and if present, whether FLT3 status would correlate with disease natural history and prognosis.

Genomic DNA was isolated from peripheral blood and bone marrow samples of 60 patients meeting international JMML diagnostic criteria. Samples were analyzed for FLT3-ITD and FLT3-ALM using polymerase chain reaction and restriction endonuclease digestion.

FLT3-ALM was found in 1/60 (1.7%) patients analyzed. Direct sequencing confirmed a C836G mutation. Clinical and laboratory characteristics of the JMML patient with the FLT3-ALM did not differ from the remainder of the cohort. No FLT3-ITD mutations were detected.

This first reported mutational analysis for both FLT3-ITD and FLT3-ALM performed in JMML documents the presence of FLT3 mutations within JMML, but at a sufficiently low prevalence as to be clinically insignificant for most patients. Despite the poor prognosis and limited therapeutic options for JMML patients with refractory disease, compassionate therapy with targeted FLT3 inhibitors should not be considered in this patient population until adequate safety and efficacy data become available.