The Clinical Landscape of NRAS- mutated Juvenile Myelomonocytic Leukemia-like Myeloproliferation Includes Children With Costello Syndrome.

Juvenile myelomonocytic leukemia (JMML) is a rare, aggressive pediatric disorder characterized by pathologic myeloproliferation because of alterations in RAS pathway genes. NRAS -mutated JMML encompasses a broad range of clinical severity. Herein we describe 4 unique cases of NRAS -mutated JMML and JMML-like myeloproliferation, 2 with somatic mutations and 2 with germline mutations. These cases illustrate the diverse clinical and hematologic presentation of this subtype of JMML, including a very unusual example presenting with Auer rods. Lastly, this is the first report of patients with phenotypic Costello syndrome presenting with JMML-like myeloproliferation, highlighting an important clinical phenomenon that has not been previously described.