Smoldering Development of Acute Megakaryoblastic Leukemia with Clonal Evolution in an Infant without Down Syndrome

Introduction

Acute myeloid leukemia (AML) accounts for about 18% of pediatric leukemias. One subgroup is acute megakaryoblastic leukemia (AMKL), which is characterized by abnormal megakaryoblasts expressing platelet-specific surface glycoproteins. While AMKL is rare in adults, it comprises between 4–15% of AML in children (Gruber TA et al., Blood 2015; 126: 943–949).

The median age of children with AMKL is notably younger (about 1.4 years) than for other AML subtypes (about 10 years) (Schweitzer J et al., Ann Hematol 2015; 94: 1327–1336). In children with Down syndrome (DS), AMKL is the most frequent type of AML with excellent prognosis. It also occurs as de novo leukemia without DS and less often as secondary malignancy. Pediatric AMKL in the absence of DS is a heterogeneous group with rather poor prognosis (Gruber TA et al., Blood 2015; 126: 943–949). Accurate diagnosis can be difficult because a subset of patients have a bone marrow (BM) blast percentage of less than 20% due to myelofibrosis (Takeda A et al., Acta Med Okayama 2014; 68: 119–123).

In the WHO classification, AMKL with translocation t(1;22)(p13;q13) is an own group of AML accounting for about 70% of AMKL in infants. The translocation results in the fusion of RNA binding motif protein 15 (RBM15) and megakaryocyte leukemia-1 (MKL1), also known as OTT-MAL fusion transcript. The fusion gene modulates HOX-induced differentiation and extracellular signaling pathways.