A case of true acute eosinophilic leukemia arising from myelodysplastic syndrome.

Acute leukemias of the myeloid lineage most commonly show myeloid and/or monocytic differentiation. Leukemias with erythroid, megakaryoblastic, and basophilic differentiation are more rare but are well documented. In contrast, acute leukemia showing marked morphologic and immunophenotypic features of eosinophilic differentiation is not a defined entity. When acute leukemias do show a prominent population of cells with eosinophilic differentiation, the eosinophil populations typically show morphologic features of mature eosinophils and the eosinophilic population may be clonally related to the neoplastic cells or may be reactive. In this case report, we describe an unusual case of acute myeloid leukemia evolving from a myelodysplastic neoplasm with low blasts in which the predominant cell population in the peripheral blood and bone marrow were cells with round nuclei and immature chromatin with prominent eosinophilic granules and immunophenotypic features suggestive of maturation arrest at an eosinophilic promyelocyte-like stage. At the time of leukemic transformation, molecular studies showed acquisition of NRAS G13R, NRAS Q61K, and KRAS G12R mutations in addition to the TET2 variants (N582fs and E1357X), del(20q), and 7q- abnormalities detected in the antecedent myelodysplastic neoplasm. No PDGFRA, PDGFRB, FGFR1, JAK2, or FLT3 rearrangements were detected by FISH or in sequencing assays. BCR::ABL1 testing was also negative. Whole-genome sequencing did not detect additional abnormalities to explain the differentiation block. In summary, this is, to our knowledge, the only reported case of an acute leukemia in which the immature cells themselves demonstrate morphologic and immunophenotypic features of eosinophilic promyelocytes.