Acute Myelomonocytic Leukemia With Abnormal Bone Marrow Eosinophils and inv(16)(p13;q22)

Abstract

Acute myeloid leukemia (AML) with abnormal bone marrow eosinophils and inv(16)(p13;q22) or t(16;16)(p13; q22) is a distinct type of AML designated as M4Eo by the French-American-British (FAB) group. AML-M4Eo is characterized by myelomonocytic differentiation accompanied by pathologic eosinophils containing abnormally large basophilic granules that are positive for myeloperoxidase, chloroacetate esterase, and periodic acid-Schiff (PAS). Immunophenotypic studies have shown that AML-M4Eo expresses myeloid markers and that some cases coexpress the T-cell-associated antigen CD2. Cytogenetic analysis of AML-M4Eo reveals a pericentric inversion, inv(16)(p13;q22); or less commonly, the translocation t(16;16)(p13;q22). These molecular abnormalities result in the juxtaposition of the CBFβ gene at 16q22 with the MYH11 gene at 16p13, creating a novel CBFβ/MYH11 fusion gene that causes leukemogenesis and a loss of function of the core binding factor protein complex. The overall prognosis for patients with AML-M4Eo is favorable. The recently proposed World Health Organization (WHO) classification of myeloid neoplasms recognizes AMLs with recurrent cytogenetic translocations as distinct clinicopathologic entities, one of which is AML-M4Eo. The differential diagnosis of AML-M4Eo is discussed.