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t(8;21) acute myeloid leukemia with trilineage phenotype diagnosed by electron microscopy

AML-M2 is a common subtype of acute myeloid leukemia (AML), usually associated with translocation t(8;21) and often accompanied by eosinophilia [1]. Bone marrow (BM) basophilia is a rarely encountered finding in this type of AML, and a single case of acute basophilic leukemia (ABL) with t(8;21) was recently reported [2]. Increased number of basophils and eosinophils, and the presence of hybrid eosinophil–basophil granulocytes are well known features in chronic myelogenous leukemia (CML) and AML associated with inv16(p13q22) [3]. We describe here a patient with AML and t(8;21), in whom electron microscopic (EM) studies of the leukemic blasts uncover the presence of granules specific for each one of the granulocytic lineages suggesting AML with very unusual hybrid trilineage phenotype. A 33-year-old previously healthy man was admitted because of fever and altered vision in October 2005. Retinal hemorrhage along with a few petechiae on the legs was found. Hemoglobin was 5.6 g/dL, leukocyte count 7.66 10/L and platelet count 816 10/L. A blood smear revealed 30% blasts with occasional Auer rods. BM aspirate showed infiltrate by small and middle size blasts with a large nucleus containing a nucleolus and pale basophilic cytoplasm. In most of the blasts small azurophilic granules were observed, and in a few of them Auer rods were identified. Numerous eosinophils in different stages of maturation were also observed. Myeloperoxidase staining was strongly positive in the blasts as was ASD-chloroacetate specific esterase, while non-specific alpha-naphthyl esterase was negative. Toluidine blue staining for demonstration of metachromatic granules was negative. On immunophenotyping, the blasts expressed HLA-DR, CD33 and CD34 antigen, whereas CD3, CD1, CD19, CD13, CD10, CD117, CD41 antigens and glycophorin were not detected. The karyotype was t(8;21)-y. Polymerase chain reaction (PCR) revealed AML1-ETO gene rearrangement. PCR for c-kit gene mutation was negative. Ultrastructural studies of BM blasts revealed the presence of cytoplasmic neutrophilic, eosinophilic and basophilic granules. In some sections the simultaneous presence of the three types of granules was detected (Figure 1). The basophilic granules exemplified the known variable morphologic appearance already described [4], including theta granules, as well as tissue mast cell-like structures. Most of the eosinophilic granules were primary ones showing a uniform and strongly electron dense appearance and a few large ones had multiple core structures surrounded by a matrix. Each one of the three types of granules showed the usual pattern of myeloperoxidase staining, which was also strongly positive in the endoplasmic reticulum (Figure 2). The small neutrophilic granules showed a characteristic uniform strong reaction, whereas the large basophilic ones had a typical patchy appearance. The primary eosinophilic granules showed a uniform strong reaction and in the secondary eosinophilic granules