An Unusual Case of Myeloperoxidase-Positive Acute Megakaryoblastic Leukemia

Dear Editor We report an unusual case of myeloperoxidase (MPO)-positive acute megakaryoblastic leukemia (AMKL). The patient was a 63-yr-old woman referred to our hospital with anemia and dyspnea. General physical examination revealed no lymphadenopathy or hepatosplenomegaly. An initial complete blood count indicated white blood cell counts of 26.78×10/L, including 47% blasts, hemoglobin of 7.2 g/dL, and platelets of 519×10/L. A peripheral blood smear showed blasts with prominent nucleoli and blue cytoplasm. Some giant platelets were also observed. A bone marrow (BM) biopsy showed hypercellularity with no fibrosis, packed with blasts, and revealed an increase in megakaryocyte number. BM aspirates showed that blasts accounted for 70% of all nucleated elements. Most blasts had medium-to-large, finely chromatinated nuclei with distinct nucleoli. Some showed multiple clear cytoplasmic projections (Fig. 1A, B) or Auer rods. No dysplastic features of hematologic precursors were observed. Cytochemical stain showed some blasts were MPO-positive (Fig. 1C) but negative for periodic acid–Schiff (PAS) stain and nonspecific esterase (NSE, α-naphthyl butyrate). Immunophenotyping showed a distinct population in the CD45 vs. SSC plot that was positive for CD7, CD11c, CD13, CD33, CD41a, CD117, cytoplasmic MPO, and HLA-DR and negative for CD2, CD3, CD5, CD10, CD14, CD19, CD20, CD22, CD34, CD56, CD64, and cytoplasmic CD79a. About 57.0% of blasts were CD41a-positive, and 57.7% were cytoplasmic MPOpositive. A blast population co-expressing CD41a and cytoplasmic-MPO was also observed (Fig. 2). Conventional karyotyping revealed a 46,XX karyotype. Molecular studies did not reveal any genetic abnormalities. As the patient refused treatment, only supportive care was provided, and the patient died from pneumonia four months later. Diagnosis of AMKL relies on multiple criteria including morphology, immunophenotyping, cytochemical stain, and ultrastructural studies [1]. The standard for diagnosis is demonstration of platelet glycoprotein (GP)-CD41 (GP IIb/IIIa) and/or CD61 (GP IIIa) by immunophenotyping. Cytoplasmic expression is more specific owing to possible contamination of platelets [2]. In cytochemical stain, megakaryoblasts are not reactive with MPO or Sudan Black B. Cells of the megakaryocytic lineage are usually positive in PAS stain owing to glycogen granules in the cytoplasm, and they are typically present in the periphery on stain, with prominent cytoplasmic blebs [3]. Reactivity with α-naphthyl acetate, but not with α-naphthyl butyrate, a different substrate for NSE, is characteristic of megakaryoblasts [4]. An ultrastructural platelet peroxidase reaction by cytochemistry, while difficult to perform, is also diagnostic for megakaryoblasts [1]. Megakaryoblasts also show acid phosphatase reactivity localizing to