Multilineage involvement of light microscopic myeloperoxidase-negative acute leukemia

The classification of acute leukemia has traditionally been based on a combination of morphology and cytochemical staining data, including myeloperoxidase (MPO) reaction; however, a recent World Health Organization (WHO) classification entails use of cytogenetic and molecular findings in addition to the classic morphological and immunophenotypic analyses. Nevertheless, there have been rare cases in which blastic cells show multilineage phenotypes. These cases may be classified as acute leukemia of ambiguous lineage in the recent WHO classification. We report the case of a 49-year-old man with acute leukemia with multilineage phenotypes. Morphological findings led to a diagnosis of acute myeloid leukemia M2 by the French-American-British classification, but at light microscopy the results of MPO staining were negative for blast cells. In contrast, results of reverse transcription polymerase chain reaction and fluorescence-activated cell sorter analyses were positive for expression of MPO messenger RNA and protein. The blast cells expressed CD4, CD19, CD22, CD33, CD38, CD79a, and HLA-DR and showed rearrangement of the immunoglobulin heavy chain and TCR-β genes. Results of immunoelectron microscopic analysis of the blast cells were positive for MPO, CD19, CD33, CD34, CD38 and glycophorin A but not for platelet peroxidase. According to these results, the blast cells had at least 4 lineage phenotypes. We concluded that the multiparameter analyses conducted in this case, including immunological and ultrastructural assays, were important in arriving at the appropriate diagnosis of acute leukemia of ambiguous lineage in the new WHO classification.