Mixed-phenotypic acute leukemia: cytochemically myeloid and phenotypically early T-cell precursor acute lymphoblastic leukemia.

Abstract

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is characterized by the specific immunophenotype of CD1a-, CD8-, and CD5weak/- with stem cell or myeloid marker expression [1]. This leukemia is associated with increased genomic instability, a high remission failure frequency, and hematological relapse. ETP-ALL patients also represent a distinct molecular T-ALL subgroup with a low frequency of NOTCH1 mutations and a high frequency of FLT3 mutations. ETP-ALL with FLT3 mutation has also been described as a new ETP-ALL subgroup with unique immunophenotypic features and high levels of CD2, myeloid antigen CD13, and CD117 expression, indicating a stem cell phenotype [2]. Mixed-phenotypic acute leukemia (MPAL), T/Myeloid subtype is rarely suspected from morphology, except in a small subset of cases, which have a distinct dual-blast population with both lymphoid and myeloid features. Therefore, final diagnoses of MPAL are based on flow cytometric immunophenotyping. Common cytogenetic abnormalities associated with MPAL include t(9;22) and MLL rearrangements, which may be restricted to B-Myeloid cases, whereas T-myeloid cases exhibit generally non-recurring chromosomal abnormalities [3]. Herein, we report a case of MPAL (T/Myeloid) with trisomy 4 and t(6;14)(q27:q22) in a 25-year-old man, the diagnosis of which was directed by the morphological presence of Auer rods and myeloperoxidase (MPO) staining, despite exhibiting a flow cytometric ETP-ALL phenotype.