TO THE EDITOR: Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is characterized by the specific immunophenotype of CD1a^-, CD8^-, and CD5^weak/- with stem cell or myeloid marker expression. This leukemia presents a particular flow cytometric diagnostic challenge, as it frequently exhibits sufficient myeloid-lineage differentiation to be classified as mixed-phenotype acute leukemia (MPAL). The 2008 World Health Organization classification has established new strict criteria for the diagnosis of MPAL; however, the exact MPO positivity cut-off percentage required for establishing a myeloid lineage was not mentioned. Herein, we report a case of ETP-ALL with trisomy 4 and t(6;14)(q27:q22) in a 25-year-old man, the final diagnosis of which was MPAL (T/Myeloid) due to the morphologic presence of Auer rods and myeloperoxidase staining, despite exhibiting a flow cytometric ETP-ALL phenotype. To the best of our knowledge, this is the first report on a case of ETP-ALL with Auer rods.

Early T-cell precursor acute lymphoblastic leukemia (ETP-ALL) is characterized by the specific immunophenotype of CD1a^-, CD8^-, and CD5^weak/- 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.

A 25-year-old man presented with multiple bilateral neck swelling along with a nasal vocal intonation, weight loss, and low-grade fever since 1 month. He was externally evaluated via neck ultrasonography and nasal endoscopy, which revealed bilateral cervical lymphadenopathy and nasopharyngeal lymphoid hyperplasia, respectively. The results of fine needle aspiration of a cervical lymph node were suggestive of a hemato-lymphoid malignancy, and the patient was referred to our hospital for further evaluation. Upon examination, he presented with bilateral cervical lymphadenopathy and splenomegaly. His lactate dehydrogenase level was elevated (1,703 U/L; reference range, 313-618 U/L) and liver and renal functions were normal. Hematological parameters revealed a hemoglobin level of 55 g/L, total leukocyte count of 193.8×10⁹ cells/L, and platelet count of 40×10⁹ cells/L. A peripheral blood smear revealed a blast frequency of 88%; these were large with scant light blue agranular cytoplasm, open nuclear chromatin, and 1-2 indistinct nucleoli. The bone marrow showed near total replacement by blasts. Most blasts exhibited the previously described morphology, but few had moderate amounts of granular cytoplasm, fine nuclear chromatin, and 2-3 prominent nucleoli, and occasional blasts contained Auer rods (Fig. 1A). Myeloperoxidase (MPO) cytochemistry revealed approximately 3% MPO-positive blasts, including an occasional Auer rod (Fig. 1B). Flow cytometric immunophenotyping revealed that the blasts were positive for CD45; the T cell markers cCD3, CD2, and CD7; and the myeloid/stem cell markers CD34, CD117, CD13, HLA-DR, CD123, and CD99, but were negative for CD1a, CD8, and CD5 as well as CD19, CD10, CD4, sCD3, CD64, CD14, CD20, CD33, CD56, CD16, CD66b, CD15, CD11b, and MPO, suggesting a diagnosis of ETP-ALL (Fig. 1C). Given the morphological presence of MPO-positive Auer rods and the flow cytometric ETP-ALL phenotype, a final diagnosis of MPAL (T/Myeloid) was made. Conventional karyotyping revealed 47, XY, +4, and t(6;14)(q27:q22). The real-time polymerase chain reaction analysis was positive for FLT3-ITD mutation and negative for NPM1 mutation.

The 2008 World Health Organization (WHO) classification established new and strict criteria for the diagnosis of MPAL [4]. The WHO definition of MPAL, T/Myeloid was based on the expression of specific T-cell (cytoplasmic/surface CD3) and myeloid (MPO) antigens; the latter is determined using flow cytometry, cytochemistry, or immunohistochemistry and/or from clear evidence of monocytic differentiation. The newly identified entity ETP-ALL, which is not yet included in the WHO classification, presents a particular flow cytometric diagnostic challenge because it frequently presents with sufficient myeloid-lineage differentiation to be classified as MPAL, T/Myeloid [5]. Recently, Borowitz [6] suggested that these 2 types of leukemia have more similarities than differences; however, they are treated differently because of the central importance of MPO in labeling a disease as myeloid and the manner, in which leukemia treatment protocols are structured. However, the exact MPO positivity cut-off percentage required to establish a myeloid lineage was not mentioned in the WHO classification. Our case fulfilled all the flow cytometric criteria for ETP-ALL, including MPO negativity [1]. However, an extensive light microscopic evaluation of the bone marrow revealed approximately 3% blasts with cytochemical MPO positivity and Auer rods (also positive for MPO); thus, this case was classified as MPAL, T/myeloid phenotype. To the best of our knowledge, this is the first case of ETP-ALL with Auer rods reported in the literature.

(A) Bone marrow aspirate showing blasts with a single Auer rod (thick arrow; Giemsa, ×100). (B) Few myeloperoxidase (MPO)-positive blasts; inset shows an MPO-positive Auer rod (thin arrow; MPO, ×100). (C) Blasts are cCD3, CD34, CD117, and CD7 positive and cMPO, CD5, and CD33 negative.