An intriguing case of bilineage B-monocytic mixed phenotype acute leukemia with BCR-ABL1 rearrangement

ABSTRACT Mixed phenotype acute leukemia (MPAL) with BCR: ABL1 rearrangement constitutes around 0.5% of all leukemias and rarely shows B-monocytic immunophenotype. We describe here a peculiar and intriguing case of a 40-year-old female who presented with anemia, leukocytosis, thrombocytopenia, and a peripheral smear showing 70% blasts of dual morphology resembling lymphoblasts and promonocytes. In cytochemistry, the promonocytes-like cells were strongly positive for non-specific esterase (NSE). Flow cytometry delineated two separate populations—the major one (57%) expressing strong CD19, CD10, CD22, cytoplasmic CD79a, CD34, CD13, CD33, CD14, and HLA-DR corresponding to the B-lineage and a minor population (10%) positive for CD14, CD64, CD11c, CD117, and dim myeloperoxidase suggestive of monocytic lineage. RT-PCR for BCR: ABL1 was positive for p210 transcript. A diagnosis of MPAL with BCR: ABL1 of B/monocytic immunophenotype was made, and the patient was managed with acute lymphoblastic leukemia (ALL) based induction chemotherapy and tyrosine kinase inhibitor. The uniqueness of the case is the presence of an additional population of bonafide promonocytes in an otherwise typical case of Philadelphia-positive B-ALL expressing CD13 and CD33. This case underscores the importance of morphology and cytochemistry in detecting and confirming minor blast populations, which may manifest as the dominant or the only clone at relapse.