The Emergence of Subclones Following Initial Chemotherapy in Mixed Phenotype Acute Leukemia

Mixed phenotype acute leukemia (MPAL) is a rare subtype of acute leukemias, and it is characterized by the immunophenotypic expression of multiple hematopoietic lineages and the potential for lineage switching post-treatment. Diagnosing MPAL can be challenging, particularly in cases with small immunophenotypically distinct subclones or subpopulations exhibiting weak antigen expression. We present two cases of MPAL where subclones or lineage switches emerged following initial treatment. Both cases exhibited a myeloperoxidase (MPO)-positive population of leukemic cells, which was initially either undervalued or overestimated, complicating their diagnosis. In one case, leukemic blasts tested negative for MPO in immunohistochemical (IHC) staining; however, flow cytometric analysis revealed a minor subclone that was weakly positive for MPO. In the other patient, leukemic blasts tested positive for MPO through IHC staining. The latter patient was diagnosed with acute myelogenous leukemia; however, the leukemic cells exhibited lymphoblastic morphological features and co-expressed both myeloid and lymphoblastic antigens in each case. Following initial treatments, selective pressure causes the proliferation of leukemic cells resistant to chemotherapy, with an immunophenotypic shift in the blasts, requiring treatment modification in both cases. These two cases highlight the diagnostic and therapeutic complexities of MPAL and underscore the crucial role of comprehensive immunophenotyping in identifying minor subclones, thereby ensuring an accurate diagnosis, informing treatment selection, and facilitating timely therapeutic adjustments.