Distinct B/myeloid and T-lymphoblast populations at separate anatomic sites in mixed-phenotype acute leukemia with BCR::ABL1 fusion.

The pluripotency of malignant blasts in acute leukemias is a growing area of scientific and clinical interest. Mixed-phenotype acute leukemias (MPALs) are defined by the presence of blasts showing evidence of differentiation along at least two lineages. Curiously, MPALs exhibit some of the same recurrent cytogenetic abnormalities (e.g., BCR::ABL1, KMT2A rearrangements) that are seen in single-lineage acute leukemias. Factors that contribute to phenotypic selection and divergence of blast populations in single-lineage and mixed-phenotype acute leukemias are incompletely understood. Optimal therapeutic management of MPAL also remains a matter of debate. Herein, we present a case of MPAL with BCR::ABL1 fusion that showed distinct T-lymphoblastic and B-lymphoblastic/myeloblastic populations at different anatomic sites (tonsil and bone marrow, respectively). Both blast populations showed clonally related TRG rearrangements with evidence of clonal evolution. The patient initially responded to tyrosine kinase inhibitor therapy, but he quickly relapsed and expired a year after diagnosis. To our knowledge, this is the first time an MPAL has been shown to have different blast lineages segregated to distinct anatomic sites in a treatment-naïve patient. This case emphasizes the importance of a multifaceted diagnostic approach to acute leukemias and highlights what is left to learn about the biology and management of these poorly understood neoplasms.