A 9-year-old male presented with lymphadenopathy, leukocytosis with white blood cell count (WBC) 277 x10⁹/L, Hemoglobin 6.1g/dL, and circulating blasts in peripheral blood. The flow cytometry analysis of peripheral blood confirmed 85% blasts expressing CD34, CD13, CD117, CD15, HLA-DR partial, CD7 and CD2. Most blasts were negative for MPO but equivocally express cytoplasmic CD3 (with an intensity lower than background T cells). In addition, a small population of discrete blast population (less than 5%) were positive for MPO. These flow cytometry findings raised differential diagnoses of MPAL, early T precursor (ETP)-ALL or AML with aberrant T cell antigen expression. A bone marrow biopsy was performed, and the flow cytometry analysis of bone marrow revealed dim cytoplasmic CD3 on blasts with the intensity equal to background T-cells. There was a discrete population of blasts (5-10%) with positive MPO expression. The findings supported a T/My MPAL diagnosis based on the current WHO diagnostic criteria. The immunostains of CD3 and MPO on bone marrow core biopsy also supported the diagnosis. The genetic and cytogenetic karyotype studies on bone marrow aspirate revealed a BCL11B copy gain (rearrangement not detected) and FLT3-ITD. The patient was treated on COGAALL1732 protocol and while leukocytosis responded to initial treatment, on day three of treatment, the patient was found on CT to have a large intraparenchymal hemorrhage and died the following day.

We report a unique case demonstrating the diagnostic and treatment challenge of MPAL with BCL11B copy gain. MPAL comprises less than 5% of pediatric acute leukemia.¹³ The current diagnostic criteria of MPAL relies on flow cytometry, immunohistochemistry or less frequently by cytochemical stain to determine the blast lineage.⁷ However, it is subjective regarding the percentage and intensity of antigen expression when assigning the lineage. For instance, the peripheral blasts in our patient revealed very dim expression of cytoplasmic CD3 and less than 5% MPO+ blasts, which together created a diagnostic challenge.¹⁴

Because of the rarity of MPAL, there are frequently diagnostic challenges that could lead to delay in treatment initiation as well as a lack of prospective trial data to guide therapy.^11,12,15 BCL11B rearrangement is known to associate with a distinct subgroup of acute leukemias with a broadly variable phenotype, including AML, ETP-ALL, and MPAL.^5,6,8,13,16 The functional consequence ofBCL11B rearrangement is overexpression ofBCL11B .⁸ Our patient had a BCL11B copy gain without known rearrangement, which might be an alternative mechanism of BCL11B overexpression. Moreover, the leukemias withBCL11B aberrations frequently contain FLT3-ITD and other high risk mutations.⁸ An ex vivo drug-sensitivity profile study revealed that FLT3 tyrosine kinase inhibitors are effective on these leukemias with BCL11B rearrangement and FLT3-ITD,⁸ which may provide a novel therapy strategy for MPAL with BCL11B rearrangement in the future.

In summary, MPAL is a rare leukemia which presents diagnostic challenges. We report a patient with an identified BCL11B copy gain without rearrangement present in T/myeloid MPAL. Identification of this BCL11B aberration may contribute to the current molecular knowledge of MPAL, and ideally additional prospective studies of patients with MPAL will more clearly define diagnostic criteria and appropriate therapy.