Association Between B-Cell Marker Expression and Lesions in Acute Myeloid Leukemia, Beyond :: Fusion: Diagnostic Pitfalls with Mixed-Phenotype Acute Leukemia-B/Myeloid.

Acute myeloid leukemia (AML) with :: fusion is well known to often demonstrate aberrant upregulation of CD19 expression. We studied the clinicopathologic and genetic features of 16 cases of AML with various lesions, including mutations, copy number gains, and translocations other than fusions with . Most of these cases were classified as AML-myelodysplasia-related or AML-post-cytotoxic therapy based on the cytogenetic and molecular work-up. These neoplasms showed partial expression of one or more B-cell antigens by flow cytometry and/or immunohistochemistry, fulfilling the criteria for mixed-phenotype acute leukemia (MPAL)-B/myeloid (i.e., ≥20% blasts expressing B and myeloid lineage antigens) in most cases. These findings suggest that AML cases with lesions including mutations, copy number gains, and translocations other than fusion, also commonly express B-cell markers, imparting a "mixed-lineage-like" immunophenotype in cases of AML that otherwise fulfill the criteria for other defined subtypes. We present these cases as to caution regarding this potential diagnostic pitfall and favor a diagnosis of AML with lesion(s) in the setting of a case of AML with myeloid/B-cell antigen expression, a history of myelodysplasia or cytotoxic therapy, the demonstration of pDC differentiation by flow cytometry (generally associated with the presence of a mutation), and the presence of a lesion (mutation, copy number gain, and/or translocation exclusive of a rearrangement with ).