CD79a expression in acute myeloid leukemia t(8;21) and the importance of cytogenetics in the diagnosis of leukemias with immunophenotypic ambiguity

Acute leukemias that express antigens associated with more than one lineage have been classified as acute lymphocytic leukemia with myeloid markers, acute myeloid leukemia with lymphoid markers, or biphenotypic acute leukemia (BAL). Antibody to cytoplasmic CD79a has been recently introduced to flow cytometry. CD79a functions in and has a high degree of specificity for B-cell differentiation. It has only recently begun to be reported in biphenotypic acute leukemias. Cases of acute leukemia submitted to the flow cytometry laboratory were retrospectively reviewed beginning from the time analysis for cytoplasmic CD79a was added to leukemia and lymphoma panels. Among 89 cases of AML, 2 showed strong coexpression of CD79a. Both cases were differentiated FAB AML-M2 and demonstrated the t(8;21) with cytogenetics and the AML1/ETO rearrangement with fluorescence in situ hybridization (FISH). These are recurring abnormalities in FAB AML-M2. The immunophenotyping met proposed scoring criteria for a diagnosis of BAL. Nevertheless, the cytogenetic and FISH findings indicate that CD79a, despite its specificity for B-cell differentiation, represented the aberrant presence of a B-cell antigen in leukemias of distinct myeloid linage. It is doubtful that, in this setting, CD79a expression should be considered a manifestation of lineage ambiguity.