Multiparameter analysis of acute mixed lineage leukemia: Correlation of a B/myeloid immunophenotype and immunoglobulin and T-cell receptor gene rearrangements with the presence of the Philadelphia chromosome translocation in acute leukemias with myeloid morphology

Morphological, immunological, cytogenetic, and molecular features of 28 cases of acute mixed lineage leukemia (AMLL), defined by the co-expression of lymphoid and myeloid cell surface antigens, were correlated in a multiparameter study. These 28 cases were identified in a series of 260 consecutive acute leukemia cases occurring predominantly in adults and were subdivided into 18 cases of AMLL with myeloid morphology and cytochemistry (AMLL-AML) and 10 cases of AMLL with lymphoid morphology and cytochemistry (AMLL-ALL). A lack of correlation was observed between the expression of B- or T-cell associated antigens with the presence of the expected immunoglobulin (Ig) or T-cell receptor (TCR) gene rearrangements in the AMLL cases with myeloid morphology. Only three of the 18 total AMLL-AML cases, each co-expressing B- and myeloid-associated cell surface antigens (B/My), had Ig heavy chain gene rearrangements with or without rearrangements of TCR genes. Ig light chain genes remained in the germline configuration. Strikingly, these three cases were the only AMLL-AML cases in our series to have the Philadelphia (Ph) chromosome translocation t(9;22)(q34;q11), suggesting that a significant percentage of acute leukemias with myeloid morphology and gene rearrangements may be Ph⁺ AMLL. The fact that three of the 10 B/My AMLL-AML cases in our series were Ph⁺ suggests that there may be an increased frequency of Ph chromosome, a translocation associated with a poor prognostic outcome, in B/My AMLL-AML occurring in the adult population. Although most AMLL cases with lymphoid morphology had Ig and TCR gene rearrangements associated with a variety of immunophenotypes and karyotypes, two Ph⁺ AMLL-ALL cases had many similar features (B/My immunophenotype; IgH with or without TCR rearrangements; Ig light chain genes germline) to their Ph⁺ AMLL-AML counterparts. However, the Ph AMLL-ALL cases differed from the Ph⁺ AMLL-AML cases by the expression of a more mature B-cell lineage immunophenotype and by their additional cytogenetic changes.