DETECTION OF MYELOPEROXIDASE GENE-EXPRESSION IN MINIMALLY DIFFERENTIATED ACUTE MYELOGENOUS LEUKEMIA (AML-M0) USING IN-SITU HYBRIDIZATION

Acute leukemias containing >3% myeloperoxidase (MPO)-positive blast ce lls, as detected cytochemically, are considered to be myelogenous in o rigin, regardless of the immunophenotypic markers expressed. Conversel y, acute leukemias that express only myeloid antigens are also conside red to be acute myelogenous leukemia (AML), even in the absence of MPO . These MPO-negative AMLs, designated AML-M0 in the FAB classification , currently require either immunophenotypic or electron microscopic st udies for identification. To examine the association of MPO and myeloi d antigen expression in AML, particularly at the early stages of myelo id cell differentiation, we have used in situ hybridization (ISH) to e valuate MPO gene expression in myeloid leukemia cell lines and a varie ty of well-characterized acute leukemias, including six cases of AML-M 0. Strong positivity for MPO mRNA was detected in the myeloid leukemia cell line HL-60 and in 22 of 27 AMLs (three AML-M0, four AML-M1, eigh t AML-M2, five AML-M4, two AML-M5a). No MPO gene expression was detect ed in three AML-M0, one AML-M5a, one AML-M7, 5 acute lymphoblastic leu kemia, the lymphoid cell lines Molt-4 and Namalwa, or in the early mye loid cell lines KG-1 and KG-1a. Ultrastructural studies for MPO activi ty were performed on four AML-M0; one leukemia showed both gene expres sion and cytochemical activity, whereas two others contained neither M PO transcripts nor enzyme. Weak MPO gene expression was evident in one AML-M0 that was negative for enzymatic activity by electron microscop y. These studies show that MPO gene expression can be detected by ISH in about half of AML-M0, supporting their presumed myelocytic derivati on. However, the blasts in some AML-M0 fail to express MPO, even at th e molecular level, suggesting that these cells are at a very early sta ge of myeloid commitment or that differentiation along some other nonl ymphoid cell line may be present.