Discordant expression of myeloid antigens and myeloperoxidase in a case of t(8;21) positive AML expressing CD7.

We describe a case of acute myeloid leukemia (AML) showing myeloperoxidase (MPO)-positive and myeloid antigens negative. Although the leukemic cells showed few granules in May-Grünwald Giemsa staining, cytochemical MPO staining revealed that most of the blast cells strongly reacted with MPO. The leukemic cells did not express myeloid antigens (CD13, CD33), nor B-lymphoid or T-lymphoid antigens on the cell surface using flow cytometry, however. The cells did express CD34 and CD7. Discordant expression of MPO and myeloid antigens was also confirmed by electron microscopic MPO staining and by immunocytochemistry using a streptoavidin-biotin alkaline phosphatase labeling technique. Cytogenetic studies showed 46, XX, t(8;21) (q22;q22), del (9) (q22) in the bone marrow cells. In addition, AML1/ETO chimeric mRNA was detected from these cells. We summarize eight reported cases of MPO positive and myeloid antigens negative AML. Five of nine cases including our case had the same chromosomal abnormality of t(8;21) (q22;q22) and showed better prognosis than the other cases.