PREVALENCE OF MYELOPEROXIDASE GENE-EXPRESSION IN INFANT ACUTE LYMPHOCYTIC-LEUKEMIA

The enzyme myeloperoxidase (MPO; donor: H2O2 oxidoreductase, EC1.11.1. 7) is a well-established marker of myeloid differentiation. Most myelo id leukemias express MPO enzyme activity at the light microscopic leve l, whereas lymphoid leukemias characteristically lack such expression. However the diagnostic significance of MPO RNA expression or of immun ohistochemically detectable MPO protein expression in leukemic blasts is unclear We studied the prevalence and diagnostic significance of MP O RNA and protein expression in 57 cases of MPO enzyme-negative infant B-precursor acute lymphocytic leukemia (ALL) since the blast cells in this condition have been reported to show a high incidence of coexpre ssion of myeloid-associated antigens. MPO expression was compared with other clinical and laboratory parameters. Of the cases examined 56% s howed detectable MPO expression at the RNA or protein level or both. M ost positive cases showed MPO protein in many leukemic blasts, whereas a few cases showed substantial MPO protein expression in only a few b last cells. MPO expression showed no significant correlation with othe r markers of myeloid differentiation. Leukemic lymphoblasts in infant ALL frequently express MPO at the RNA or protein level; this expressio n does not imply an overall myeloid phenotype. The leukemic blasts in infant ALL may derive from an immature hematopoietic precursor cell no t fully committed to lymphoid differentiation.