Human neutrophil microbicidal activity is largely mediated by reactive species generated by the oxygen-dependent myeloperoxidase (MPO) system. Peroxidase-negative neutrophils from many patients with hereditary MPO deficiency possess a 90-kDa MPO-related protein. We recently identified a missense mutation, R569W, in the MPO gene of many subjects with MPO deficiency. In these studies we examined the consequences of R569W on MPO biosynthesis and processing, using stably transfected K562 cells expressing normal MPO or the R569W mutation. K562 cells expressing normal MPO mimicked faithfully many features of MPO biosynthesis in myeloid cells. 1) apopro-MPO was synthesized; 2) a functional heme group was inserted into apopro-MPO, and enzymatically active pro-MPO was thereby generated; 3) pro-MPO underwent proteolytic processing to mature MPO; and 4) hemin augmented the processing of pro-MPO. pREP-R569W cells synthesized apopro-MPO, but heme was not inserted. Neither enzymatically active pro-MPO nor mature MPO was synthesized by transfectants expressing mutated cDNA, confirming our hypothesis that the R569W mutation results in a form of apopro-MPO which does not undergo post-translational processing to enzymatically active MPO species. In addition, these data support previous suggestions that heme insertion into apopro-MPO is necessary for its subsequent proteolytic processing into mature MPO subunits.

Myeloperoxidase (MPO) is an essential component of the oxygen-dependent microbicidal system of neutrophils and monocytes. Hereditary deficiency of MPO occurs in 1 in 2,000 to 4,000 individuals in the general population and has been generally considered an autosomal recessive trait. Previous studies have used the peroxidase activity of blood leukocytes to assess the phenotype of affected family members. Eosinophil peroxidase (EPO) also contributes to the peroxidase activity of blood leukocytes. Because EPO expression is normal in MPO-deficient subjects, eosinophil contamination can significantly contribute to peroxidase activity in leukocytes from family members of an MPO-deficient subject and thereby undermine correct interpretation of the inheritance pattern. To avoid this potential problem, we used cytochemical, immunochemical, and genetic techniques to assess the inheritance pattern of MPO deficiency in sixteen individuals from five unrelated kindreds. Each kindred had an index case with MPO deficiency and the R569W missense mutation, a genotype that causes MPO deficiency. Our analysis demonstrated that MPO deficiency was not inherited as a simple autosomal recessive trait. Most subjects were compound heterozygotes with respect to the R569W mutation and demonstrated a spectrum of phenotypes. Our data demonstrate the broad phenotypic impact of compound heterozygosity on the expression and function of a multimeric protein such as MPO.

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