HEREDITARY EOSINOPHIL PEROXIDASE DEFICIENCY - IMMUNOCHEMICAL AND SPECTROSCOPIC STUDIES AND EVIDENCE FOR A COMPOUND HETEROZYGOSITY OF THE DEFECT

Hereditary eosinophil peroxidase (EPO; EC 1.11.1.7) deficiency is a ra re abnormality of eosinophil granulocytes characterized by decreased o r absent peroxidase activity and decreased volume of the granule matri x. The molecular basis of the defect is not known. We report here its molecular characterization in an EPO deficient subject and his family members. The EPO-deficient eosinophils contained EPO-related material as determined immunochemically using either monoclonal or polyclonal a nti-EPO antibodies but had no spectroscopic evidence of EPO. Eosinophi l precursors from the EPO-deficient subject contained normally sized E PO mRNA, which was reverse transcribed into the corresponding cDNA clo nes encompassing the whole gene. Sequencing of these clones disclosed two mutations, a G --> A transition causing a nonconservative replacem ent of an arginine residue with a histidine and an insertion causing a shift in the reading frame with the appearance of a premature stop co don. The two mutations were located on different chromosomes indicatin g a compound heterozygosity for the defect. Both the son and the daugh ter of the proband inherited the G --> A transition, and their eosinop hils contained a peroxidase activity intermediate between that of cont rol subjects and the proband, suggesting that the transition is a defi ciency-causing mutation, Eosinophil precursors from the EPO-deficient subject were found to actively synthesize an EPO that was apparently n ormal in terms of cytochemical reaction for peroxidase and immunoreact ivity with monoclonal and polyclonal anti-EPO antibodies, but spectros copically abnormal. The cytochemical reaction for peroxidase tended to decrease or disappear in the eosinophil precursors of the EPO-deficie nt subject but not of a normal subject as differentiation went on, sug gesting that the Arg --> His substitution causes the production of an unstable EPO that undergoes progressive degradation as the cells matur e.