Aberrant restriction endonuclease digests of DNA from subjects with hereditary myeloperoxidase deficiency

Myeloperoxidase (MPO) is a critical component in the oxygen-dependent microbicidal activity of neutrophils. Hereditary deficiency of MPO occurs commonly, but its genetic basis has not been determined. Previously we have reported the presence of an 89-kilodalton protein, likely pro-MPO, in normal and MPO-deficient neutrophils and hypothesized that the absence of peroxidase activity in neutrophils from affected subjects was the result of defective posttranslational processing of pro-MPO. In this study we analyzed nucleic acids from three completely and two partially MPO-deficient individuals by using a cDNA probe for MPO. The affected individuals studied are unrelated to one another. Neutrophils from all affected subjects lacked mature MPO subunits; however, a monospecific antibody for MPO identified in these cells a high-molecular weight protein that is the same size as pro-MPO. Northern blots demonstrated that the amount and size of RNA (3.3 kilobases [kb]) in a completely deficient subject was normal. BglII digests of genomic DNA from control individuals (n = 14) contained three fragments that hybridized with cDNA for MPO under very stringent conditions. In contrast, BglII digests of genomic DNA from completely MPO-deficient individuals contained an extra fragment of 2.1 kb that was not present in DNA from controls. In addition, two different endonuclease digest patterns were found in MPO-deficient subjects who were biochemically and phenotypically identical. We conclude from these studies that (a) hereditary MPO deficiency is not associated with a major deletion or rearrangement of the MPO gene; (b) myeloid precursors in an MPO-deficient individual contain normal amounts of an mRNA that is the same size as that for MPO in normal individuals; and (c) the genetic basis for MPO deficiency may be heterogeneous, with at least two genotypes generating the same phenotype. These findings are consistent with the hypothesis that the genetic defect in MPO deficiency results in synthesis of a modified pro-MPO that undergoes defective posttranslational processing.