BIOCHEMICAL AND MOLECULAR CHARACTERIZATION OF HEREDITARY MYELOPEROXIDASE DEFICIENCY

Hereditary myeloperoxidase (MPO) deficiency is a neutrophil disorder c haracterized by the lack of peroxidase activity. Cytochemical, biochem ical, spectroscopic, immunocytochemical, and genetic studies were carr ied out on a fi-year-old MPO-deficient subject and on her parents. The father was also NPO-deficient, whereas the mother had 24% of normal M PO activity. Although the typical absorption spectrum of MPO was absen t in both the father and daughter, the father's neutrophils, and not t hose of the daughter, contained material antigenically related to MPO. In the MPO gene of the father, two mutations were found, each located in a different allele: a T --> C transition, causing the nonconservat ive replacement M251T and a 14-base deletion within exon 9. The M251T substitution occurred in the carboxy-terminal region of the light chai n that is included in the heme pocket. The daughter inherited the 14-b ase deletion from her father. The study of the MPO mRNAs present in li quid cultures of granulocyte precursors surprisingly showed that the s ame genetic defect, ie, the 14-base deletion, seemed to exhibit differ ent mRMA phenotypes in the father and the daughter. In fact, mRNA deri ved from the 14-base-deleted allele was not found in the father and an aberrantly spliced MPO mRNA with a 77-base deletion of exon 9, which includes the 14-base deletion and leads to the generation of a prematu re stop codon, was found in the daughter. The possibility that Delta 7 7 mRNA could derive from other mutations linked to the Delta 14 allele was dismissed because no sequence differences were found in the regio n (exons and exon-intron junctions). Our data indicate that the altera tion of the mRNA context caused by the 14-base deletion provide a basi s for the 77-base deletion in the mRNA processing. Since the granulocy te precursors from the liquid cultures of the father were more differe ntiated than those from the daughter, the observed different behavior of the 14-base-deleted allele in the father and daughter may be the re sult of a differentiation-stage dependent control of altered spliced m RNA, which may be tolerated during the early stages of differentiation but degraded at later stages. In the liquid cultures of the daughter' s cells, in addition to the mRNA with the 77-base deletion, a mRNA wit h the wild type sequence was also found. This mRNA was inherited from the mother, since no mutations were found in her MPO cDNA and MPO gene . The MPO defect might be caused by a regulatory mutation that induces the MPO gene switch off at an early stage of granulocyte differentiat ion. (C) 1997 by The American Society of Hematology.