Characterization of mutant neutrophil elastase in severe congenital neutropenia

Severe congenital neutropenia is a heritable human disorder characterized b y neutropenia and acute myelogenous leukemia. me recently determined that t he majority of cases result from de novo or autosomal dominantly inherited heterozygous mutations in ELA2, encoding neutrophil elastase. Neutrophil el astase is a chymotryptic serine protease localized in granules of neutrophi ls and monocytes and is the major target of inhibition of the serpin alpha (1)-antitrypsin. The mutations causing severe congenital neutropenia consis t of amino acid missense substitutions, in-frame deletion, splice donor mut ation producing a deletion, splice acceptor mutation causing insertion of n ovel residues, and protein truncating mutations of the carboxyl terminus re sulting from nonsense substitutions and deletions leading to frameshifts. W e have expressed 14 mutant forms of neutrophil elastase in vitro and have c haracterized their biochemical properties. The mutations have variable effe cts on proteolytic activity, eliminating the possibility that the disease r esults from haploinsufficiency. There is no evidence that the mutant enzyme s are cytotoxic. The mutant enzymes retain vulnerability to inhibition by a i-antitrypsin, but demonstrate variable avidity for interaction with this s erpin. Somewhat surprisingly, the mutant enzymes inhibit the wild type enzy me when both are coexpressed within the same cell, suggesting the potential to interfere with normal subcellular trafficking or post-translational pro cessing.