Neutropenia-associated ELANE mutations disrupting translation initiation produce novel neutrophil elastase isoforms
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Hereditary neutropenia usually results from heterozygous germline mutations of ELANE, encoding neutrophil elastase (NE). How mutations cause disease remains uncertain. Two hypotheses have been proposed. In one, ELANE mutations lead to mislocalization of NE. In the other, ELANE mutations disturb protein folding, inducing an unfolded protein response (UPR) in the endoplasmic reticulum (ER). Here we describe new types of mutations disrupting the translational start site. At first glance, they should block translation and are incompatible with either the mislocalization or misfolding hypotheses, which require mutant protein for pathogenicity. We find that start site mutations, instead, force translation from downstream, in-frame initiation codons, yielding amino-terminally truncated isoforms lacking ER-localizing (pre) and zymogen-maintaining (pro) sequences, yet retain essential catalytic residues. Patient-derived induced pluripotent stem cells (iPSC) recapitulate hematopoietic and molecular phenotypes. Expression of the amino-terminally deleted isoforms in vitro reduces myeloid cell clonogenic capacity. We define an internal ribosome entry site (IRES) within ELANE and demonstrate that adjacent mutations modulate IRES activity, independently of protein-coding sequence alterations. Some ELANE mutations therefore appear to cause neutropenia via production of amino-terminally deleted NE isoforms rather than by altering the coding sequence of the full-length protein.