Inducible expression of a disease-associated mutation impairs granulocytic differentiation, without eliciting an unfolded protein response.

Severe congenital neutropenia (SCN) is characterized by a near absence of neutrophils, rendering individuals with this disorder vulnerable to recurrent life-threatening infections. The majority of SCN cases arise because of germline mutations in the gene elastase, neutrophil-expressed () encoding the neutrophil granule serine protease neutrophil elastase. Treatment with a high dose of granulocyte colony-stimulating factor increases neutrophil production and reduces infection risk. How mutations produce SCN remains unknown. The currently proposed mechanism is that mutations promote protein misfolding, resulting in endoplasmic reticulum stress and activation of the unfolded protein response (UPR), triggering death of neutrophil precursors and resulting in neutropenia. Here we studied the mutation p.G185R, often associated with greater clinical severity ( decreased responsiveness to granulocyte colony-stimulating factor and increased leukemogenesis). Using an inducible expression system, we observed that this mutation diminishes enzymatic activity and granulocytic differentiation without significantly affecting cell proliferation, cell death, or UPR induction in murine myeloblast 32D and human promyelocytic NB4 cells. Impaired differentiation was associated with decreased expression of genes encoding critical hematopoietic transcription factors (, , , and ), cell surface proteins ( and ), and neutrophil granule proteins ( and ). Together, these findings challenge the currently prevailing model that SCN results from mutant , which triggers endoplasmic reticulum stress, UPR, and apoptosis.