Induction of the Unfolded Protein Response but Normal Basal Granulopoiesis in Mice Expressing G192X ELA2

Severe congenital neutropenia (SCN) is an inborn disorder of granulopoiesis characterized by chronic neutropenia, a block in granulocytic differentiation at the promyelocyte/myelocyte stage, and a marked propensity to develop acute myeloid leukemia. Most cases of SCN are associated with germline heterozygous mutations of ELA2, encoding neutrophil elastase (NE). To date, 59 different, mostly missense, mutations of ELA2 have been reported. A unifying mechanism by which all of the different ELA2 mutants disrupt granulopoiesis is lacking. We and others previously proposed a model in which the ELA2 mutations result in NE protein misfolding, induction of the unfolded protein response (UPR), and ultimately apoptosis of granulocytic precursors. Testing this (and other) models has been limited by the rarity of SCN and difficulty in obtaining clinical samples for testing. Herein, we report the preliminary description of a novel transgenic mouse line that expresses G192X Ela2, reproducing the G193X ELA2 mutation found in some patients with SCN. The G192X mutation was introduced into the murine Ela2 locus by homologous recombination in embryonic stem cells. Heterozygous or homozygous G192 Ela2 “knock-in” mice were healthy with no apparent developmental defect. While expression of Ela2 mRNA was normal, no mature NE protein was detected in the neutrophils of homozygous G192X Ela2 mice. However, in granulocytic precursors (mainly promyelocytes/myelocytes) a small amount of heavily glycosylated mutant NE protein was detected. Together, these observations suggest that G192X NE protein is retained in the endoplasmic reticulum (ER) and rapidly degraded. Consistent with ER stress and induction of the UPR, a significant increase in BiP/GRP78 and ATF6 mRNA expression in mutant granulocytic precursors were observed. Surprisingly, G192X Ela2 mice have normal basal granulopoiesis. The number of circulating neutrophils, granulocytic differentiation in the bone marrow, and number and cytokine responsiveness of myeloid progenitors were comparable to wild type mice. In summary, the G192X Ela2 mice appear to reproduce the NE protein misfolding and UPR activation observed in human SCN granulocytic precursors. However, expression of G192X Ela2 is not sufficient to disrupt basal granulopoiesis in mice. Studies of stress granulopoiesis are underway.