Comparative Analysis of Aberrant Autophagy in HAX1-Mutated and ELANE/ELA2-Mutated Neutrophil Granulocytes.

Abstract 550

Severe congenital neutropenia is a heterogeneous disorder with the phenotypic hallmark of ‘myeloid maturation arrest‘. Recent work in our group identified that HS-1 associated protein X1 (HAX1), a mitochondrial protein is essential for survival of neutrophils through maintenance of mitochondrial membrane potential (MMP). A large group of patients have mutations in ELANE/ELA2 or HAX1, respectively, causing increased susceptibility to apoptosis. Although phenotypically indistinguishable, it is controversial whether HAX1 and ELA2 mutant cells share a joint pathway. To further address the biological abnormalities, we measured reactive oxygen species (ROS) production and autophagy in primary human neutrophil granulocytes from SCN patients. HAX1 deficient neutrophils, but not SCN neutrophils expressing functional HAX1 showed evidence of enhanced production of ROS, a continuous byproduct of the oxidative phosphorylation. Antioxidant defense system in a cell constitutes proteins that restrain ROS mediated cellular damage. The degradation of catalase, a primary antioxidant defense protein was accelerated in HAX1 deficient neutrophils as compared to healthy controls. This effect was not observed in ELA2 deficient neutrophils or in SCN neutrophils expressing functional HAX1. Oxidative phosphorylation coupled ATP synthesis; a prime function of the mitochondria is hampered with mitochondrial dysfunction, resulting from loss of MMP. We observed that the HAX1-deficient neutrophils but not ELA2-deficient neutrophils have an elevated AMP/ATP ratio as measured by the increased activation of AMP activated protein kinase α (AMPKα). In view of increased ROS production hampering the cellular organelles, and decreased levels of cellular ATP, we hypothesized that autophagy may play a role in HAX1-deficient neutrophils. Autophagy describes a process of organelle digestion within a cell aiming at sparing energy and disposal of damaged organelles. In case these rescue mechanisms are futile, the cell undergoes apoptosis. Expression of beclin-1, an autophagy essential protein was increased in HAX1-deficient neutrophils, accompanied by an increase in the ATG12-5 complex formation as compared to healthy control cells. Furthermore, transmission electron microscopy studies revealed evidence of increased autophagy in HAX1-deficient neutrophils. These observed effects are specific to HAX1 deficiency, as these biochemical aberrations were not observed in ELA2-deficient patients or SCN patients expressing functional HAX1. Moreover the phenotype was observed only in neutrophils and not in HAX1-deficient lymphoid or monocytic cells. Our results unravel a novel role of the antiapoptotic protein HAX1 in maintenance of cellular homeostasis in neutrophil granulocytes by regulating the autophagic machinery through control over ROS production.