Multiple genes core to ERAD, macro-autophagy and lysosomal degradation pathways participate in the proteostasis response in α1-antitrypsin deficiency.

BACKGROUND: In the classical form of α1-antitrypsin deficiency (ATD), the misfolded α1-antitrypsin Z (ATZ) variant accumulates in the endoplasmic reticulum (ER) of liver cells. A gain-of-function proteotoxic mechanism is responsible for chronic liver disease in a sub-group of homozygotes. Proteostatic response pathways, including conventional ERAD and autophagy, have been proposed as the mechanisms that allow cellular adaptation and presumably protection from the liver disease phenotype. Recent studies have concluded that a distinct lysosomal pathway called ERLAD completely supplants the role of the conventional macro-autophagy pathway in degradation of ATZ. Here we used several state-of-the-art approaches to more fully characterize the proteostatic responses in cellular systems that model ATD.

METHODS: We used CRISPR-mediated genome editing coupled to a cell selection step by FACS to carry out screening for proteostasis genes that regulate ATZ accumulation and combined that with selective genome editing in two other model systems.

RESULTS: ERAD genes are key early regulators and multiple autophagy genes, from classical as well as from ERLAD and other newly described ER-phagy pathways, participate in degradation of ATZ in a manner that is temporally regulated and evolves as ATZ accumulation persists. Time-dependent changes in gene expression are accompanied by specific ultrastructural changes including dilation of the ER, formation of globular inclusions, budding of autophagic vesicles and alterations in the overall shape and component parts of mitochondria.

CONCLUSIONS: Macro-autophagy is a critical component of the proteostasis response to cellular ATZ accumulation and it becomes more important over time as ATZ synthesis continues unabated. Multiple sub-types of macro-autophagy and non-autophagic lysosomal degradative pathways are needed to respond to the high concentrations of misfolded protein that characterizes ATD and these pathways are attractive candidates for genetic variants that predispose to the hepatic phenotype.