ERAD defects and the HFE-H63D variant are associated with increased risk of liver damages in Alpha 1-Antitrypsin Deficiency

Abstract

BackgroundThe most common and severe disease causing allele of Alpha 1-Antitrypsin Deficiency (1ATD) is Z-1AT. This protein aggregates in the endoplasmic reticulum, which is the main cause of liver disease in childhood. Based on recent evidences and on the frequency of liver disease occurrence in Z-1AT patients, it seems that liver disease progression is linked to still unknown genetic factors.MethodsWe used an innovative approach combining yeast genetic screens with next generation exome sequencing to identify and functionally characterize the genes involved in 1ATD associated liver disease.ResultsUsing yeast genetic screens, we identified HRD1, an Endoplasmic Reticulum Associated Degradation (ERAD) associated protein, as an inducer of Z-mediated toxicity. Whole exome sequencing of 1ATD patients resulted in the identification of two variants associated with liver damages in Z-1AT homozygous cases: HFE H63D and HERPUD1 R50H. Functional characterization in Z-1AT model cell lines demonstrated that impairment of the ERAD machinery combined with the HFE H63D variant expression decreased both cell proliferation and cell viability, while Unfolded Protein Response (UPR)-mediated cell death was hyperstimulated.ConclusionThis powerful experimental pipeline allowed us to identify and functionally validate two genes involved in Z-1AT-mediated severe liver toxicity. This pilot study moves forward our understanding on genetic modifiers involved in 1ATD and highlights the UPR pathway as a target for the treatment of liver diseases associated with 1ATD. Finally, these findings support a larger scale screening for HERPUD1 R50H and HFE H63D variants in the sub-group of 1ATD patients developing significant chronic hepatic injuries (hepatomegaly, chronic cholestasis, elevated liver enzymes) and at risk developing liver cirrhosis.

Alpha 1-Antitrypsin Deficiency: A Disorder of Proteostasis-Mediated Protein Folding and Trafficking Pathways

“…In this cellular pathway, EDEM3 (ER degradation-enhancing α-mannosidase-like protein 3), degradation enhancer of misfolded glycoproteins in the ER, is implicated in the recognition of ERAD substrates [30,32]. After recognition, ERAD substrates are retro-translocated into the cytosol through a channel formed by some proteins including ubiquitin ligase HMG-CoA reductase degradation 1 homolog (HRD1) and homocysteine inducible ER protein with ubiquitin-like domain 1 (HERPUD1), and are extracted from the ER to the cytosol in order to be transported and degraded by the proteasome (Figure 3) [33]. Finally, in contrast to the WT-AAT, neither Z-AAT nor another Z variant such as the well-known null Hong Kong variant, bind to ERGIC-53, suggesting that ERGIC-53-AAT interaction is conformation-dependent [31].…”

“…Moreover, the main classic environmental factors such as alcohol associated with liver injury cannot be advanced during childhood. Thus, the presence of modifier genes in the onset of severe pediatric AATD liver disease is now well recognized but still remains to be identified [9,33,54]. Among all the potential candidates, several publications suggest that genetic factors affecting the efficiency of the disposal pathways (UPS and/or autophagy) might act as potential modifiers of AATD liver disease outcome.…”

“…This suggests that Z-AAT aggregates indirectly impair degradation of polyubiquitinated proteins at the 26S proteasome level, compromising substrate delivery to the proteolytic core [52]. In addition, two genetic studies, a candidate gene-sequencing strategy and a genome-wide association study have pinpointed some potential genetic modifiers in the ERAD pathway as potential candidates in AATD liver disease [33,54]. Difference in ERmanI expression is associated with an earlier age-of-onset for end-stage liver disease, and HERPUD1 R50H and HFE H63D variants are associated with the advanced liver disease component of AATD [33,54].…”

SERPINA1 and MAN1B1 polymorphisms are not linked to severe liver disease in a French cohort of alpha‐1 antitrypsin deficiency children

“…In that way, sortilin (and other human homologs of the Vps10 protein) would be a very good candidate since this endosomal protein‐sorting receptor plays an established role in trafficking specific cargo proteins to the lysosome from the Golgi and the plasma membrane . The HERPUD1 R50H and HFE H63D mutations are also to be tested for their association with severe liver disease since they have been recently pointed out in our recent WES study focusing on five sibling couples homozygous for the Z mutation but with very different liver involvement …”

“…11 The HERPUD1 R50H and HFE H63D mutations are also to be tested for their association with severe liver disease since they have been recently pointed out in our recent WES study focusing on five sibling couples homozygous for the Z mutation but with very different liver involvement. 12…”