Alpha‐1 antitrypsin (AAT) deficiency (AATD) is an inherited disease caused by mutations in the serpin family A member 1 ( SERPINA1 , also known as AAT ) gene. The most common variant, PI*Z (Glu342Lys), causes accumulation of aberrantly folded AAT in the endoplasmic reticulum (ER) of hepatocytes that is associated with a toxic gain of function, hepatocellular injury, liver fibrosis, and hepatocellular carcinoma. The unfolded protein response (UPR) is a cellular response to improperly folded proteins meant to alleviate ER stress. It has been unclear whether PI*Z AAT elicits liver cell UPR, due in part to limitations of current cellular and animal models. This study investigates whether UPR is activated in a novel human PI*Z AAT cell line and a new PI*Z human AAT (hAAT) mouse model. A PI*Z AAT hepatocyte cell line (Huh7.5Z) was established using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 gene editing of the normal ATT (PI*MM) gene in the Huh7.5 cell line. Additionally, novel full‐length genomic DNA PI*Z hAAT and PI*M hAAT transgenic mouse models were established. Using these new models, UPR in Huh7.5Z cells and PI*Z mice were comprehensively determined. Robust activation of UPR was observed in Huh7.5Z cells compared to Huh7.5 cells. Activated caspase cascade and apoptosis markers, increased chaperones, and autophagy markers were also detected in Z hepatocytes. Selective attenuation of UPR signaling branches was observed in PI*Z hAAT mice in which the protein kinase R‐like ER kinase and inositol‐requiring enzyme1α branches were suppressed while the activating transcription factor 6α branch remained active. This study provides direct evidence that PI*Z AAT triggers canonical UPR and that hepatocytes survive pro‐apoptotic UPR by selective suppression of UPR branches. Our data improve understanding of underlying pathological molecular mechanisms of PI*Z AATD liver disease.

We agree that the mechanisms of up-regulation of C/ EBP homologous protein (CHOP) in this model remain unclear. [1,2] Interestingly, we saw highly variable CHOP expression even in litter mates. The up-regulation of CHOP is not related to mouse age and activating transcription factor 6α (ATF6α) protein level in this model.We are aware of the age-related changes in the number of Z alpha-1 antitrypsin globules found in the liver of murine models of alpha-1 antitrypsin deficiency, and as such, we studied mice only between the ages of 6 and 12 months. In our murine models, we have noted a decrease in Z alpha-1 antitrypsin globules while the alpha-1 antitrypsin levels in plasma remain constant as the mice age. This observation leads us to believe that decreases in globules are caused by cellular adaptation to Z alpha-1 antitrypsin disposal rather than downregulation of alpha-1 antitrypsin gene expression. This hypothesis needs to be explored further.We appreciate Professors Piccolo and Brunetti-Pierri's recognition that M control mice open new avenues of study, and we are submitting both M and Z mice to the Jackson Laboratory so they will be available to other researchers.