Liver injury in homozygous PIZZ alpha1-anti-trypsin (alpha1-AT) deficiency is due to the hepatotoxic effect of the abnormally folded alpha1-ATZ molecule that is retained in the endoplasmic reticulum (ER). However, only a subgroup of children with this deficiency develops liver disease. We have predicted that this subgroup is predisposed to liver injury by other genetic traits that affect the fate of mutant secretory protein within the ER. In fact, using a novel genetic complementation system in which the mutant alpha1-ATZ gene is expressed in cell lines from PIZZ individuals with liver disease (susceptible hosts) and those from PIZZ individuals without liver disease (protected hosts), we found that alpha1-ATZ, retained in the ER, is only degraded efficiently in the protected hosts. The lag in ER degradation in susceptible hosts results in greater net retention of the hepatotoxic alpha1-ATZ molecule within the ER. In recent studies, we have found that the fate of alpha1-ATZ within the ER is determined by multiple interactions with components of the quality control apparatus. The hypothesis of this project is that the fate of alpha1-ATZ within the ER misfolding, aggregation, degradation is a major determinant of susceptibility to liver disease in alpha-AT deficiency. It is influenced by biochemical conditions within the ER, by interaction with chaperones, by interaction with the ubiquitin-dependent proteasomal degradative system and by the cell morphological (e.g. formation of vesicular degradative compartments) and biochemical response to retention of unfolded proteins within the ER (e.g. of the ER membrane). In this application we propose the generation of cell lines and animal models in which alpha1-ATZ expression and ER retention can be induced experimentally over specific time intervals and in specific concentrations so that we can study the conditions which determine its fate in the ER and how the cell responds morphologically and biochemically to its accumulation in the ER. We will also use this experimental paradigm to determine whether the rules which apply to alpha1-ATZ are generalizable to other mutant proteins which are retained in the ER in inherited deficiency states.
