Alpha-1-antitrypsin (AT) deficiency and the serpinopathies: pathobiology and new therapeutic strategies

Antitrypsin is found at high concentrations in the bloodstream, where its main role is to protect the lungs against tissue damage from inflammation. Liver cells (hepatocytes) normally release individual molecules of antitrypsin into the circulation. Antitrypsin deficiency results when an individual inherits two genes with small changes in the antitrypsin protein. The Z variant, found in about 4% of people of North European decent, is the most common cause of severe antitrypsin deficiency. The Z variant causes antitrypsin to form long chains of linked molecules (called "polymers") that are trapped inside liver cells. The build-up of polymers damages the cell and increases the chance of developing liver cirrhosis and liver cancer. The reduced amount of properly formed protein in the circulation means the lungs are not as well protected against inflammation and so individuals develop emphysema. We have shown that a similar process occurs in mutants of other members of the protein family to which antitrypsin belongs. These include polymerisation of mutants of neuroserpin in the brain to cause dementia. We have grouped all the conditions together as a single class of disease that we have called 'the serpinopathies'. The application builds on 25 years of work by our group and has 5 interlinking projects within a programme of work. We propose to: (i) define the structure of the pathological polymer using biophysical analysis and cryo-electron microscopy. We will define the structure of the pathological polymer isolated from human tissues and use this information to develop strategies to block the abnormal protein-protein linkage that underlies antitrypsin deficiency and the serpinopathies. (ii) establish the cellular response to intracellular serpin polymers. We will use the imaging of cells that express polymers to define the effect of polymer formation on cell function. We will also establish whether cell dysfunction can be reversed by small molecules that block polymer formation and so determine whether it is the polymers themselves that cause cell dysfunction and disease. (iii) define disease mechanism in a multicellular context and identify new therapeutic targets using a worm (C. elegans) model of antitrypsin deficiency. We have developed a novel worm (C. elegans) model in which Z antitrypsin is retained as polymers in association with impaired motility, thin body structure and delayed development. We will assess the effect of small molecules and polymer blocking antibodies on intracellular polymerisation and the motility of the worm, dissect the mechanism by which intercellular antitrypsin polymers signal toxicity within the cell and undertake a chemical screen for novel agents that can reduce the toxicity of polymers in the worm. These will be used as the basis to design new therapies for use in man. (iv) define the dynamics of antitrypsin polymers in vivo and their utility as a biomarker of disease. We will study individuals with antitrypsin deficiency undergoing liver and lung transplantation to evaluate the changes in antitrypsin polymers within the lung and circulation. We will also evaluate circulating polymers as the first biomarker that is specific for predicting and diagnosising antitrypsin deficiency related liver disease. (v) develop a diagnostic technology for imaging Z antitrypsin polymers in vivo. We will use cell penetrating monoclonal antibodies or small molecules that are specific for polymers to image intracellular polymers in mouse models of disease. Our longer term aim is to develop this for use in man so we can assess whether there is a correlation between polymer burden and liver disease and if this new imaging test will be useful to accelerate drug development in man. Taken together this work will increase our understanding of mechanism of antitrypsin deficiency and the serpinopathies and allow the development of new approaches to treatment.