Hypersensitive mousetraps, α1-antitrypsin deficiency and dementia

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α1-Antitrypsin functions as a ‘mousetrap’ to inhibit its target proteinase, neutrophil elastase. The common severe Z deficiency variant (Glu342 → Lys) destabilizes the mousetrap to allow a sequential protein-protein interaction between the reactive-centre loop of one molecule and β-sheet A of another. These loop-sheet polymers accumulate within hepatocytes to form inclusion bodies that are associated with juvenile cirrhosis and hepatocellular carcinoma. The lack of circulating protein predisposes the Z α1-antitrypsin homozygote to emphysema. Loop-sheet polymerization is now recognized to underlie deficiency variants of other members of the serine proteinase inhibitor (serpin) superfamily, i.e. antithrombin, C1 esterase inhibitor and α1-anti-chymotrypsin, which are associated with thrombosis, angio-oedema and emphysema respectively. Moreover, we have shown recently that the same process in a neuron-specific protein, neuroserpin, underlies a novel inclusion-body dementia, known as familial encephalopathy with neuroserpin inclusion bodies. Our understanding of the structural basis of polymerization has allowed the development of strategies to prevent the aberrant protein-protein interaction in vitro. This must now be achieved in vivo if we are to treat the associated clinical syndromes.