Genetic Variants of α 1-Antitrypsin

Parveen      Salahuddin

Abstract

α1-antitrypsin (α1-AT) is a 52 kDa sialoglycoprotein. The function of α1-antitrypsin is to protect the lower respiratory tract of lungs from proteolytic degradation by neutrophil elastase. Severe genetic deficiency of α1-AT is associated with early onset emphysema and liver diseases. α1-AT also exhibits anti-inflammatory activities independent of its protease inhibitor function. There are over 90 genetic variants of human α1-antitrypsin. These variants occur due to amino acid substitution / deletion which results in charge differences. Based on charge differences these variants have been identified by isoelectric focusing. The two most common deficiency variants are S and Z. The S variant migrates anodal to Z variant. The Z variant migrates most cathodal in isoelectric focusing, hence named Z. In Z variant, the β-sheet A undergoes expansion , therefore it can easily accept the reactive site loop of a second α1-AT molecule and consequently form polymers of α1-AT. These polymers of α1-AT aggregate in the hepatocytes and show liver and lungs diseases. Contrary to this, the S variant of α1-AT is not associated with any significant clinical disease because the conformation of the inhibitor is not altered significantly. The Z related pathologies could be treated by liver transplantation, augmentation therapy, gene therapy, peptide therapy and chemical chaperone therapy. In addition to common deficiency variants, there are several rare deficiency variants of α1-AT like Siiyama , Mmalton , Mprocida, Mheerlen, Mmineral springs, Mnichinan, Pduarte, Wbethesda Zaugsberg, and Zbristol. In Siiyama , Mmalton , Mnichinan and Zaugsberg, the β-sheet A is present in an open state, therefore these variants readily undergo polymerization and consequently show aggregation in the hepatocytes. In Mprocida, Mheerlen, Mmineral springs, Pduarte and Wbethesda the conformation is altered significantly, therefore these variants become conformationally less stable and thereby undergo intracellular proteolysis. These rare genetic variants show lungs and / or liver disease. There are several null variants of α1-AT that are not detected either at the stage of transcription or translation. The examples of some of the null variants are QOcardiff, QOhong kong, QOgranite falls, QObellingham, QOmattawa, QObolton, and QOludwigshafen. The molecular basis of deficiency of these variants also form the theme of this review.

Keywords: α1-antitrypsin, aggregation, emphysema, genetic variants of α1-antitrypsin, intracellular proteolysis, liver diseases, serpins superfamily