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The most common genotype associated with severe α-1-antitrypsin deficiency (AATD) is the Z homozygote. The Z variant (Glu342Lys) of α-1-antitrypsin (AAT) undergoes a conformational change and is retained within the endoplasmic reticulum (ER) of hepatocytes leading to the formation of ordered polymeric chains and inclusion bodies. Accumulation of mutated protein predisposes to cirrhosis whilst plasma AAT deficiency leads to emphysema. Increased risk of liver and lung disease has also been reported in heterozygous subjects who carry Z in association with the milder S allele (Glu264Val) or even with wild-type M. However, it is unknown whether Z AAT can co-polymerize with other AAT variants in vivo. We co-expressed two AAT variants, each modified by a different tag, in cell models that replicate AAT deficiency. We used pull-down assays to investigate interactions between co-expressed variants and showed that Z AAT forms heteropolymers with S and with the rare Mmalton (Phe52del) and Mwurzburg (Pro369Ser) mutants, and to a lesser extent with the wild-type protein. Heteropolymers were recognized by the 2C1 mAb that binds to Z polymers in vivo. There was increased intracellular accumulation of AAT variants when co-expressed with Z AAT, suggesting a dominant negative effect of the Z allele. The molecular interactions between S and Z AAT were confirmed by confocal microscopy showing their colocalization within dilated ER cisternae and by positivity in Proximity Ligation Assays. These results provide the first evidence of intracellular co-polymerization of AAT mutants and contribute to understanding the risk of liver disease in SZ and MZ heterozygotes.

Haptoglobin (HP) groups were determined in Chinese patients suffering from eight different diseases and compared to those of 552 control subjects. Patients with cirrhosis of the liver showed a significant difference from controls in the distribution of HP types (p = 2 x 10(-4)), with increases in the HP*1 allele and HP1-1 phenotype frequencies. The relative risk for the HP1-1 phenotype was 3.3.

Severe lung disease and liver disease are not recognised features of the PiMZ phenotype, which is associated with alpha 1 antitrypsin deficiency. A 31 year old woman with this phenotype was found to have emphysema and complete heart block and showed evidence of hepatic cirrhosis, although her three sisters, all of whom had the same phenotype, were clinically normal. This case supports the possibility of a causal relation between the PiMZ phenotype and chronic lung and liver disease, but an association between alpha 1 antitrypsin deficiency and complete heart block could not be proved in this patient.