Clinical meaning
Alpha-1 antitrypsin (AAT) deficiency is one of the most common hereditary conditions affecting the lungs and liver, with an estimated prevalence of 1 in 2,000-5,000 individuals of European descent. It is an autosomal codominant disorder caused by mutations in the SERPINA1 gene on chromosome 14, which encodes alpha-1 antitrypsin, a serine protease inhibitor (serpin) primarily produced by hepatocytes. AAT deficiency leads to two distinct organ pathologies through two different mechanisms: destructive lung disease from unopposed protease activity and liver disease from toxic accumulation of misfolded AAT protein within hepatocytes.
Alpha-1 antitrypsin is the most abundant circulating serine protease inhibitor in human plasma, with normal serum levels of 100-220 mg/dL. Its primary physiological function is to protect the lower respiratory tract from destruction by neutrophil elastase, a powerful serine protease released by activated neutrophils during their normal antimicrobial function. Neutrophil elastase is capable of degrading virtually all components of the extracellular matrix, including elastin (the structural protein that gives the lung its elastic recoil), collagen, and proteoglycans. Under normal circumstances, AAT rapidly binds to and inactivates neutrophil elastase through a suicide-substrate mechanism: AAT's reactive center loop mimics the natural substrate of neutrophil elastase, but when the protease cleaves the reactive loop, AAT undergoes a dramatic conformational change that traps the enzyme in an irreversible complex, permanently inactivating it.
The SERPINA1 gene has over 120 identified variants, designated by letters based on their isoelectric focusing migration pattern. The normal allele is designated Pi*M (protease inhibitor, medium migration), and the normal genotype is Pi*MM. The two most clinically significant deficiency alleles are Pi*Z (the most common severe deficiency allele, found in 1-3% of Northern Europeans) and Pi*S (a milder deficiency allele). The Pi*Z mutation (Glu342Lys) causes the AAT protein to misfold in the endoplasmic reticulum of hepatocytes. Approximately 85% of the Z-variant AAT polymerizes within the hepatocyte ER, forming insoluble aggregates that are retained in the liver rather than being secreted into the circulation. This has two consequences: first, the serum AAT level drops to only 10-15% of normal (typically 20-50 mg/dL), leaving the lungs unprotected from neutrophil elastase; second, the accumulated polymer aggregates in hepatocytes cause a toxic gain-of-function liver injury through ER stress, autophagy activation, and eventually hepatocyte death.