Alpha 1-antitrypsin Deficiency - Diagnosis

Diagnosis

A1AT deficiency remains undiagnosed in many patients. Patients are usually labelled as having COPD without an underlying cause. It is estimated that about 1% of all COPD patients actually have A1AT deficiency. Thus, testing should be performed for all patients with COPD, asthma with irreversible air-flow obstruction, unexplained liver disease, or necrotizing panniculitis. The initial test performed is serum A1AT level. A low level of A1AT confirms the diagnosis and further assessment with A1AT protein phenotyping and A1AT genotyping should be carried out subsequently.

As protein electrophoresis does not completely distinguish between A1AT and other minor proteins at the alpha-1 position (agarose gel), antitrypsin can be more directly and specifically measured using a nephelometric or immunoturbidimetric method. Thus, protein electrophoresis is useful for screening and identifying individuals likely to have a deficiency. A1AT is further analysed by isoelectric focusing (IEF) in the pH range 4.5-5.5, where the protein migrates in a gel according to its isoelectric point or charge in a pH gradient. Normal A1AT is termed M, as it is migrates toward the center of such an IEF gel. Other variants are less functional, and are termed A-L and N-Z, dependent on whether they run proximal or distal to the M band. The presence of deviant bands on IEF can signify the presence of alpha 1-antitrypsin deficiency. Since the number of identified mutations has exceeded the number of letters in the alphabet, subscripts have been added to most recent discoveries in this area, as in the Pittsburgh mutation described above. As every person has two copies of the A1AT gene, a heterozygote with two different copies of the gene may have two different bands showing on electrofocusing, although a heterozygote with one null mutant that abolishes expression of the gene will only show one band. In blood test results, the IEF results are notated as, e.g., PiMM, where Pi stands for protease inhibitor and "MM" is the banding pattern of that person.

Other detection methods include use of enzyme-linked-immuno-sorbent-assays in vitro and radial immunodiffusion. Alpha 1-antitrypsin levels in the blood depend on the genotype. Some mutant forms fail to fold properly and are, thus, targeted for destruction in the proteasome, whereas others have a tendency to polymerise, thereafter being retained in the endoplasmic reticulum. The serum levels of some of the common genotypes are:

• PiMM: 100% (normal)
• PiMS: 80% of normal serum level of A1AT
• PiSS: 60% of normal serum level of A1AT
• PiMZ: 60% of normal serum level of A1AT
• PiSZ: 40% of normal serum level of A1AT
• PiZZ: 10-15% (severe alpha 1-antitrypsin deficiency)

PiZ is caused by a glutamate to lysine mutation at position 342, while PiS is caused by a glutamate to valine mutation at position 264. Other rarer forms have been described; in all there are over 80 variants.