Alpha 1-antitrypsin deficiency
|Alpha 1-antitrypsin deficiency|
|Classification and external resources|
Structure of Alpha 1-antitrypsin
Alpha 1-antitrypsin deficiency (α1-antitrypsin deficiency, A1AD) is a genetic disorder that causes defective production of alpha 1-antitrypsin (A1AT), leading to decreased A1AT activity in the blood and lungs, and deposition of excessive abnormal A1AT protein in liver cells. There are several forms and degrees of deficiency, principally depending on whether the sufferer has one or two copies of the affected gene because it is a co-dominant trait. Severe A1AT deficiency causes panacinar emphysema or COPD in adult life in many people with the condition (especially if they are exposed to cigarette smoke), as well as various liver diseases in a minority of children and adults, and occasionally more unusual problems. It is treated by avoidance of damaging inhalants, and in severe cases by intravenous infusions of the A1AT protein, by transplantation of the liver or lungs. It usually produces some degree of disability and reduced life expectancy.
Signs and symptoms
Symptoms of alpha-1 antitrypsin deficiency include shortness of breath, wheezing, rhonchi, and rales. The patient's symptoms may resemble recurrent respiratory infections or asthma that does not respond to treatment. Individuals with A1AD may develop emphysema during their thirties or forties even without a history of significant smoking, though smoking greatly increases the risk for emphysema. A1AD also causes impaired liver function in some patients and may lead to cirrhosis and liver failure (15%). It is a leading indication for liver transplantation in newborns.
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α1-antitrypsin deficiency has been associated with a number of diseases:
- Wegener's granulomatosis
- Pelvic organ prolapse
- Primary sclerosing cholangitis
- Autoimmune hepatitis
- Emphysema, predominantly involving the lower lobes and causing bullae
- Secondary Membranoproliferative Glomerulonephritis
Alpha 1-antitrypsin (A1AT) is produced in the liver, and one of its functions is to protect the lungs from neutrophil elastase, an enzyme that can disrupt connective tissue. Normal blood levels of alpha-1 antitrypsin may vary with analytical method, but are typically around 1.0-2.7 g/l. In individuals with PiSS, PiMZ and PiSZ phenotypes, blood levels of A1AT are reduced to between 40 and 60% of normal levels. This is usually sufficient to protect the lungs from the effects of elastase in people who do not smoke. However, in individuals with the PiZZ phenotype, A1AT levels are less than 15% of normal, and patients are likely to develop panacinar emphysema at a young age; 50% of these patients will develop liver cirrhosis, because the A1AT is not secreted properly and instead accumulates in the liver. A liver biopsy in such cases will reveal PAS-positive, diastase-resistant granules. Unlike glycogen and other mucins which are diastase sensitive (i.e., diastase treatment disables PAS staining), A1AT deficient hepatocytes will stain with PAS even after diastase treatment - a state thus referred to as diastase resistant.
Cigarette smoke is especially harmful to individuals with A1AD. In addition to increasing the inflammatory reaction in the airways, cigarette smoke directly inactivates alpha 1-antitrypsin by oxidizing essential methionine residues to sulfoxide forms, decreasing the enzyme activity by a factor of 2000.
Serpin peptidase inhibitor, clade A, member 1 (SERPINA1) is a gene that translates the protein Alpha 1-antitrypsin. SERPINA 1 has been localized to chromosome 14q32. Over 75 mutations of the SERPINA1 gene have been identified but clinically significant effects for Alpha 1-antitrypsin deficiency have only been attributed to a missense mutation in exon 5. The single base-pair substitution produces the allele "Z" which causes alpha 1-antitrypsin deficiency. The non-mutated SERPINA1 gene produces the “M” allele which produces normal levels of alpha 1-antitrypsin. Genetic diagnosis of Alpha 1-antitrypsin deficiency is performed through Polymerase Chain Reaction (PCR) of the localized mutated region of SERPINA1 and allele-specific oligonucleotides (ASO probes). A hybridized ASO probe containing the mutated sequence for the “Z” allele and an ASO probe bearing the normal DNA sequence allow direct determination of the individual's genotype, the identification of the “Z” allele in the individual and the genetic diagnosis of Alpha 1-antitrypsin deficiency.
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.
In the United States, Canada, and several European countries, lung-affected A1AD patients may receive intravenous infusions of alpha-1 antitrypsin, derived from donated human plasma. This augmentation therapy is thought to arrest the course of the disease and halt any further damage to the lungs. Long-term studies of the effectiveness of A1AT replacement therapy are not available. It is currently recommended that patients begin augmentation therapy only after the onset of emphysema symptoms.
Augmentation therapy is not appropriate for liver-affected patients; treatment of A1AD-related liver damage focuses on alleviating the symptoms of the disease. In severe cases, liver transplantation may be necessary.
A1AD was discovered in 1963 by Carl-Bertil Laurell (1919–2001), at the University of Lund in Sweden. Laurell, along with a medical resident, Sten Eriksson, made the discovery after noting the absence of the α1 band on protein electrophoresis in five of 1500 samples; three of the five patient samples were found to have developed emphysema at a young age.
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