Alpha-1 antitrypsin (AAT) deficiency is a clinically under-recognized genetic disorder that causes the defective production of alpha-1 antitrypsin protein. AAT protein protects the body from the neutrophil elastase enzyme which is released from white blood cells to fight infection. This inherited disorder leads to decreased AAT activity in the blood and lung and deposition of excessive abnormal AAT protein in the liver.
Both deficiency and an abnormal form of AAT are due to mutations in the SERPINA1 gene. Without enough functional AAT, neutrophil elastase destroys alveoli and causes lung disease. Abnormal AAT can also accumulate in the liver and cause damage to this organ. People with AAT deficiency usually develop the first signs and symptoms of lung disease between ages 20 and 50. It is well-documented that the rate of decline in lung function is strongly dependent on cigarette smoking.
AAT deficiency is inherited by the autosomal co-dominant transmission which means that affected individuals have inherited an abnormal AAT gene from each parent. The gene that encodes AAT is called SERPINA1 and is located on the long arm of chromosome 14.
At least 150 alleles of AAT (SERPINA1) have been identified, and each has a letter code based upon electrophoretic mobility of the protein produced. The normal allele is referred to as “M,” and it is the most common version (allele) of the SERPINA1 gene. Most people in the general population have two copies of the M allele (MM) in each cell. Other versions of the SERPINA1 gene lead to reduced levels of alpha-1 antitrypsin. For instance, the S allele produces moderately low levels of this protein, and the Z allele produces very little AAT. Individuals with two copies of the Z allele (ZZ) in each cell are likely to have AAT deficiency. Those with the SZ combination have a higher risk of developing lung diseases (such as emphysema), particularly if they smoke.
Worldwide, it is estimated that 161 million people have one copy of the S or Z allele and one copy of the M allele in each cell (MS or MZ). Individuals with an MS (or SS) combination usually produce enough alpha-1 antitrypsin to protect their lungs. People with MZ alleles have a slightly increased risk of impaired lung or liver function.
AAT phenotypes are based on the electrophoretic mobility of the proteins produced by the various abnormal AAT alleles. Genotyping is performed by identifying specific alleles in DNA.
Based on this, variants of AAT can be categorized into four basic groups:
Environmental factors, such as exposure to tobacco smoke, chemicals, and dust, likely impact the severity of alpha-1 antitrypsin deficiency.
Severe deficiency of AAT has a strong risk factor for early-onset emphysema, but not every severely deficient individual would develop emphysema. Risk factors for emphysema include cigarette smoking, dusty occupational exposure, a parental history of chronic-obstructive pulmonary disease (COPD), and a personal history of asthma, chronic bronchitis, or pneumonia.
AAT deficiency occurs worldwide, but its prevalence varies by population. This disorder affects about one in 1500 to 3500 individuals with European ancestry. It is uncommon in people of Asian descent.
Although AAT deficiency is considered to be rare, estimates that 80,000 to 100,000 individuals in the United States have a severe deficiency of AAT suggest that the disease is under-recognized. It is estimated that more than three million people worldwide have allele combinations associated with severe deficiency of AAT.
Emphysema in AAT deficiency is considered due to an imbalance between neutrophil elastase in the lung, which destroys elastin, and the elastase inhibitor AAT, which protects against proteolytic degradation of elastin. This mechanism is called a “toxic loss of function.” Specifically, cigarette smoking and infection increase elastase production in the lung, therefore increasing lung degradation. Also, the polymers of “Z” antitrypsin are chemotactic for neutrophils, which may contribute to local inflammation and tissue destruction in the lung.
The pathogenesis of the liver disease is quite different and is called a “toxic gain of function.” The liver disease results from the accumulation within the hepatocyte of un-secreted variant AAT protein. Only those genotypes associated with pathologic polymerization of AAT within the endoplasmic reticulum of hepatocytes produce disease. Most patients are homozygous for the Z allele (i.e., PI*ZZ); liver disease does not occur in null homozygotes who have severe AAT deficiency but no intra-hepatocytic accumulation.
The main clinical manifestation of AAT deficiency is related to the involvement of three separate organs: the lung, the liver, and rarely, the skin.
Clinical presentation of lung diseases namely of emphysema due to AAT deficiency has many features in common with usual COPD. Dyspnea is the most common presenting symptom, and many patients have a cough, sputum production, and wheezing, either chronically or with upper respiratory tract infections. Spontaneous secondary pneumothorax may be the presenting manifestation of AAT deficiency or a complication of the known disease. Bronchiectasis has also been associated with severe deficiency of AAT.
Clinical presentation of extrapulmonary disease in patients with at-risk alleles (e.g., Z, S[iiyama], and M[malton]) may develop adult-onset chronic hepatitis, cirrhosis, or hepatocellular carcinoma.
Other extrapulmonary manifestations of AAT deficiency include necrotizing panniculitis, hot, painful, erythematous nodules or plaques on the thigh or buttocks that are the major dermatologic manifestation of AAT deficiency. Others are systemic vasculitis, psoriasis, urticaria, angioedema and possibly inflammatory bowel disease, intracranial and intra-abdominal aneurysms, fibromuscular dysplasia, and glomerulonephritis.
Imaging: chest x-ray is used to determine the pattern and extent of emphysema and exclude other causes of dyspnea. The “classic” pattern of emphysema in AAT deficiency is basilar predominant emphysematous bullae although a range of patterns from basilar predominant to apical predominant emphysema may be seen. Some clinicians perform chest computed tomography (CT) scans for initial assessment.
Additional features that should lead clinicians to test for AAT deficiency include:
The diagnosis of severe AAT deficiency is confirmed by demonstrating a serum level below 11 micromols/L (approximately 57 mg/dL by nephelometry) in combination with a severe deficient phenotype assessed by testing for the most common deficient alleles (i.e., S, Z, I, F).
If the AAT serum level is greater than 20 micromol/L (80 mg/dL), it is unlikely that the patient has clinically significant AAT deficiency, but if we are evaluating for the presence of particular mutations, genotyping is necessary to identify heterozygotes and mutations that have incomplete penetrance.
The normal plasma concentration of AAT ranges from 80 mg/dL to 220 mg/dL (20 to 48 micromol/L using nephelometry or 150 mg/dL to 350 mg/dL by radial immunodiffusion). However, given the variability in reference ranges, patients with a serum AAT level below 100 mg/dL (18.4 micromols/L) should be evaluated further with isoelectric focusing or genotyping.
Isoelectric focusing is the gold standard blood test for identifying AAT variants and is considered a phenotype test.
Genotyping of the protease inhibitor (Pi) locus is performed on a blood sample using polymerase chain reaction (PCR) technology or restriction fragment length polymorphisms. These tests detect the most common known variants (F, I, S, Z). Gene sequencing of exonic DNA can be used if both tests fail to determine the genetic variant.
In monitoring asymptomatic patients, i.e., those with no respiratory symptoms and a normal baseline spirometry (i.e., FEV1 80% or greater of predicted), a spirometry should be repeated when symptoms change or at 6 to 12-month intervals. An unexplained decrease in the post-bronchodilator FEV1 to less than 805 predicted is an indication to initiate augmentation therapy.
Guidelines are lacking regarding monitoring for liver disease in patients homozygous for PiZ, PiS[iiyama] or PiM[malton]. It is advised to assess serum aminotransferases, alkaline phosphatase, and bilirubin on an annual basis. Some clinicians also obtain a complete blood count (CBC), looking for thrombocytopenia, and an abdominal ultrasound looking for cirrhosis every 6 to 12 months.
Intravenous augmentation via the infusion of pooled human alpha-1 antitrypsin which is alpha-1 proteinase inhibitor. This therapy is the most direct and efficient means of elevating AAT levels in the plasma and the lung interstitium.
The American Thoracic Society suggests weekly augmentation therapy with human pooled AAT for individuals who have plasma levels of AAT less than 11 micromols/L and established airflow obstruction, defined as a FEV1 less than 80% predicted. On the other hand, the Canadian Thoracic Society suggests keeping AAT augmentation therapy for AAT-deficient patients (AAT level less than 11 micromols/L) with a FEV1 of 25% to 80% predicted who have quit smoking and are on optimal medical therapy.
The selection criteria for augmentation therapy include:
Augmentation therapy is not recommended for patients with heterozygous phenotypes whose plasma AAT level exceeds 11 micromols/L.
Side effects associated with IV AAT infusion are uncommon, and no long-term reactions have been noted. Some side effects can occur including: