Chapter 516. Alpha-1 Antitrypsin Deficiency Lung Disease

Alpha1-antitrypsin (AAT) circulates in the plasma as a 52-kD glycoprotein. It is synthesized primarily in hepatocytes and to a lesser extent in macrophages and monocytes. AAT is produced at a basal level, which results in plasma concentrations of 11 micromole or greater. AAT is also an acute phase reactant, with levels increasing dramatically during periods of stress, fever, or infection.1-6 Some individuals with AAT deficiency manifest with neonatal liver disease, as discussed in Chapter 421. This chapter discusses the pulmonary manifestations of AAT deficiency.

Epidemiology and Genetics

Approximately 4% of northern Europeans and North Americans of European descent possess at least one mutant AAT allele. This predicts that 1 in 2500 live births in this population will possess two mutant alleles, either being homozygous mutant alleles or compound heterozygous mutant alleles. Up to 100,000 Americans could be AAT deficient, but only approximately 6000 Americans have been diagnosed as such. This discrepancy may represent the incomplete penetrance of the disorder or a failure to diagnose affected individuals. There is evidence that a combination of both of these explanations may be true.

Several studies have tested the utility of targeted detection of AAT deficiency within populations of adult chronic lung disease patients. These studies have consistently shown that 3% to 4% of such populations are AAT deficient. Given that approximately 11 million adults are symptomatic with chronic obstructive pulmonary disease, this targeted detection could yield a total of 30,000 to 40,000 American AAT-deficient patients. Thus, the best estimate is that there is an as-yet-undiagnosed population of 25,000 to 35,000 symptomatic AAT-deficient individuals. There may then be another 60,000 or more asymptomatic “healthy” AAT-deficient individuals.7,8

As mentioned above, mutations of the AAT gene are quite common in individuals of northern European ancestry. In particular, the one missense mutant genotype (Glu 342→Lys) that results in the PiZ phenotype on isoelectric focusing (IEF) gel electrophoresis accounts for 95% of AAT mutants in this population. Several other AAT alleles have been associated with deficiency states (eTable 516.1). These include numerous missense and null alleles. Environmental factors may play a role in determining which AAT-deficient individuals develop clinically evident deficiency disease. Tobacco smoke exposure (either directly from smoking or from environmental exposure) is a key risk factor for the development and severity of lung disease in AAT-deficient individuals.9 Infections have also been implicated, including both viral and bacterial infections. A number of studies have attempted to identify gene modifiers, which might significantly affect the disease phenotype in AAT-deficient individuals. Only recently has a study identified IL-10 as a potential modifier gene for the development of chronic obstructive pulmonary disease (COPD) in individuals who are PiZ homozygotes. The study suggests an association with patients carrying the high IL-10-producing allele having higher lung function; conversely, the lower IL-10-producing allele is strongly associated with lower lung function.10,11 To date, no other modifiers have yet been confirmed.

Pathophysiology

Alpha1-antitrypsin (AAT), alternatively known as alpha1-proteinase inhibitor or as the GenBank designation SERPINA1, is a member of the serine proteinase inhibitor (serpin) class of proteins, which have evolved over time to serve a variety of crucial roles in the maintenance of homeostasis, particularly in the context of physiological cascades such as those mediating inflammation, immunity, thrombosis, and thrombolysis. AAT itself is a serpin with multiple substrates. Its functionality in the inhibition of trypsin is the basis for its common name. Abundant evidence supports a key role in inhibition of neutrophil elastase (NE). Other neutrophil products, including proteinase 3, cathepsins, and alpha-defensins are prominent among the substrates for AAT activity.1 The various normal inhibitory capacities possessed by AAT all include regulating the inflammatory response and limiting the downstream consequences of that response. Specifically, AAT appears to limit the action of neutrophil-derived proteases to the local environment of neutrophil accumulation, such as at a localized nidus of infection or foreign body reaction. This is particularly crucial within the lung parenchyma, where neutrophil accumulation within the pulmonary capillaries lie in close juxtaposition with interstitial elastin fibers.

Many neutrophil-derived products appear to contribute to the pathology seen in the airways and alveoli of AAT-deficient patients. These include neutrophil elastase, cathepsin G, proteinase 3, human neutrophil, and peptide (alpha defensins). These factors, unopposed by AAT, contribute to a proinflammatory state in the airways of AAT-deficient patients and to destruction of the extracellular matrix both in the pulmonary parenchyma and in the airways. There is also evidence that such factors are injurious to the cellular components of the lung, including airway and alveolar epithelial cells and endothelial cells.

Recent evidence has also pointed to an important antiapoptotic role of AAT in the lungs. AAT has the ability to enter alveolar capillary endothelial cells and inhibit caspase-3, a key element in the final common pathway to endothelial cell apoptosis. One pharmacologically induced mouse model of emphysema was produced by an inhibitor of the vascular endothelial growth factor receptor, which resulted in activation of caspase-3-mediated cell death in the endothelial cells of the alveoli.12-21 AAT, when delivered by protein replacement or gene therapy, was able to prevent this apoptosis-induced emphysema phenotype. This suggests that AAT’s cytoprotective role may stem from inactivation of extracellular factors and from its ability to inhibit intracellular caspase-3-mediated apoptosis.

The molecular mechanism by which AAT inhibits NE has been well described. In brief, the metastable native conformation of AAT has a reactive center methionine residue on an exposed loop of the molecule; the scissile bond of the reactive site loop is the ideal substrate for neutrophil elastase. The interaction of one AAT molecule with one molecule of NE begins when AAT binds to NE. The attached NE proceeds with hydrolytic cleavage of the reactive site loop of AAT. This cleavage results in a conformational change in AAT, which then binds irreversibly to NE, resulting in a translocation of NE from one pole of AAT to the opposite side; this causes the protease to be crushed and inactivated much like a spring-loaded mousetrap.5

The molecular consequences of the single nucleotide change that comprises the PI*Z genotype have now been well delineated. The substitution of a lysine residue in place of glutamate in the A sheet of the alpha1-antitrypsin (AAT) molecule disrupts a key salt bridge. This allows for insertion of the reactive loop of a neighboring AAT molecule into this beta-sheet structure. A series of such “loop-sheet” interactions can result in long Z-polymers, the crystal structure of which have been solved (eFig. 516.1).

The previously described alteration of the secondary structure in the A sheet of AAT may influence Z-AAT’s function in two respects. First, the mutation may favor the misfolded conformation of Z-AAT, resulting in secretion impairment and degradation enhancement of Z-AAT. Second, the formation of loop-sheet polymers may further stabilize the misfolded conformation, inhibit secretion, and enhance accumulation of Z-AAT within hepatocytes.12

In general, it is thought that impaired secretion of Z-AAT leads to serum deficiency, which in turn is primarily responsible for AAT-deficient lung disease, while excessive retention and accumulation of Z-AAT within hepatocytes triggers local events in susceptible individuals that ultimately result in hepatocellular injury, hepatitis, and clinically evident liver disease.13

A significant amount of Z-AAT successfully exits the endoplasmic reticulum and moves through the secretory pathway to the extracellular space. However, the Z mutation also renders the protein partially functional. Specific activity estimates are consistently below 50% (Fig. 516-1). Hundreds of other AAT mutations have been discovered. These mutations vary with respect to how much they affect proper trafficking of AAT, proper secretion, and the antielastase specific activity. One particular mutation, PiS, is often associated with a variable lung disease phenotype in the compound heterozygous state (PI*SZ).

Clinical Features

The majority of patients with alpha1-antitrypsin (AAT) lung disease present in adulthood. The average age of diagnosis is 52 years old. By the time of diagnosis, most patients have had pulmonary symptoms for at least 10 years. Pulmonary symptoms often include many elements in common with asthma (including bronchodilator responsive airway obstruction), emphysema, and chronic bronchitis. Patients often experience exacerbations of cough, tachypnea, and respiratory distress, which are often triggered by intercurrent illnesses, including viral respiratory tract infections.22 The radiographic appearance of the chest is quite variable but typically includes variable zones of chronic hyperinflation to a more advanced degree in the lower lobes, along with an increase in the AP diameter of the chest wall. As noted above, some individuals have coexisting lung and liver disease at the time of initial diagnosis. One or the other of these manifestations may have been underappreciated or undiagnosed prior to the definitive diagnosis of AAT deficiency. Other extrapulmonary manifestations may be seen in AAT-deficient patients, including panniculitis, which is a rare but characteristic finding.

Diagnosis

The key features for diagnosing AAT deficiency include a total plasma level less than 11 micromole (approximately 570 micrograms per mL, or 57 mg/dL) and an isoelectric focusing (IEF) gel phenotype indicating either the presence of an abnormal migration pattern or the absence of the normal M band, or both. In recent years, genotyping technology has simplified and enhanced the ability to diagnose patients rapidly and accurately in many cases. By convention, the phenotype as determined by IEF is designated as the Pi type, with the letters indicating all visible bands. Thus, a Z homozygote is PiZ, a normal homozygote is PiM, and a heterozygous individual is PiMZ. The corresponding genotypes would be indicated as PI*ZZ, PI*MM, and PI*MZ, respectively. The serum levels of AAT associated with various Pi phenotypes are shown in Figure 516-1. The lung pathology in AAT deficiency is classically described as panacinar emphysema, which is greater in the bases of the lung than in the apices. This type of emphysema includes loss of the entire alveolar septae and an increase in alveolar diameter. Inflammatory changes, particularly in the airways, are also typical.

Treatment

Patients with alpha1-antitrypsin (AAT)-deficient lung disease should be cared for by a pulmonary subspecialist with a multidisciplinary team in place. Anticipatory guidance should include the avoidance of tobacco smoke exposure, which can substantially alter the outcome for these individuals. Other environmental elements such as wood-burning stoves and occupational exposure to smoke or other noxious inhaled substances should also be avoided. In addition, AAT-deficient patients should receive the pneumococcal polysaccharide vaccine and yearly influenza immunizations. Careful attention to nutrition is also important in these patients, as is early detection of hypoxemia, including nocturnal or exercise-related hypoxemia. The appropriate use of oxygen supplementation in this context is essential (eTable 516.2).

Several plasma-derived and recombinant AAT protein-replacement therapies are now approved by the US Food and Drug Administration for replacement of plasma levels of AAT in deficient patients. Each of these available products has been designed for weekly IV infusion. Numerous studies have indicated that these products are very safe and are effective for restoring plasma levels above the threshold value of 11 micromole, which is generally accepted as being critical to avoid progression of lung disease23 (Fig. 516-1). Despite that, it has been difficult to show convincing evidence of therapeutic effect on the lung disease symptomatology in prospective, randomized clinical trials of IV AAT replacement. While the reasons for this are not entirely clear, the episodic nature of pulmonary symptoms, the variability of pulmonary function measurements within individual patients, and the very slow rate of decline in measurements of pulmonary function have made it difficult to design a study with sufficient statistical power to demonstrate a difference in PFT variables. Nonetheless, retrospective registry data have demonstrated a trend toward significant benefits from AAT replacement, including a substantial decrease in mortality.

While there would be a theoretical advantage to localized AAT replacement by aerosol delivery of the protein replacement, this modality of therapy remains investigational. The primary, well-established end point for IV replacement is restoration of plasma levels. This end point is not as relevant for aerosol protein replacement, since relatively little of the protein is absorbed. While several inflammatory markers have been shown to be decreased in bronchoalveolar lavage fluid samples after aerosol protein replacement, there is not a clear indication of when sufficient replacement would be achieved. As with IV replacement, measurement of clinical pulmonary outcomes, such as PFTs, have not shown consistent benefits.

Patients with severe emphysema, including those with AAT deficiency, may benefit from partial resection of the lung in selected circumstances. This intervention appears to provide some benefit by reversing the severe overexpansion of the chest wall with the corresponding mechanical disadvantage for the diaphragm and other muscles of respiration. Other theoretical benefits have also been hypothesized, including the stimulation of repair processes by lung progenitor cells.

Patients who progress to respiratory failure due to AAT-deficient lung disease may be candidates for bilateral lung transplantation. It is important to note that unlike lung transplant recipients with cystic fibrosis, these patients remain fully insufficient for AAT and require continued IV protein replacement.

Novel approaches to treatment or prevention of AAT lung disease include both small molecule pharmaceutics and gene-augmentation strategies, none of which has yet been proven to be safe and effective. In the former category are approaches such as the chemical chaperone strategy mentioned above, which could be therapeutic for lung disease if it resulted in a net increase in secretion of endogenous AAT from hepatocytes into the circulation. This class of approaches also includes several small molecule protease inhibitors, such as specific inhibitors of neutrophil elastase. In the latter category, there have been clinical trials of cationic liposome mediated AAT gene delivery and two different serotypes of recombinant adeno-associated virus (rAAV) vectors, the latter one of which has not yet been completed.24,25 To date, plasma levels from these forms of gene augmentation have not approached the therapeutic threshold of 11 micromole. However, the favorable safety profile of these vector types, as compared with some others, make it likely that additional trials with higher doses and more efficient delivery systems may be undertaken. Other vector classes, including helper-dependent adenovirus vectors and novel rAAV serotypes, may also move forward to clinical trials, as their efficiency of expression appears to be superior to earlier version vectors, as judged by preclinical proof-of-concept data.