Alpha-1 antitrypsin deficiency: an update on clinical aspects of diagnosis and management

Clinical heterogeneity has been demonstrated in alpha-1 antitrypsin deficiency (AATD), such that clinical suspicion plays an important role in its diagnosis. The PiZZ genotype is the most common severe deficiency genotype and so tends to result in the worst clinical presentation, hence it has been the major focus of research. However, milder genotypes, especially PiSZ and PiMZ, are also linked to the development of lung and liver disease, mainly when unhealthy behaviors are present, such as smoking and alcohol use. Monitoring and managing AATD patients remains an area of active research. Lung function tests or computed tomography (CT) densitometry may allow physicians to identify progressive disease during follow up of patients, with a view to decision making about AATD-specific therapy, like augmentation therapy, or eventually surgical procedures such as lung volume reduction or transplant. Different types of biological markers have been suggested for disease monitoring and therapy selection, although most need further investigation. Intravenous augmentation therapy reduces the progression of emphysema in PiZZ patients and is available in many European countries, but its effect in milder deficiency is less certain. AATD has also been suggested to represent a risk factor and trigger for pulmonary infections, like those induced by mycobacteria. We summarize the last 5–10 years’ key findings in AATD diagnosis, assessment, and management, with a focus on milder deficiency variants.


Introduction
Alpha-1 antitrypsin deficiency (AATD) is an autosomal co-dominant disease, usually underdiagnosed owing to its variable penetrance and clinical heterogeneity. The alpha-1 antitrypsin (AAT) protein is encoded by the SERPINA1 gene on chromosome 14, and its main function is to inactivate neutrophil elastase (NE) upon insult to the lungs, such as smoking. In its absence, there is an imbalance of proteinases and anti-proteinases, which leads to the progression of emphysema and deterioration of lung function, resulting in chronic obstructive pulmonary disease (COPD). In some mutations, polymerization of AAT in alveolar macrophages and the presence of pro-inflammatory AAT polymers, previously reported to be obtained in bronchoalveolar lavage in PiZZ patients, contribute to the pathogenesis in AATD lungs 1 . This mini-review summarizes key findings in this disease's diagnosis, assessment, and management from the last 5-10 years.

Which patients develop clinically relevant disease?
A number of genetic mutations cause AATD. It has long been accepted that the Z allele, and in particular the PiZZ genotype, is linked to emphysema and early onset COPD 2 . There is also limited evidence that patients with null mutations have worse prognosis 3 .
In recent years, there has been growing interest in the relative risk conferred by genotypes causing milder deficiency, such as the S allele. The S protein forms fewer polymers than does the Z protein; therefore, it is retained less within hepatocytes and leads to less endoplasmic reticulum protein overload. Consequently, the S allele is only a minor risk factor or co-factor for cirrhosis in specific subpopulations such as chronic alcohol abusers. On the other hand, alcohol stimulates AAT production in hepatocytes, which may aggravate liver function in carriers of a single abnormal allele, in particular in carriers of the more pathogenic Z allele 4 . Circulating AAT is inversely proportional to the amount of liver polymerization/ retention of each type of AAT; Table 1 shows some of the milder deficiency genotypes, levels, and risks of disease.
Whilst their milder genetic profile when compared with PiZZ makes PiSZ, SS, and MZ patients less likely to develop adverse effects linked to AATD, such genotypes are much more prevalent than ZZ in the world 5-7 , and in the presence of unhealthy behaviors they become big risk groups for the development of lung disease. This enhances public health need to increase diagnosis and implement preventive measures in these patients 7,8 .

SZ genotype
More than 700,000 PiSZ patients have been reported in Europe 7 . The major clinical risk in PiSZ is the development of COPD, which is three times higher compared with PiMM 9 , less so in never-smoking patients 10 . When PiSZ patients develop emphysema, usually it has an apical dominance 5 ; physicians' cognitive bias to screen for AATD mainly in basal emphysema may exclude them from testing and follow-up, thus leading to a greater proportion of undiagnosed patients relative to PiZZ. Reversibility has also been observed in a large number of patients, which is frequently associated with more severe airflow obstruction 10 . Abnormalities in forced expiratory volume in 1 second (FEV1) are associated with basal-predominant emphysema, usually present in PiZZ, while abnormality in diffusing capacity of lung for carbon monoxide (DLCO) is associated with upper-zone emphysema 11,12 , which is often seen in PiSZ patients. Since these types of emphysema may be driven by different mechanisms 2 , we can speculate that the pathophysiology of emphysema differs between PiSZ and PiZZ genotypes such that therapy applicable to PiZZ cannot be assumed to be effective in PiSZ. Although disease progression in PiSZ patients has been reported to be similar to that in PiZZ patients, the evidence for this is inconsistent 10 . Furthermore, the survival rate seems to be better in PiSZ; the decline in FEV1 can be up to 169% faster in PiSZ when compared with PiMM but may not be a good predictor of survival 19 . It is possible that computed tomography (CT) densitometry or DLCO would be more informative regarding survival given that upper zone density decline is relevant to mortality 11 and is common in PiSZ patients.
Just like in lung disease, PiSZ patients express a milder form of liver disease than PiZZ patients, since liver toxicity is proportional to the amount of retained protein (PiZZ > PiSZ). The Z allele in PiSZ genotype confers an increased risk for cirrhosis in chronic metabolic injury (six times higher), such as in non-alcoholic fatty liver disease (NAFLD) and chronic alcohol abuse 4 . The association between PiSZ heterozygosity and risk of developing other complications of AATD such as panniculitis and granulomatosis with polyangiitis is controversial but smaller than PiZZ homozygosity 7 .

SS genotype
PiSS genotypes are rarely diagnosed in clinical practice. Although the S allele is more common than the Z allele, interestingly, PiSS is not as commonly found as other genotypes 6,20,21 . For that reason, it is difficult to get accurate results regarding clinical phenotype. However, it has been noticed in a small cohort that COPD and asthma had a higher prevalence than expected 18 . As for liver disease, it remains undetermined if there is any clinical association, although the incidence was higher than predicted in one cohort study 18 .

MZ and MS genotypes
PiMZ and PiMS are the most frequent AATD genotypes 6,20,21 . PiMS is the least studied group, since many assume that it has no clinical relevance, given that AAT levels are close to normal. Limited evidence suggests that when smoking history is controlled, this group is not at risk for COPD when compared with the general population 9 . The PiMZ genotype is especially important when it comes to current or ex-smokers, as their risk for COPD becomes similar to that of PiSZ 14 . Furthermore, decline of lung function and an increased risk for emphysema development have been shown 15 .
Whether or not PiMZ individuals are at risk of developing liver disease is controversial. The presence of the Z allele was associated with higher transaminase levels, increased risk of progression of alcoholic cirrhosis and non-alcoholic liver disease, higher rates of decompensation of cirrhosis, and increased risk of liver transplantation 16 . As for the risk of liver cancer in PiMZ individuals, this is even more controversial, with some studies suggesting a risk for cholangiocarcinoma 22 and others reporting no association at all 23 . The presence of the Z allele might enhance susceptibility for carcinogenesis, as pre-neoplastic and neoplastic lesions were largely found to arise from PAS-D-devoid areas in PiZ mice 24 , similar to lesions found in AATD patients with hepatocellular carcinoma 23 . Further studies are still needed to confirm these assumptions.

Health behaviors
Health behaviors also play an important part in the presentation and management of patients with AATD. In order to present clinically with significant disease, milder deficiency genotypes require more intense environmental exposures to manifest. A summary of these health behavior differences is presented in Table 2.
Smoking cessation is the most important protective measure in AATD, even though there are studies reporting a minor effect when comparing PiSZ with PiZZ. Nevertheless, a faster rate of decline in lung function has been observed in both genotypes, which indicates that tobacco cessation must be a priority 25,26 . PiSZ patients exhibit a lower risk of lung disease and are less susceptible to smoking effects when compared with PiZZ patients 10 ; however, because of their higher AAT levels, they may have less concern that their genotype presents Emphasizing smoking cessation and behavioral interventions among PiSZ is likely to be highly beneficial, as they have an increased risk of developing COPD when compared to PiMM smokers 14 . Regardless of genotype, additional education about moderation of alcohol consumption should be considered because of the increased risk of liver disease among individuals with AATD. Reduction of harmful inhaled substances from occupational exposure should also be advised.

Recommendations for AATD diagnosis
AATD testing is recommended for all adults with emphysema, COPD, or asthma, whenever airflow obstruction is present or incompletely reversible, after optimized treatment with bronchodilators 14,25,27,28 . Other rarer forms of AATD might be present, so unexplained bronchiectasis, granulomatosis with polyangiitis, necrotizing panniculitis, and liver disease of unknown etiology should also prompt further AATD testing 14,25,27,28 . Once the diagnosis is made, familial testing is advocated, since AATD is a heritable disease.
AAT levels alone are inaccurate for identifying these patients since equivalent AAT levels could represent different milder AATD genotypes 13 , as demonstrated in Figure 1. Confirmatory testing, through phenotyping and genotyping, are strongly recommended to identify normal, deficient, or non-functioning alleles, or even rarer AAT alleles, which otherwise would go unrecognized 14,27,28 .

New diagnostic modalities
A delay in diagnosis has been associated with worsened clinical status 29,30 , so there has been a focus on ways to make diagnostic testing easier and more efficient. AATD screening usually starts by measurement of the level of AAT in the blood and, if it is low, followed by phenotype or genotype for definitive confirmation. Phenotyping refers to testing the speed of protein migration by isoelectric focusing, whilst genotyping is usually done for specific mutations (usually for the S and Z mutations). Newer approaches which allow home testing or testing in primary care are desirable and include the Alphakit® Quickscreen (Diagnostic Grifols, Barcelona, Spain) for the identification of the Z protein using lateral-flow paper-based technologies 31 . A positive result should prompt further investigation. A limitation of this approach is that a negative result (absence of Z protein in blood) may lead to underdiagnosis of non-Z AATD genotypes. A newer Luminexbased algorithm capable of detecting 14 different AATD mutations simultaneously, compared to the two traditional mutations (S and Z), in a shorter time has also been developed 32 . This can be performed from drops of blood from a fingerstick or a buccal swab and covers >98% of mutation combinations known to cause AATD. Table 3 shows the methods of diagnosis reported to date.

Pulmonary involvement
Emphysema and COPD are the main clinical features of AATD; severity depends on genotypes and health behaviors (discussed above). AATD lung disease is characterized by basal pan-lobular emphysema at an early age, though a range of other phenotypes have been recognized (Figure 2). Reversibility of airflow obstruction is observed in up to 80% of AATD patients 2,35 . This has prognostic impact, since the degree of reversibility associates with rapid decline of lung function 36 . Chronic bronchitis (CB) affects approximately 40% of patients with AATD 37 . CB, as part of the spectrum of neutrophilic inflammation in the lungs, might be one of the clinical features that should draw attention to AATD diagnosis 2 . Nevertheless, clinical heterogeneity makes AATD a challenging diagnosis.
The relationship between asthma and AATD is unclear, although it has been proposed 38 that patients tested and diagnosed with AATD at an early age are more likely to be labeled as asthmatic 28 . This uncertainty, and the presence of asthma symptoms, with fixed or reversible obstruction in lung function in significant numbers of AATD patients, is a factor behind the recommendation to test for AATD in a wide range of respiratory patients 39 . Allergic asthma is usually more common in younger AATD patients, and AAT serum levels have been shown to be lower in asthmatic carriers of a Z allele 40 . However, no significant association was observed between common SERPINA1 SNPs and the risk of developing school-age Requires sophisticated bioinformatics systems to analyze and clinically interpret the data asthma, the presence of a deficient allele (S or Z) did not affect the risk of wheezing in childhood and further development of asthma in adolescence 41 , and no association was made between AATD genotypes or lung function severity with allergic asthma severity 40 . Future research is needed, as there are inconsistent data regarding an association between AATD and asthma.
Bronchiectasis is found in many AATD patients, although it is usually encountered in patients who already have emphysema, suggesting that there is a shared pathophysiological process underway 2 . Bronchiectasis may also present as part of pulmonary Langerhans cell histiocytosis (PLCH). PLCH is strongly linked with cigarette smoking, manifests in young adults, and is characterized by the presence of polycystic lung lesions. It has been speculated that AATD patients might be at a greater risk for developing PLCH, as cystic pulmonary lesions have been observed 42,43 .
The pulmonary microbiota in AATD patients differs from that of usual COPD smoking patients. AATD patients on augmentation therapy (AT) have lower sputum neutrophils and a lower specific bacterial load (Moraxella catarrhalis and Streptococcus pneumonia) 44 . Among bronchiectasis patients, the risk of non-tuberculous mycobacteria (NTM) infection seems to be higher in AATD patients when compared to primary ciliary dyskinesia and common variable immunodeficiency 45 , perhaps because AAT inhibits rapid growth of mycobacterial infection in macrophages, thus enhancing macrophage immunity against NTM 46,47 . A potential link between AATD and invasive infections, like invasive pulmonary aspergillosis, has also been postulated 48 .

Extrapulmonary involvement
Diseases such as panniculitis and vasculitis are observed, albeit rarely. Necrotizing panniculitis and systemic vasculitis with positive c-ANCA should prompt testing for AATD, since an association between them has been established 28 . Other reported associations of AATD from cases and small cohort studies include inflammatory bowel disease, glomerulonephritis, rheumatoid arthritis, fibromyalgia, vascular abnormalities (fibromuscular dysplasia of the arteries, abdominal and brain aneurysms, and arterial dissection), psoriasis, chronic urticaria, pancreatitis, and multiple sclerosis (Figure 3). Although these are rare associations, they are plausible, since AAT is anti-inflammatory and immunomodulatory 47,49 ; thus, in AATD, enhanced risk of inflammatory and autoimmune diseases could occur. It has even been proposed that AT could help to prevent these issues, though it is controversial 50 .

Imaging markers
Usually lung function is used to evaluate the progression and deterioration of AATD 14 . The measurement of pulmonary emphysema through CT densitometry has become more common in recent research. CT density has been associated with clinically relevant parameters, such as FEV1 and quality of life (Saint George's Respiratory Questionnaire [SGRQ]), and has a clear and consistent relationship with mortality 51 in COPD, which showed that density could be a valid surrogate outcome for disease severity. Use of CT densitometry in disease monitoring has been vital in proving an effect of AT in emphysema 10 , and lower CT density has also been related to mortality in AATD patients with basal emphysema, while FEV1 and DLCO alone have a weaker relationship 11 . This suggests that densitometry may be a useful clinical tool in AATD; however, clinical heterogeneity, lack of longitudinal data, and inter-individual lung volume variation are some of the limitations in the wide adoption of this technology.

Biological markers
Desmosine and isodesmosine (lung elastin degradation products usually elevated in COPD patients but also in AATD patients) were reduced after long-term intravenous AT and possibly with nebulized therapy 52 . The plasma degradation product of fibrinogen (Aα-Val360) was a disappointing marker, lacking a linear progression with time when considering its relationship between disease activity and severity, although it does reduce with augmentation 53 . The presence of elevated free light chains could also play a role in risk stratification in AATD patients, since they independently predict mortality in patients with severe AATD and usual COPD. At present, they are a more important pathogenic theme in usual COPD, but contribution to immune activation within the disease process in AATD is not excluded 54 . More recently, complement component C3d was proposed, since it correlates with both radiographic emphysema and severity of the emphysema in AATD, but not in usual COPD; also, in PiZZ AATD after intravenous AT, AAT disrupts C3 activation, thereby decreasing C3d plasma levels. The role of C3d in AATD is still unknown; however, a potential role for the complement system is emerging in the pathogenesis of emphysema 55 . Finally, interleukin (IL)-27, a cytokine released by macrophages and neutrophils, has been proposed, as its levels appear to reflect sputum neutrophilia

Treatment and management of patients with AATD General COPD treatment
Most AATD patients' management is based on COPD prevention and maintenance therapy. It is important to initiate and maintain bronchodilator therapy, with a good inhaler technique, such as long-acting β-adrenergic receptor agonists (LABA) and long-acting muscarinic receptor agonists (LAMA) 25 . It is conceivable that targeting pro-inflammatory pathways with inhaled corticosteroids (ICS) would be more beneficial in AATD patients, since exacerbation rates are higher and longer than in usual COPD 56 , but this remains unproven. Evidence is present that the response to ICS in AATD is associated with blood eosinophil count 57 , as in usual COPD, implying that a blanket approach would be inappropriate. Macrolides reduce the risk of exacerbations in usual COPD 58 . We might speculate that there would be the same effect on AATD patients with COPD, although data are lacking in this area. In severe AATD patients with established emphysema, AT should also be offered, according to guidelines 14,28 .
Influenza and pneumococcal vaccination should occur, as AATD patients have a high susceptibility for lower respiratory tract infections 44, 56,59 . Clinical benefits of pulmonary rehabilitation (PR) have been questioned in AATD patients, as unfavorable muscle response to exercise has been proposed 60 . Nevertheless, PR has improved health status, exercise tolerance, and quality of life, all problems that AATD patients experience, thus is reasonable to recommend. In cases of severe chronic hypoxemia at rest, long-term oxygen therapy improves survival, and if chronic hypercapnia is also present, long-term non-invasive ventilation might decrease hospitalizations and mortality, as in usual COPD 58 . Palliative approaches should always be initiated in cases of refractory symptoms.
Although recommendations for general treatment in AATD are based on usual COPD management, the majority of COPD pharmacotherapy clinical trials exclude these patients 58,61-65 .

Augmentation therapy
The use of AAT-AT is highly variable throughout Europe owing to variable health policies, product registration, and reimbursement issues. France and Germany have the most patients receiving AAT-AT (around 60%), whereas in Spain only approximately 20% of patients are receiving treatment 25 . Several countries such as the UK do not cover AAT-AT. AAT-AT has produced beneficial consequences, like ameliorating lung function decline and emphysema progression, prolonging survival, and delaying the decline in quality of life, especially in severe AATD, i.e. in ZZ or Z null patients 14,56,66-68 . Controversy remains over the effect on exacerbations, since a meta-analysis of randomized controlled trials (RCTs) revealed a small statistically significant increase in annual exacerbations (0.29/year) on AAT-AT 56 , shown in Table 3. However, evidence of this is inconsistent. AAT-AT was related to a significant reduction in exacerbation rate 69 and a reduction in exacerbation severity 70 in cohort studies. The potential benefits of AAT-AT in PiZZ patients are summarized in Table 4.
It should be noted that most of the evidence relates to PiZZ patients. In addition, most guidelines recommend the presence of emphysema, a specified level of FEV1, and a specific level of AAT, which excludes almost 90% of PiSZ patients 10,28,71 . While there may be a small proportion of PiSZ patients who might benefit from AAT-AT, such as those rapidly declining with AAT levels below threshold limit (11 µM), scientific evidence supporting clinical efficacy continues to be vague. In several European countries, health authorities have funded AT despite a lack of evidence of benefit in PiSZ patients. Close follow-up in rapid decliners and a wait-and-see approach should be maintained, restricting therapy to those most at risk and aiming for a better quality of life for the patient.
Although dosage has been established at 60 mg/kg/week, it has been proposed that doubling the dosage (120 mg/kg/week) could be even more beneficial because it leads to serum trough AAT levels at physiologic values. A more pronounced impact on slowing disease progression, an overall reduction of anti-proteolytic effect, with significant reductions of collagenase (matrix metalloproteinase-1 [MMP1]) and gelatinase (MMP9), and a reduction in inflammatory effects, namely a significant decrease in IL-10, an anti-inflammatory cytokine important in limiting local host immune responses, have been reported 72 . Further studies are still required.

Lung volume reduction and transplantation
More invasive approaches like lung volume reduction surgery (LVRS) can be offered; LVRS has demonstrated benefits in AATD, but it seems to be inferior when compared with usual COPD, since it has a higher short-term mortality 56 . Bronchoscopic interventions, like endobronchial valves and lung coils, can improve health status and lung function at least for 6-12 months following treatment, although a small study has reported a 2-year beneficial period 56 . Although these approaches are possible in selected patients, their long-term benefits remain to be elucidated. In addition, the usual approach targeting apical disease is not very useful for patients with AATD; perhaps newer coil procedures may be more useful, though data are lacking to prove this at present.
AATD patients represent 5% of lung transplants performed worldwide, but outcomes and survival rates in a post-transplant phase are still unknown. A recent retrospective study evaluated the incidence of complications and survival of AATD recipients with a control group of COPD recipients 73 . They observed (i) early bronchial anastomotic complications and (ii) late bowel complications. Anastomotic complications with dehiscence were seen only in AATD patients who were under AT and discontinued it before the transplant. This was associated with a probable rebound phenomenon characterized by increased neutrophil activity on bronchoalveolar lavage. Conversely, AATD patients who did not receive AT had better lung outcomes and greater survival rate. Bowel inflammation associated with ischemia was observed too but only in AATD recipients, not in COPD recipients 73 . Since a probable link between timing of withdrawal of replacement therapy in AATD patients and anastomotic complications might be present, new strategies should be considered when referring these patients for lung transplant. Nevertheless, significant health status benefits have been generally observed after transplant, indicating that it is appropriate when quality of life is poor 56 .
When comparing survival rates after lung transplantation, between AATD recipients and usual COPD, no difference in long-term survival was observed in the majority of the studies, albeit AATD patients are usually younger and have fewer comorbilities 56 . Only two studies have reported otherwise, with a 10-year survival superior in COPD patients then in AATD patients 77,78 .

Products in development
A recent review has examined the different experimental approaches being pursued in trials in AATD 79 , and covering them in detail is beyond the scope of this review. These approaches are summarized in Table 5.

Conclusion
Diagnostic techniques for AATD are improving, but milder genotypes (PiSZ and PiMZ) remain underdiagnosed in the general population. AAT-AT confers decreased emphysema progression and may need to be stopped prior to transplantation if disease progresses to this point. Whilst we can speculate that these potential benefits might be extended to milder forms like PiSZ, further investigations are still needed.