Alpha-1 Antitrypsin Deficiency (AATD)


In 1989, the American Thoracic Society published the first statement regarding the diagnosis and treatment of adults with Alpha-1 Antitrypsin Deficiency (AATD).1 This was updated in 2003 when the American Thoracic Society and European Respiratory Society issued a joint statement based on evidence-based research, which strongly recommended diagnostic testing for AATD in all symptomatic adults with chronic obstructive pulmonary disease (COPD).2 Since 2003, there has been continuous development and progression of more accurate and cheaper methods of genetic testing for diagnosis.3 Most recently, the European Respiratory Society issued an updated statement in 2017 on the diagnosis and treatment of pulmonary disease in AATD.3

Testing Methods for Diagnosis

The first step to obtaining an accurate AATD diagnosis is via blood testing to determine the serum concentration of alpha-1 antitrypsin (AAT) protein. Originally, this was performed using methods, such as paper electrophoresis in 1963, when Laurell and Eriksson published their first study, and then eventually radial immunodiffusion and rocket electrophoresis.3 

Currently, the quantification of AAT levels in the blood is done primarily via nephelometry, or secondarily via a comparable latex-enhanced immunoturbidimetric assay.3,4 Over the last decade, use of dried blood samples to measure AAT serum concentration levels has become more prevalent.5,6,7 

To officially diagnose an individual with AATD, the serum concentration levels of AAT must differentiate the Pi*MM normal genotype from other genotypes with at least 1 deficient S or Z allele. This threshold level of serum AAT is 24.4 µM (or 1.1 g/L) with 73.4% sensitivity and 88.5% specificity.8 The AAT serum plasma levels are an important first step as it is used to initially differentiate the severely deficient individuals (Pi*ZZ, Pi*ZNull, Pi*NullNull, and most Pi*SZ) from the intermediate deficiencies.3

Given the high degree of overlap of serum AAT concentrations among the varying genotypes, plasma levels of this protein alone are insufficient to definitively diagnose intermediate deficiency in M heterozygous genotypes (Pi*MZ, Pi*MS, and Pi*MNull).8 

To circumvent this overlap, protein genotyping, which is completely independent of AAT serum levels, detects specific AAT gene mutations, specifically S and Z variants, but it can also detect null and rare variants (E, F, G, I, M, and P) using PCR (or polymerase chain reaction) assays.9,10,11 PCR assays are limited by the prerequisite for specific primers and known sequence defects for accurate detection of mutations. 

Whole gene sequencing, some of which can be completed in a day like Next Generation Sequencing (NGS), are able to detect stop mutations and recognize rarer and even unknown variants without the need for a primer.12 

To summarize, official AATD diagnosis requires confirmation of low AAT serum concentration levels AND either genetic testing that confirms a disease-causing mutation in both copies of the SERPINA1 gene OR detecting that an AAT protein variant does not functional properly by using isoelectric focusing.13 Isoelectric focusing is a methodology by which proteins are separated into their respective isoelectric points by migrating at varying speeds in a matrix according to the pH level. The isoelectric point of a protein changes if there is a mutation causing an amino acid substitution.14 

Clinical Practice Guidelines, Recommendations for Testing

It is currently recommended by the Medical and Scientific Advisory Committee of the Alpha-1 Foundation that the following individuals be tested for AATD regardless of age of symptom onset, severity of symptoms, or ethnicity:

  • All individuals with symptomatic airflow obstruction, whether labeled as asthma, emphysema, or COPD15
  • All individuals with unexplained chronic liver disease15
  • All individuals with unexplained bronchiectasis15 
  • All individuals with necrotizing panniculitis and granulomatosis with polyangiitis15 
  • First-degree relatives as well as extended family members should be tested if one individual in the family is diagnosed with an abnormal AATD gene as it is passed down through families15
  • AAT serum level testing alone is not recommended for familial testing since it does not fully reflect disease risk from AATD as genotyping would15
  • For symptomatic individuals, genotyping is recommended.15

Early and accurate diagnosis is paramount to improving quality of life and patient outcomes in individuals with AATD. Due to the nature of the varying severities, disease presentations, and possibly even mild or asymptomatic individuals, diagnosis of AATD has been difficult and poorly understood by primary care physicians and even pulmonologists. 

However, as detection of AATD improves, diagnostic techniques involving biomarkers, such as desmosine and isodesmosine, which are both products of elastin degeneration, and home testing methods have been developed and are  improving rates of early diagnosis.16 Moreover, computed tomography (CT) scans can identify AATD-related emphysema since it manifests with predominantly more basilar alveolar destruction than the centrilobular presentation of emphysema caused by smoking.16,17 Although not recommended, serial chest CT scanning could map disease progression to indicate efficacy of treatment.15,16 

References

  1. Buist AS, Burrows B, Cohen A, et al. Guidelines for the approach to the patient with severe hereditary alpha-1-antitrypsin deficiency. Am Rev Respir Dis. 1989; 140(5):1494-1497. doi:10.1164/ajrccm/140.5.1494
  2. American thoracic society/european respiratory society statement: standards for the diagnosis and management of individuals with alpha-1 antitrypsin deficiency. Am J Respir Crit Care Med. 2003; 168(7):818-900. doi:10.1164/rccm.168.7.818
  3. Miravitlles M, Dirksen A, Ferrarotti I, et al. European respiratory society statement: diagnosis and treatment of pulmonary disease in α1-antitrypsin deficiency. European Respiratory Journal. 2017; 50(5). doi:10.1183/13993003.00610-2017 
  4. Ledue TB, Collins MF. Development and validation of 14 human serum protein assays on the roche cobas® c 501. J Clin Lab Anal 2011; 25: 52e60. doi:10.1002/jcla.20430 
  5. Costa X, Jardi R, Rodriguez F, et al. Simple method for alpha1-antitrypsin deficiency screening by use of dried blood spot specimens. European Respiratory Journal. 2000; 15(6):1111-1115. 
  6. Gorrini M, Ferrarotti I, Lupi A, et al. Validation of a rapid, simple method to measure α1-antitrypsin in human dried blood spots. Clinical Chemistry. 2006; 52(5):899-901. doi:10.1373/clinchem.2005.062059 
  7. Zillmer LR, Russo R, Manzano BM, et al. Validation and development of an immunonephelometric assay for the determination of alpha-1 antitrypsin levels in dried blood spots from patients with COPD. J Bras Pneumol. 2013; 39(5):547-554. doi:10.1590/S1806-37132013000500004
  8. Ferrarotti I, Thun GA, Zorzetto M, et al. Serum levels and genotype distribution of α1-antitrypsin in the general population. Thorax. 2012; 67(8):669-674. doi:10.1136/thoraxjnl-2011-201321
  9. Orrù G, Faa G, Pillai S, et al. Rapid PCR real-time genotyping of m-malton alpha1-antitrypsin deficiency alleles by molecular beacons. Diagn Mol Pathol. 2005;14(4):237-242. doi:10.1097/01.pas.0000178221.44474.b3
  10. Denden S, Lakhdar R, Boudawara Keskes N, Hamdaoui MH, Chibani JB, Khelil AH. PCR-based screening for the most prevalent alpha 1 antitrypsin deficiency mutations (PI S, Z, and Mmalton) in COPD patients from eastern tunisia. Biochem Genet. 2013;51(9):677-685. doi:10.1007/s10528-013-9597-6
  11. Belmonte I, Montoto L, Miravitlles M, et al. Rapid detection of Mmalton α1-antitrypsin deficiency allele by real-time PCR and melting curves in whole blood, serum and dried blood spot samples. Clinical Chemistry and Laboratory Medicine (CCLM). 2016;54(2). doi:10.1515/cclm-2015-0297
  12. Behjati S, Tarpey PS. What is next generation sequencing? Arch Dis Child Educ Pract Ed. 2013;98(6):236-238. doi: 10.1136/archdischild-2013-304340
  13. Alpha-1 antitrypsin deficiency. Genetic and Rare Diseases Information Center (GARD) -an NCATS Program. Accessed June 4, 2021.  
  14. Isoelectric focusing – an overview. Science Direct Topics. Accessed June 4, 2021.
  15.  Sandhaus R, Gerard T. The diagnosis and management of alpha-1 antitrypsin deficiency in the adult. Chronic Obstructive Pulmonary Diseases: Journal of the COPD Foundation. 3(3):668-682.
  16. Quinn M, Ellis P, Pye A, Turner AM. Obstacles to early diagnosis and treatment of alpha-1 antitrypsin deficiency: current perspectives. Ther Clin Risk Manag. 2020; 16:1243-1255. doi:10.2147/TCRM.S234377 
  17. Anariba, DEI. How is alpha1-antitrypsin deficiency (AATD) characterized in chest x-rays? Medscape. Accessed on May 28, 2021.

Article reviewed by Kyle Habet, MD, on July 1, 2021.

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