Improved screening for better patient management
Alpha1-antitrypsin, a protein synthesised by the liver, was discovered in 1955 by Jacobsson, and the first cases of deficiency were described in 1963 by Laurell and Eriksson.
Alpha1-antitrypsin deficiency is a fairly common genetic disorder, affecting 1/1500 to 1/5000 people in Europe, mostly of Caucasian origin. It manifests as a significant drop in serum alpha1-antitrypsin concentration, pulmonary emphysema, cirrhogenic liver disease and, much more rarely, panniculitis. Largely under-diagnosed, this disease would benefit greatly from wider screening, since the earlier it is diagnosed, the better the prognosis; early introduction of hygienic and dietary measures, particularly smoking cessation, has a major impact on preventing complications, and treatment is available for severe forms.
What is alpha-1 antitrypsin deficiency?
Role and characterisation of alpha1-antitrypsin deficiency
Alpha1-antitrypsin (AAT) is a 394 AA glycoprotein encoded by the SERPINA1 gene, of which over 300 variants are known. The protein is polymorphic, with numerous isoglycoforms. Its normal plasma concentration is between 0.9 and 2 g/L, and it is mainly synthesised by the liver. It is a protein of the acute phase of inflammation, and its primary role in humans is to inhibit proteases, in particular elastase produced by neutrophils, mainly in the lungs.
The conformation of the protein, which determines its function, and the quantity of circulating alpha1-antitrypsin is determined by the codominant expression of alleles of parental origin. Numerous variants have been described for the “Protease Inhibitor” or PI phenotype. The “normal” PI phenotype (functional protein, normal AAT levels) is PI*MM.
Alpha1-antitrypsin deficiency is an autosomal recessive genetic disorder, a conformational disease associated with pathogenic variants of the SERPINA1 (Serine protease inhibitor 1) gene, on chromosome 14, encoding AAT. The two most frequent pathogenic variants are:
- Variant Z (PI*ZZ); frequency worldwide 1%, the most pathogenic, characterised by a p.Glu366Lys substitution. It accounts for 95% of severe deficiencies, with a decreasing North-South frequency gradient;
- Variant S (PI*SZ); worldwide frequency 2-3%, characterised by a p.Glu264Val substitution, with no clinical impact unless associated with variant Z.
However, many other SERPINA1 gene variants have been described, some of which, like MMalton, have effects similar to those of variant Z; others, like S, are of no consequence.
The pathophysiology of lung damage is linked to a reduction in anti-elastatic protection in the lungs; liver damage is secondary to polymerization of abnormal AAT molecules and their accumulation in hepatocytes.
Epidemiology of alpha-1 antitrypsin deficiency
Although considered a rare disease, alpha1-antitrypsin deficiency is currently one of the most common genetic disorders in Europe, with a prevalence ranging from 1/1,500 to 1/5,000.
In France, around 10,000 people suffer from the severe form of the disease.
If it is considered rare, it is because it is unrecognised and largely under-diagnosed. It is estimated that an average of 7.5 years elapses between the appearance of the first symptoms and diagnosis.
However, an early diagnosis means that sufferers can modify their lifestyles to reduce the risk of respiratory complications, in particular giving up smoking, the main risk factor for the development of these complications, and to monitor patients, which improves their prognosis. In fact, the severity of lung damage, mortality and survival without transplantation are correlated with delayed diagnosis; but too little screening is carried out.
What are the clinical signs of alpha-1 antitrypsin deficiency?
AAT deficiency mainly manifests as pulmonary emphysema, cirrhogenic liver disease and, in rare circumstances, panniculitis.
Pulmonary involvement
Lung involvement is characterised by pulmonary emphysema, only in adults, but which can occur early, sometimes before the age of 40, with or without obstructive ventilatory disorder. Characteristic emphysema abnormalities are found on CT scans. The disease usually presents as chronic obstructive pulmonary disease (COPD), sometimes in the form of asthma or bronchial dilatation, with breathing difficulties, wheezing and a chronic cough.
Smoking is the main risk factor, as cigarette smoke increases protease activity. Hence the importance of screening these patients for the disease, as smoking cessation is effective in preventing or delaying the onset of respiratory failure.
Hepatic damage
Structural changes in certain AAT variants lead to their polymerization and accumulation in hepatocytes, resulting in hepatic manifestations. The disease is sometimes observed in children in the first few months of life, in the form of neonatal cholestasis, which may progress to cirrhosis, or disappear spontaneously after the first year. In adults, it manifests itself as hepatic steatosis, with the development of fibrosis and the risk of cirrhosis and hepatocellular carcinoma. Alcohol and obesity are the main risk factors.
Similarly, screening for the disease in these patients enables the implementation of the necessary simple and effective dietary hygiene measures, as well as regular follow-ups.
Clinical symptoms develop in homozygous ZZ patients. Heterozygotes MZ or SZ present a moderate decrease in AAT (serum concentration between 0.5 and 1.1 g/L), an increased risk of emphysema only in the case of associated smoking, and a high risk of cirrhogenic liver disease only in the case of alcoholism or obesity.
How is alpha-1 antitrypsin deficiency diagnosed?
Screening for the disease is based on serum AAT levels, in the absence of an inflammatory syndrome. There is currently no scientific consensus on the strategy to adopt, but an expert opinion from the Société de Pneumologie de Langue Française (SPLF) suggests that, if AAT is less than 1.1 g/L, SERPINA1 gene variants should be determined by phenotyping or (targeted) genotyping. In case of discrepancy between AAT assay and phenotyping or genotyping and/or in case of strong clinical suspicion, gene sequencing will be proposed.
Biochemical assay
Biochemical determination of AAT is performed by immunonephelometry or immunoturbidimetry, on serum or plasma (heparinized tube or EDTA). Standardisation of assays with a reference standard ensures consistency of results between laboratories. Its main advantage is that it is simple and inexpensive.
However, AAT may be increased in cases of inflammatory syndrome (hence the recommendation to measure it at a distance), as well as in women who are pregnant or taking oral contraceptives. Conversely, it decreases in cases of hepatocellular insufficiency, nephrotic syndrome, digestive losses (exudative enteropathy, chronic inflammatory bowel disease or IBD) and, of course, congenital deficiency.
Phenotyping
Phenotyping uses isoelectrofocusing to characterise AAT isoforms according to their electrophoresis gel migration profile (M, S, Z, etc.). The most frequent PI*M, PI*S or PI*Z variants are thus identified, but certain variants such as Null (absence of protein) or M-deficient variants that do not alter the isoelectric point of AAT (e.g., MMalton) are not identified and require genotyping.
Genotyping
Genetic diagnosis is based on targeted genotyping (targeted detection of specific variants) using EDTA whole blood. It requires extraction of leukocyte DNA and can be performed using allele-specific PCR (M, S, Z probes). However, this technique can only identify the alleles specifically sought, in homo or heterozygous form, so there is a risk of false negatives.
In the second intention, the SERPINA1 gene is sequenced using the Sanger or Next Generation Sequencing (NGS) technique: the sequence is compared with the reference sequence (M variant) to characterise the variant of interest, enabling the search for and identification of rare variants.
How is alpha-1 antitrypsin deficiency treated?
Treatment is based primarily on non-specific therapeutic measures, the most important of which is smoking cessation. Other recommended measures for ZZ homozygotes and heterozygotes carrying a single copy of variant Z, are to moderate alcohol consumption and avoid or limit obesity.
Treatment and follow-ups are similar to those for patients with COPD or “classic” cirrhosis.
All patients, whether homozygous or heterozygous, should be referred to a pulmonologist or hepatologist. Relatives should be informed of the family’s genetic risk, and screening for the disease is recommended around the index case (parents, children, grandchildren and siblings).
A specific treatment is now available for patients with a severe form of the disease: human alpha-1 antitrypsin administered intravenously. Three specialties are available: Alfalastin®, Prolastin® and Respreeza®, and are indicated for patients under 70 years of age, who do not smoke or smoke no more, with AAT deficiency, emphysema and obstructive ventilatory disorder (FEV1 between 35 and 70% of the theoretical value), after validation by a multidisciplinary consultation meeting.
As a last resort, lung or liver transplantation may be necessary.
A number of promising new therapies are currently being developed, including fazirsiran, an RNA interferon that reduces AAT production in liver cells (currently in Phase 3 trials), molecules blocking variant Z polymerization, activation of autophagy in hepatocytes by carbamazepine (currently in Phase 2 trials), hepatic transplantation of healthy or genetically modified stem cells, and CRIPR/Cas9 genome editing.
How can we improve screening for alpha-1 antitrypsin deficiency?
The first recommendations from the American Thoracic Society (ATS) date back to 1989, and advocate screening for the disease only in cases of strong clinical suspicion.
In 2003, much more comprehensive recommendations were issued by the ATS/ERS (European Respiratory Society), advocating screening for AAT deficiency in the following situations:
- Disappearance of alpha-1 globulins on serum protein electrophoresis.
- Early-onset emphysema.
- Early-onset COPD.
- Cortico-resistant asthma.
- Unexplained bronchiectasis.
- Unexplained hepatopathies.
- Unexplained panniculitis and vasculitis.
- Respiratory symptoms spanning several generations.
- Family screening around a case.
In 2022, the “Société de Pneumologie de langue française (SPLF)” took up all the indications of ATS, broadening them and emphasising all patients with emphysema and all those with COPD.
For biologists, it should be emphasised that since 2003, it has been recommended to systematically perform a weighted AAT assay in the event of a decrease in alpha-1-globulins on serum protein electrophoresis.
