Application of a diagnostic algorithm for the rare deficient variant Mmalton of alpha-1-antitrypsin deficiency: a new approach

Testing Alpha-1 Antitrypsin Deficiency in Black Populations

Alpha-1 antitrypsin (AAT) deficiency (AATD) is an under-recognized hereditary disorder and a significant cause of chronic obstructive pulmonary disease (COPD), a disease that contributes to global mortality. AAT is encoded by the SERPINA1 gene, and severe mutation variants of this gene increase the risk of developing COPD. AATD is more frequently screened for in non-Hispanic White populations. However, AATD is also observed in other ethnic groups and very few studies have documented the mutation frequency in these other ethnic populations. Here, we review the current literature on AATD and allele frequency primarily in Black populations and discuss the possible clinical outcomes of low screening rates in a population that experiences poor health outcomes and whether the low frequency of AATD is related to a lack of screening in this population or a truly low frequency of mutations causing AATD. This review also outlines the harmful SERPINA1 variants, the current epidemiology knowledge of AATD, health inequity in Black populations, AATD prevalence in Black populations, the clinical implications of low screening of AATD in this population, and the possible dangers of not diagnosing or treating AATD.

Alpha-1 antitrypsin deficiency hidden in allegedly normal variants

INTRODUCTION

Rare variants of Alpha-1 antitrypsin (AAT) deficiency (AATD) have been described by the Spanish registry of patients with AATD. The great majority of these rare variants are Mmalton alleles and many recent case series of them have been identified in the Canary Islands. The objective of this study was to analyze the distribution of Mmalton mutations in a Canarian population previously studied for the most common deficient alleles, namely PI*S (S) and PI*Z (Z), with PI*M (M) being the normal variant.

METHODS

A cross-sectional study of 648 patients with allergic asthma was carried out. Mmalton mutation of the SERPINA1 gene was assayed by real-time PCR.

RESULTS

Of the 648 patients, 3 (0.46%) were carriers of a Mmalton allele. All of them had low levels of AAT (53.9 mg/dL, 90 mg/dL, and 61 mg/dL, respectively) and were asymptomatic, showing normal lung function, radiological images, and levels of hepatic transaminases.

CONCLUSION

In conclusion, although the most frequent AATD genotypes are Z and S alleles, it is important to consider other rare variants, particularly when low AAT serum levels are observed. Although individuals with the Mmalton mutation usually have a heterogenous clinical presentation and very low levels of AAT, all the patients in this study were asymptomatic.

Characterization of Novel Missense Variants of SERPINA1 Gene Causing Alpha-1 Antitrypsin Deficiency

The SERPINA1 gene is highly polymorphic, with more than 100 variants described in databases. SERPINA1 encodes the alpha-1 antitrypsin (AAT) protein, and severe deficiency of AAT is a major contributor to pulmonary emphysema and liver diseases. In Spanish patients with AAT deficiency, we identified seven new variants of the SERPINA1 gene involving amino acid substitutions in different exons: PiSDonosti (S+Ser14Phe), PiTijarafe (Ile50Asn), PiSevilla (Ala58Asp), PiCadiz (Glu151Lys), PiTarragona (Phe227Cys), PiPuerto Real (Thr249Ala), and PiValencia (Lys328Glu). We examined the characteristics of these variants and the putative association with the disease. Mutant proteins were overexpressed in HEK293T cells, and AAT expression, polymerization, degradation, and secretion, as well as antielastase activity, were analyzed by periodic acid-Schiff staining, Western blotting, pulse-chase, and elastase inhibition assays. When overexpressed, S+S14F, I50N, A58D, F227C, and T249A variants formed intracellular polymers and did not secrete AAT protein. Both the E151K and K328E variants secreted AAT protein and did not form polymers, although K328E showed intracellular retention and reduced antielastase activity. We conclude that deficient variants may be more frequent than previously thought and that their discovery is possible only by the complete sequencing of the gene and subsequent functional characterization. Better knowledge of SERPINA1 variants would improve diagnosis and management of individuals with AAT deficiency.

Genetic diagnosis of α1-antitrypsin deficiency using DNA from buccal swab and serum samples

BACKGROUND

α1-Antitrypsin deficiency (AATD) is associated with a high risk of developing lung and liver disease. Despite being one of the most common hereditary disorders worldwide, AATD remains under-diagnosed and prolonged delays in diagnosis are usual. The aim of this study was to validate the use of buccal swab samples and serum circulating DNA for the complete laboratory study of AATD.

METHODS

Sixteen buccal swab samples from previously characterized AATD patients were analyzed using an allele-specific genotyping assay and sequencing method. In addition, 19 patients were characterized by quantification, phenotyping and genotyping using only serum samples.

RESULTS

The 16 buccal swab samples were correctly characterized by genotyping. Definitive results were obtained in the 19 serum samples analyzed by quantification, phenotyping and genotyping, thereby performing the complete AATD diagnostic algorithm.

CONCLUSIONS

Buccal swab samples may be useful to expand AATD screening programs and family studies. Genotyping using DNA from serum samples permits the application of the complete diagnostic algorithm without delay. These two methods will be useful for obtaining more in depth knowledge of the real prevalence of patients with AATD.

Laboratory Diagnosis by Genotyping

Alpha-1 antitrypsin (AAT) genotyping is useful to confirm the clinical diagnosis of AAT deficiency and determine the specific allelic variant. Genotyping is the reference standard procedure for identifying rare allelic variants and characterizing new variants. It is also useful when there is a discrepancy between the patients' AAT levels and their phenotypes. AAT genotype is determined by an allele-specific genotyping assay for the S, Z, and Mmalton variants and by exome sequencing.