Normal Alpha-1 Antitrypsin Physiology

Background

• Alpha-1 Antitrypsin (A1AT) is a 52 kD (394 AA) Glycoprotein Serine Protease Inhibitor (From the “Serpin” Family)
  • “Serpinopathies”: collection of neurodegenerative diseases, angioedema-related disorders, and coagulation disorders

Synthesis and Trafficking of Alpha-1 Antitrypsin

• Sites of Alpha-1 Antitrypsin (A1AT) Synthesis
  • Liver: predominant site of synthesis
  • Lung (Macrophages and Bronchial Epithelial Cells): minor site of synthesis
• Trafficking
  • A1AT Synthesized in the Liver Reaches the Lungs Via Diffusion from the Circulation

Normal Alpha-1 Antitrypsin Level

• Normal Serum Alpha-1 Antitrypsin (A1AT) Level: 100-300 mg/dL (20-60 µM)
• Threshold (Protective) Serum A1AT Level Below Which There is an Increased Risk of Emphysema: <80 mg/dL (<11 µM)

Normal Biologic Functions of Alpha-1 Antitrypsin

• Inhibition of Pancreatic Trypsin/Chymotrypsin
• Inhibition of Elastase/Granulocytic Elastase
  • Main Site of Action is Neutrophil Elastase
• Inhibition of Collagenase/Synovial and Skin Collagenases
• Inhibition of Microorganism Proteases

Interactive Effect of Tobacco Smoke Exposure

• Smokers Have Increased Neutrophils and Increased A1AT
  • However, the A1AT is Inactivated by Tobacco Smoke
• There is Greater Desmosine Excretion (Derived from Elastin Cross-Links) in Smokers with More Rapid Decline in Lung Function (as Compared to Smokers with Normal Rates of Decline)
  • This Supports the Role of Elastin Degradation in the Decline in Lung Function
• Basilar-Predominance of Lung Disease
  • Basilar Predominance of Lung Disease is Likely Related to Greater Blood Flow to Lung Bases, with More Neutrophil Elastase-Mediated Damage in this Region of the Lung

Alpha-1 Antitrypsin Deficiency Genetics

General Information

• Pi = Protease Inhibitor
• SERPINA1 Gene is Located on the Long Arm of Chromosome 14 and Encodes for A1AT
  • At Least 150 Different Alleles of SERPINA1 Have Been Identified
    • Alleles Have Letter Codes Based on the Electrophoretic Mobility of the Protein Which is Produced
  • M Allele: normal allele
  • Z Allele: point mutation Glu342Lys (substitution of lysine for glutamic acid at position 342) -> increased polymerization and aggregation of A1AT protein
  • S Allele: single amino acid substitution of valine for glutamic acid at position 264 -> enhanced intracellular degradation of A1AT protein
• Mode of Genetic Transmission
  • Autosomal Co-Dominant Transmission
    • Affected Patients Inherit an Abnormal Gene from Each Parent

Genotypes

• Pi-MM (Heterozygosity for Normal M Allele)
  • Pi-MM is the Genotype Present in 90% of European Descendants
  • A1AT Level: normal
  • Risk of Disease
    • No Risk of Disease
• Pi-MZ
  • Pi-MZ is the Genotype Present in 2-3% of Caucasians in the US
  • A1AT Level: mean level is approximately 57% of normal
    • Rarely Have A1AT Levels Below the Serum Lower Cutoff Value
  • Risk of Lung Disease
    • From Family Study Data: minimally increased risk of lung disease
    • From Population Study Data: probably no increased risk of lung disease
  • Risk of Liver Disease
    • Associated with Cryptogenic Cirrhosis in Adults (Hepatology, 1998) [MEDLINE]
• Pi-SS
  • A1AT Level: mean level is approximately 52% of normal
  • Risk of Disease
    • No Risk of Disease
• Pi-SZ (Compound Heterozygosity = Carries Two Different Mutations of the Gene)
  • A1AT Level: mean level is approximately 37% of normal
    • Approximately 10% of Pi-SZ cases have A1AT levels <80 mg/dL (<11 µM)
  • Risk of Disease
    • Rarely Develop Emphysema: this group provides the basis for the serum level cutoff of 11 µM
• Pi-ZZ
  • A1AT Level: mean level is approximately 16% of normal
  • Risk of Disease
    • See Below
• Pi-Null
  • A1AT Level: undetectable serum A1AT level
• Pi-Null-Null
  • A1AT Level: decreased serum A1AT level
• PiZ-Null
  • A1AT Level: decreased serum A1AT level
• Dysfunctional A1AT
  • A1AT Level: normal levels of dysfunctional A1AT

End-Organ Dysfunction in Alpha-1 Antitrypsin Deficiency

• Lung Disease
  • Lung Disease Results from an Imbalance Between the Amount of Neutrophil Elastase in the Lung (Which Destroys Elastin) and the Amount of Elastase Inhibitor A1AT (Which Protects Against the Proteolytic Degradation of Elastin)
• Liver Disease
  • Liver Disease Results from an Accumulation of Unsecreted Variant A1AT Protein with the Hepatocytes

Pulmonary Function Tests (PFT’s) (see Pulmonary Function Tests)

• Obstructive Pattern (see Obstructive Lung Disease)
  • FEV1: decreased
  • FVC: normal
  • FEV1/FVC: decreased

Perfusion (Q) Scan (see Ventilation-Perfusion Scan)

• Demonstrates that Earliest Involvement Occurs in the Lung Bases

Bronchoscopy (see Bronchoscopy)

• Usually Not Necessary
• Bronchoalveolar Lavage (BAL): A1AT present in normals

Open Lung Biopsy (see Open Lung Biopsy)

• Usually Not Necessary: panlobular basilar-predominant emphysema

Chest X-Ray/Chest CT Pattern (see Chest X-Ray and Chest Computed Tomography)

• Hyperinflation with Basilar-Predominant Emphysema: this is strongly suggestive, but not necessary, for the diagnosis
  • This is in Contrast to Normal (Pi-MM) COPD, Which Typically Manifests Upper Lobe Predominance
  • Pi-ZZ Patients Typically Have More Evidence of Emphysema than PI-MM Patients with COPD
  • Approximately 85% of Pi-ZZ Cases Have Some Radiologic Evidence of Emphysema
    • Of These, 99.8% Have Some Basilar Emphysema, While 24% Have Emphysema Confined to the Lung Bases

Serum Alpha-1 Antitrypsin Level (see Serum Alpha-1 Antitrypsin)

Background

• A1AT is an Acute Phase Reactant Which Increases During Acute Illness
  • Subnormal Alpha-1 Antitrypsin May Potentially Increase into the Normal Range in Patients with PiSZ and PiMZ Phenotypes: for this reason, testing should be deferred until acute illness resolves
• Etiology of Elevated Serum Alpha-1 Antitrypsin (see Elevated Serum Alpha-1 Antitrypsin)
  • Active Vasculitis (see Vasculitis)
  • Following Intravenous Typhoid Vaccine Administration (see Typhoid Fever)
  • Infection
  • Malignancy
  • Pregnancy (see Pregnancy)
  • Severe Burns (see Burns)
  • Tobacco Use (see Tobacco): smoking elevates the A1AT level approximately 20%

Assays

• General Comments
  • Alpha-1 Antitrypsin Level in mg/dL x 0.185 = Alpha-1 Antitrypsin Level in μmol/L
• Immunoturbidimetry
  • ARUP Labs
• Immunodiffusion
• Nephelometry
• Rocket Immunoelectrophoresis

Interpretation

• Normal Alpha-1 Antitrypsin Level (ARUP Labs, Using Quantitative Immunoturbidimetry Assay): 90-200 mg/dL (16.65-37 μmol/L)
  • Note that the Normal Alpha-1 Antitrypsin Levels Differ for Each Assay
• Threshold Level for “Deficiency”: <90 mg/dL (<16.65 µmol/L)
  • The Threshold Represents a Level of About 35% of Normal
  • Most Patients Below this Level are Homozygous for the Z Allele (PiZZ Phenotype)

Clinical Data

• Small Dutch Study Evaluating Alpha-1 Antitrypsin Phenotypes in Severe Asthma (Respir Med, 2006) [MEDLINE]
  • A1AT Heterozygosity Does Not Seem to Be an Important Risk Factor of Persistent Airflow Limitation in Patients with Asthma

Recommendations

• Targeted Testing for Alpha-1 Antitrypsin Deficiency is Recommended for the Following Indications (Canadian Thoracic Society Alpha-1 Antitrypsin Testing and Alpha-1 Antitrypsin Replacement Guidelines, 2012) (Can Respir J, 2012) [MEDLINE]
  • Patient with COPD Diagnosed Before Age <65
  • Patient with COPD Diagnosed with a <20 Pack-Year Smoking History
• Targeted Testing is Not Recommended for the Following Indications (Canadian Thoracic Society Alpha-1 Antitrypsin Testing and Alpha-1 Antitrypsin Replacement Guidelines, 2012) (Can Respir J, 2012) [MEDLINE]
  • Patients with Bronchiectasis or Asthma (Grade 2C Recommendation)

Genetic Testing

• Isoelectric Focusing: gold standard test for identifying A1AT variants
  • Assesses Distinct Protein Migration Speed of the A1AT Variants
    • S Migrates “Slowly”
    • M Migrates in the “Middle”
    • Z Migrates the Most Slowly: “Z” was used to designate that it was “last”
• Genotype PCR (or Restriction Fragment Length Polymorphism, RFLP)
  • Detects the Most Common A1AT Alleles (F, I, S, Z)
• Gene Sequencing
  • Used When PCR and RFLP Fail to Detect the More Rare Variants or Null Alleles

Liver Biopsy (see Liver Biopsy)

• PAS-Positive Granules in Hepatocytes
  • Probably Represent Unsecreted Incompletely Glycosylated Z-Protein

Epidemiology

• Genotype Present in 2-3% of Caucasians in the US

Diagnosis

• A1AT Level: usually 12-35 µmol/L (mean level: 57% of normal)
  • Since this Group of Patients Rarely Have A1AT Level <80 mg/dL (<11 µM), They are Not Considered Candidates for A1AT Replacement Both for the Reasons Related to Their A1AT Level and Because They are Not Believed to Be at Increased Risk for Lung Disease

Clinical Manifestations

• Risk of Lung Disease
  • From Family Study Data: minimally increased risk of lung disease
  • From Population Study Data: probably no increased risk of lung disease
• Cryptogenic Cirrhosis (in Adults) (Hepatology, 1998) [MEDLINE]
• Necrotizing Panniculitis (see Panniculitis)
  • Epidemiology
    • Cases Have Been Reported

Epidemiology

• Rare Genotype

Diagnosis

• A1AT Level: usually 8-19 µmol/L (mean: 37% of normal)
  • Approximately 10% of Pi-SZ Cases Have A1AT Level <80 mg/dL (<11 µM)

Clinical Manifestations

• Emphysema: rarely occurs in this population
  • This Group Provides the Basis for the Serum Level Cut-Off of 80 mg/dL (11 μM)

Epidemiology

• Frequency of Z Allele in US and Europe: 1 in 1600 to 1 in 4000
  • Usually Whites of European Descent: as the Z allele is rare in asians and blacks
• Relationship of Pi-ZZ Genotype to Disease
  • Pi-ZZ Genotype Accounts for <10% of COPD Cases in North America
  • Pi-ZZ Genotype Accounts for >95% of Severely Deficient A1AT Deficiency Patients
  • Homozygous Pi-ZZ Disease is the Most Common Genetic Liver Disease in Children
    • Occurs in 1:1600 to 1:2800 Infants in the US
  • Approximately 10-20% of Pi-ZZ Cases Have Significant Liver Disease During Childhood
  • Family History of COPD is Usually Present in Pi-ZZ Cases, Due to the Genetic Basis of A1AT Deficiency

Diagnosis

• A1AT Level (see Serum Alpha-1 Antitrypsin): usually 2.5-7.0 µmol/L (mean level: 16% of normal)

Clinical Manifestations

General Comments

• Clinical Features Which Should Raise Suspicion of Alpha-1 Antitrypsin Deficiency (American Thoracic Society, ATS/European Respiratory Society, ERS, Alpha-1 Antitrypsin Deficiency Guidelines, 2003) (Am J Respir Crit Care Med, 2003) [MEDLINE]
  • Anti-Proteinase 3-Positive Vasculitis (C-ANCA-Positive Vasculitis)
  • Early-Onset Emphysema (Age ≤45 y/o)
  • Emphysema in the Absence of a Recognized Risk Factor (Smoking, Occupational Dust Exposure, etc)
  • Emphysema with Prominent Basilar Hyperlucency
  • Family History of Any of the Following
    • Bronchiectasis (see Bronchiectasis)
    • Emphysema (see Chronic Obstructive Pulmonary Disease)
    • Liver Disease (see Cirrhosis)
    • Panniculitis (see Panniculitis)
  • Necrotizing Panniculitis (see Panniculitis)
  • Unexplained Bronchiectasis (see Bronchiectasis)
  • Unexplained Liver Disease (see Cirrhosis)

Dermatologic Manifestations

• Necrotizing Panniculitis (see Panniculitis)
  • Epidemiology
    • Rare: <50 cases reported
    • Mean Age of Onset: 40 y/o (Am J Respir Crit Care Med, 2003) [MEDLINE]
  • Clinical
    • Hot, Painful, Erythematous Nodules or Plaques
      • Typically Located on Thighs or Buttocks
      • May Have an Oily, Yellow Discharge

Gastrointestinal/Hepatic Manifestations

• General Comments
  • Hepatic Manifestations May Occur in Infancy in Pi-ZZ Cases, But Usually Resolve: may progress to cirrhosis by time of mid-late adulthood
  • Approximately 10-20% of Pi-ZZ Cases Have Significant Liver Disease During Childhood
  • Homozygous Pi-ZZ Disease is the Most Common Genetic Liver Disease in Children: occurs in 1:1600 to 1:2800 infants in the US
  • Approximately 10-15% of Adults with A1AT Deficiency Develop Liver Disease (Am J Respir Crit Care Med, 2003) [MEDLINE]
  • Approximately 40% of Adults with Pi-ZZ Phenotype (or M[malton] Phenotype) Have Histologically-Significant Liver Injury or Cirrhosis (Best Pract Res Clin Gastroenterol, 2010) [MEDLINE]
    • Prevalence of Liver Disease at Death in A1AT Deficiency is Even Higher in Never Smokers: 28% (Thorax, 2008) [MEDLINE]
• Chronic Hepatitis/Elevated LFT’s (see Elevated Liver Function Tests)
  • Physiology
    • Pathologic Polymerization of Variant Form of A1AT, Resulting in Accumulation within Hepatocytes: appears as inclusions which stain positively with periodic acid-Schiff (PAS) reagent, but resist digestion by diastase
    • Note: this is in contrast with the proteolytic mechanism, which is responsible for the pulmonary manifestations of the disease
  • Clinical: may occur in adults without antecedent childhood hepatitis
• Cholestasis (see Elevated Liver Function Tests)
  • Physiology
    • Pathologic Polymerization of Variant Form of A1AT, Resulting in Accumulation within Hepatocytes: appears as inclusions which stain positively with periodic acid-Schiff (PAS) reagent, but resist digestion by diastase
    • Note: this is in contrast with the proteolytic mechanism, which is responsible for the pulmonary manifestations of the disease
  • Clinical
    • Disproportionate Elevation in the Alkaline Phosphatase, as Compared to the Serum Aminotransferases
    • Variable Elevation in the Serum Bilirubin
    • Variable Abnormalities in Tests of Synthetic Function
• Cirrhosis (see Cirrhosis)
  • Epidemiology
    • In Patients with Pi-ZZ Related Lung Disease, Approximately 63.2% Had a History or Clinical Findings Suggestive of Liver Disease or Had Liver Function/Ultrasound Abnormalities (Am J Respir Crit Care Med, 2013) [MEDLINE]
    • In Patients with Pi-ZZ Related Lung Disease, Approximately 17.5% Had Severe Fibrosis/Cirrhosis on Liver Biopsy (Am J Respir Crit Care Med, 2013) [MEDLINE]
      • Clinical Features Present in This Subset of Patients: higher body mass index (BMI), elevated ALT, elevated alkaline phosphatase, elevated PT (INR), maximal vital capacity, thrombocytopenia, and abnormal liver echogenicity/splenomegaly on hepatic ultrasound
  • Physiology
    • Pathologic Polymerization of Variant Form of A1AT, Resulting in Accumulation within Hepatocytes: appears as inclusions which stain positively with periodic acid-Schiff (PAS) reagent, but resist digestion by diastase
    • Note: this is in contrast with the proteolytic mechanism, which is responsible for the pulmonary manifestations of the disease
  • Recommended Diagnostic Screening
    • Liver Function Tests (LFT’s) (see Liver Function Tests)
      • Hepatic Biosynthetic Activity (Serum Albumin, Prothrombin Time) is Usually Preserved
      • Bilirubin May Be Normal
    • Platelet Count (see Complete Blood Count)
    • Hepatic Ultrasound (see Abdominal-Pelvic Ultrasound)
• Hepatic Vein Thrombosis (Budd-Chiari Syndrome) (see Hepatic Vein Thrombosis)
• Hepatocellular Carcinoma (Hepatoma) (see Hepatocellular Carcinoma)
  • Physiology
    • Pathologic Polymerization of Variant Form of A1AT, Resulting in Accumulation within Hepatocytes: appears as inclusions which stain positively with periodic acid-Schiff (PAS) reagent, but resist digestion by diastase
    • Note: this is in contrast with the proteolytic mechanism, which is responsible for the pulmonary manifestations of the disease
• Hepatosplenomegaly (see Hepatomegaly and Splenomegaly)
  • Physiology
    • Pathologic Polymerization of Variant Form of A1AT, Resulting in Accumulation within Hepatocytes: appears as inclusions which stain positively with periodic acid-Schiff (PAS) reagent, but resist digestion by diastase
    • Note: this is in contrast with the proteolytic mechanism, which is responsible for the pulmonary manifestations of the disease

Pulmonary Manifestations

• General Comments
  • Pi-ZZ Genotype Accounts for <10% of COPD Cases in North America
  • Pi-ZZ Genotype Accounts for >95% of Severely Deficient A1AT Deficiency Patients
  • Family History of COPD is Usually Present in Pi-ZZ Cases, Due to the Genetic Basis of A1AT Deficiency
• Pi-ZZ Patients Manifest a Highly Variable Extent of Lung Disease: rate of decline in Pi-ZZ patients varies from 23-316 mL per year
  • Premature Emphysema: hallmark of A1AT deficiency
  • Onset of Dyspnea (see Dyspnea)
    • Non-Smokers: median age of onset of dyspnea is 53 y/o
    • Smokers: median age of onset of dyspnea is 40 y/o
  • Symptoms of Lung Disease: rarely develop before age 25
• Risk Factors for Decline in FEV1 (Development of Lung Disease) in the Setting of A1AT Deficiency
  • Age 30-44 y/o
  • Atopy: possible, but unproven, risk factor
  • Bronchodilator Responsiveness
  • Decreased Serum A1AT Level (see Serum Alpha-1 Antitrypsin)
  • FEV1 35-79% Predicted
  • Gas/Fume Exposure
  • Interleukin-10 (IL-10) Gene Polymorphisms: maybe be associated with development of COPD in the setting of severe A1AT deficiency
  • Lower Respiratory Infections
  • Male Sex
  • Occupational Exposure to Siliceous Dusts, Metal Gases, Ozone, Pollution, Particulate Matter <10 um, and Welding
  • Recurrent Respiratory Infections
  • Tobacco Smoke Exposure (see Tobacco): severity of obstruction and age of onset are most strongly related to smoking
    • Non-Smoker Pi-ZZ Patients Rarely Develop Symptoms Prior to Age 40
    • Non-Smoker Pi-ZZ Patients Rarely Develop Obstruction Prior to Age 50’s-60’s
• Bronchiectasis (see Bronchiectasis): present in approximately 40% of the cases first reported in 1963 and in some later studies
• Chronic Bronchitis (see Chronic Obstructive Pulmonary Disease): present in about 50% of symptomatic cases
  • Bronchospasm: 27% of cases have bronchial hyperresponsiveness (asthma-like symptoms occur in 4-34% of cases)
  • Chronic Sputum Production
• Emphysema (see Chronic Obstructive Pulmonary Disease)
  • Premature Emphysema is the Hallmark of A1AT Deficiency

Rheumatologic Manifestations

• Granulomatosis with Polyangiitis (GPA) (Wegener’s Granulomatosis) (see Granulomatosis with Polyangiitis)
  • Epidemiology
    • Due to the Strong Association Between c-ANCA Positive Vasculitis and A1AT Deficiency, It Has Been Suggested that All Wegener’s Granulomatosis Patients Be Tested for A1AT (Phenotyping or Genotyping May Be Required, Since A1AT is an Acute Phase Reactant Which Increases During Active Vasculitis)

Disease Manifestations with Unclear Association with A1AT Deficiency

• Bladder Cancer (see Bladder Cancer)
• Celiac Disease (see Celiac Disease)
• Chronic Pancreatitis (see Chronic Pancreatitis)
• Colorectal Cancer (see Colorectal Cancer)
• Glomerulopnephritis (see xxxx)
  • IgA Nephropathy (see IgA Nephropathy)
  • Proliferative Glomerulonephritis
• Inflammatory Bowel Disease
  • Ulcerative Colitis (UC) (see Ulcerative Colitis)
• Vascular Manifestations
  • Fibromuscular Dysplasia (see Fibromuscular Dysplasia)
  • Intraabdominal Aneurysm
  • Intracranial Aneurysm (see Intracranial Aneurysm)
• Lung Cancer (see Lung Cancer)