Alpha Thalassemia

  • Preferred first-tier genetic test for confirmation of suspected α-thalassemia or α-thalassemia trait
  • Detect common, rare, and novel deletions or duplications in the α-globin gene cluster (HBZ, HBM, HBA1, HBA2, HBQ1) and its HS-40 regulatory region
  • Acceptable first-tier genetic test for confirmation of suspected α-thalassemia or α-thalassemia trait
  • Assesses for seven common deletions of HBA1 and HBA2 (-α3.7, -α4.2, -(α)20.5, --SEA, --MED-I, --FIL, and --THAI)
  • Comprehensive genetic test for detection of α-thalassemia or α-thalassemia trait
  • Detect deletional and nondeletional variants in HBA1 and HBA2
  • Optimal test for the initial and confirmatory diagnosis of any suspected hemoglobinopathy
  • Cascade reflex testing may include electrophoresis, solubility testing, and/or molecular analysis of the globin genes
  • Effective test for screening and follow-up of individuals with known hemoglobinopathies
  • Optimal test for the initial diagnosis of a suspected hemoglobinopathy is Hemoglobin Evaluation Reflexive Cascade

Useful when a pathogenic familial variant identifiable by sequencing is known

Fetal test to detect a previously characterized sequence variant in a family member

Alpha (α)​ thalassemia is the most common inherited disorder of hemoglobin (Hb) worldwide and is caused by HBA1 and HBA2 gene variants. Decreased or absent synthesis of the hemoglobin (Hb) α-chain results in clinical presentations ranging from asymptomatic silent carriers to severe anemia and fetal lethality. The two clinically significant forms of α-thalassemia are Hb Bart hydrops fetalis syndrome and hemoglobin H (HbH) disease. Alpha thalassemia is found more often in certain ethnicities, including African, African American, Mediterranean, Middle Eastern, and Southeast Asian.

Disease Overview

Prevalence and/or Incidence

  • Most common inherited disorder of Hb worldwide
  • Carrier frequencies in high-risk populations
    • African, African American: 1/3
    • Middle Eastern, Southeast Asian: 1/20
    • Mediterranean: 1/30-50
  • Hb Bart hydrops fetalis syndrome and HbH disease more frequent in Southeast Asian, Asian Indian, and Mediterranean populations than in African populations

Symptoms

Phenotype Associated Symptoms

α-thalassemia silent carrier

Typically asymptomatic, though borderline anemia or mild microcytosis may be present

Often misdiagnosed as iron deficiency

Normal Hb electrophoresis

α-thalassemia trait

Mild microcytic anemia may be present

Often misdiagnosed as iron deficiency

Normal Hb electrophoresis

HbH disease

Moderate to severe form of α-thalassemia

Moderate microcytic hypochromic anemia

Hemolysis with Heinz bodies

Splenomegaly

Rare extramedullary hematopoiesis

Propensity for acute hemolysis after oxidative stress, drug therapy, or infection

Hb Bart hydrops fetalis syndrome

Most severe form of α-thalassemia

Risk for fetus

  • Lethal in fetal or early neonatal period
  • Generalized edema, ascites, pleural and pericardial effusions
  • Severe hypochromic anemia
  • Usually detected on ultrasound at 22-28 weeks gestation

Maternal complications during pregnancy

  • Preeclampsia
  • Polyhydramnios or oligohydramnios
  • Antepartum hemorrhage
  • Premature delivery
α, alpha

Pathophysiology​

Typically, individuals have four functioning α-globin genes (αα/αα). Two genes, HBA1 and HBA2, are present on each copy of chromosome 16, and α-globin chains function as subunits of fetal Hb  (HbF: α2γ2) and adult Hb (HbA: α2β2). The number of α-globin genes deleted or inactivated correlates with different α-thalassemia phenotypes. Genotype/phenotype correlations in α-thalassemia are complex and may be influenced by coinheritance of other Hb variants or α-globin gene duplications. 

Phenotype Genotype(s)

α-thalassemia silent carrier

-α/αα

α-thalassemia trait

-α/-α

--/αα

HbH disease

--/-α

Hb Bart hydrops fetalis syndrome

--/--

Genetics

Genes

HBA1 and HBA2

Inheritance

Autosomal recessive

Variants

  • HBA1 and HBA2 large gene deletions account for approximately 90% of pathogenic α-thalassemia variants
    • -α3.7 and -α4.2 deletions result in the deletion of a single gene
    • -(α)20.5, --SEA, --MED-I, --FIL, and --THAI deletions result in the deletion of the HBA1 and HBA2 genes from the same chromosome
  • Sequence variants and regulatory region variants occur mainly in HBA2 and account for up to 15% of causative variants
    • Nondeletional variants include
      • Sequence variants that inactivate the gene
      • Small insertions/deletions
      • Variants that result in unstable α-globin protein (eg, Hb Constant Spring)
    • Nondeletional α-globin variants may be pathogenic or benign
      • Both may result in an abnormal protein detectable by Hb evaluation
      • Pathogenic nondeletional variants often have a more severe effect than single gene deletions
  • α-globin gene duplication results in three or more active α-globin genes on a single chromosome
    • Typically benign
    • May alter expected clinical phenotypes and hematological features when coinherited with beta (β) thalassemia

Test Interpretation

Sensitivity/Specificity

  • Analytical sensitivity/specificity: 99% for both duplication/deletion analysis and sequencing
  • Clinical sensitivity: most pathogenic HBA1 and/or HBA2 gene variants are large deletions not detectable by sequencing
    • Deletion: up to 90%, depending on ethnicity 
    • Sequencing: up to 15%, depending on ethnicity 
Results and Limitations

 

Alpha Thalassemia (HBA1 and HBA2) 7 Deletions

Alpha Globin (HBA1 and HBA2) Deletion/Duplication

Alpha Globin (HBA1 and HBA2) Sequencing

Negative result

No common α-globin gene deletions were detected

  • Risk for α-thalassemia is reduced but not excluded

No large α-globin deletions or duplications were detected

  • Risk for α-thalassemia is reduced but not excluded

No pathogenic variants were detected

  • Risk for α-thalassemia is reduced
  • Large deletions of the α-globin genes, which account for the majority of variants, are not detected by sequencing

Positive result

Predicted genotype (-α/αα)

  • Individual is predicted to be a silent carrier

Predicted genotype (-α/-α) or (--/αα)

  • Individual is predicted to have α-thalassemia trait

Predicted genotype (--/-α)

  • Individual is predicted to be affected with HbH disease

Predicted genotype (--/--)

  • Result is consistent with Hb Bart hydrops fetalis syndrome

Predicted genotype (-α/αα)

  • Individual is predicted to be a silent carrier

Predicted genotype (-α/-α) or (--/αα)

  • Individual is predicted to have α-thalassemia trait

Predicted genotype (--/-α)

  • Individual is predicted to be affected with HbH disease

Predicted genotype (--/--)

  • Result is consistent with Hb Bart hydrops fetalis syndrome

Predicted genotype (ααα/αα)

  • An extra functional α-globin gene present

1 pathogenic variant detected

  • Individual is predicted to be a silent carrier or carrier of α-thalassemia
  • A more severe disorder is possible if another undetected α-globin variant is present

2 pathogenic variants detected

  • Individual is predicted to be a carrier of α-thalassemia; mild microcytic anemia often present
  • Homozygosity or compound heterozygosity for nondeletional variants results rarely in HbH disease

Inconclusive result

n/a

Deletion or duplication of unknown clinical significance detected

Variant of unknown clinical significance detected

Limitations

Rare α-globin gene deletions, nondeletional variants, gene duplications and variants of the regulatory region will not be detected

Diagnostic errors can occur due to rare sequence variations

Rare syndromic or acquired forms of α-thalassemia will not be detected

 

Breakpoints of large deletions/duplications will not be determined; therefore, it may not be possible to distinguish variants of similar size

This assay does not assess for nondeletional variants within the coding or regulatory regions of the α-globin cluster genes

Individuals carrying both a deletion and duplication within the α-globin gene cluster may appear to have a normal number of α-globin gene copies

Rare syndromic or acquired forms of α-thalassemia associated with ATRX variants will not be detected

Diagnostic errors can occur due to rare sequence variations

Large deletions/duplications and some variants of the regulatory regions will not be detected

The phase of identified variants may not be determined

Diagnostic errors can occur due to rare sequence variations

Sequencing of both HBA1 and HBA2 may not be possible in individuals harboring large α-globin deletions on both alleles

Rare syndromes associated with α-thalassemia, such as ATR-X and ATR-16, will not be detected

n/a, not applicable

References