Alpha Thalassemia

Content Review: January 2021 Last Update:
  • Preferred first-tier genetic test for confirmation of suspected α thalassemia or α thalassemia trait
  • Detects common, rare, and novel deletions or duplications in the α-globin gene cluster and its HS-40 regulatory region
  • Multiplex ligation-dependent probe amplification (MLPA) used to detect Hb Constant Spring (HbCS) (HBA2 c.427T>C; p.Ter143Gln); targeted Sanger sequencing is performed to assess HbCS copy number in absence of a concurrent HBA2 deletion.
  • First-tier genetic test for confirmation of suspected α thalassemia or α thalassemia trait
  • Detects common, rare, and novel deletions or duplications in the α-globin gene cluster (HBZ, HBM, HBA1, HBA2, HBQ1) and its HS-40 regulatory region
  • Comprehensive genetic test for detection of α thalassemia or α thalassemia trait
  • Detect deletional and nondeletional variants in HBA1 and HBA2
  • Diagnostic testing for α thalassemia in fetus with suggestive clinical findings or at risk for α thalassemia due to familial HBA1/HBA2 deletions or HbCS variant
  • Detects common, rare, and novel deletions or duplications in the α-globin gene cluster and its HS-40 regulatory region
  • MLPA is used to detect HbCS (HBA2 c.427T>C; p.Ter143Gln); targeted Sanger sequencing is performed to assess HbCS copy number in absence of a concurrent HBA2 deletion.

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 are 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

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 both 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 or HBcS)
    • 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.
  • Alpha-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

Alpha Globin (HBA1 and HBA2) Sequencing and Deletion/Duplication

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: at least 90%, varies by ethnicity 
    • Sequencing: up to 15%, varies by ethnicity 

Results

Result Variant(s) Detected Interpretation

Positive

Genotype (-a/aa)

 

Silent carrier

Genotype (-a/-a) or (--/aa)

 

α Thalassemia trait

1 pathogenic sequence variant

 

Silent carrier or α thalassemia trait

Genotype (--/-a)

HbH disease

 

Genotype (--/--)

 

Hb Bart hydrops fetalis syndrome

Genotype (aa/aaa)

Extra functional alpha globin gene copy is present

Negative

No pathogenic variants detected

Greatly decreased probability that the individual is affected with, or a carrier of, α thalassemia

Inconclusive

Variant(s) of uncertain clinical significance identified

Unknown clinical significance

Limitations

  • Diagnostic errors can occur due to rare sequence variations.
  • Sequence analysis will not detect all regulatory region variants or variants in α-globin cluster genes other than HBA1 and HBA2.
  • Sequencing of both HBA1 and HBA2 may not be possible in individuals harboring large α-globin deletions on both alleles.
  • This assay is unable to sequence HBA2-HBA1 fusion genes; thus, HBA1 or HBA2 sequence variants occurring in cis with a 3.7 kb deletion or other HBA2-HBA1 hybrid gene will not be detected.
  • It may not be possible to determine the phase of identified sequence variants.
  • Specific breakpoints of large deletions/duplications will not be determined; therefore, it may not be possible to distinguish deletion variants of similar size.
  • 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.

References