Gamma Globin (HBG1 and HBG2) Sequencing

  • Assess for gamma globin gene variants resulting in neonatal hemolytic anemia, cyanosis, or methemoglobinemia in symptomatic infants when other etiologies have been excluded.
  • Assess for nondeletional HPFH in individuals with elevated fetal hemoglobin.
  • Characterize abnormal hemoglobins identified by electrophoresis suspected to represent gamma chain variants.
Related Tests
  • Comprehensive test for diagnosis of thalassemia or hemoglobinopathy in an individual with hematologic or clinical findings.
  • Not recommended for routine carrier screening in healthy adults for purposes of reproductive decision-making.
  • Determine etiology of unexplained hemolytic anemia or family history of unexplained hemolytic anemia.
  • Determine etiology of unexplained hyperbilirubinemia in neonates.  
  • Effective test for screening and follow-up of individuals with known hemoglobinopathies.
  • The optimal test for the initial diagnosis of a suspected hemoglobinopathy is Hemoglobin Evaluation Reflexive Cascade.

Fetal hemoglobin (Hb F) is the predominant hemoglobin in the fetus and is comprised of two alpha globin chains and two gamma chains. A-gamma is expressed from the HBG1 gene and G-gamma is expressed from the HBG2 gene. Promoter variants in either HBG1 or HBG2 can result in nondeletional hereditary persistence of fetal hemoglobin (HPFH), a clinically benign condition but can ameliorate disease severity in sickle cell disease and thalassemia. The majority of genetic variants in the gamma genes are clinically benign; however, rare variants that result in qualitative defects may produce a clinical phenotype in neonates. Unstable variants may result in hemolytic anemia/hyperbilirubinemia, high- or low-oxygen affinity variants may present as erythrocytosis or cyanosis, and M-hemoglobin variants may cause methemoglobinemia. Such clinical symptoms related to gamma chain variants typically resolve after 6 months of age due to the gamma- to beta-globin switch.

Disease Overview

Expression of variant gamma proteins is related to the overall expression of Hb F and will vary over time.

Clinical symptoms related to gamma chain variants commonly resolve after the first 6 months of life, given the normal switch from fetal hemoglobin expression to adult hemoglobin expression that occurs at that time.

Clinical presentations in neonates include:

  • Hemolytic anemia/hyperbilirubinemia
  • Erythrocytosis
  • Cyanosis
  • Methemoglobinemia

HPFH may occur in adults:

  • A clinically benign disorder characterized by elevated Hb F into adulthood
  • When coinherited with sickle cell disease or beta thalassemia, HPFH may ameliorate disease severity

Genetics

Genes

HBG1 (A-gamma) and HBG2 (G-gamma)

Inheritance

Autosomal dominant

Variants

Over 100 gamma globin variants have been described, many of which are clinically benign.

  • Qualitative defects:
    • Unstable variants may result in hemolytic anemia/hyperbilirubinemia
    • High- or low-oxygen affinity variants may result in erythrocytosis or cyanosis, respectively
    • M hemoglobin variants may cause methemoglobinemia
  • Quantitative defects
    • Promoter region variants may be associated with non-deletional HPFH, a clinically benign condition
    • Polymorphisms may influence expression of the gamma genes

Test Interpretation

Sensitivity/Specificity

  • Clinical sensitivity: Unknown
    • Gamma globin variants are a rare cause of neonatal hemolytic anemia, cyanosis, erythrocytosis, or methemoglobinemia
  • Analytical sensitivity/specificity: >99%

Test Methodology

  • Long-range polymerase chain reaction (PCR) followed by nested PCR and bidirectional sequencing of all coding regions, intron/exon boundaries, proximal promoter, and 5’ proximal promoters of the HBG1 and HBG2 genes

Results

  • No pathogenic variants detected
    • Reduces the likelihood of a gamma globinopathy or nondeletional gamma HPFH
  • Pathogenic variant detected
    • Consistent with a diagnosis of a gamma globinopathy or nondeletional gamma HPFH
    • Clinical manifestations are variable and dependent on the effect of the identified variant on protein function/expression and on the age of the patient
    • Correlation with hematologic parameters and hemoglobin electrophoresis results is strongly recommended
  • Gamma globin gene sequencing may identify variants of unknown clinical significance

Limitations

  • Diagnostic errors can occur due to rare sequence variations or repeat element insertions
  • Large deletions/duplications, distal regulatory region variants, deep intronic variants, and hybrid gene events will not be detected
Additional Resources