- 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.
High Performance Liquid Chromatography/Electrophoresis/RBC Solubility/Polymerase Chain Reaction/Fluorescence Resonance Energy Transfer/Sequencing
- 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.
Massively Parallel Sequencing
- Determine etiology of unexplained hemolytic anemia or family history of unexplained hemolytic anemia.
- Determine etiology of unexplained hyperbilirubinemia in neonates.
High Performance Liquid Chromatography/Electrophoresis/RBC Solubility
- 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.
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
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
HBG1 (A-gamma) and HBG2 (G-gamma)
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
- Clinical sensitivity: Unknown
- Gamma globin variants are a rare cause of neonatal hemolytic anemia, cyanosis, erythrocytosis, or methemoglobinemia
- Analytical sensitivity/specificity: >99%
- 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
- 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
- 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
Manca L, Masala B. Disorders of the synthesis of human fetal hemoglobin. IUBMB Life. 2008;60(2):94-111.
Patrinos GP, Giardine B, Riemer C, et al. Improvements in the HbVar database of human hemoglobin variants and thalassemia mutations for population and sequence variation studies. Nucleic Acids Res. 2004;32(Database issue):D537-D541.