Hemoglobin, Gamma G

Alternative Names

  • HBG2
  • Hemoglobin--Gamma Locus, 136 Glycine
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OMIM Number

142250

NCBI Gene ID

3048

Uniprot ID

P69892

Length

252,418 bases

No. of Exons

3

No. of isoforms

1

Protein Name

Hemoglobin Subunit Gamma-2

Molecular Mass

16126 Da

Amino Acid Count

147

Genomic Location

chr11:5,253,187-5,254,780

Gene Map Locus
11p15.4

Description

The solution to the fetus's problem of getting oxygen from its mother's blood involves the development of a fetal hemoglobin. The hemoglobin in fetal red blood cells differs slightly from that in adult corpuscles. Two of the four peptides of the fetal and adult hemoglobin chains are identical - the alpha chains - but adult hemoglobin has two beta chains, while the fetus has two gamma chains. Normal beta-chains bind the natural regulator diphosphoglycerate, which assists in the unloading of oxygen. The gamma-chain isoforms do not bind diphosphoglycerate as well and therefore have a higher affinity for oxygen. In the low-oxygen environment of the placenta, oxygen is released from adult hemoglobin. In this same environment, fetal hemoglobin does not give away oxygen, but binds it. This small difference in oxygen affinity mediates the transfer of oxygen from the mother to the fetus. Within the fetus, the myoglobin of the fetal muscles has an even higher affinity for oxygen, so oxygen molecules pass from fetal hemoglobin for storage and use in the fetal muscles. Fetal hemoglobin is not deleterious to the newborn, and in humans, the replacement of fetal hemoglobin-containing blood cells with adult hemoglobin-containing blood cells is not complete until about 6 months after birth.

Molecular Genetics

The gamma locus determines the gamma chain of fetal hemoglobin (alpha-2/gamma-2). Two types of gamma polypeptide chains do exist. Although not distinguishable by most of the physical methods used, sequencing has shown at least 1 amino acid difference: at position 136 one type has glycine (G-gamma) and the second type has alanine (A-gamma). The ratio of G-gamma to A-gamma is fairly constant (about 7:3) during the fetal period. The ratio declines progressively during the postnatal gamma-to-beta switch, leading to an average value of 2:3 in the small residual amount of Hb F detectable in normal adult blood. In a normal human genome there are usually four gamma structural loci, two on each autosome that presumably arose by gene duplication.

A number of mutations have been identified that interfere with the normal process of hemoglobin switching and result in hereditary persistence of fetal hemoglobin. Several single-base substitutions located within the promoter regions of the gamma genes appear to be responsible for the hereditary persistence of fetal hemoglobin phenotype.

Hematologic correlations with restriction mapping suggest that a region of DNA near the 5-prime end of the delta gene may be involved in the cis-suppression of gamma-globin gene expression in adults. The beta-globin locus control region is a powerful regulatory element required for high-level globin gene expression. Nucleotide sequences of the beta-globin locus control region were originally described as a cluster of DNase I hypersensitive sites 6 to 18 kb upstream to the epsilon-globin gene. The hemoglobin beta locus control region is thought to organize the entire 60-kb beta-globin gene cluster into an active chromatin domain and allows a high level of position-independent, copy number-dependent, and erythroid-specific globin gene expression.

 

Epidemiology in the Arab World

View Map
Variant NameCountryGenomic LocationClinical SignificanceCondition(s)HGVS ExpressionsdbSNPClinvar
Emirates NM_000184.3:c.178A>GUnited Arab Emirateschr11:5254429PathogenicFetal Hemoglobin Quantitative Trait Locus 1NG_000007.3:g.43187A>G; NM_000184.3:c.178A>G; NP_000175.1:p.Lys60Glu2893307814997
NM_000184.2:c.-167C>AAlgeriachr11:5254895PathogenicFetal Hemoglobin Quantitative Trait Locus 1NG_000007.3:g.42721C>A; NM_000184.2:c.-167C>A480944915001
NM_000184.2:c.-211C>TSaudi Arabiachr11:5254939Uncertain SignificanceFetal Hemoglobin Quantitative Trait Locus 1NG_000007.3:g.42677C>T; NM_000184.2:c.-211C>T748214414984

Other Reports

Saudi Arabia

Wood et al. (1980) studied fetal hemoglobin (HbF) synthesis in 22 cases of sickle cell anemia from Saudi Arabia and compared with an equal number of cases of African origin. Among the Saudi Arabs gamma chain synthesis ranged from 4.0% to 19.9% of the total non-alpha chain synthesis (mean 8.1%) while the corresponding range for the Negro cases was < 0.3% to 4.6% (mean 1.7%). In both groups the peripheral blood HbF level was on average 3-4 times higher than the proportion synthesized, indicating that the selective survival of HbF containing cells (F cells) was an important factor in determining the final HbF levels. Among the Saudi Arab cases there was a significant negative correlation between the degree of F cell enrichment and either the HbF level of the percentage gamma chain synthesis. No such correlation was observed among the Negro cases. Wood et al. (1980) realized that a high proportion of the cases in both groups were carriers of alpha thalassemia in addition to sickle cell, but did not observe any effect of alpha thalassemia on HbF production.

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