The human β-globin locus is a cluster of genes located on chromosome 11. It is responsible for creating the β-chains found in hemoglobin. This cluster consists of 5 genes: ϵ, Gγ, Aγ, δ, and β genes, that respective order downstream. These genes are controlled by a locus control region (LCR) and are differentially expressed throughout development. Expression of these genes is regulated in embryonic erythropoiesis by many transcription factors, including KLF1, which is associated with the upregulation of adult hemoglobin in adult definitive erythrocytes, and KLF2, which is vital to the expression of embryonic hemoglobin.
Mechanism The human beta globin (HBB) locus is located on the short arm of chromosome 11 in an AT-rich region and is part of the olfactory receptor (OR) gene superfamily. The locus is crucial to the production of the β-globin chain of hemoglobin A (HbA), which, alongside the α-globin chain derived from the human alpha globin (HBA) locus, constitutes healthy adult hemoglobin. The locus, spanning across approximately 70 kilobases, includes five individual beta globin-like genes: ϵ, Gγ, Aγ, δ, and β, arranged in that order. The HBB locus is controlled by a 34kb long locus control region (LCR) upstream of the ϵ-gene. The position of each gene from the LCR dictates its temporal regulation, with the genes closer to the LCR being expressed earlier in development, and those further being expressed later in development. Overall, the output of the HBB locus is regulated, regardless of how the individual genes are differentially expressed, to equal the output of its counterpart HBA locus. This ensures that the synthesis of beta globin locus products is in line with the other components that form hemoglobin, the final functional product.
Regulation
Fetal to adult switching
In the early first trimester, an embryonic form of β-globin, ϵ-globin, is produced from the expression of the ϵ-gene. As the fetus develops, tetrameric fetal hemoglobin (HbF) is formed from the fusion of the two γ-globin chains encoded by the γ-genes found in the β-globin locus with adult α-globin chains. This occurs as soon as the first definitive erythrocytes are being produced by the fetal liver from stem and progenitor cells. In healthy humans, the production of HbF drops off at around 9 months of gestational age contrasted by a simultaneous increase of adult hemoglobin (HbA). This switching of hemoglobin production from γ-globin to β-globin in healthy humans is a result of a shift from Gγ and Aγ gene expression to β- and δ-gene expression.
Note that this process is neither complete nor irreversible as adults are able to produce γ-globin at quantities lower than 1% of their total hemoglobin.
Expression The regulatory genes required for basal transcription and developmental regulation are found within the first 500 base pairs from the 5'-end of the β-globin locus. Three regulatory elements within the promoter region of the gene cluster have been found to exhibit significantly reduced transcription levels following induced mutation indicating their role in gene regulation. These elements consist of a CACCC box, a CCAAT box, and a TATA box located between base pairs -95 to -30 relative to the start site of transcription. The promoter region of the adult β-globin gene has a regulatory element difference compared with the fetal γ-globin promoter region, consisting of a duplicate of the CACCC box, a single CAAT box, and a single TATA box.
Higher levels of β-locus gene expression necessarily involves the β LCR gene located 6-22 kilobase pairs from the 5'-end. The LCR gene consists of five DNase I hypersensitive sites (HS1-HS5). Each site is capable of being bound by different proteins with varying outcomes. Transcription factors include the proteins NF-E2, GATA-1, and EKLF which bind to sites HS1 to HS4. Specific HS binding patterns also vary widely between species. For example, NF-E2 and GATA-1 are highly abundant at HS2 and HS1/HS4 binding sites, respectively, during the expression of adult β gene and fetal mouse with transgenic γ-globin genes. Nonetheless, there is still much discrepancy surrounding the role and extent to which each HS site of the LCR gene plays due to conflicting results across various studies.
Structure
Each gene within the beta globin locus produces its own protein product, a globin chain, when transcribed. For essentially all types of hemoglobin (Hb) at all stages of development, half of the globin chains come from the alpha globin locus (HBA), and half will come from the beta globin locus (HBB). Thus, two globin chains from the HBB locus will form a tetramer alongside its counterpart globin changes from the HBA locus, ferrous iron (Fe2+) in heme groups, and other groups to produce a functioning hemoglobin molecule. The ferrous iron is able to combine with a single oxygen molecule, becoming oxidized to ferric iron (Fe3+). Since there are 4 such ferrous iron ions in hemoglobin, a single molecule is able to bind 4 oxygen molecules. Furthermore, cooperativity, the mechanism under which hemoglobin undergoes structural changes to better bind subsequent oxygen molecules, enables progressively easier binding. This is a reversible process, allowing hemoglobin to bind oxygen in the lungs, and reversing the binding, releasing the oxygen when it reaches its target destination. Cooperativity also works inversely.
Globin chains range from 141-146 amino acids in length, with different chains, each being designated by a greek letter, being the result of varying amino acid sequences. Each globin change is subsequently divided by eight helices, separated by seven non helical segments. The 8 helices, which are rigid and linear, receive an alphabetical designation from A to H. On the other hand, the non helical, connecting segments are designated by the helices they connect. For example, the segment connecting helices A and B would be designated as AB.
Function Throughout different developmental stages, different globin chains are used to produce different types of hemoglobin suitable for what that developmental stage aims to accomplish. Thus, genes are subject to temporal regulation in both the HBB or HBA locus to ensure that during the proper globin chains are synthesized in their respective developmental stages. Generally, the 3 developmental stages are considered as embryonic, fetal, and adult. The embryonic stage features 4 types of hemoglobin: HbE Portland 1, composed of two ζ-globin and two γ-globin chains, HbE Portland 2, composed of two ζ-globin and two β-globin chains, HbE Gower-1, composed of two ζ-globin and two ε-globin chains, and HbE Gower-2, composed of two α-globin and two ε-globin chains. The fetal stage features 2 types: HBF, composed of two α-globin and two γ-globin chains, and HBA, composed of two α-globin and two β-globin chains. In the adult stage, there are 3 types of hemoglobin: both types of hemoglobin from the fetal stage and HBA2, composed of two α-globin and two δ-globin chains. While these may be all the possible types of hemoglobins that may be found at a certain development stage, the actual distribution of these types can be quite extreme. For instance, nearly all hemoglobin in adults is of the HbA variant, with only small amounts of detectable HbA2 and HbF.
Different chains also have varying affinities for one another, accomplished through differences in charge. Globin chains are held together at tetramer-dimer interfaces, which vary in strength depending on the combination of chains. As development transitions from one stage to the next, globin chains will compete with one another to tetramize into hemoglobin. Later stage development globin chains having stronger affinity, allowing for a smooth transition between hemoglobin types as the higher affinity new globin chains outcompete the lower affinity globin chains from the previous developmental stage.
The affinity of the ferrous iron for oxygen is also reliant on the type of globin chain that composes the hemoglobin. Embryonic phase hemoglobins have a very high affinity for oxygen in comparison to those of later developmental stages; this is necessary for the growing fetus to adapt to both its changing environment and its own changing physiology. The vast number of environmental conditions it must account for at this stage of development is the reasoning behind the embryonic stage possessing the most types of hemoglobin - each serves a different purpose to allow for oxygen transport. Diffusional oxygen transport in the amniotic fluid to placental circulation is just an example of the dramatic environmental changes faced by the embryo in this early stage of life. As the embryo develops into a fetus and the circulatory system takes shape, these embryonic hemoglobins are discarded in favor of more mature hemoglobins, although the specialized fetal hemoglobin (HbF) is still an adaptation for the fetus' dependency on placental blood for gas exchange. The fetal hemoglobin necessarily needs to have higher affinity for oxygen in order to outcompete their mother's adult hemoglobin for oxygen. Finally, as the development continues and birth occurs, the higher HbF oxygen affinity is no longer a necessity as the baby can now supply its own oxygen, leading to a final transition to the adult hemoglobin variants.
Timing and localization ϵ-globin, is first found in the yolk sac from the first three to eight weeks of pregnancy. ϵ-globin production drops off while there is a sharp increase in fetal hemoglobin (HbF) beyond the sixth week of pregnancy. HbF is first found in the liver starting from week six lasting week thirty, then in the spleen from weeks nine to twenty-eight, and finally in the bone marrow from weeks twenty eight to birth. After birth, most of the HbF is replaced by HbA by six to twelve months of age. In adults, the β- and δ-globins are produced in the bone marrow.
In transgenic mice with the human β-globin gene cluster, ϵ-globin and HbA from the γ-genes are expressed in the embryonic yolk sac stage. The γ-genes are also expressed in the fetal liver and adult bone marrow while β-globin is only expressed during the fetal liver and adult bone marrow.
Evolution
The genes of premature β-like globins, ϵ-globin and HbA, are not unique to humans and similar analogs can be found in all jawed vertebrates. All jawed vertebrates are capable of producing an embryonic specific globin in the yolk sac and adult hemoglobin in the bone marrow but only some species are able to produce a fetal globin that is formed in the liver. Notably, the β-globin locus belonging to birds are distinct from those found in mammals suggesting a shared ancestor or convergent evolution.
In most amniotes, the β-like globin genes and the ɑ-like globin genes are located on different chromosomes requiring coordination of expression to maintain a balance. For fish species, however, orthologs of the ɑ-like globin gene cluster contains both the β-like globin genes and the ɑ-like globin genes on the same chromosome. The larger cluster also requires its ɑ and β components to be differentially expressed during development.
ɑ-like and β-like genes can found in the locus LA found between the three genes LCMT1, AQP8, and ARHGAP17 in species of fish. However, in humans, platypus, chickens, and frogs, the globin genes are missing from LA suggesting either the common ancestor of jawed vertebrates had ɑ-like and β-like genes and tetrapods lost them or that the globin genes were novelly introduced into that region. In another locus MN between genes MPG and NPRL3, placental mammals and chickens only have ɑ-like globin genes whereas platypus and marsupials have ɑ-like and β-like globin genes in the region. Regardless of the difference, with the presence of globin genes in MN, it is highly likely that the last common ancestor of vertebrates had globin genes at the MN locus.