Embryonic hemoglobin

Embryonic hemoglobin is a tetramer produced in the blood islands in the embryonic yolk sac during the mesoblastic stage (from 3rd week of pregnancy until 3 months). The protein is commonly referred to as hemoglobin ε.

In humans, the earliest blood islands containing primitive erythroid cells appear in the yolk sac at roughly 17 days of development.^[1] These first erythrocytes are large, nucleated cells that produce globin chains and begin circulating in the embryo around day 22, coinciding with the onset of cardiac activity.^[1]

Although adult hemoglobin is well known to science, embryonic hemoglobin is considered little-known due to its difficult nature being acquired and given ethical contradictions.^[2] In recent years, transgenic mice have been able to overcome those difficulties and allow research of embryonic hemoglobins in their native states.^[2] Comparing embryonic globins and hemoglobins in humans and mice allow scientists to study their evolutional purposes by looking at when each form is expressed, which globin subunits they contain, and which erythroid or non-erythroid cell types they are associated with.^[3]

The physiologic properties of embryonic hemoglobin include high oxygen affinity. Embryonic hemoglobins are proteins that are able to effectively bind oxygen in environments, embryonic tissue and placental barrier, where there are little amounts of oxygen.^[2][3] The function of embryonic hemoglobin is carried out in large part to the Bohr Effect, an effective exchange between carbon dioxide and oxygen. The HbE protein is capable of oxygen-binding avidity regardless of acidosis.^[3] Since both fetal and embryonic hemoglobins are exposed to lower amounts of oxygen, they bind more readily than adult Hb.^[2]

The human embryonic haemoglobins were discovered in 1961.^[4][5] These include Hb-Gower 1, consisting of 2 zeta chains and 2 epsilon chains, and Hb-Gower 2, which consists of 2 αlpha-chains and 2 epsilon-chains, the zeta and epsilon chains being the embryonic haemoglobin chains. Not found in embryonic hemoglobins are adult type β chains.^[3]

Similar to other mammalian hemoglobins, human embryonic hemoglobins are composed of four globin subunits. These subunits each average about 17,000 Daltons and consist of two α-like and two β-like.^[3]

Hemoglobin Gower 1 (also referred to as ζ₂ε₂ or Hb Gower-1) is a form of hemoglobin existing only during embryonic life, and is the primary embryonic hemoglobin. It is composed of two zeta chains and two epsilon chains, and is relatively unstable, breaking down easily.^[6]

Hemoglobin Gower 2 (also referred to as α₂ε₂ or Hb Gower-2) is a form of hemoglobin existing at low levels during embryonic and fetal life. It is composed of two alpha chains and two epsilon chains, and is somewhat unstable, though not as much as hemoglobin Gower 1.^[6] Due to its relative stability compared to hemoglobin Gower 1 and hemoglobin S, it has been proposed as a subject for reactivation in the adult in cases of severe β thalassemia and hemoglobinopathies in subjects for which the reactivation of hemoglobin F is contraindicated due to toxicity concerns.^[6] β thalassemia results from diminished or absent or absent β-globin synthesis, leading to chronic anemia.^[1] Patients who maintain higher levels of γ-globin expression typically experience a less severe form of the disease.^[1]

Hemoglobin Portland I (also referred to as ζ₂γ₂ or Hb Portland-1) is a form of hemoglobin existing at low levels during embryonic and fetal life, composed of two zeta chains and two gamma chains.^[6]

Hemoglobin Portland II (also referred

to as ζ₂β₂ or Hb Portland-2) is a form of hemoglobin existing at low levels during embryonic and fetal life, composed of two zeta chains and two beta chains. It is very unstable, even more so than hemoglobin Gower 1, and breaks down very rapidly under stress.^[6] Despite this, it has been proposed as a candidate for reactivation in cases of severe α thalassemia or hemoglobinopathies afflicting the alpha chain.^[6][7]

Hemoglobin Portland III (also referred to as ζ₂δ₂ or Hb Portland-3) is a form of hemoglobin which has only been detected in stillborn infants with α thalassemia major, in which the alpha chain is completely absent. It is composed of two zeta chains and two delta chains.^[8][9] Hemoglobin Portland III is quite unstable, though less so than hemoglobin Portland II.^[8]

In humans, as a baby gestates, the body creates and utilizes three different versions of hemoglobin. The first version acts in the embryonic stage, in the baby's yolk sac, to create ε-globin.^[10] The onset of yolk sac erythropoiesis occurs as mesenchymal cells transition into hematopoietic stem cells capable of producing blood cells.^[3] The three embryonic human hemoglobins present during the first few months after conception are Portland-1, Gower-1, and Gower-2.^[2] This is embryonic hemoglobin and focuses on effectively binding to oxygen in a low oxygen environment.^[2] Embryotic chains seen in the early embryos are mostly composed of one α-like and the other β-like.^[3]

Shifting to the fetal stage, with fetal Hb synthesis detected as young as 37-day-old embryos, the yolk sac erythroid cell population of γ genes and α genes are expressed together (both definitive erythropoiesis).^[3] The body focuses on the fetal liver and starts making γ-globin which is more efficient at binding oxygen from the carrier's blood during this stage.^[2][10] Within the β-globin gene cluster, two duplicated γ-globin form the necessary molecules. Then, the γ-globin chains and adult α-globin chains combine to form stable tetramers (HbF).^[10]

Fetal hemoglobin is predominant until birth, where after, an expression switch is made from HbF to HbA (adult hemoglobin). This is a switch from γ-globin to β-globin.^[10] At a pH of 7.25 and below, oxygen affinity of fetal and adult hemoglobin decrease, stimulated by tissue acidosis.^[3]

These developmental expressions can be seen as the subunit interfaces of the hemoglobins strengthen progressively during development using low concentration gel-filtered conditions, showing evidence of shifting tetramer, dimer, and monomer proportions.^[11] This maturation reflects competition among globin chains for partners that form more stable interactions. Therefore, intrinsic biochemical properties of globin proteins themselves can influence the course of developmental transitions.^[11]

Both experimental and clinical observations have shown that embryonic hemoglobin genes can become active outside their normal developmental period. This causes the appearance of erythroid cells of newborns and adults. These disruptions in globin gene regulation often occur in conditions involving chromosomal abnormalities and suggest that structural changes in genomic DNA can lead to atypical patterns of globin expression.^[3] The mechanisms that shut down expression of specific hemoglobin, reflected in the disappearance of certain hemoglobin types, are not well define.^[11] Differences in the stability of embryonic, fetal, and adult globin mRNAs are thought to contribute to developmental silencing.^[11]

For example, chromosomal abnormalities can lead to a delay in switching from embryonic hemoglobin.^[12] The coordinated switching of globin genes, driven by scientific promoter activity and repressor mechanisms, is generally used to explain how the first three hemoglobin types are normally turned on and off during development.^[2] The genes that encode α-like and β-like globins are organized into two distinct clusters located on different chromosomes.^[3] Within each cluster, the genes are arranged linearly from an embryonic globin gene at the 5' end to an adult globin at the 3' end, mirroring the sequence in which they are activated during development.^[3] It is important to note that all hemoglobin molecules can undergo post-transitional changes that give rise to minor hemoglobin variants.^[13]

Naturally occurring mutations that elevate γ-globin production are found within promoters of the γ-globin genes, and these altered sites overlap with binding motifs for the transcription factors BCL11A and ZBTB7A.^[1] This has produced difficulties in efforts to bind a therapeutic strategy for reactivating fetal γ-globin in individuals with β thalassemia.^[1] Recent work has shown that the LIN28B pathway influences BCL11A translation in humans, and additional regulators such as ATF4 and MYB have been reported to enhance BCL11A expression.^[1] Modulating these factors may then support the re-expression of γ-globin.^[1]

In embryonic erythroid cells, regulation of globin gene expression alters the relative synthesis rates of different globin chains as cells progress through terminal differentiation.^[3] During embryonic and fetal development, additional regulatory mechanisms shift production away from embryonic globins and toward fetal and adult forms.^[3] Although these developmental transitions are well documented, the specific signals that drive them in mammals are still largely unknown, and only a limited number of molecular pathways have been identified that many contribute to the control of embryonic globin gene expression.^[3]

^[8][14]