Fetal hemoglobin


Fetal hemoglobin, or foetal haemoglobin is the main oxygen carrier protein in the human fetus. HemoglobinF is found in fetal red blood cells, and is involved in transporting oxygen from the mother's bloodstream to organs and tissues in the fetus. It is produced at around 6weeks of pregnancy and the levels remain high after birth until the baby is roughly 2–4months old. HemoglobinF has a different composition than adult forms of hemoglobin, allowing it to bind oxygen more strongly; this in turn enables the developing fetus to retrieve oxygen from the mother's bloodstream, which occurs through the placenta found in the mother's uterus.
In the newborn, levels of hemoglobin F gradually decrease and reach adult levels usually within the first year, as adult forms of hemoglobin begin to be produced. Diseases such as beta thalassemias, which affect components of the adult hemoglobin, can delay this process, and cause hemoglobin F levels to be higher than normal. In sickle cell anemia, increasing the production of hemoglobin F has been used as a treatment to relieve some of the symptoms.

Structure and genetics

Hemoglobin F, like adult hemoglobin, has four subunits or chains. Each subunit contains a heme group with an iron element which is key in allowing the binding and unbinding of oxygen. As such, hemoglobin F can adopt two states: oxyhemoglobin and deoxyhemoglobin. As hemoglobin F has 4 heme groups, it can bind to up to four oxygen molecules. It is composed of two α subunits and two γ subunits, whereas hemoglobin A is composed of two α and two β subunits.
In humans, the α subunit is encoded on chromosome 16 and the γ subunit is encoded on chromosome 11. There are two very similar genes that code for the α subunit, HBA1 and HBA2. The protein that they produce is identical, but they differ in gene regulatory regions that determine when or how much of the protein is produced. This leads to HBA1 and HBA2 contributing 40% and 60%, respectively, of the total α subunits produced. As a consequence, mutations on the HBA2 gene are expected to have a stronger effect than mutations on the HBA1 gene.
There are also two similar copies of the gene coding for the γ subunit, HBG1 and HBG2, but the protein produced is slightly different, just in one protein unit: HBG1 codes for the protein form with an alanine at position 136, whereas HBG2 codes for a glycine
BCL11A and ZBTB7A are major repressor proteins of hemoglobin F production, by binding to the gene coding for the γ subunit at their promoter region. This happens naturally as the newborn baby starts to switch from producing hemoglobin F to producing hemoglobin A.
Some genetic diseases can take place due to mutations to genes coding for components of hemoglobin F. Mutations to HBA1 and HBA2 genes can cause alpha-thalassemia and mutations to the promoter regions of HBG1 and HBG2 can cause hemoglobin F to still be produced after the switch to hemoglobin A should have occurred, which is called hereditary persistence of fetal hemoglobin.

Production

During the first 3 months of pregnancy, the main form of hemoglobin in the embryo/fetus is embryonic hemoglobin, which has 3 variants depending on the types of subunits it contains. The production of hemoglobin F starts from week 6, but it's only from 3 months onwards that it becomes the main type found in fetal red blood cells.
The switch to produce adult forms of hemoglobin starts at around 40 weeks of gestation, which is close to the expected time of birth. At birth, hemoglobin F accounts for 50-95% of the infant's hemoglobin and at around 6 months after birth, hemoglobin A becomes the predominant type. By the time the baby is one year old, the proportions of different types of hemoglobin are expected to approximate the adult levels, with hemoglobin F reduced to very low levels. The small proportion of red blood cells containing hemoglobin F are called F-cells, which also contain other types of hemoglobin.
In healthy adults, the composition of hemoglobin is hemoglobin A, hemoglobin A2 and hemoglobin F.
Certain genetic abnormalities can cause the switch to adult hemoglobin synthesis to fail, resulting in a condition known as hereditary persistence of fetal hemoglobin.

Binding to oxygen

Factors affecting oxygen affinity

The four hemes, which are the oxygen-binding parts of hemoglobin, are similar between hemoglobin F and other types of hemoglobin, including hemoglobin A. Thus, the key feature that allows hemoglobin F to bind more strongly to oxygen is by having γ subunits. In fact, some naturally existing molecules in our body can bind to hemoglobin and change its binding affinity for oxygen. One of the molecules is 2,3-bisphosphoglycerate and it enhances hemoglobin's ability to release oxygen. 2,3-BPG interacts much more with hemoglobin A than hemoglobin F. This is because the adult β subunit has more positive charges than the fetal γ subunit, which attract the negative charges from 2,3-BPG. Specifically, the β-subunit of haemoglobin A has a His143 residue that can complex the phosphate; in haemoglobin F this is substituted for a serine residue. Due to the preference of 2,3-BPG for hemoglobin A, hemoglobin F binds to oxygen with more affinity, in average, and there is less of an allosteric inhibition of oxygen binding by 2,3-BPG.

Even higher oxygen affinity – hemoglobin Barts (four γ subunits)

is an abnormal form of hemoglobin produced in hemoglobin Barts syndrome or alpha-thalassemia major, the most severe form of alpha-thalassemia. Alpha-thalassemia is a genetic blood disorder and one of the most common hemoglobin-related diseases, affecting the production of α subunits from hemoglobin. Depending on how many genes coding for the α subunit are impacted, patients with this disease can have reduced to no production of the α subunit of the hemoglobin. As a consequence, less hemoglobin is available and this affects oxygen supply to the tissues.
Hemoglobin Barts syndrome manifests when all four genes coding for α subunit are deleted. This is often fatal for the fetus carrying the disorder, as in the absence of α subunits, a form of hemoglobin with four γ subunits, hemoglobin Barts, is produced. This form of hemoglobin isn't fit for oxygen exchange precisely due to its very high affinity for oxygen. While hemoglobin Barts is very efficient at binding oxygen, it doesn't release oxygen to the organs and tissues. The disease is fatal for the fetus or newborn unless early diagnosis and intervention is carried out during pregnancy, and the child will be dependent on lifelong blood transfusions.

Quantification of oxygen binding

To quantify how strongly a certain type of hemoglobin binds to oxygen, a parameter called P50 is often used. In a given situation, P50 can be understood as the partial pressure of oxygen at which Hb is 50% saturated. For example, Hemoglobin F has a lower P50 than hemoglobin A. This means that if we have the same amount of hemoglobin F and hemoglobin A in the blood and add oxygen to it, half of hemoglobin F will bind to oxygen before half of hemoglobin A manages to do so. Therefore, a lower P50 means stronger binding or higher affinity for oxygen.
For reference, the P50 of fetal hemoglobin is roughly 19 mmHg, whereas adult hemoglobin is approximately 26.8 mmHg.

Oxygen exchange in the womb

During pregnancy, the mother's circulatory system delivers oxygen and nutrients to the fetus and carries away nutrient-depleted blood enriched with carbon dioxide. The maternal and fetal blood circulations are separate and the exchange of molecules occurs through the placenta, in a region called intervillous space which is located in between maternal and fetal blood vessels.
Focusing on oxygen exchange, there are three important aspects that allow oxygen to pass from the maternal circulation into the fetal circulation. Firstly, the presence of hemoglobin F in the fetus allows a stronger binding to oxygen than maternal hemoglobin. Secondly, the mother's bloodstream is richer in oxygen than that of the fetus, so oxygen naturally flows towards the fetal circulation by diffusion. The final factor is related to the effects of pH on maternal and fetal hemoglobin. As the maternal blood acquires more carbon dioxide, it becomes more acidic and this favors the release of oxygen by the maternal hemoglobin. At the same time, the decrease in carbon dioxide in fetal blood makes it more alkaline and favors the uptake of oxygen. This is called the Bohr effect or Haldane effect, which also happens in the air exchange in the lungs. All of these three factors are present simultaneously and cooperate to improve the fetus' access to oxygen from the mother.

F-cells

F-cells are the subpopulation of red blood cells that contain hemoglobin F, in amongst other types of hemoglobin. While common in fetuses, in normal adults, only around 3-7% of red blood cells contain hemoglobin F. The low percentage of F-cells in adults owes to two factors: very low levels of hemoglobin F being present and its tendency to be produced only in a subset of cells rather than evenly distributed amongst all red blood cells. In fact, there is a positive correlation between the levels of hemoglobin F and number of F-cells, with patients with higher percentages of hemoglobin F also having a higher proportion of F-cells. Despite the correlations between hemoglobin F levels and F-cell numbers, usually they are determined by direct measurements. While the amount of hemoglobin F is calculated using cell lysates, which are fluids with contents of cells that were broken open, F-cell numbers are done by counting intact red blood cells.
Due to the correlation between the amount of hemoglobin F and F-cells, F-cell numbers are higher in some inherited hemoglobin disorders, including beta-thalassemia, sickle cell anemia and hereditary persistence of fetal hemoglobin. Additionally, some acquired conditions can also have higher F-cell numbers, such as acute erythropoietic stress and pregnancy.
F-cells have similar mass of haemoglobin per cell compared to red blood cells without haemoglobin F, which is measured mean cell haemoglobin values.