Physiology, Fetal Hemoglobin

Daniel Kaufman; Jasmin Khattar; Sarah Lappin

Physiology, Fetal Hemoglobin

Introduction

Fetal hemoglobin (HbF) is the dominant form of hemoglobin present in the fetus during gestation. HbF is produced by erythroid precursor cells from 10 to 12 weeks of pregnancy through the first six months of postnatal life. HbF contains two alpha and two gamma subunits, while the major form of adult hemoglobin, hemoglobin A (HbA), contains two alpha and two beta subunits. The genes that express gamma chain proteins are present in the beta chain locus on chromosome 11. The gamma subunit differs from its adult counterpart in that it contains either an alanine or a glycine at position 136, both of which are neutral, nonpolar amino acids. This difference introduces conformational changes to the protein that gives rise to several physiological differences in oxygen delivery that are important in fetal circulation.[1][2][3]

Cellular

Fetal hemoglobin has a vital role in the transport of oxygen from maternal to fetal circulation. Oxygen transfer from the maternal circulation to the fetal circulation is made possible by HbF having a high oxygen affinity but decreased affinity to 2,3-bisphosphoglycerate relative to HbA. The HbF oxygen dissociation curve is left-shifted in comparison to HbA. The partial pressure at which HbF is half saturated with oxygen (P50) is 19 mm Hg, compared to 27 mm Hg for HbA. This value indicates that HbF has a high affinity for oxygen, giving HbF the ability to bind oxygen more readily from the maternal circulation. HbF also shows a decreased affinity for 2,3-bisphosphoglycerate (2,3-DPG), a metabolic intermediate produced in tissues with high energy use (low ATP, high acid production). A higher binding affinity to 2,3-DPG causes a right shift in HbA, favoring the unloading of oxygen. 2,3-DPG is essential for proper oxygen unloading in the postnatal circulation. Another property of fetal circulation, allowing for oxygen transfer to the fetus, is fetal hematocrit. Fetal hematocrit (15g/dL) is higher than that in the mother (12g/dL), yielding a higher potential oxygen content per liter of blood. In the fetal systemic circulation, the low oxygen tension allows for proper unloading of oxygen, despite HbF's oxygen affinity. The lower oxygen tension in the fetus is important for development, particularly angiogenesis.[4][5][6]

Development

The evolution of hemoglobin follows gene mutations, such as gene duplication, gene conversion, and translocation of genes in ancient hemoproteins. Mutations that resulted in changes to the primary structure of globin altered its properties and genetic regulatory regions. The gamma-globin genes of fetal hemoglobin are products derived from duplications of beta-globin gene clusters. In the fetus, HbF is preceded by the embryonic hemoglobins, whose production in the yolk sac (weeks 3 through 8) decreases shortly after HbF is produced in the liver (weeks 6 through 30), followed by the spleen (9 through 28), and finally the bone marrow (28 through birth). Approximately all HbF is replaced by HbA by 6 to 12 months of age unless hemoglobinopathy is present in the individual; the average adult has less than 1% of HbF as a result. The switch from gamma to beta chain occurs through a transcriptional switch in erythroid precursor cells in the bone marrow.[7]

Related Testing

Fetal hemoglobin is useful in evaluating various conditions in pregnancy and the neonate. The hemoglobin alkaline denaturation test (Apt test) can help to differentiate maternal and fetal blood. In this test, a blood sample of 0.1 mL is added to a glass tube with an alkali reagent containing potassium hydroxide, and the solution is shaken gently for 2 minutes. HbA will bind hydroxide to form hematin, turning the sample a dark-green brown, which indicates the presence of maternal blood in the sample. If only fetal blood is present, the solution will remain pink. The Kleihauer-Betke test assesses the extent of maternal-fetal hemorrhage and the required dose of RhoD immunoglobulin for Rh-negative mothers, for the prevention of maternal Rh antibody formation leading to Rhesus disease in the fetus/neonate. The test utilizes HbF's property of acid resistance. A blood smear taken from the mother is exposed to an acidic pH solution; maternal red blood cells become "ghost cells," as HbA is unstable at a low pH, and the cell membrane is denatured. Since fetal blood cells contain HbF, they remain pink, as HbF is stable at this pH range. The appropriate dose of RhoD immunoglobulin is calculated based on the percentage of fetal blood cells in the maternal blood.[8][9][10]

Pathophysiology

Fetal hemoglobin is of great significance in the pathophysiology of hemoglobinopathies. In alpha thalassemia, one or more of the four alpha-chain genes are deleted on chromosome 16, leading to decreased HbA production and/or abnormal hemoglobin production from beta or gamma chains. The severity is dependent upon the number (1 to 4) of deleted genes. Deletion of a single gene is largely asymptomatic, while the deletion of three genes presents with chronic microcytic anemia and hemolysis that may present with symptoms of fatigue, hepatosplenomegaly, and pigmented gallstones. When all four genes are deleted, there is the production of abnormal hemoglobin composed of four gamma chains; the term for this is Hb Barts. Hb Barts is incompatible with life because the gamma chains have too great an affinity for oxygen, and therefore oxygen delivery is significantly impacted. Affected fetuses suffer from a condition known as hydrops fetalis, marked by accumulations of fluid throughout the body, causing ascites, pleural effusions, pericardial effusions, and scalp edema. This condition results in spontaneous abortion in nearly all cases.[11][12][13]

Clinical Significance

One medical application of the properties of HbF is in the management of sickle cell anemia. At baseline, HbF accounts for 2% to 20% of hemoglobin in sickle cell disease, depending on various patient-dependent factors, and this elevation appears to be due to the greater oxygen affinity of HbF; therefore, HbF is less likely to deoxygenate, sickle, and cause pain crises in these patients. Indeed, sickle cell disease patients do not manifest symptoms in infancy due to elevated HbF, but as HbF decreases, patients may become symptomatic. HbA shows a decreased half-life in sickle cell disease because vaso-occlusive crises that occur during sickling and deoxygenation induce hemolysis. Through an unknown mechanism, the pharmacologic drug hydroxyurea increases the fraction of HbF found in adults. Treatment with hydroxyurea is indicated in patients that have frequent pain crises, acute chest syndrome, or severe anemia. By increasing HbF, hydroxyurea reduces the requirement for transfusions in patients with sickle cell anemia.[14][15]

Article Details

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