Hemoglobin D

Hemoglobin was discovered as some sort of crystal formed from earthworm body fluid and animal blood by German biochemist Friedrich Ludwig Hünefeld at Leipzig University in 1840. When the protein nature was established another German Felix Hoppe-Seyler gave the name hemoglobin (literally "blood protein") in 1864. Its role as an oxygen transporter was later established. In 1950, Itano and James V. Neel discovered a slightly different case in which individuals had sickled red blood cells but not anemia. The hemoglobin was named hemoglobin III, but later known as hemoglobin C (HbC). In 1934, Jean V. Cooke and J. Keller Mack, pediatricians at St. Louis, USA, reported a case of white American family which had some member suffering from sickle cell anemia. Of six siblings, two children had anemia, while others, including their parents, were healthy. Blood tests indicated the two children had sickled red blood cells, but with uncharacteristically slow process of sickling. The father, who had no disease, was found to have sickled re blood cells.In the paper published in the Proceedings of the National Academy of Sciences of the United States of America, Itano presented the need to have naming convention for the different types of hemoglobin, and introduced the alphabet-coding system such as hemoglobin a (for normal adult type), b (sickle cell type), c (sickle cell-associated type) and d (for the novel type); as he explained:In order to facilitate the discussion in the present paper and to avoid confusion in future works, it seems desirable at this time to establish a system of symbols for identifying the various forms of adult human hemoglobin... normal hemoglobin, sickle cell hemoglobin, the abnormal hemoglobin reported by Itano and Neel, and the abnormal hemoglobin reported in the present paper will be designated adult human hemoglobins a, b, c and d, respectively, more briefly as hemoglobins a, b, c and d. Although the nomenclature system became a convention, hemoglobin D, in particular, became known by various names, generally based on their origin of identification; like hemoglobin D-Los Angeles for the first discovered, Around the same time, Corrado Baglioni of Massachusetts Institute of Technology identified the exact abnormality that substitution of glutamic acid with glutamine at position 121 in the β-chain was the basis of HbD, the findings which he reported in 1962. == Structure ==

Hemoglobin D has the basic structure and composition of normal adult hemoglobin. It is a globular protein containing prosthetic (non-protein) group called heme. There are four individual peptide chains, namely two α- and two β-subunits, each made of 141 and 146 amino acid residues, respectively. One heme is associated with each chain and responsible for binding free oxygen in the blood. A single HbD is therefore a tetramer (containing four molecules), denoted as α2β2. Each subunit has a molecular weight of about 16,000 Da (daltons), making the tetramer about 64,000 Da (64,458 g/mol) in size. HbD is different from HbA only on the β-subunit where the amino acid glutamic acid at 121 position is replaced with glutamine (α2β2121Glu→Gln). It has the same chemical characteristic as HbS (a hemoglobin of sickle cell trait), with one fewer negative charge at an alkaline pH than HbA. However, unlike HbS, it does not produce sickled RBC on its own under low level of oxygen. == Genetics and diseases ==

Hemoglobin D is synthesised due to mutation in HBB, the gene that produces β-subunits of hemoglobin and is present on human chromosome 11. A point mutation in the first base of the 121 codon that normally has GAA sequence for normal hemoglobin is changed to CAA. GAA codes for glutamic acid, while CAA for glutamine. This gene mutation makes HbD, which can further give rise to several genetic and disease conditions. The specific mutations can occur at different sites of the gene. According to the Globin Gene Server database, there are other types of HbD such as HbD-Agri (HBB:c.29C→A;364G→C), HbD-Bushman (HBB:c.49G→C), HbD-Ouled Rabah (HBB:c.60C→A or 60C→G), HbD-Iran (HBB:c.67G→C), HbD-Granada (HBB:c.68A→T), HbD-Ibadan (HBB:c.263C→A) and HbD-Neath (HBB:c.365A→C). Depending on the nature of inheritance of HbD mutation there are four conditions, some of which can be deadly diseases: • Heterozygous HbD/HbA trait, which does not affect the individuals. • Heterozygous HbD-thalassemia, which causes the symptoms of thalassemia generally with mild anemia. • Heterozygous HbS-D, which gives rise to sickle cell anemia, but generally milder and slower symptoms. • Homozygous HbD/HbD, which is the rarest form, but is associated with HbD disease. == Effects and symptoms ==

Hemoglobin in combination with normal hemoglobin (heterozygous HBD/HbA) is asymptomatic, causing no effects. Individuals have normal hemoglobin level and their red blood cells are normal spherical structure. Homozygous HbD/HbD causes mild hemolytic anemia and chronic non-progressive splenomegaly (enlargement of spleen). HPLC coupled with mass spectrometry (HPLC-ESI-MS/MS) can accurately detect the protein but the procedure is costly and time consuming. Genetic screening can be done with polymerase chain reaction that can identify HbS from other hemoglobin variants. Management Hemoglobin D conditions such as homozygous and HbD/HbA heterozygous do not require medical intervention. HbD/HbS and HbD-thalassemia conditions are managed like the typical cases of sickle cell anemia and thalassemia. Dietary supplementation of folic acid is recommended by the WHO. In 2019, Crizanlizumab, a monoclonal antibody was approved by the United States FDA for reducing the frequency of blood vessel blockage in 16 years and older individuals. For thalassemia, regular lifelong blood transfusions is the usual treatment. Bone marrow transplants can be curative for some children. Medications like deferoxamine, deferiprone and luspatercept. Gene therapy, exagamglogene autotemcel is approved for medical use in the United Kingdom since November 2023. == Prevalence ==

Hemoglobin D is most abundant among Sikhs, with occurrence of 2% in Punjab and 1% in Gujarat. It is also found in small number of individuals among Africans, Americans and Europeans who usually had close ethnicity with Indians in the past. It is sporadically recorded in some Turkish, Algerian, West African, Saudi Arabian, native American, English, and Irish population. HbD/ HbQ, and HbD/Hb Fontainebleau are also found in India. A rare case of HbS/HbD is reported from Pakistan in which individuals are diagnosed with bone infection (osteomyelitis). An isolated condition of HbD/HbC is recorded in US. == References ==