What Is Hemoglobinopathy?
Hemoglobin is a binding protein with a molecular weight of 64,000 and is composed of globin and heme. Protoporphyrin
Hemoglobinopathy Hemoglobinopathy
- Hemoglobinopathy is a group of hereditary blood diseases caused by abnormal molecular structure of hemoglobin (abnormal hemoglobin disease) or abnormal rate of synthesis of globin peptide chains (anemia of globin production, also known as marine anemia). Clinical manifestations of hemolytic anemia, methemoglobinemia, or cyanosis caused by hypoxia or increased red blood cells due to increased or decreased hemoglobin oxygen affinity.
Structure and type of hemoglobinopathy
Hemoglobinopathy structure
- Hemoglobin is a binding protein with a molecular weight of 64,000 and is composed of globin and heme. Protoporphyrin
- Hemoglobinopathy
- The quaternary structure of hemoglobin: The peptide chain structure arranged by amino acid sequence is called the primary structure of hemoglobin. Amino acids in the peptide chain can be divided into hydrophilic polarized amino acids (the side chains of which are carboxyl and amino) and non-polarized amino acids (the side chains of which are aromatic). The side chains of various amino acids in the peptide chain are tightened with each other to form an alpha helix, and the spiral segments are connected by short rather than spiral segments. Spiral segments are denoted by AH from the N-C end, and non-spiral segments are denoted by AB, CD, etc., which is called the secondary structure of hemoglobin. The iron atom of heme has 6 coordination bonds, and the 5th coordination bond is bound to the amino acid at position 8 of the F segment of the peptide chain (ie, the imidazolyl group at the 87th position of the chain or the 92th position of the chain) The 6th coordination bond binds oxygen and indirectly binds to the 7th amino acid of the E segment of the peptide chain (that is, the imidazolyl group at the 58th position of the alpha chain or the 63th position of the -chain) to make the peptide chain The three-dimensional structure that constitutes a two-layer spiral serpentine coil inside and outside around heme as the center is called a tertiary structure (Figure 20-12). Hydrophilic amino acids are distributed in the outer layer, so that hemoglobin can be dissolved in water without precipitation; hydrophobic amino acids are distributed in the inner layer, so that water molecules cannot enter the interior of the heme cavity, and the oxidation of Fe2 + of heme to Fe3 + is avoided. Four hemoglobin monomers (the tertiary structure of the peptide chain plus heme) are combined into a tetramer according to a certain spatial relationship, such as HbA (or HbA1, 22), HbA2 (22), and HbF (22), which are called heterogeneous types Tetramer; a tetramer composed of two pairs of the same tertiary structure hemoglobin monomers, such as HbH (4) and HbBart (4), called homotetramers. The tetramer is a quaternary structure of hemoglobin. Through X-ray diffraction to study the spatial relationship of tetramers, it was found that the contact surfaces of 11 and 22 are larger, the mutual mobility is smaller, and the hydrophobicity is beneficial to the stability of the hemoglobin molecular configuration. The contact surfaces of 12 and 21 are small but not strong, and have large mobility, which is conducive to the normal uptake and release of oxygen by hemoglobin. Tetramers dissociate, first dissociating into 11 and 22. In summary, the external structure of hemoglobin and the molecule must be complete and negatively charged; the binding sites of the and chains must be fixed, and the sequence of amino acids surrounding the heme cavity must be complete; otherwise, hemoglobin cannot maintain the molecular structure stability and normality. Transports oxygen physiological functions and is easily damaged.
Hemoglobinopathy type
- There are three kinds of hemoglobin in normal people after birth: HbA, which consists of a pair of chains and a pair of chains (22), is the main hemoglobin of normal people, accounting for more than 95% of the total hemoglobin. HbA appears in small amount at two months of the embryo, accounting for 10% to 40% at birth, reaching human level 6 months after birth; hemoglobin A2 (HbA2), which consists of a pair of chains and a pair of chains (22 ). From 6 to 12 months after birth, it accounts for 2 to 3% of hemoglobin; fetal hemoglobin (HbF) is composed of a pair of chains and a pair of chains (22), which account for 70% to 90% of hemoglobin in the body at birth. It will gradually decrease. By June, the content had dropped to about 1% of the total hemoglobin. Different peptide chains of hemoglobin are controlled by different genetic genes. Alpha chain genes are located on chromosome 16 and , , and chain genes are located on chromosome 11 in a chain relationship. Deletion or deficiency of the alpha globin gene results in a decrease or lack of alpha globin chain synthesis, which is called alpha marine anaemia. Defects in the beta globin gene, resulting in reduced or absent beta globin chain synthesis, are called beta anaemia. The globin gene mutation results in the replacement or absence of single or multiple amino acids in the peptide chain, resulting in a change in the molecular structure of globin, which is called abnormal hemoglobin. The amount of abnormal hemoglobin confirmed by structural analysis is increasing all over the world. By the early 1990s, there were more than 600 abnormal hemoglobin, but less than one third of the abnormal hemoglobin was accompanied by clinical symptoms. The World Health Organization estimates that about 150 million people worldwide carry the hemoglobin disease gene, and has listed hemoglobin disease as one of the six common diseases that seriously endanger human health. Abnormal hemoglobin disease has a high incidence in China, Yunnan, Guizhou, Guangxi, Xinjiang and other places. 67 types of abnormal hemoglobin have been found, including alpha chains (34), beta chains (26), gamma chains (4), etc. Anomalies, 19 of which are the first of our kind. Thalassaemia is common in South China and Southwest China. According to the census data of nearly 1 million people in 28 provinces, cities and autonomous regions in China in the past 10 years, the incidence of abnormal hemoglobin disease is 0.33%, the incidence of alpha marine anemia is 2.64%, and the incidence of beta marine anemia The rate is 0.66%.
Molecular Genetics of Hemoglobinopathy
- The molecular genetic changes of hemoglobin can be roughly classified into the following six categories:
Hemoglobinopathy (a) single base substitution
- Due to the substitution of a single base in the genetic code, the amino acid determined by the base changes accordingly, forming abnormal hemoglobin, such as HbS, HbC, etc., which is replaced by a single amino acid in the peptide chain. Among the abnormal hemoglobins found so far, this type is the most common, accounting for about 90%.
Hemoglobinopathy (2) Mutation of termination code
- Due to the mutation of the stop code (UAA, UAG), the globin peptide chain does not terminate at the normal position, leading to the lengthening or shortening of the peptide chain. For example, the base of amino acid 145 of the chain of Hb McKees Rock changed from UAU to UAA (termination Code), which prematurely ends the beta chain and contains only 144 amino acids. Another example is the stop code at the 142nd position of the Hb ConstantSpring chain, UAA becomes CAA, and the stop code does not appear until the 173rd position, so the Hb Constant Spring chain has 32 amino acids more than the normal chain.
Hemoglobinopathy (3) frameshift mutation
- For example, in the genetic code of the normal hemoglobin peptide chain, the insertion or deletion of 1 or 2 bases can change the base composition of the normal triplet codon. Amino acid, and caused the chain to extend to amino acid 157, which is 11 amino acids more than the normal chain.
Hemoglobinopathy (4) codon deletion or insertion
- When germ cells undergo meiosis, the chromosomes in the union undergo mismatches or unequal exchanges, forming two globin genes. A peptide chain that loses a part of codons and synthesizes a part of amino acids, such as HbLyon. The 17th to 18th positions of the beta chain lack lysine and valine. Corresponding codons are embedded in another chromatid to synthesize a peptide chain with some amino acids inserted. Another example is the Hb Grady alpha chain between the 119th and 120th of the three amino acids (phenylene-threonine-proline).
Hemoglobinopathy (5) Fusion Gene
- During meiosis, unequal exchanges between different globin genes occur, and abnormal hemoglobin of the fusion chain is synthesized, such as the erroneous combination of the and chain genes, resulting in unequal exchange, forming the fusion genes (Hb Lepore) and (Hb Anti Lepore).
Hemoglobinopathy (6) other
- Due to the deficiency of alpha globin gene, the synthesis of alpha chain is reduced or absent, and the excess beta chain forms a tetramer with the gamma chain, such as beta 4-HbH, gamma 4Hb Barts; Defects reduce or absent -chain synthesis, resulting in reduced HbA, and increased HbF and HbA2. The aforementioned globin peptide chain itself does not change the amino acid sequence.
Molecular pathology and hemolytic mechanism of hemoglobinopathy
- There are many types of abnormal hemoglobin disease, and the clinical symptoms are diverse. However, the abnormalities caused by structural changes can be summarized into the following categories:
- 1. The abnormal hemoglobin produced by the replacement of the internal amino acid in the molecule, the non-polar amino acid inside the hemoglobin molecule, which constitutes the contact between the hemoglobin and the globin chain in the hemoglobin molecule, the contact between the spiral segments of the peptide chain and the contact between the hemoglobin monomers, such as Substitution by amino acids with different physicochemical properties will affect the configuration and stability of the molecule. Such abnormal hemoglobins include hemoglobin M (HbM), unstable hemoglobin (UHb), and hemoglobin with altered oxygen affinity.
- (1) HbM: The histidine linked to the heme iron atom in the peptide chain is replaced by tyrosine. The most common is that histidine of E7 or F8 is replaced by tyrosine. Oxygen forms an ionic bond with the iron atom of heme, which makes the iron atom appear in a stable high iron state, which affects the normal oxygen release function of hemoglobin, makes the tissue insufficiently supplied with oxygen, and increases cyanosis and red blood cells. Heme is easily separated from the globin chain, making the hemoglobin molecular structure unstable and hemolysis.
- (2) UHb: The amino acid in the peptide chain that is closely bound to the heme is replaced or deleted, which affects the three-dimensional structure of the peptide chain or weakens the binding force with the heme to form a UHb molecule. Water easily enters the hemoglobin bag to oxidize heme to methemoglobin; the sulfhydryl group of cysteine at position 93 of is oxidized to produce sulfide to form sulfide hemoglobin, which separates the globin chain from heme. The free globin chain is unstable at 37 ° C, and the tetramer is easily dissociated into monomers. It aggregates and precipitates in the red blood cells to form inclusions, which makes the cell membrane stiff. When the microcirculation occurs, the membrane part is often lost and eventually becomes spherical red blood cells. It is blocked in the spleen and destroyed. There are many types of proteins that generally have no significant effect on molecular configuration, function, and stability. HbE is the replacement of glutamic acid at position 26 of the beta chain by lysine. Because the physical and chemical properties of the two amino acids are the same, although the substitution position is on the 11 contact surface, it has little effect on the stability and function of the hemoglobin molecule. A small number of these abnormal hemoglobins can cause solubility changes. For example, HbS and HbC both change their external shape or charge, and their solubility decreases during hypoxia. HbS polymerizes into a spiral and twists into sickle-shaped fibers. HbC polymerizes as a Para-crystallization; both make the cell membrane hard, difficult to pass through the microcirculation, lose part of the red blood cell membrane, form spherical red blood cells, and block and dissolve in the splenic sinus.
- In patients with beta-thalassemia, excess alpha peptide chains form multimers, causing damage to the red blood cell membrane, resulting in the ineffective production of a large number of young red blood cells. Alpha thalassemia, excess and chains form HbH (4) or HbBart (4). HbH is an unstable hemoglobin. HbH inclusions bind to the membrane of red blood cells, which changes the membrane's permeability to cations. Potassium and water gradually penetrate from the inside of the red blood cells to the outside of the cells. Potassium-deficient red blood cells have a shortened life span and are easily destroyed in the monocyte / phagocytic system, leading to hemolysis. Hb Bart has an increased affinity for oxygen, resulting in tissue hypoxia.
Hemoglobinopathy diagnosis
- The distribution of this disease varies from region to region and ethnic group, so you should ask the patient's nationality and ethnicity in detail, whether there is jaundice, anemia, hepatosplenomegaly, retarded growth or cyanosis, and increased erythrocytes in the clinic. Are there any patients with the same history in the family? Laboratory tests include reticulocyte counts, hematocrit, peripheral red blood cell morphology, and red blood cell fragility tests to understand the presence of low pigment and small cell anemia. If the above examinations indicate the possibility of hemoglobinopathy, the following laboratory examinations should be performed on patients and their families to further confirm the diagnosis.
- China's Beijing, Shanghai and other big cities have established advanced genetic diagnosis technology, and successfully performed genetic diagnosis and prenatal diagnosis on a variety of genetic blood diseases such as hemophilia, alpha marine anemia, beta-sea anemia, and abnormal hemoglobin disease. Genetic diagnosis:
- 1. Commonly used genetic diagnostic methods are extraction of whole blood, dried paper blood, amniotic fluid cells, and villous cell DNA for DNA dot hybridization, which is suitable for diagnosing genetic disorders of genetic diseases, such as alpha globin gene deletions in patients with alpha marine anemia.
- 2. Restriction enzyme zymography is suitable for diagnosing genetic diseases in which a mutation in a gene changes the restriction enzyme cut point or DNA deletion changes the size of an enzymatic digestion fragment.
- 3 Restriction fragment polymorphism analysis (RFLP). RFLP is inherited in Mendelian way. If a certain genetic disease gene is closely linked to specific RFLP, this polymorphic fragment can be used as a "genetic marker", which can be inferred by RFLP linkage analysis. Whether the family member and the fetus carry a genetic disease gene, RFLP linkage analysis is suitable for the diagnosis of any single gene genetic disease.
- 4 Oligonucleotide hybridization is a direct genetic diagnostic technique. For genetic diseases whose base sequence has been identified at the mutation site, the mutant genes can be directly detected and identified.
- 5. Polymerase chain reaction (PCR) DNA amplification in vitro. This highly efficient DNA analysis technique can be used for genetic diagnosis directly by electrophoretic analysis of PCR products, and is suitable for diagnosis of genetic diseases caused by gene deletion or partial DNA deletion.
- 6. Non-deleted mutant genes can be combined with restriction site changes. If linked to RFLP sites, PCR amplification products can be digested with restriction enzymes and analyzed directly by electrophoresis, without the need for molecular probes for molecular hybridization, which greatly simplifies The experimental operation allows genetic diagnosis to be completed in half a day.
Hemoglobinopathy treatment instructions
- There is currently no cure. For patients' families and areas with high incidence of this disease, a hemoglobin disease screening, genetic counseling and premarital examination should be done. Perform prenatal diagnosis if necessary, do a good job of eugenics, and prevent the birth of children with severe hemoglobin disease. For severe patients, excessive blood transfusions can be given, and iron chelating agents can be used to reduce hemosiderin deposits. Spleen can be excised when the spleen is hyperactive. As for regulating globin synthesis and controlling the correct and effective expression of exogenous globin genes in host cells, it is still in the experimental research stage and has not been used clinically as a gene therapy method for marine anemia.
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