Information

What is the structural difference between beta and gamma globin chains of Hb?


Hemoglobins are tetramers composed of pairs of two different polypeptide subunits. The subunit composition of the principal hemoglobins are α2β2 (HbA; normal adult hemoglobin), α2γ2 (HbF; fetal hemoglobin) and α2δ2 (HbA2; a minor adult hemoglobin)

The total number of amino acids in beta chain and gamma chain is the same-146. The higher affinity of HbF to oxygen is due to presence of serine(in gamma chain) in place of histidine(in beta chain), which inhibits its binding with 2,3-bisphosphoglyceric acid.

Is this the only difference, or are there more such differences in the amino acid sequence?


Here's a pairwise sequence alignment of the human hemoglobin beta (HBB) and gamma (HBG1) chains:

HBB_HUMAN 1 VHLTPEEKSAVTALWGKVNVDEVGGEALGRLLVVYPWTQRFFESFGDLST 50 .|.|.|:|:.:|:|||||||::.|||.|||||||||||||||:|||:||: HBG1_HUMAN 1 GHFTEEDKATITSLWGKVNVEDAGGETLGRLLVVYPWTQRFFDSFGNLSS 50 HBB_HUMAN 51 PDAVMGNPKVKAHGKKVLGAFSDGLAHLDNLKGTFATLSELHCDKLHVDP 100… |:||||||||||||||.:… |.:.|||:||||||.||||||||||||| HBG1_HUMAN 51 ASAIMGNPKVKAHGKKVLTSLGDAIKHLDDLKGTFAQLSELHCDKLHVDP 100 HBB_HUMAN 101 ENFRLLGNVLVCVLAHHFGKEFTPPVQAAYQKVVAGVANALAHKYH 146 |||:|||||||.|||.||||||||.|||::||:|… ||:||:.:|| HBG1_HUMAN 101 ENFKLLGNVLVTVLAIHFGKEFTPEVQASWQKMVTAVASALSSRYH 146

You can see the H143S (or S143H?) substitution, as well as many more.


The gamma globin genes (HBG1 and HBG2) are normally expressed in the fetal liver, spleen and bone marrow. Two gamma chains together with two alpha chains constitute fetal hemoglobin (HbF) which is normally replaced by adult hemoglobin (HbA) in the year following birth. In the non-pathological condition known as hereditary persistence of fetal hemoglobin (HPFH), gamma globin expression is continued into adulthood. Also, in cases of beta-thalassemia and related conditions, gamma chain production may be maintained, possibly as a mechanism to compensate for the mutated beta-globin. The two types of gamma chains differ at residue 136 where glycine is found in the G-gamma product (HBG2) and alanine is found in the A-gamma product (HBG1). The former is predominant at birth. The order of the genes in the beta-globin cluster is: 5' - epsilon – gamma-G – gamma-A – delta – beta - 3'. [4]

  1. ^ abcGRCh38: Ensembl release 89: ENSG00000213934 - Ensembl, May 2017
  2. ^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  3. ^
  4. Higgs DR, Vickers MA, Wilkie AO, Pretorius IM, Jarman AP, Weatherall DJ (May 1989). "A review of the molecular genetics of the human alpha-globin gene cluster". Blood. 73 (5): 1081–104. doi: 10.1182/blood.V73.5.1081.1081 . PMID2649166.
  5. ^
  6. "Entrez Gene: HBG1 hemoglobin, gamma A".
  • Huisman TH, Kutlar F, Gu LH (1992). "Gamma chain abnormalities and gamma-globin gene rearrangements in newborn babies of various populations". Hemoglobin. 15 (5): 349–79. doi:10.3109/03630269108998857. PMID1802881.
  • Gelinas R, Yagi M, Endlich B, et al. (1985). "Sequences of G gamma, A gamma, and beta genes of the Greek (A gamma) HPFH mutant: evidence for a distal CCAAT box mutation in the A gamma gene". Prog. Clin. Biol. Res. 191: 125–39. PMID2413469.
  • Giardina B, Messana I, Scatena R, Castagnola M (1995). "The multiple functions of hemoglobin". Crit. Rev. Biochem. Mol. Biol. 30 (3): 165–96. doi:10.3109/10409239509085142. PMID7555018.
  • Anderson NL, Anderson NG (2003). "The human plasma proteome: history, character, and diagnostic prospects". Mol. Cell. Proteomics. 1 (11): 845–67. doi: 10.1074/mcp.R200007-MCP200 . PMID12488461.
  • Chang JC, Kan YW (1979). "beta 0 thalassemia, a nonsense mutation in man". Proc. Natl. Acad. Sci. U.S.A. 76 (6): 2886–9. doi:10.1073/pnas.76.6.2886. PMC383714 . PMID88735.
  • Saglio G, Ricco G, Mazza U, et al. (1979). "Human T gamma globin chain is a variant of A gamma chain (A gamma Sardinia)". Proc. Natl. Acad. Sci. U.S.A. 76 (7): 3420–4. doi:10.1073/pnas.76.7.3420. PMC383837 . PMID291015.
  • Poon R, Kan YW, Boyer HW (1979). "Sequence of the 3'-noncoding and adjacent coding regions of human gamma-globin mRNA". Nucleic Acids Res. 5 (12): 4625–30. PMC342777 . PMID318163.
  • Grifoni V, Kamuzora H, Lehmann H, Charlesworth D (1975). "A new Hb variant: Hb F Sardinia gamma75(E19) isoleucine leads to threonine found in a family with Hb G Philadelphia, beta-chain deficiency and a Lepore-like haemoglobin indistinguishable from Hb A2". Acta Haematol. 53 (6): 347–55. doi:10.1159/000208204. PMID808940.
  • Proudfoot NJ, Brownlee GG (1976). "3' non-coding region sequences in eukaryotic messenger RNA". Nature. 263 (5574): 211–4. doi:10.1038/263211a0. PMID822353. S2CID4211839.
  • Marotta CA, Forget BG, Cohne-Solal M, et al. (1977). "Human beta-globin messenger RNA. I. Nucleotide sequences derived from complementary RNA". J. Biol. Chem. 252 (14): 5019–31. PMID873928.
  • Frier JA, Perutz MF (1977). "Structure of human foetal deoxyhaemoglobin". J. Mol. Biol. 112 (1): 97–112. doi:10.1016/S0022-2836(77)80158-7. PMID881729.
  • Ahern E, Holder W, Ahern V, et al. (1975). "Haemoglobin F Victoria Jubilee (alpha 2 A gamma 2 80 Asp-Try)". Biochim. Biophys. Acta. 393 (1): 188–94. doi:10.1016/0005-2795(75)90230-5. PMID1138921.
  • Waye JS, Cai SP, Eng B, et al. (1993). "Clinical course and molecular characterization of a compound heterozygote for sickle hemoglobin and hemoglobin Kenya". Am. J. Hematol. 41 (4): 289–91. doi:10.1002/ajh.2830410413. PMID1283810. S2CID35045351.
  • Bailey WJ, Hayasaka K, Skinner CG, et al. (1994). "Reexamination of the African hominoid trichotomy with additional sequences from the primate beta-globin gene cluster". Mol. Phylogenet. Evol. 1 (2): 97–135. doi:10.1016/1055-7903(92)90024-B. PMID1342932.
  • Gottardi E, Losekoot M, Fodde R, et al. (1992). "Rapid identification by denaturing gradient gel electrophoresis of mutations in the gamma-globin gene promoters in non-deletion type HPFH". Br. J. Haematol. 80 (4): 533–8. doi:10.1111/j.1365-2141.1992.tb04569.x. PMID1374633. S2CID27249036.
  • Berry M, Grosveld F, Dillon N (1992). "A single point mutation is the cause of the Greek form of hereditary persistence of fetal haemoglobin". Nature. 358 (6386): 499–502. doi:10.1038/358499a0. hdl: 1765/2476 . PMID1379347. S2CID4235661.
  • Loudianos G, Moi P, Lavinha J, et al. (1993). "Normal delta-globin gene sequences in Sardinian nondeletional delta beta-thalassemia". Hemoglobin. 16 (6): 503–9. doi:10.3109/03630269208993118. PMID1487421.
  • Fucharoen S, Shimizu K, Fukumaki Y (1990). "A novel C-T transition within the distal CCAAT motif of the G gamma-globin gene in the Japanese HPFH: implication of factor binding in elevated fetal globin expression". Nucleic Acids Res. 18 (17): 5245–53. doi:10.1093/nar/18.17.5245. PMC332148 . PMID1698280.
  • Plaseska D, Kutlar F, Wilson JB, et al. (1991). "Hb F-Jiangsu, the first gamma chain variant with a valine----methionine substitution: alpha 2A gamma 2 134(H12)Val----Met". Hemoglobin. 14 (2): 177–83. doi:10.3109/03630269009046959. PMID1703137.
  • Overview of all the structural information available in the PDB for UniProt: P69891 (Hemoglobin subunit gamma-1) at the PDBe-KB.

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Epidemiology

Approximately 5 percent of the world's population has a globin variant, but only 1.7 percent has alpha or beta thalassemia trait.2 Thalassemia affects men and women equally and occurs in approximately 4.4 of every 10,000 live births. Alpha thalassemia occurs most often in persons of African and Southeast Asian descent, and beta thalassemia is most common in persons of Mediterranean, African, and Southeast Asian descent. Thalassemia trait affects 5 to 30 percent of persons in these ethnic groups.2

SORT: KEY RECOMMENDATIONS FOR PRACTICE

Persons with anemia from thalassemia trait should not take iron supplements unless they have coexistent iron deficiency.

Persons with beta thalassemia major require periodic lifelong blood transfusions to maintain hemoglobin levels higher than 9.5 g per dL (95 g per L) and sustain normal growth.

Persons with beta thalassemia major require chelation therapy for iron overload.

Persons at risk of having a child with thalassemia should be offered preconception genetic counseling.

A = consistent, good-quality patient-oriented evidence B = inconsistent or limited-quality patient-oriented evidence C = consensus, disease-oriented evidence, usual practice, expert opinion, or case series. For information about the SORT evidence rating system, go to https://www.aafp.org/afpsort.xml .

SORT: KEY RECOMMENDATIONS FOR PRACTICE

Persons with anemia from thalassemia trait should not take iron supplements unless they have coexistent iron deficiency.

Persons with beta thalassemia major require periodic lifelong blood transfusions to maintain hemoglobin levels higher than 9.5 g per dL (95 g per L) and sustain normal growth.

Persons with beta thalassemia major require chelation therapy for iron overload.

Persons at risk of having a child with thalassemia should be offered preconception genetic counseling.

A = consistent, good-quality patient-oriented evidence B = inconsistent or limited-quality patient-oriented evidence C = consensus, disease-oriented evidence, usual practice, expert opinion, or case series. For information about the SORT evidence rating system, go to https://www.aafp.org/afpsort.xml .


Hemoglobin: Normal, High, Low Levels and Causes

Hemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs.

Hemoglobin is made up of four protein molecules (globulin chains) that are connected together. The normal adult hemoglobin (abbreviated Hgb or Hb) molecule contains two alpha-globulin chains and two beta-globulin chains. In fetuses and infants, beta chains are not common and the hemoglobin molecule is made up of two alpha chains and two gamma chains. As the infant grows, the gamma chains are gradually replaced by beta chains, forming the adult hemoglobin structure.

Each globulin chain contains an important iron-containing porphyrin compound termed heme. Embedded within the heme compound is an iron atom that is vital in transporting oxygen and carbon dioxide in our blood. The iron contained in hemoglobin is also responsible for the red color of blood.

Hemoglobin also plays an important role in maintaining the shape of the red blood cells. In their natural shape, red blood cells are round with narrow centers resembling a donut without a hole in the middle. Abnormal hemoglobin structure can, therefore, disrupt the shape of red blood cells and impede their function and flow through blood vessels.

Anemia Symptoms

Anemia is a medical condition in which the red blood cell count or hemoglobin is less than normal. Symtoms of anemia include

  • Fatigue
  • Feeling of unwellness
  • Heart palpitations
  • Hair loss
  • Shortness of breath

How is hemoglobin measured?

Hemoglobin is usually measured as a part of the routine complete blood count (CBC) test from a blood sample.

Several methods exist for measuring hemoglobin, most of which are done currently by automated machines designed to perform different tests on blood. Within the machine, the red blood cells are broken down to get the hemoglobin into a solution. The free hemoglobin is exposed to a chemical containing cyanide that binds tightly with the hemoglobin molecule to form cyanomethemoglobin. By shining a light through the solution and measuring how much light is absorbed (specifically at a wavelength of 540 nanometers), the amount of hemoglobin can be determined.

What are normal hemoglobin values?

The hemoglobin level is expressed as the amount of hemoglobin in grams (gm) per deciliter (dL) of whole blood, a deciliter being 100 milliliters.

The normal ranges for hemoglobin depend on the age and, beginning in adolescence, the gender of the person. The normal ranges are:

  • Newborns: 17 to 22 gm/dL
  • One (1) week of age: 15 to 20 gm/dL
  • One (1) month of age: 11 to 15 gm/dL
  • Children: 11 to 13 gm/dL
  • Adult males: 14 to 18 gm/dL
  • Adult women: 12 to 16 gm/dL
  • Men after middle age: 12.4 to 14.9 gm/dL
  • Women after middle age: 11.7 to 13.8 gm/dL

All of these values may vary slightly between laboratories. Some laboratories do not differentiate between adult and "after middle age" hemoglobin values. Pregnant females are advised to avoid both high and low hemoglobin levels to avoid increased risks of stillbirths (high hemoglobin &ndash above the normal range) and premature birth or low-birth-weight baby (low hemoglobin &ndash below the normal range).

What does a low hemoglobin level mean?

A low hemoglobin level is referred to as anemia or low red blood count. A lower than a normal number of red blood cells is referred to as anemia and hemoglobin levels reflect this number. There are many reasons (causes) for anemia.

Some of the more common causes of anemia are:

  • loss of blood (traumatic injury, surgery, bleeding, colon cancer, or stomach ulcer),
  • nutritional deficiency (iron, vitamin B12, folate),
  • bone marrow problems (replacement of bone marrow by cancer),
  • suppression by red blood cell synthesis bychemotherapy drugs, , and
  • abnormal hemoglobin structure (sickle cell anemia or thalassemia).

QUESTION

What does a high hemoglobin level mean?

Higher than normal hemoglobin levels can be seen in people living at high altitudes and in people who smoke. Dehydration produces a falsely high hemoglobin measurement that disappears when the proper fluid balance is restored.


Beta Thalassemia

The thalassemias are a group of genetic (inherited) blood disorders that share in common one feature, the defective production of hemoglobin, the protein that enables red blood cells to carry and deliver oxygen. There are many different mechanisms of defective hemoglobin synthesis and, hence, many types of thalassemia.

What is the most common type of thalassemia?

The most familiar type of thalassemia is beta thalassemia. It involves decreased production of normal adult hemoglobin (Hb A), the predominant type of hemoglobin from soon after birth until death. (All hemoglobin consists of two parts: heme and globin). The globin part of Hb A has 4 protein sections called polypeptide chains. Two of these chains are identical and are designated the alpha chains. The other two chains are also identical to one another but differ from the alpha chains and are termed the beta chains. In persons with beta thalassemia, there is reduced or absent production of beta globin chains.

What is the difference between thalassemia minor and major?

There are two forms of beta thalassemia. They are thalassemia minor and thalassemia major (which is also called Cooley's anemia).

Thalassemia minor: The individual with thalassemia minor has only one copy of the beta thalassemia gene (together with one perfectly normal beta-chain gene). The person is said to be heterozygous for beta thalassemia.

Persons with thalassemia minor have (at most) mild anemia (slight lowering of the hemoglobin level in the blood). This situation can very closely resemble that with mild iron-deficiency anemia. However, persons with thalassemia minor have a normal blood iron level (unless they are iron deficient for other reasons). No treatment is necessary for thalassemia minor. In particular, iron is neither necessary nor advised.

Thalassemia major (Cooley's anemia): The child born with thalassemia major has two genes for beta thalassemia and no normal beta-chain gene. The child is homozygous for beta thalassemia. This causes a striking deficiency in beta chain production and in the production of Hb A. Thalassemia major is a significant illness.

The clinical picture associated with thalassemia major was first described in 1925 by the American pediatrician Thomas Cooley. Hence, the name Cooley's anemia in his honor.

At birth the baby with thalassemia major seems entirely normal. This is because the predominant hemoglobin at birth is still fetal hemoglobin (HbF). HbF has two alpha chains (like Hb A) and two gamma chains (unlike Hb A). It has no beta chains so the baby is protected at birth from the effects of thalassemia major.

Anemia begins to develop within the first months after birth. It becomes progressively more and more severe. The infant fails to thrive (to grow normally) and often has problems feeding (due to easy fatigue from lack of oxygen due to the profound anemia), bouts of fever, diarrhea, and other intestinal problems.

SLIDESHOW

What is Mediterranean anemia?

The gene for beta thalassemia is not evenly distributed among peoples. It is, for example, relatively more frequent in people of Italian and Greek origin, both of which are peoples from the Mediterranean. Because of this, thalassemia major has been called Mediterranean anemia.

The name thalassemia was coined at the University of Rochester in upstate New York by the Nobel Prize-winning pathologist George Whipple and the professor of pediatrics William Bradford from the Greek thalassa for sea and -emia, meaning the blood. Thalassemia means "sea in the blood." But for the Greeks, the sea was the Mediterranean, so thalassemia also conveys the idea of the Mediterranean in the blood.

The reason that the gene for beta thalassemia is relatively common, for example, among people of Italian and Greek origin is that parts of Italy and Greece were once full of malaria. The presence of thalassemia minor (like sickle cell trait in Africa) afforded protection against malaria, and therefore, this gene thrived.

What is the genetic pattern of inheritance of beta thalassemia?

The pattern of genetic transmission of beta thalassemia (and sickle cell disease) was deciphered by James V. Neel when he was at the University of Rochester (and later at the University of Michigan). Dr. Neel recognized that the parents of children with thalassemia major had thalassemia minor with one beta thalassemia gene. When these parents had children, they have a 25% chance of having a thalassemia major child (with both genes for beta thalassemia), a 50% chance of having children with thalassemia minor (with only one gene for beta thalassemia), and a 25% chance of having a child without thalassemia major or minor (with both genes for normal beta chains). This form of inheritance is medically referred to as an autosomal recessive pattern.


Alpha and Beta Thalassemia and Laboratory Tests

Thalassemia

Thalassemia is a type of disorder that is blood inherited (passed down from parents to their children). This condition affects the amount and type of hemoglobin produced by the body.

Hemoglobin (abbreviated as Hgb or Hb) is a component that is found in the red blood cells (abbreviated as RBCs). The red blood cells need to function properly because it carries oxygen to different parts of the body. Hemoglobin is a vast compound, with different portions like:

  • Heme – This is a molecule with iron at the center
  • Globins – This is another portion that is made up of four protein chains (each globin in the chain holds a heme group, which contains one iron atom.)

Depending on the structure, globin chains are labeled as delta, gamma, beta, or alpha. It is important to note that not all hemoglobin is the same. Each is classified depending on the globin chain type. Also, it is important to note that the type of globin chain plays a huge role in hemoglobin’s ability to transport oxygen.

Normal Hemoglobin Types Include:

  • Hemoglobin A – This Type of Hb is predominant in adults, and it makes up about 95%-98% of hemoglobin. It contains two beta and two alpha protein chains.
  • Hemoglobin A2 – This type makes up about 2%-3.5% of Hgb found in adults. This type also has two delta and two alpha protein chains.
  • Hemoglobin F – This type of Hgb makes up 2% of hemoglobin found in adults. This type also has two gamma and two alpha protein chains.

Did you know that hemoglobin F is primarily produced by developing babies (fetus), while in the womb? Hb F production decreases to low levels within the first year after birth.

A person with Thalassemia usually has one or multiple genetic mutations, which they have inherited. For this reason, it decreases the production of normal hemoglobin. The moment the body is not producing enough hemoglobin, the red blood cells will not function normally or deliver oxygen to the body effectively. This problem may cause anemia with symptoms and signs, ranging from mild to severe. It all depends on the type of thalassemia one has.

Signs and symptoms may include:

There are four genes in our DNA when it comes to the hemoglobin, which code for the gamma-globin chains, two genes delta, two genes beta, and alpha-globin chains. Because everyone inherits a set of chromosomes from both parents, everyone receives two alpha globulin genes and a beta globulin gene. For this reason, a person can inherit a mutation in either beta or alpha-globin genes.

The production of a low amount of a globin chain is caused by a mutation in one or several globin genes. Of course, when this happens, one should expect an imbalance of alpha to beta chains, which results in an unusual form of hemoglobin or an increased number of minor hemoglobin like Hgb F or Hgb A2. These thalassemias are generally classified by the type of globin chain whose synthesis is low.

For example, the common alpha chain related condition is known as alpha thalassemia. The seriousness of this condition highly depends on the number of genes affected. Other types of mutations, such as globin chain genes coding, may result in a structurally altered globin. This may result in hemoglobin S, which causes sickle cell. This inherited condition, which causes the production of abnormal hemoglobin molecules, is fully described in an article talking about Hemoglobin Abnormalities. Both hemoglobin abnormalities and thalassemia are known as Hemoglobinopathies.

Alpha Thalassemia

Alpha thalassemia is a result of either mutation or depletion of one or several alpha -globin genetic factor copies. It is important to note that alpha-globin production decreases due to mutation. The bigger the number of genes affected, the low alpha-globin the body will produce. All four types of alpha thalassemia are classified based on the number of genes affected. They include:

Those who have a mutation(s) in only one alpha-globin gene are simply silent carriers. To such a person, they will have a normal hemoglobin level as well as a red cell profile. However, they can still pass on the affected gene to their children. Such a person will also not experience any signs or symptoms of the condition, and they are only identified after having a child with thalassemia. The only way to know if you are a carrier is through DNA analysis (check thalassemia tests).

A person with alpha thalassemia traits will have red blood cells that are hypochromic (paler) and microcytic (smaller) than that of a normal person. The red blood cell will also have a decreased mean corpuscular volume (MCV), which is a measurement of the average size of a single RBC. The person will also have mild chronic anemia. In most cases, a person with alpha thalassemia trait will not experience any other sings and a times lack symptoms.

This form of anemia does not respond well to iron supplements. Alpha thalassemia trait should be done by eliminating other causes of microcytic anemia. Confirmatory testing through DNA analysis is available, but not mainly done.

With this condition, because there is a huge decrease in the alpha-globin chain, the number of beta chains becomes high. This ten comes together into a group of 4 beta chains, known as Hemoglobin H., this becomes visible in the red blood cells on a specially stained blood smear. Hgb H disease may cause moderate to severe anemia, which results in health problems like fatigue, bone deformities, and an enlarged spleen. Its symptoms or signs vary greatly. Some people are asymptomatic, while other people get serious anemia, needing constant medical care. This condition is mostly found in people of Mediterranean descent or Southwest Asian.

Hydrops Fetalis is the most severe form of alpha thalassemia. With this condition, the body does not produce any alpha globin, which means the body does not have normal hemoglobin. A majority of the unborn affected by alpha thalassemia during pregnancy become anemic. They frequently have larger hearts and livers. They also retain hydropic (excessive fluids). Pregnancy diagnosis is often conducted during the last months of the pregnancy.

Pregnant mothers are also at risk. Research shows that mothers are at high risk of getting toxemia (high blood pressure, protein in the urine, swollen ankle, and feet). The mother may also develop severe hemorrhage (postpartum bleeding). Most fetuses with the severe case of alpha thalassemia are often miscarried, stillborn, or die shortly after birth. It is rare for a child with the alpha thalassemia major to survive through extensive medical care and Utero blood transfusions. This condition is most common to individuals of Mediterranean descent, African, Indian, middle eastern, southern Chinese, and Southeast Asian.

Beta thalassemia is a condition that is a result of a mutation in one or more beta-globin genes. There are more than 250 mutations that have been identified. However, only about 20 are the most common. The seriousness of anemia as a result of beta-thalassemia highly depends on the mutation itself and the degree of beta production. The different types of beta-thalassemia include:

A person with this condition will have one gene with a mutation and the other one normal. This causes a mild decrease in beta-globin production. In most cases, this condition does not bring any health problems other than abnormal small red blood cells and positive mild anemia that does not respond to iron supplements. Remember, this condition can be inherited.

A person with this condition has two abnormal genes, which causes moderate to a severe decrease in beta-globin production. Such a person may develop symptoms later in life than those of the thalassemia major, and the symptoms are mild. Such a person rarely requires treatment with blood transfusion. The seriousness of the health problems and anemia the person will receive will generally depend on the type of mutation. The dividing line between thalassemia major and thalassemia intermedia is the degree of anemia, the frequency, and the number of blood transfusions needed. However, blood transfusion is needed regularly to a person with thalassemia intermedia.

This is the most severe condition of beta-thalassemia. A person suffering from this condition has two abnormal genes, which causes either a serious decrease or lack of beta-globin production. This prevents the production of high numbers of normal hemoglobin A. this condition will appear within the first two years of life and mainly leads to life-threatening conditions. It does also affect growth and skeletal abnormalities during infancy. These conditions need regular blood transfusions and considerable ongoing medical care.

With time, the frequent transfusions cause an excessive amount of iron in the body. If it is left untreated, the excessive iron can be deposited in the heart, liver, and other vital body organs, which may lead to organ failure. For this reason, a person undergoing transfusion will need chelation therapy to reduce iron overload.

This condition is commonly found in Africans, Mediterranean, and southeast Asian descendants in the US. This is because it is associated with the incidence of malaria in those areas because thalassemia can increase malarial tolerance. Therefore, in those areas, malaria thalassemia incidences are being as high as 10%.

Other types of thalassemia happen when a gene for beta-thalassemia is inherited in combination with the hemoglobin gene. The most important of these are:

This is one of the most common hemoglobin variations. This condition is found specifically in people from the African and Southeast Asian descendants. Therefore, if a person inherits one beta-thalassemia gene and one Hb E gene, the combination produces Hb E-beta thalassemia, which causes moderate to severe anemia, which is like beta-thalassemia intermedia.

This is a well-known condition of hemoglobin variants. Those who inherit one beta-thalassemia gene and one Hb S gene results in Hb S-beta thalassemia. With this condition, the severity depends on the amount of beta-globin that is produced by the beta gene. If beta-globin is not produced, clinical pictures are like sickle cell disease.

Tests and Diagnosis

Few laboratory tests can be used to detect and diagnose thalassemia:

This form of diagnosis is an evaluation of cells in the blood. Aside from other things, CBC determines the number of red blood cells and how much hemoglobin is in them. This diagnosis is used to evaluate the shape and size of the red blood cells available and reported as red cell indices. Diagnosis will include MCV (mean corpuscular volume) and a measurement of the red blood cells. The first indication of thalassemia is a low MCV. Howe? Well, if iron deficiency has been ruled out, but still the MCV is low, then a physician will consider thalassemia next.

2.Blood Smear (similarly known as a peripheral smear and manual differential)

With this laboratory test, the expert will examine a thin layer of blood which has been treated with a special stain under a microscope. From there, the professional will consider the number and types of platelets, red blood cells, and white blood cells to see if they are normal and mature. It is important to note that a person with thalassemia, the red blood cells will appear smaller than usual. It is also important to remember that red cells may also:

  • anisocytosis and poikilocytosis (vary in size and shape)
  • hypochromic (appear paler than normal)
  • have uneven hemoglobin distribution (producing cells that look like a bull’s eye)
  • be nucleated (cells being normal, matured but do not have a nucleus)

The higher the percentage the cells are found to be abnormal, the higher the chances of a person having the disorder and, therefore, cells losing its ability to circulate oxygen.

This form of diagnosis or test may include ferritin, iron, UIBC (unsaturated iron-binding capacity), percentage saturation of transferrin, and TIBC (total iron-binding capacity). This diagnosis measures the ability of the body to store and use iron. This test is important because it helps determine if iron deficiency is the root cause of anemia. With this test, one or more tests may be conducted simply to monitor the degree of iron overload in a person with thalassemia.

Often, iron deficiency anemia is confused with alpha thalassemia because both have similar cell characteristics. However, it is wise to note that iron levels are not expected to be low when someone has been diagnosed with thalassemia. As such, the person with alpha thalassemia will not benefit from iron therapy, and infarct may cause major body organs to fail due to iron overload.

To differentiate beta-thalassemia minor from lead poisoning or iron deficiency: erythrocyte porphyrin test may be needed. A person will have normal porphyrin levels even if they have beta-thalassemia, but those with either lead poisoning or iron deficiency will have an elevated porphyrin reading.

4. Hemoglobin Electrophoresis (Hemoglobinopathy (Hb) Evaluation)

This test aims to evaluate the kind, and the relative number of hemoglobin is present in the red blood cells. Hb A (Hemoglobin A) contains both the beta and alpha-globin, and it is a type of hemoglobin, which normally makes up about 97% of the hemoglobin in adults. Hemoglobin F usually makes up less than 2%, while Hb A2 (hemoglobin A2) usually takes up about 3% of hemoglobin in adults.

People with beta-thalassemia major often have larger percentages of Hgb F. That is because beta-thalassemia affects the balance of alpha and beta hemoglobin chain formation greatly. It causes an increase in minor hemoglobin components. Also, remember that a person with beta-thalassemia minor often has a high number of Hgb A2. Hb S is dominant in persons with sickle cell disease.

This test is important and helps identify and confirm the mutation in beta and alpha globin-producing genes. This test is not routinely done but can be used to aid thalassemia diagnosis, as well as determine carrier status if indicated.

The hemoglobin beta gene may be sequenced or analyzed to confirm the presence of mutations that may cause beta-thalassemia. Remember, there are more than 250 mutations that have been associated with beta-thalassemia, even though some do not come with signs or symptoms. On the other hand, some decrease the amount of beta-globin production while others prevent its production. The confirmation or discovery of those mutations is what confirms the diagnosis.

The main molecular test available for alpha thalassemia helps confirm common mutations such as deletions. Remember, everyone has two copies of these genes known as alleles. One of the functions of alleles is governing the production of alpha-globin. Therefore, if mutation leads to functional loss of either one or more alpha genes, alpha thalassemia will occur.

Because thalassemia is a condition that is passed down the generation, family education is wise so as they can evaluate or carry out studies to identify the types of mutations found within the family if found necessary by a HealthCare professional.

Amniotic fluid genetic testing is used in rare cases or situations if found a fetus is at risk for thalassemia. This testing is crucial, especially when both parents are carriers of a mutation that puts the infants at risk. In a nutshell, this test often takes place if the case is severe.


BETA-THALASSEMIA CARRIER IDENTIFICATION

The typical phenotype of the beta-thalassemia trait, essentially characterized by reduced MCV, MCH, and increased HbA2, may be modified by several coinherited genetic factor, which may cause problems in carrier identification (Table 3).

Coinheritance of heterozygous beta-thalassemia and alpha-thalassemia may raise the MCV and the MCH, high enough to determine normal values at least in some of these double heterozygotes. This may occur as a result of either a deletion of two alpha globin structural genes or as a nondeletion lesion affecting the major alpha globin gene (the two functional alpha genes, denominated as alphal and alpha2, have a relative expression of 1:3). Fortunately, these carriers may be easily identified for their high HbA2 levels. 93,94

Elevation of HbA2 is the most important feature in the detection of heterozygous beta-thalassemia, but a substantial group of beta-thalassemia heterozygotes may have normal HbA2. The first mechanism to account for the abnormally low HbA2 levels in a beta-thalassemia carrier is the presence of a specific mild beta-thalassemia mutation, such as the beta + IVS-I nt 6 mutation. 95 A second common mechanism is the coinheritance of heterozygous beta-thalassemia and delta-thalassemia. The decreased output of the delta globin chains may result in normalization of HbA2 levels. 96,97 Also, gammadeltabeta- and deltabeta-thalassemia carriers have normal HbA2. However, all these normal-HbA2 atypical heterozygotes have low MCV and MCH. Because of this phenotype, normal HbA2 beta-thalassemia heterozygotes should be differentiated from alpha-thalassemia heterozygotes by globin chain synthesis analysis and/or by alpha, beta, and delta globin gene analysis. Deltabeta-thalassemia, in addition, may easily be defined by the variable but markedly increased HbF.

Another major problem in carrier screening is the identification of silent beta-thalassemia or the triple or quadruple alpha globin gene arrangement, both of which may lead to the production of intermediate forms of beta-thalassemia by interacting with typical heterozygous beta-thalassemia. Silent beta-thalassemias are characterized by normal MCV and MCH values and normal HbA2 and by the fact that they are defined only by the slight imbalance in the alpha/nonalpha globin synthesis.

Nevertheless, on examining the hematological features of these carriers, one may find borderline HbA2 or MCV and MCH values, which may alert for the presence of atypical beta-thalassemia, thus requiring further studies (globin chain synthesis or gene analysis). The most common silent beta-thalassemia is the beta + −101 C→T mutation others are very rare. 98 The triple-quadruple alpha globin gene arrangement may show a slight imbalance of alpha/nonalpha chain synthesis or, more commonly, may be completely silent. An extreme, although rare, instance of thalassemia gene combination, which may result in carrier identification, is the coinheritance of alpha, delta, and beta-thalassemia, which may lead to a completely silent phenotype. 99

Carrier detection procedure

Several procedures have been proposed for beta-thalassemia carrier screening. 100 The cheapest and simplest is based on MCV and MCH determination, followed by HbA2 quantitation for subjects showing microcytosis (low MCV) and reduced Hb content per red blood cell (low MCH). However, because with this procedure a considerable proportion of double heterozygotes for beta- and alpha-thalassemia may be missed (these are found in many populations, such as Sardinians, where both disorders are common), it can only be used in populations with a low frequency of alpha-thalassemia. At our center in Cagliari, in the first set of examinations, we include MCV and MCH determination and Hb chromatography by HPLC, which can quantitate HbA2 and HbF and can detect the most common Hb variants (HbS, HbC, and HbE) that may result in a Hb disorder by interacting with beta-thalassemia (Fig. 4). It should be stated that HPLC is also capable of detecting Hb Knossos, a mild beta-thalassemia allele, which is not identified by using common procedures for Hb analysis. In the presence of low MCV and MCH and elevated HbA2 levels, a diagnosis of heterozygous beta-thalassemia is made. A phenotype characterized by microcytosis, hypochromia, normal-borderline HbA2, and normal HbF may result from iron deficiency, alpha-thalassemia, gammadeltabeta-thalassemia, beta + delta-thalassemia, or mild beta-thalassemia. After excluding iron deficiency through appropriate studies (red blood cell, zinc protoporphyrin determination, and transferrin saturation), the different thalassemia determinants leading to this phenotype are discriminated by globin chain synthesis analysis and eventually by alpha, beta, and delta globin gene analysis. 100 In the presence of normal MCV and borderline HbA2 levels, we are inclined to suspect the presence of a silent mutation or the triple or quadruple alpha globin gene arrangement and, therefore, proceed directly to alpha- and beta globin gene analysis, because the alpha/beta globin chain synthesis ratio could also be normal. 101 Definition of the type of thalassemias in these carriers is solely recommended when they mate with a carrier of a typically high HbA2 beta-thalassemia or an undetermined type of thalassemia. In those rare cases showing normal or low MCV-MCH, normal or reduced HbA2 levels, and high HbF, we suspect the presence of deltabeta-thalassemia, which should be differentiated from HPFH. This distinction is performed by globin chain synthesis analysis (normal in HPFH and unbalanced in deltabeta-thalassemia) or beta-cluster gene analysis or both.

Flowchart for thalassemia carrier identification.

Molecular diagnosis of modifying genes

Molecular diagnosis is carried out in patients affected by homozygous beta-thalassemia for defining the genotype, which may be useful for predicting the severity of the disease, and in carriers identified by hematological analysis.

The procedures available to detect the beta-thalassemia mutation have been already described. 23,24 As previously mentioned, delta globin gene analysis may be necessary to define double heterozygotes for delta- and beta-thalassemia that may be mistaken for alpha-thalassemia trait. The suspicion of interacting delta-thalassemia may arise when borderline HbA2 levels are found or when family studies show segregating delta-thalassemia (characterized by normal MCV-MCH and low HbA2) and beta-thalassemia. However, identification of delta and beta double heterozygotes may be accomplished by globin chain synthesis analysis and/or alpha, beta, and delta globin gene analysis. 101,102

Definition of the delta-thalassemia mutation may be carried out using one of the previously mentioned PCR-based methods. As in beta-thalassemia, also in delta-thalassemia, each population at risk has its own spectrum of common delta-thalassemia mutations that may be defined through a limited number of specific primers/probes. In Sardinians, for instance, few delta-thalassemia mutations have been so far detected. The list of delta-thalassemia mutations is available at the repository of the human beta and delta globin gene mutation. Although most of the delta-thalassemia determinants are in trans (on opposite chromosomes) to beta-thalassemia, some have also been detected in cis (on the same chromosome). 103

Definition of the alpha globin gene arrangement may be performed to discriminate between heterozygosity for alpha-thalassemia and double heterozygosity for delta- and beta-thalassemia or gammadeltabeta-thalassemia. This analysis could also be useful in defining coinherited alpha-thalassemia in homozygous beta-thalassemia, which may lead to the prediction of a mild clinical condition. Deletion alpha 0 - or alpha + -thalassemias are detected by PCR using two primers flanking the deletion breakpoint, which amplify a DNA segment only in presence of specific deletions. 104 Nondeletion alpha-thalassemia may be detected by restriction endonuclease analysis or allelic oligonucleotide specific probes on selectively amplified alpha1 and alpha2 globin genes. Alpha globin gene triplication and quadruplication may be detected by the MPLA procedure.

Definition of coinherited HPFH determinants can be useful in predicting the severity of the phenotype of an affected fetus. As mentioned above, in fact, on increasing the gamma chain output, coinherited HPFH with homozygous beta-thalassemia may lead to a milder phenotype. The presence of high HbF in the parents may lead to the suspicion of double heterozygosity for beta-thalassemia and HPFH. The 196 C→T in the A-gamma gene and −158 C→T in the G-gamma gene mutations have been proved to be capable of ameliorating the clinical phenotype of homozygous beta-thalassemia. The HPFH determinants may easily be detected through restriction endonuclease or dot blot analysis with oligonucleotide-specific probes on PCR-amplified DNA. Furthermore, definition of the polymorphisms at the BCL11A and HBS1L-MYB region may lead to predict the development of a specific mild phenotype.

If targeted mutation analysis fails to detect the mutation, mutation scanning or sequence analysis can be used to detect mutations in the HBB coding region (mutations in the noncoding region would not be detected by this analysis). Sensitivity of both mutation scanning and sequence analysis is 99%.

Deletions of variable extent of the beta gene or of the HBB cluster that result in beta-thalassemia or in the complex beta-thalassemias, called gammadeltabeta-thalassemia and deltabeta-thalassemia, are rare causes of beta-thalassemia and testing that deletions is available clinically by using MPLA.


Hemoglobin structure

Heme is an iron-porphyrin compound composed of Porphyrins and iron, P orphyrins are cyclic compounds derived from the porphin nucleus made of 4 pyrrole rings linked by 4 methenyl bridges (-CH=) labelled α, β, γ, and δ. The porphyrins found in nature are compounds in which side chains are substituted for the hydrogen atoms in the porphin nucleus. These are different types of porphyrins. Only types I and III occur in nature:

Type I isomer: The substituent groups attached to the 4 pyrrole rings are symmetrically arranged such as (AP.AP.AP.AP). Type III isomer: The substituent groups attached to the 4 th pyrrole ring are arranged in the reverse order, e.g. (AP, AP, AP, PA). The biologically important porphyrin in heme and cytochrome are type III isomers.

Iron is present in the ferrous state (Fe ++ ) and is linked to 4 nitrogen atoms of 4 pyrrole rings. Also, there are 2 additional bonds called 5 th and 6 th coordination bonds. These two bonds are located on each side of the heme plane (perpendicular to the heme plane).

Hemoglobin structure

The 5 th position is linked to the nitrogen atom of the imidazole ring of proximal histidine. while the 6 th position is bound to oxygen in HbO2 and empty in deoxyhemoglobin (Hb) (unoccupied). The 4 pyrrole rings are attached to side chains called methyl, Vinyl, methyl, Vinyl, methyl, propionyl, propionyl, methyl. (M, V, M, V, M, P, P, M).. The transport of O2 is based on a physical interaction between molecular O2 and iron of heme to provide reversible association.

Globin (protein part or apoprotein) is a simple protein (histone) which is characterized by its high content of histidine and lysine, Globin is composed of four polypeptide chains 2α & 2β chains, The α-chain contains 141 amino acids & β-chain contains 146 amino acids.

Each β-polypeptide chain is folded into 8 right-handed termed A-H starting from NH2-terminal, while α-subunit is folded into 7α-helices. The ratio of heme to globin is 4: 1. So, each heme moiety is linked to one peptide chain.

The myoglobin-hemoglobin family of proteins has produced a way in which Fe ++ can be bounded to the proteins so as to produce an O2 binding site. Hemoglobin protects the O2 binding Fe ++ from irreversible oxidation by providing environments in which the first step of an oxidation reaction ( the binding of oxygen) is permitted, but the final step (oxidation) is blocked.

Types of normal hemoglobin

Adult hemoglobin. There are 2 types of HbA1 and HbA2. Major adult hemoglobin: Hb A12 β2) contains 2 alpha chains and 2 beta chains. This hemoglobin A1 constitutes 95-97% of the total hemoglobin. Minor adult hemoglobin: Hb A22 δ2) contains 2 α-chains and 2 δ-chains. Hb A2 forms about 2-4% of total hemoglobin. In the δ-chains, there is more than one amino acid different than those in β-chains. e.g. arginine residue at the position 16 instead of glycine which is normally present in the beta chain.

Glycosylated hemoglobin (Hb A1c) is a modified form of hemoglobin similar to hemoglobin A1 but it contains glucose linked to ε amino group present on lysyl residues and at the NH2-terminal ends. The reaction is non-enzymatic and its rate depends on the concentration of glucose.

It is present in normal value 5% of the total hemoglobin. This percentage is increased in prediabetic and diabetic patients up to 8-14 %. Thus, glycohemoglobin gives an idea about the blood glucose level during the last three months and is useful in the assessment of diabetic control.


Sickle hemoglobin (HbS) allele and sickle cell disease: a HuGE review

Sickle cell disease is caused by a variant of the beta-globin gene called sickle hemoglobin (Hb S). Inherited autosomal recessively, either two copies of Hb S or one copy of Hb S plus another beta-globin variant (such as Hb C) are required for disease expression. Hb S carriers are protected from malaria infection, and this protection probably led to the high frequency of Hb S in individuals of African and Mediterranean ancestry. Despite this advantage, individuals with sickle cell disease exhibit significant morbidity and mortality. Symptoms include chronic anemia, acute chest syndrome, stroke, splenic and renal dysfunction, pain crises, and susceptibility to bacterial infections. Pediatric mortality is primarily due to bacterial infection and stroke. In adults, specific causes of mortality are more varied, but individuals with more symptomatic disease may exhibit early mortality. Disease expression is variable and is modified by several factors, the most influential being genotype. Other factors include beta-globin cluster haplotypes, alpha-globin gene number, and fetal hemoglobin expression. In recent years, newborn screening, better medical care, parent education, and penicillin prophylaxis have successfully reduced morbidity and mortality due to Hb S.