Hemoglobin Gene Structure & Hemoglobin Disorder: Lecture Notes


Hemoglobin Gene Structure and Arrangement

The human hemoglobins are encoded in two tightly linked structure:
Alpha Gene Family: 
- Alpha gene cluster is encoded by genes located in chromosome 16
- The alpha gene cluster contains 
  • zeta gene that is expressed in the embryonic stage, 
  • the alpha-like genes that are not express (pseudogenes)
  • Alpha genes that encode for the alpha subunit of hemoglobin
Beta Gene Family
- The gene for the beta-globin chain is located on chromosome 11
- The beta gene cluster consists of 
  • beta gene encodes for beta-subunit of adult hemoglobin
  • two gamma genes (Gγ and Aγ) that encode γ chain of fetal hemoglobin
  • the delta gene that codes for the subunit present in HbA2
 

Expression of Hemoglobin During Various Developmental Stages

Embryonic Hemoglobin: Red blood cells first appear at about six weeks after conception, these RBC contain the embryonic hemoglobin synthesized by the yolk sac. These include
  • Hb Portland (ζ2γ2)
  • Hb Gower I (ζ2ε2)
  • Hb Gower II (α2ε2)

Fetal Hemoglobin: After 10 to 11 weeks, fetal hemoglobin (HbF, α2γ2) becomes predominant. Initially, the HbF is synthesized by liver cells until bone marrow is developed. 
- HbF is the predominant hemoglobin found in the fetus and newborn

Adult Hemoglobin: At about 38 weeks, it switches to adult hemoglobin (HbA, α2β2) and synthesized throughout life. 

Hemoglobinopathies

Hemoglobinopathies are the group of disorders caused by the mutation of genes encoding globin chain resulting in structural or functional abnormalities. The example includes

Sickle Cell Anemia/Sickle Cell Disease

- Sickle cell disease is a genetic disorder caused by a single nucleotide substitution in the sixth position of the beta chain of hemoglobin. 
- In sickle cell disease, the glutamate is replaced by valine at the sixth position of the beta chain
- Sickle cell disease is generally presented with intermittent vaso-occlusive events and chronic hemolytic anemia. 

Inheritance Pattern: Autosomal Recessive

Genetic/Biochemical Basis of Disease:
- The homozygous mutation in point mutation in the beta-globin resulting Glu ->Val replacement in the six positions of the beta-chain of hemoglobin
- In addition, the co-inheritance of one hemoglobin S allele and the second pathogenic variant of the beta chain are also presented as sickle cell disease. eg. Hb S/C. In these patients, the normal (wildtype) beta chain of hemoglobin is absent 
- The substitution of the nonpolar valine at the sixth positions of the beta chain forms a protrusion on the beta-globin chain that fits into the complimentary site of the alpha chain of another hemoglobin chain
- In low oxygen tension, HbS polymerized inside the RBC, forming a network of fibrous fibers that stiffens and distort the cell. These cells are seen as sickled-shape under the microscope


Sickle Cell Trait and Selective Advantage to Malaria 
- Heterozygous mutation resulting in HbS and HBA
- Milder phenotype, and individual are not anemic and have normal red blood cell indices. 
- HbA/S confers a survival advantage in the region with malarial epidemic
- Short life span of red blood than normal red blood cells and the malaria parasite cannot complete the intracellular life cycle

Pathological Manifestation
- Intermittent episode of microvascular occasion leading to tissue ischemia/reperfusion injury and chronic hemolysis
- Vaso-occlusive events (blockage of blood vessels) lead to pain and acute or chronic injury of the affected organ system ( bone marrow, spleen, liver, joints etc.)
-Other complication includes: acute chest syndrome, chronic hemolysis (anemia, jaundice, cholelithiasis), pulmonary hypertension
 
Diagnosis 
- Normocytic anemia, Sickle cells, nucleated red blood cells on the peripheral blood smear
- Presence of HbS using HPLC, Electrophoresis etc.
Molecular diagnosis
-Detection of point mutation characteristic of sickle cell disease
 
Treatment
Glutamine, Hydroxyurea etc

Thalassemias: Disorders of defective globin chain synthesis 

-Thalassemias are hereditary hemolytic disease with the defective synthesis of the globin chain. Thalassemias are categorized into 
  • α thalassemia
  • β thalassemia 

α thalassemia

Inheritance Pattern: Autosomal Recessive

Genetic/Biochemical Basis
- α thalassemias are the genetic disorders in which the synthesis of α globin are decrease or absent
- α globin has copies of four genes 
- Defect in only one gene are manifest as a silent carrier without any pathological manifestation
- Defect in two copies of the gene are known as α-thalassemia trait
- Defect in three copies of α globin gene (HbH disease) have mild to moderately severe hemolytic anemia. HbH is characterized by the presence of  β4 tetramers with a hyperbolic oxygen dissociation curve
- Defect in all four copies α globin gene are known are Hb Bart, and manifest as the most severe form of α thalassemia. Hb Bart is characterized by the presence of  tetramers with a hyperbolic oxygen dissociation curve

Pathological Manifestation
- Hb Bart syndrome is the most severe clinical condition related to α-thalassemia. Affected fetuses are either delivered stillborn at 30-40 weeks' gestation or die soon after birth. The main clinical features include generalized edema and pleural and pericardial effusions as a result of congestive heart failure induced by severe anemia.
- HbH disease. The phenotype of HbH disease varies. The majority of individuals have enlargement of the spleen and less commonly of the liver, mild jaundice, and sometimes mild-to-moderate thalassemia-like skeletal changes (e.g., hypertrophy of the maxilla, bossing of the skull, and prominence of the malar eminences) that affect the facial features.

Diagnosis

Analysis of Hemoglobin variants in α thalassemias (Ref: Genereviews)
Hemoglobin Type NormalAffected
Hb Bart syndromeHbH disease 
HbA96%-98%060%-90%
HbF<1%0<1.0%
Hb Bart085%-90%2%-5%
HbH000.8%-40%
HbA22%-3%0<2.0%

Molecular Diagnosis:
Detection of targeted deletion analysis, sequencing analysis, and deletion/duplication analysis of the coding and regulatory regions of α globin gene

Treatment: No treatment available to treat the disease. Blood transfusion, stem cell transplantation are available options

β-thalassemia 

β thalassemia is characterized by the reduced synthesis of the beta subunit of hemoglobin that results in microcytic and hypochromic anemia. 

Inheritance Pattern: Autosomal Recessive

Genetic/Biochemical Basis
Thalassemia Major: These individuals have both copies of the defective beta-globin gene. 
Thalassemia Trait: These individuals have a single copy of the defective beta-globin gene. 

Pathological Manifestation
- Because the beta-globin chain is not expressed until late in fetal gestation, the pathological manifestation occurs after birth. 
- The pathological manifestation include: severe microcytic anemia, mild jaundice, hepatosplenomegaly, iron overload cirrhosis, 

Diagnosis
- Decreased Hemoglobin
- Decreased mean corpuscular volume and mean corpuscular hemoglobin
- Microcytic & hypochromic anemia
- Peripheral blood spear shows the presence of nucleated RBC (erythroblast) & poikilocytosis
- Quantitative analysis of alpha and beta-globin chains
Molecular Diagnosis 
- Detection of mutation of the beta-globin gene, numerous gene defects at the coding region and regulatory regions are identified.

Treatment
 No available to cure the disorder. Treatment is available to reduce iron overload, and blood transfusion to replenish RBC

Hemoglobin C (HbC)

In individuals with HbC, the sixth position glutamate of a beta chain is replaced by a lysine. These individuals have mild chronic hemolytic anemia. 

Methamoglobinopathies (HbM)

A disorder associated with the increased susceptibility of oxidation of Ferrous ions present in hemoglobin. The oxidized Fe+3 Heme cannot bind to the oxygen leading to cyanosis. In Hemoglobin M disease, a mutation in the gene coding for one of the globin proteins usually results in the substitution of amino acid with tyrosine. A deficiency of NADH-methemoglobin reductase that converts methemoglobin (Fe+3) to hemoglobin (Fe+2) also causes HbM.

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