1. Molecular basis of hemoglobinopathies

2. Hemoglobinopathies
A quarter of a million people are born in the world each year with one of the disorders of of the structure or synthesis of hemoglobin. The hemoglobinopathies therefore have the greatest impact on mobidity and mortality of any single group of disorders following mendelian inheritance.

3. Hemoglobinopathies, occupy a unique position in
medical genetics.
most common single-gene disorders in humans
the human globin genes were the first disease-related genes to be cloned, their molecular and biochemical pathology is better understood than any other group of genetic diseases.
The globins also cast light on the process of evolution at both the molecular and the population levels and provide a model of gene action during development

4. Structure and Function of Hemoglobin
Hemoglobin is the oxygen carrier in red blood cells. The molecule contains four subunits: two α chains and two β chains. Each subunit is composed of a polypeptide chain, globin, and a prosthetic group, heme, which combines with oxygen to give the molecule its oxygen-transporting ability

5. The Human Hemoglobin In normal adult hemoglobin, hemoglobin A (HbA),
these globin chains are designated α and β. (α2β2 )
The two types of chains are almost equal in length; the α chain has 141 amino acids,
and the β chain has 146.
In addition to Hb A, there are five other normal
human hemoglobins, each of which has two α or α
like chains and two non-α chains.

6. Globin Chains α Globin β Globin δ Globin γ Globin ζ Globin
141 amino acids
Coded for on Chromosome 16
Found in normal adult hemoglobin, A1 and A2
β Globin
146 amino acids
Coded for on Chromosome 11,
found in Hgb A1
δ Globin
Found in Hemoglobin A2--small amounts in all adults
γ Globin
Found in Fetal Hemoglobin
ζ Globin
Found in embryonic hemoglobin

7. Hemoglobin Types Hemoglobin Type Globin Chains
Portland — ζ 2 γ 2
Gower I — ζ 2ε 2
Gower II — α 2 ε 2
Hgb F — <1% α 2 γ 2
Hgb A —95% α 2 β 2
Hgb A2—2.5% α 2 δ 2

8. Hemoglobin Genes Alpha globins are coded on chromosome 16
Two genes on each chromosome (α1 and α2)
Four genes in each diploid cell
Gene deletions result in Alpha-Thalassemias
Also on chromosome 16 are Zeta globin genes—Gower’s hemoglobin (embryonic)
Beta globins are coded on chromosome 11
One gene on each chromosome
Two genes in each diploid cell
Point mutations result in Beta-Thalassemias
Also on chromosome 11 are Delta (Hgb A2) and Gamma (Hgb F) and Epsilon (Embryonic)

10. Developmental Expression of Globin Genes and Globin Switching
The change in the expression during development of the various globin genes, sometimes referred to as globin switching is a classic example of the ordered regulation of developmental gene expression.

11. Hb F (α2γ2) is the predominant hemoglobin throughout fetal life and constitutes approximately 70% of total hemoglobin at birth, but in adult life, Hb F represents less than 1% of the total hemoglobin. Although β chains can be detected in early gestation, their synthesis becomes significant only near the time of birth; by 3 months of age, almost all the hemoglobin present is of the adult type, Hb A. Synthesis of the δ chain also continues after birth, but Hb A2 (α2δ2) never accounts for more than about 2% of adult hemoglobin

12. Ontogeny of Hemoglobin
Embryonic Hemoglobin
Hgb Gower 1 (ζ2,ε2)
Hgb Gower 2 (α2, ε2)
Fetal Hemoglobin
Hgb F (α2,γ2)
Adult Hemoglobin
Hgb A (α2,β2)
Hgb A2 (α2,δ2)
Hgb F (α2,γ2) (<1%)
Early embryogenesis
Yolk sac erythroblasts
Major Hemoglobin of
Intra-uterine life
Adult Hemoglobin

13. THE HEMOGLOBINOPATHIES
The hereditary disorders of hemoglobin can be
divided into two groups,
structural variants, which alter the globin polypeptide without affecting its rate of synthesis;
disorders of synthesis of the globin chains, arise from inadequate hemoglobin production and unbalanced accumulation of globin subunits.

14. Hemoglobin Structural Variants
Most variant hemoglobins result from point mutations in one of the globin structural genes. More than 400 abnormal hemoglobins have been described, and approximately half of these are clinically significant.

15. Hemoglobinopathy Type of Mutation Single amino acid substitution
Hgb S, Hgb C
Deletion
Hgb Gun Hill
Abnormal Hybridization
Hgb Lepore
Abnormal elongation
Hgb Constant Spring
Examples of Hemoglobinopathy
α Chain
Hgb Gphiladelphia
Β Chain
Hgb S, Hgb C
γ Chain
Hgb FTexas
δ Chain
Hgb A2Flatbush

16. The hemoglobin structural variants can be separated into three classes, depending on the clinical phenotype
Variants that cause hemolytic anemia. The great majority of mutant hemoglobins that cause hemolytic anemia make the hemoglobin tetramer unstable.
Sickle cell globin (HbS) and Hb C, are not unstable but cause the mutant globin proteins to assume unusual rigid structures.
Mutants with altered oxygen transport, due to increased or decreased oxygen affinity or to the formation of methemoglobin, a form of globin incapable of reversible oxygenation.

17. Sickle Cell Disease
Sickle-cell disease, an autosomal recessive condition, is the most common hemoglobinopathy and the clinical manifestations are cerebral symptoms, kidney failure, heart failure, weakness and lassitude. Sicle-cell hemoglobin (HbS) is less soluble than normal hemoglobin and tends to polymerize, causing the sickle- shaped deformation of the red cells.

18. HbS
Sickle cell hemoglobin (Hb S) is due to a single nucleotide substitution that changes the codon of the sixth amino acid of β globin from glutamic acid to valine (GAG → GTG: Glu6Val) Homozygosity for this mutation is the cause of sickle cell disease, a serious disorder that is common in some parts of the world; most frequently in equatorial Africa and less commonly in the Mediterranean area and India.

21. Disorders of Hemoglobin Synthesis
The thalassemias, collectively the most common human single-gene disorders, are a heterogeneous group of diseases of hemoglobin synthesis in which mutations reduce the synthesis or stability of either the α-globin or β-globin chain to cause α-thalassemia or β-thalassemia

22. α-Thalassemia
α-Thalassemia results from an underproduction of the α-globin chains. Genetic disorders of α-globin production affect the formation of both fetal and adult hemoglobins and therefore cause intrauterine as well as postnatal disease. There are four α chain loci, with two nearly identical copies of the α globin gene on each chromosome 16. The spectrum of α-thalassemia therefore reflects whether the patient lacks one, two, three, or all four α globin genes. The various forms of α thalassemia have been shown to be mostly due to deletions of one or more of these structural genes

23. Patients with loss of a single α chain gene are silent carriers and have a normal hematocrit and MCV. (α-/αα)
Patients with deletion of two α chains (α-thalassemia trait), either on the same chromosome (- -/αα ) or on different chromosomes (α-/α-), are microcytic and mildly anemic.
Patients who inherit only one α chain gene (- -/α-) have hemoglobin H disease.
Inheritance of the homozygous deletions at four allele (- -/- -) results in no functional α chain loci, and is incompatible with life (hydrops fetalis).

24. α Thalassemia Four possible α thalassemia syndromes:
α-thalassemia-2 trait (silent carrier): One of the four α-globin gene loci fails to function.
α-thalssemia-1 trait (α Thal 1 trait): Two of the four α-globin gene loci fails to functions.
Hgb H disease: Three of the four α-globin gene loci fails to functions.
Bart’s Hydops Fetalis: Four of the four α-globin gene loci fails to functions.

25. β-Thalassemia
The β-thalassemias share many features with α-thalassemia. Decreased β-globin production causes a hypochromic, microcytic anemia, and the imbalance in globin synthesis leads to precipitation of the excess α chains, which in turn leads to damage of the red cell membrane. In contrast to α-globin, however, the β chain is important only in the postnatal period. The onset of β-thalassemia is not apparent until a few months after birth, when β-globin normally replaces γ-globin as the major non-α chain, and only the synthesis of the major adult hemoglobin, Hb A, is reduced.

26. Because the δ gene is intact, Hb A2 production continues, and in fact, elevation of the Hb A2 level is unique to β-thalassemia heterozygotes.
The level of Hb F is also increased, not because of a reactivation of the γ-globin gene expression that was switched off at birth, but because of selective survival and perhaps also increased production of the minor population of adult red blood cells that contain Hb F.

27. β-thalassemias are usually due to single-base pair substitutions rather than to deletions. In many regions of the world where β-thalassemia is common, there are so many different β-thalassemia mutations that persons carrying two β-thalassemia alleles are more likely to be genetic compounds than to be true homozygotes for one allele.

28. Infants with homozygous β-thalassemia (thalassemia major) present with anemia once the postnatal production of Hb F decreases, generally before 2 years of age. Carriers of one β-thalassemia allele are clinically well and are said to have thalassemia minor. The diagnosis of thalassemia minor can be supported by hemoglobin electrophoresis, which generally reveals an increase in the level of Hb A2 (α2δ2).