Download genetic disorder of haemoglobin

GENETIC DISORDER OF
HAEMOGLOBIN
Haemoglobinopathies and
Thalassaemias
1
Haemoglobin
 A conjugated protein consisting of iron-containing heme
and protein (globin)
 Globin chains are of different types:
-chains and non  -chains
 Each molecule is a tetramer of two - and non  chains.
 Each globin binds a haem in a haem binding site.
 Haemoglobin binds and transports oxygen from lungs to
the tissues, while it transports CO2 from tissues to the lungs.
2
3
Types of Hemoglobin in adults
Globin genes
in
Chromosome
16
11
Gene product Tetramers
Name of
Conc.
(globin)in RBCs
haemoglobin
adult


, -chain
2 2
Hb A
96-97


, -chain
2 2
Hb A2
2.3-3.5
Hb F
<1.0


 ,-chain
2 2
-----------------------------------------------------------------
4
Chromosome 11
5’

G
A



3’
Chromosome 16


2 1
2
1
5’
3’
Structure of each Globin gene
5’
Exon 1 Intron 1 Exon 2 Intron 2
3’
Exon 3
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HEMOGLOBIN SYNTHESIS
25%
25%
0.5%
1.5%
48%
α
α
γ
δ
β
α
α
γ
δ
β
25%
25%
0.5%
1.5%
48%
Chromosome 16
Chromosome 11
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HEMOGLOBINOPATHY; DEFINITION
An inherited mutation of the globin genes leading
to a qualitative or quantitative abnormality of
globin synthesis.
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STRUCTURAL HEMOGLOBINOPATHY
Amino acid substitution in the globin
chain e.g. sickle hemoglobin (HbS)
8
THE THALASSEMIAS
Syndromes in which the rate of synthesis of a
globin chain is reduced: beta thalassemia – reduced beta chain
synthesis
alpha thalassemia – reduced alpha chain
synthesis
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Disorders of Haemoglobin
Haemoglobinopathies
(Structural disorder
of Hb)
Thalassaemias
(Biosynthetic
disorder
of Hb)
Co-existing
structural /
biosynthetic
disorders
Constitute a major health problem in several
populations of the world
(particularly those residing in malaria
endemic region)
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Haemoglobinopathies
•
•
•
•
Genetic structural disorder.
Due to mutation in the globin gene of haemoglobin.
Mostly autosomal recessive inheritance.
Result in haemoglobin variants with altered structure
and function.
• Altered functions include:
 Reduced solubility
 Reduced stability
 Altered oxygen affinity- increased or decreased
 Methaemoglobin formation
11
HEMOGLOBINOPATHIES
Decrease, lack of, or abnormal globin
 May be severe hemolytic anemia
 Abnormal Hb with low functionality
 Mutation may be deletion, substitution, elongation
 Hb electrophoresis may be helpful

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Types of Mutations in Haemoglobin
•
Point mutation: a change of a single nucleotide base in a DNA
giving rise to altered amino acids in the polypeptide chains
(e.g. Hb S , Hb C)
•
Deletions and additions: Addition and deletion of one or more bases
in the globin genes
•
Unequal crossing over: as in Hb-lepore and Hb-antilepore
associated with -thalassaemias.
________________________________________________________
*Most abnormal Hbs are produced by mutations in the structural
genes which determine the amino acid sequence of the globin
chains of the Hb molecule.
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Geographical distribution of common Hb variants
Variant
Occurrence predominantly in:
Hb S (6GluVal)
Africa, Arabia, Black Americans
Hb C (6Glulys)
West Africa, China
Hb E (26Glulys)
South East Asia
Hb D (121GluGln)
Hb O (121GluVal)
Asia
Turkey and Bulgury
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Sickle Cell
Haemoglobin


GAG
6

RBC


GTG
Sickle Cell
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Haemolysis
Inheritance of Sickle Cell Anaemia
AR
AS
AS
AS
SS
AA
AS 16
Red cell sickling
Lungs
↑pO2
Tissues
↓pO2
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Major abnormalities & problems in SCA
Sickling of the red cell during deoxygenation, as the HbS
has low solubility at low O2 partial pressure and precipitates.
 Chronic haemolytic anaemia due to repeated sickling in tissues and
unsickling in the lungs.
 Plugging of microcapillaries by rigid sickled cells leading to sickle cell
crises i.e severe pain and edema. This causes significant damage to
internal organs, such as heart, kidney, lungs and endocrine glands.
 Repeated infections.
 Frequent cerebrovascular accidents.
 Hand-foot syndrome (in small,i.e.around age of 3y); Swollen hands and
feet may be the first signs of sickle cell anemia in babies. The swelling is
caused by sickle-shaped red blood cells blocking blood flow out of their
hands and feet
 Bone deformation – bossing of the forehead.
 Hepato-spleenomegaly.
 Growth retardation.
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 Frequent blood transfusion requirements.
 Psychosocial problems.
Site of
Sickling
Clinical
Features
Management
Bone
Painful crises
Lung
Acute chest
syndrome
Stroke
Myocardial
infarction
Acute splenic
sequestration:
Proliferative
retinopathy
Pain relief and hydration.
Hydroxyurea
Transfusion regimen, pain
relief and hydration
Transfusion regimen.
Transfusion regimen, pain
relief and hydration
Transfusion, pain relief
and hydration
Retinal surveillance. Laser
Brain
Heart
Spleen
Retina
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SICKLE CELL TRAIT
 Heterozygous
state for HbS (HbAS)
 No serious clinical consequences
 Sudden death during intensive training
 Hematuria,
isosthenuria
(renal
papillary
necrosis)
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Thalassaemias
Genetic disorders resulting from
decreased biosynthesis of globin chains
of haemoglobin.
21
Thalassaemias
•
•
•
•
A group ( not single identity) of Genetic defects.
Due to mutations in and around the globin genes.
Decreased production of one or more of the globin chains.
Result in an imbalance in the relative amounts of
the - and non  -chains. Altered /non-  ratio.
• As a consequences of thalassaemias there is excess
production of the other chains, and a decreased over all
haemoglobin synthesis.
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Types of Thalassaemias
- Thalassaemia*
-Thalassaemia*
- Thalassaemia
- Thalassaemia
- Thalassaemia
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* Most common
α Thalassemia
 Deletion
of one or more alpha genes from
chromosome 16
 -a/aa: silent career with little signs
 --/aa: cis double deletion more common in SEA
 -a/-a: trans double deletion
 --/-a: Hb H disease
 --/--: Hb Bart’s hydrops fetalis
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ALPHA THALASSEMIA
αα/αα
Normal
αα/α-
Mild microcytosis
αα/- α-/αα-/- -
Mild microcytosis
- -/- -
Hemoglobin Barts – Hydrops Fetalis
Hemoglobin H disease
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- Thalassaemia
- Decreased / ratio
Hb




In - Thalassaemia Decreased
production
of - chains
Normal = 

- Thalassaemia


Accumulation of 



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Point Mutation producing - Thalassaemia
Less Frequent
Introns
Chromosome 16
5’
exon1
exon2
3’
exon3
Base Substitution
2bp del
5bp del
Chain Termination
Defect
Poly A signal
Mutation
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Mutations Producing - Thalassaemia
Deletions
Most frequent:
Chromosome 16
/
Normal
-/
-/-
--/
--/-
--/--
-thal 2 -thal 2 -thal 1
HbH Hydrops
hetero
homo
hetero Disease fetalis
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- Thalassaemia-2
•
One  -gene deletion.
•
-chain production is only about 75% of normal.
•
May be homo- (- /- ) or heterozygous (- / )
•
The patient usually shows a normal phenotypic
appearance but there might be mild thalassaemia
symptoms.
•
Hypochromic-microcytic RBC’s due to partial reduction
of -chain.
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- Thalassaemia-1
•
Two  -genes deletion- (o ) thal.
•
The patient synthesizes -chain but it is
decreased to about 50% of normal.
•
Anaemic symptomsanaemia.
•
May be homozygous (- -/- -) or heterozygous(--/
). If the patient is homozygous than there is
no -chain synthesis, and if heterozygous then
there is decreased synthesis of the -chain to
half normal level.
hypochromic
microcytic
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Hb H Disease
•
Three -gene (three alleles) deletion.
•
The Hb present during foetal life is “Hb Bart’s” (4), while
during adulthood the Hb present is “Hb H” (4).
•
Some of the symptoms include:
hepatosplenomegaly, impairment of erythropoiesis, and
hypochromic-microcytic haemolytic anaemia.
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Hydrops foetalis
•
Homozygous o-thalassaemia.
•
There is a complete absence of -chain (all -genes are
deleted).
•
The Hb produced at birth is Hb Barts (4).
•
•
Hydrops foetalis is lethal and the baby is born dead.
Hydrops fetalis is a severe, life-threatening problem of
severe edema (swelling) in the fetus and newborn. It is
also called hydrops.
•
Symptoms include:
Hepatosplenomegaly,
hypochromic- microcytic anaemia.
severe
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-
Thalassaemia
Increased / ratio
Hb
 
 
In - Thalassaemia Decreased
production
of - chains
Normal = 
- Thalassaemia



Accumulation of 



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BETA THALASSEMIA
 Impaired
production of beta chain
 beta thalassemia minor – heterozygous (or trait)
 beta thalassemia major - homozygous
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TYPES OF - THALASSAEMIA
Thalassemia Major
(Cooley's anemia)
-severe form of beta
thalassemia
- presence of two
abnormal genes that
cause either a severe
decrease or complete
lack of beta globin
production.
Thalassemia Minor
- presence of one normal
gene and one with a
mutation
- causes mild to
moderate mild
anemia.
ETIOLOGY OF - THALASSAEMIA
 Beta
thalassemia is caused by a deficiency of Beta
globin inherited in an autosomal recessive pattern,
which means both copies of the HBB(Hemoglobin
beta) gene in each cell have mutations.
 The
parents of an individual with an autosomal
recessive condition each carry one copy of the mutated
gene, but they typically do not show signs and
symptoms of the condition.
BETA THALASSEMIA TRAIT
No symptoms
 Mild microcytic anemia

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BETA THALASSEMIA MAJOR
No beta chain produced (no HbA)
 Severe microcytic anemia occurs gradually in the first
year of life
 Marrow expansion
 Iron overload
 Growth failure and death

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- Thalassaemia
Usually point mutation in the control region.
 The absence of beta-globin is referred to as beta-zero (B0)
thalassemia.
 Other HBB gene mutations allow some beta-globin to be
produced but in reduced amounts. A reduced amount of
beta-globin is called beta-plus (B+) thalassemia β+ has
minimal production.
 β+/β+ or βo/βo is  thal major or Cooley’s anem
 Often not apparent at birth until β chain takes over γ
chain production.
 High Hb A2, Hb F.
 Related: Hb Lepore (δ-β usion), High Persistence of Foetal
40
Hb; (HPFH).

- Thalassaemia
• It is characterized by either no -chain synthesis (i.e. o)
or decreased synthesis of -chain (+).
• Excess -chains precipitate in RBC’s causing severe
ineffective erythropoiesis and haemolysis.
• The greater the -chains, the more severe the anaemia.
• Production of -chains helps to remove excess -chains
and to improve the -thalassaemia. Often HbF
level is increased.
• Majority of -thalassaemia is due to point mutation.
41
o-Thalassaemia
The -chain is totally absent.
There is increase in HbF with absence of HbA.
This is combined with ineffective erythropoiesis.
In majority of the cases, -gene is present but there is
complete absence of mRNA.
• Characteristics of this disorder are:
• Skeletal deformities (e.g. enlargement of upper jaw,
bossing of skull and tendency of bone fractures).
• Severe hypochromic- microcytic anaemia.
• Survival depends on regular blood transfusion.
• This leads to iron overload (iron accumulates in the
blood and tissues, causing tissue damage).
• Death usually occurs in the 2nd decade of life (i.e. at
age of about 20 years) if measures are not taken to
avoid iron overload by chelation therapy.
•
•
•
•
Mutations affecting the -Globin gene.
Chromosome 11
1.
2.
3.
4.
5.
Mutations affecting transcription initiation
Mutations affecting RNA splicing
Mutations affecting translation initiation
Non-sense Mutations.
Mutations of polyadenylation site.
>200 -Thal
mutations reported
to-date
Worldwide
43
Clinical Classification of Thalassaemias
1. Thalassaemia major:
The patient depends on blood transfusions especially if he is
homozygous.
2. Thalassaemia intermediate:
• Homozygous mild +-thalassaemia.
• Co-inheritance of -thalassaemia.
• Heterozygous -thalassaemia.
• Co-inheritance of additional -globin genes.
•  -thalassaemia and hereditary persistence of foetal Hb
• Homozygous Hb lepore
• Hb H disease.
3. Thalassaemia minor (trait):
• o-thalassaemia trait.
• +-thalassaemia trait.
• Hereditary persistence of foetal Hb only.
• -thalassaemia trait.
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• o- and +-thalassaemia trait.
BETA THALASSEMIA MAJOR
TREATMENT
 Transfusion
 Iron
chelation
 Stem cell transplant
45
Hb-Lepore
• This is an abnormal Hb due to unequal crossing-over of
the - and -genes to produce a polypeptide chain
consisting of the - chain at its amino end and - chain at
its carboxyl end.
• The -fusion(hybrid) chain is synthesized inefficiently
and normal  and -chain production is abolished.
• The homozygotes show thalassaemia intermediate and
heterozygotes show thalassaemia trait.
Chromosome 11
5’

G
A



3’
HEMOGLOBINOPATHY-ANTENATAL
DIAGNOSIS
Test partners of heterozygous or affected individuals
 Antenatal diagnosis from DNA obtained by chorionic
villus sampling, or by amniocentesis

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