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The Hemoglobin E Thalassemias

Naeim Ehtesham

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The nature of Hemoglobin E

HbE is the β -globin chain structural variant caused by a G-A substitution at

condon 26 of the β -globin gene, leading to replacement of lysine for glutamic

acid at this position

Abnormal sequence also activates a cryptic 5′ splice site that causes abnormal pre-

mRNA splicing

The normal donor splice site competes with this new cryptic splice site and

consequently the level of correctly spliced βE -globin mRNA is decreased while

the aberrant splicing leads to a 16 nucleotide deletion of the 3′ end in exon I and

creates a new inframe stop codon

The abnormally spliced mRNA is non-functional , as a result, HbE is synthesized

at a reduced rate

The synthetic rate of this defective β -globin chain was shown to be slightly lower

than normal, leading to the phenotype of a mild form of β -thalassemia

It is therefore classified as a β+-thalassemia allele

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The interactions of Hemoglobin E

with different forms of Thalassemia

Although HbE alone does not cause any significant clinical problems, its

interactions with various forms of α and β thalassemia produce a very wide range

of clinical syndromes of varying severity

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HEMOGLOBIN Eβ THALASSEMIA

The compound heterozygous state for HbE and β thalassemia

In general, HbE β thalassemia is β thalassemia syndrome of intermediate severity

with a very heterogeneous clinical spectrum

Two types have been described, depending on the presence or absence of HbA:

In HbE, βo thalassemia, βA-globin chains are not synthesized and the condition is

characterized by the production of HbE and HbF without detectable HbA

In HbE β+ thalassemia,different β thalassemia mutations result in variable

severity of the disease, reflecting different levels of HbA

The condition results from co-inheritance of a beta thalassaemia allele from one

parent, and the structural variant haemoglobin E from the other

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EPIDEMIOLOGY

Hemoglobin E beta-thalassemia (Hb E thalassemia) accounts for approximately

half of all the cases of severe thalassemia in the world population.

Hb E thalassemia occurs widely throughout the eastern half of the Indian

subcontinent, Bangladesh, Burma, and throughout Southeast Asia

HbE/ β -thalassemia incidence is also increasing and becoming a health problem

in North America and European countries because of immigration and interracial

marriage

Haemoglobin Eβ -thalassaemia is the commonest form of severe thalassaemia in

many Asian countries

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Pathophysiology

The pathophysiology of HbE β thalassemia reflects both the reduced output of

HbE together with the added globin-chain imbalance consequent on the

coinheritance of β thalassemia

Globally, the intermediate forms of beta-thalassaemia do not cause a major public

health problem except for the case of hemoglobin E beta-thalassaemia

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Clinical Features

One of the most striking features of HbE β thalassemia is its remarkable clinical heterogeneity

The condition may present as a mild, asymptomatic anemia or a life-threatening disorder that may lead to death from anemia in the early first years of life

At one end of the spectrum, there are patients whose clinical course is almost indistinguishable from that of severe β -thalassemia major; whereas at the other end, there are patients who grow and develop normally without the need for blood transfusion

At birth, infants with severe HbE β thalassemia are asymptomatic because HbFlevels are high. As HbF production decreases and is replace by HbE at 6–12 months of age, anemia with splenomegaly develops

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Genotype–Phenotype Interaction

Definition of Severity

β -Thalassemia Mutations

Coinheritance of α Thalassemia

Association with Increased HbF

Amount of Alternatively Spliced βE -Globin mRNA

Pyrimidine 5′ Nucleotidase Deficiency

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β -Thalassemia Mutations

βo thalassemia is more severe than β+ thalassemia, in which a wide range of β -

globin chain production is observed

In early studies Thai investigators27 suggested that patients who coinherit a mild

β -thalassemia allele with HbE might have mild disease, whereas those who

coinherited severe β+ or βo -thalassemia alleles might have more severe disease.

More recent studies suggest that the severity of the β mutation is an important, but

uncommon, cause of the clinical diversity of HbE/ β -thalassemia

Therefore, the β -thalassemia mutation alone does not account for the wide

phenotypic variation and other modifying genetic factors must be responsible

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Coinheritance of α Thalassemia

The concomitant inheritance of α thalassemia may be responsible for less severe

anemia and a milder phenotype in HbE βo thalassemia

The coinheritance of the heterozygous state for α thalassemia has been found to

result in a remarkable degree of amelioration of the clinical severity of HbE β

thalassemia

Patients with HbE/ β -thalassemia who coinherit a determinant for α -thalassemia

should, in theory, have fewer unmatched α chains, leading to the more balanced

globin chain synthesis, and resulting in a milder phenotype

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Association with Increased HbF

Coinheritance of determinants that increase HbF expression can ameliorate the

severity of HbE β thalassemia

Inheritance of a chromosome with the C → T polymorphism that results in an

Xmn-1 cleavage site at position -158 to the γ-globin gene is associated with

increased HbF and milder anemia

Two copies of this allele (Xmn-1+ /+) are necessary to produce a significant

clinical effect. Increased expression of the γ-globin gene was also detected in the

Xmn-1+ /+ patients. This increase of γ-globin gene activity reduces the overall

globin chain imbalance and thus ameliorates the anemia.

The relationship between this polymorphism and the steady-state level of

haemoglobin F, its association with a later age of presentation, and its increased

representation in the older age groups indicate that it is a major modifier of the

haemoglobin E thalassaemic phenotype

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Amount of Alternatively Spliced

βE -Globin mRNA

The percentage of alternative spliced βE - globin mRNA was determined by RT-

PCR in 14 patients with the same thalassemia mutation . Preliminary results

showed abnormally spliced βE -globin mRNA in patients with severe symptoms

and low hemoglobin levels between 2.9% and 6.1%, whereas those with higher

hemoglobin levels had values from 1.6% to 2.6%

Recently, Tubsuwan et al. used the allele-specific RT-qPCR to study βE -globin

gene expression and found that the correctly to aberrantly spliced βE -globin

mRNA ratio in 30% of mild HbE b-thalassemia patients was higher than that of

the severe patients. It appears therefore that the splicing process of βE -globin pre

mRNA differs among HbE β -thalassemia patients and serves as one of the

modifying factors for disease severity

A decrease of aberrantly spliced βE -globin mRNA levels prevent accumulation of

potentially toxic truncated proteins, which can have a deleterious effect on the

cell.

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Pyrimidine 5′ Nucleotidase Deficiency

In a Bangladeshi family, an individual homozygous for both HbE and pyrimidine

5′ nucleotidase deficiency was found. The patient had a severe hemolytic anemia

in contrast to HbE homozygotes

Globin chain synthesis experiments showed that the mechanism underlying the

interaction between these two genotypes was a marked decrease in the stability of

HbE in pyrimidine 5′ nucleotidase-deficient red blood cells

This individual was shown to have a mutation in the P5N-1 gene affecting

expression of the P5N-1 enzyme, believed to be susceptible to free radical damage

In these cells, free α -globin chains but not βE -globin chains accumulated on the

membrane. It was hypothesized that the marked instability of HbE in the enzyme-

deficient cells resulted from oxidant damage to mildly unstable HbE .Clearly this

interaction also has the potential to modify the phenotype of HbE β thalassemia

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Bilirubin metabolism

Chronic hyperbilirubinemia, gallstone formation, and gall bladder disease in

patients with HbE/ β -thalassemia may significantly worsen the phenotype of the

disease

The increased level of bilirubin in these disorders has been related to a

polymorphism of the promoter of the UGT1*1gene

Premawardhena and colleagues showed that the UGT*1 genotype is also

important in the genesis of gallstones in patients with HbE/ β -thalassemia

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Variation in iron loading

Patients with HbE/ β -thalassemia suffer mostly from chronic hemolytic anemia.

In severe cases, regular blood transfusion is needed to sustain an acceptable

quality of life. This lifelong need for blood transfusion causes an accumulation of

iron in several vital organs, leading to organ malfunction and premature death

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Variation in Adaptation to Environmental Factors

Patients with HbE β thalassemia are more susceptible to malaria infection,

particularly that caused by P. vivax, than age-matched controls in the population

Those who have been exposed to malaria tend to have larger spleens and fall into

the more severe phenotypic categories

Significantly higher level of malarial antibodies,in those with HbE/ β -thalassemia

compared with the control population.

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Treatment & management Because HbE β thalassemia has such a variable phenotype it is vital to observe

babies and young children with this condition after presentation for a reasonable

period before deciding on the best approach to management

They may present with a particularly low hemoglobin level consequent to a recent

infection, and it is particularly important therefore not to establish them on a

regular transfusion until their steady-state hemoglobin level and level of growth

and degree of splenomegaly has been assessed

Hydroxyurea therapy may increase HbF levels

For those who present early with severe disease, bone marrow transplantation

remains an important option

In malarious areas it will be very important to conduct trials of malaria

prophylaxis

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The restoration of correctly spliced βE -globin mRNA by antisense

oligonucleotides specific to the aberrant splice site may improve the disease

severity among these patients

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THANKS FOR YOUR ATTENTION

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