The hemoglobin E thalassemias is a rare disease and...

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2. The Hemoglobin E Thalassemias
Naeim Ehtesham
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3. 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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5. 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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6. 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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7. 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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8. 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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9. 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 HbF
levels 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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10. 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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11. 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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12. 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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13. 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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14. 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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15. 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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16. 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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17. 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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18. 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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19. 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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20.  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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21. THANKS FOR YOUR ATTENTION
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