The hemoglobin E thalassemias

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

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2. The Hemoglobin E Thalassemias Naeim Ehtesham 2

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 β E - 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 3

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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 5

6. HEMOGLOBIN E β 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 , β o thalassemia , β A -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 6

7. EPIDEMIOLOGY H emoglobin 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 E β - thalassaemia is the commonest form of severe thalassaemia in many Asian countries 7

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 8

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 9

10. Genotype–Phenotype Interaction Definition of Severity β -Thalassemia Mutations Coinheritance of α Thalassemia Association with Increased HbF Amount of Alternatively Spliced β E -Globin mRNA Pyrimidine 5′ Nucleotidase Deficiency 10

11. β - Thalassemia Mutations β o 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 β o - 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 11

12. Coinheritance of α Thalassemia The concomitant inheritance of α thalassemia may be responsible for less severe anemia and a milder phenotype in HbE β o thalassemia T he 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 12

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 13

14. Amount of Alternatively Spliced β E - Globin mRNA The percentage of alternative spliced β E - globin mRNA was determined by RT-PCR in 14 patients with the same thalassemia mutation . Preliminary results showed abnormally spliced β E -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 β E -globin gene expression and found that the correctly to aberrantly spliced β E -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 β E -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 β E -globin mRNA levels prevent accumulation of potentially toxic truncated proteins , which can have a deleterious effect on the cell. 14

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 β E - 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 15

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 16

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 17

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 T hose 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. 18

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 19

20. The restoration of correctly spliced β E -globin mRNA by antisense oligonucleotides specific to the aberrant splice site may improve the disease severity among these patients 20

21. THANKS FOR YOUR ATTENTION 21