Haemoglobin molecule is the oxygen-carrying pigment of the red blood cell which consists of a heam group and globin chains. A normal adult blood contains three types of haemoglobin depending on the type of globin chain that paired with the haem group. These include; Haemoglobin A, Haemoglobin A2 and HbF2.
Normal haemoglobin molecules in adult blood.
HbA HbF HbA2
Structure a2β2 α2Y2 α2β2
Normal % 96-98 0.5-0.8 1.52-3.2
(From Hoffbrand and Moss, 2011)
Each Haemoglobin A molecule of an adult contains four linked polypeptide (globin) chains which consists of two alpha () chains containing 141 amino acids and two beta chains containing 146 amino acids that pairs with haem molecules. (Chessbrough, 2004).
Synthesis of haemoglobin molecules starts during the embryonic life and takes place mostly in the Yolk sac, liker, and spleen. The Genes responsible for synthesis of the globin chains occur in two clusters; є,, and β on chromosome II and and a on chromosomes 16. Two types of chain occur; G and A, which differ by a glycine or alanine amino acid at position 136 of the gamma polypeptide chain (see illustration/diagram pg 89).
During embryonic, fetal and adult life different genes are activated or suppressed. The different globin chains are synthesized independently and then combine with each other to produce the different haemoglobin.
Synthesis of globin chain of a haemoglobin molecule follow the normal central dogma while the haem synthesis occurs largely in the mitochondria
by a series of biochemical reactions, that finally leads to incorporation of iron (Fe2+) into proto porphyrin to form haem.
(The expression of a human globin gene from transcription, excision of introns splicing of exons and translation to ribosome. (Hoffbrand and Moss, 2011).
During the synthesis of beta-globin chain, any mutation at conserved sequence gives rise to Beta-thalassaemia. The physiology structure of the haemoglobin molecule enables it to transport oxygen to the tissue and carry carbondioxide back to the lungs.
Pathophysiology Of Beta-Thalassaremia
The clinical severity of the Beta-thalassaemias depends on the extent of alpha globin chain/non-alpha globin chain (i.e. β+and) imbalance. The non-assembled alpha globin chains that result from unbalanced alpha globin/non-alpha globin chain synthesis precipitate in the form of inclusion in erythroid cells.
However, the degree of precipitation of these inclusion depends on the type of Beta-thalassaemia, for instance, in heterozygous β+ thalassaemia (i.e Beta-thalassaemia minor/trait, there is synthesis of beta-globin gene with just a small reduction of the chain, therefore the degree of precipitation of the alpha –globin chains is usually small, thereby making the individuals to be asymptomatic. In Beta-thalassaemia major, the absence of beta globin chains results in a relative excess of unbound alpha globin chains that precipitate in erythroid precursors in the bone marrow, leading to their premature death and hence to ineffective erythropoiesis (Gallanelo and Rafaelle, 2010).
The degree of globin chain reduction is determined by the nature of the mutation at the beta globin gene located on chromosome II. Peripheral haemolysis contributing to Jaundice and anaemia is less prominent in thalassaemia major than in thalassaemia intermedia and occurs when insoluble alpha globin chains induce membrane damage to the peripheral erythrocytes, anaemia stimulates the production of erythropoietin with consequent intensive but in effective expansion of the bone marrow (up 25-30 times normal), which in turn cause the typical previously described bone deformities. Prolonged and severe anaemia with jaundice and increased enthropoietic drive also result in hepatosplenomegaly and extramedullary exythropeiesis (Gallanelo and Raffaella, 2010).
Any inherited or acquired conditions that reduces the alpha/non-alpha globin chain imbalance in Beta thalassaemia results in a lesser degree of alpha globin chain precipitation and leads to a mild β-thalassaemia phenotype. One of the most common and consistent mechanism is homozygosity or compound heterozygosity for two β+ thalassaemia mild and silent mutations, coinheritance of alpha thalassaemia which may normalize the red cell indices.
However, certain conditions can also worsen beta-thalassaemia conditions, these includes;
- Compound heterozygosity for a mild/silent β+ and a severe mutation produces a variable phenotype ranging from thalassaemia intermedia to thalassaemia major.
- Haemoglobin E (HbE); which is a thalassaemia structural variant characterized by the presence of an abnormal structure as well as biosynthetic defect, is also included in this group. This is based on the fact that the nucleotide substitution at codon 26 produces the HbE variant which activates a potential cryptic RNA splice region, resulting in alternative splicing at this position. The homozygous state for HbE results in a mild hemolytic microcytic anaemia, but compound heterozygosity for beta-thalassaemia and HbE results in a wide range of often severe but sometimes mild or even clinically asymptomatic clinical phentypes. (Gallanello and Raffaella, 2010).
Genetic Modifiers of Beta-Thalassaemia
The coinheritance of some genetic determinants able to sustain a continuous production of gamma globin chains (HbF) in adult life may reduce the extent of alpha/non-alpha globin chain imbalance:
- The beta-thalassaemia mutation per se increases the gamma globin chain (HbF) output. This occurs in the following two situations:
1. -thalassaemia which is caused by deletion of variable size in the HBB gene cluster.
2. Deletions remove only the 51 region of the HBB promoter, which also results in high levels of HbA2.
- Co-transmission of hereditary persistence of fetal hemoglobin (HPFH), which is as a result of single point mutation of the hemoglobin G(HBG2) or hemoglobin A(HBG1) gene promoter. The most common is a single-base substitution of cytosine to Thymine at position 158 upstream of the transcription start site of the HBG2 gene, which is silent in normal individuals and in β-thalassaemia heterozygote, but leads to increased HbF production in individuals with erythropoietic stress, as occurs in homozygous beta-thalassaemia. This HBG2 mutation is sometimes referred to as G-158C<T. It is in linkage disequilibrium (in cis configuration) with some HBB mutation. This explains the mild phenotype that may result from the inheritance of these mutations.
- Coinheritance of heterocellular HPFH may or may not be linked to the HBB gene cluster but results to increased synthesis of HbF. Recent studies using genome-wide association studies (G-WAS) have identified two quantitative trait loci (QTLs) (BCL11A on chromosome 2p16 and HBSIL-MYB intergenic region on chromosome 6q23) that account for 20-30% of the common variation in HbF levels in healthy adults and person with beta-thalassaemia and sickle cell diseases (Uda et al, 2008, Thein et al, 2009). BCL11A seems to b involved in the regulation of the globin gene switching process (Sankaran et al, 2008). The ameliorating effect of QTLs and thalassaemia on the phenotypic severity of homozygous beta- thalassaemia has recently been reported (Galanello et al, 2009). Recently, an additional potential locus has been identified when point mutations in KLF1 were found to be associated with HPFH.KLF1 is a zinc-finger erythroid transcriptional regulator that binds to the critical promoter elements of the adult β-thalassaemia globin.It plays a critical role in regulating the switch between fetal and adult haemoglobin expression both by direct activation indirect repression of -globin gene expression in adult erythroid progenitors via regulation of BCL11A. Recently, several KLF1 mutations have been identified in individual with a thalassaemia carrier phenotype and a particularly mild form of sickle cell disease (Gallinne et al, 2012). It is likely that many other HbF-associated QTLs also exist.