Epidemiology: Beta-thalassaemia is prevalent in Mediterranean countries, Middle East, Central Asia, India, Southern china, and the far East as well as countries along the north coast of Africa and in south America. The highest carrier frequency is reported in Cyprus (14%), sardine (10.3%), and south east Asia (Flint etals., 1988),
it is suspected that there is an increase in prevalence of B-thalassaemia in West African countries (Willcox, 1975, Ringelham et al, 1968, Ademowo et al, 2002).
The high gene frequency of beta-thalassaemia in these regions is most likely related to the selective pressure from plasmodium falciparum malaria endemicity (Flint et al., 1988). Thalassaemia provides some protection against malaria resulting in more thalassaemia carriers surviving malaria epidemics than non thalassaemia, thereby inflating the percentage of those population carrying the thalassaemia genes population migration and intermarriage between different ethnic groups has introduced thalassaemia in almost every country of the world, including Northern Europe where thalassaemia was previously absent. It has been estimated that about 1.5% of the global (80 to 90 million people) are carriers of beta- thalassaemia with about 60,000 symptomatic individuals born annually, the great majority in the developing countries.
However, accurate data on carrier rates in many population are lacking, particularly in areas of the world known or expected to be heavily affected (Vichinsky, 2005) According to Thalassaemia International federation second edition report in 2008 only about 200,000 patients with thalassaemia major are alive and registered as receiving regular treatment around the world.
Etiology and Genetic Mutation of Haemoglobin Submit Beta Locus (Hbb)
The etiology of beta- thalassaemia is mutation of the haemoglobin submit Beta (HBB) is the normal allelic variant, which spans 1.6kb, contains three exons (coding region) and two introns (non-coding region, in which the DNA is not represented in the finished protein). The initial RNA is transcribed from both introns and exons, and from this transcript the RNA derived from the introns is removed by a process known as splicing which is performed by proteins called spliceosome.
The introns usually starts with a G-T dinucleotide and end with an A-G dinucleotide. The splicing machinery recognizes these sequences as well as conserved sequences. (Hoffbrand and Moss, 2011).
Examples of mutations that produce β-thalassaemia. These include single base changes, small deletions and insertions of one or two bases affecting introns, exons or the flanking regions of the β-globin gene. FS, ‘frameshifts’ deletion of nucleotide(s) that places the reading frame out of phase downstream of the lesion; Ns, ‘non-sense’: premature chain termination as a result of a new translational stop codon (e.g UAA); SPL, ‘splicing’: inactivation of splicing or new splice sites generated (aberrant splicing) in exons or introns; promoter, CAP, initiation: reduction of transcription or translation as a result of lesion in promoter, CAP or initiation regions; Poly A, mutations on the poly A addition signal resulting in failure of poly addition and an unstable mRNA. (From Hoffbrand and Moss, 2011).
In the nucleus of the erythroid precursors, RNA is also “capped” by addition of methyl-guanosine groups at the 51 end; this enables the MRNA to attach to the ribosome. There is poly-adenylation at the 31 end of the newly formed MRNA, this helps in stabilization of the MRNA.
Therefore, mutations occurring at any of these sequences may give rise to β-thalassaemia, also mutation of a number of other conserved sequences important in regulation of beta globin synthesis will result to β-thalassaemia.
For instance; HBB is regulated by an adjacent CACCC boxes, and an upstream regulatory element dubbed the locus control region (LCR). A number of transcription factors also regulate the function of HBB, the most important of which is the erythroid kruppel-like factor (eklf), which binds the proximal CACCC box, and whose knockout in the mouse, leads to a thalassaemia-like clinical picture. (Gallanelo and Raffaelle, 2010).
More than 200 disease causing mutations have been identified to date. The large majority are point mutation in functionally important region of beta globin chain. (Giardine et al, 2007, Huisman et al, 1997).
- Single nucleotide substitution
- Deletion and insertion
- Frame shift mutation
- Nonsense codon mutation
- Incorrect splicing
- Mutation the promoter, cap and initiation region
- Mutation of the poly A region
Single nucleotide substitution:
Substitution of a single nucleotide for another in the beta-globin gene may result in coding of an amino acid that has no biological role in synthesis of the β-globin chains.
Small deletions and insertions:- When there is a small deletion or insertion of nucleotides in the MRNA that affected introns, exons or the flanking regions of the β-globin gene, this will lead to formation of a different amino acid with a different function altogether.
Frameshift: - This occurs when deletion of one or more nucleotide causes a shift in the arrangement of the bases, thereby giving rise to a different amino acid other than the original one that forms the beta-globin chain. Β-thalassaemia results.
Nonsense: - This occurs as a result of translation of a stop codon (eg UGA, UAA) thereby leading to a premature chain termination. These stop codons usually come at the end of a gene sequence but mutation may cause insertion or deletion of a base thereby resulting to coding of a stop codon prematurely.
Nonsense mutation results in production of truncated protein. These proteins produced due to this type of mutation may or may not be functional depending on the degree of shortening (Mr. Ukwa’s note, Unpublished). This type of mutation gives rise to β-thalassaemia.
Incorrect splicing: - Splicing is the process of removing the non-coding regions of a gene (introns) and joining of the coding region (exons). The splicing machinery called spliceosomes performs this function by correctly recognizing these sequences as well as the conserved sequences (Hoffbrand and Moss, 2011).
Mutations of the splicing machinery or the region between the exons and intron results to decrease in production of the beta globin chain. Addition of a contiguous length of non-coding instruction into the MRNA or adding just a discontinuous fragment of it atimes leads to normal haemoglobin because of the fact that all the correct instruction can be present, however, addition of intronic regions produces pathology which interferes with the regulation of desired levels of protein production enough to ultimately produce anemia. β or β+-thalassaemia usually results in incorrect splicing.
Mutation at promoter, CAP, initiation region: - Promoter region of a DNA is just a sequence that comes before the transcription initiation sequence. At -10 base pairs of the promoter region is a sequence that comprises of TATA AT which is called the pribnow box. (Mr. Ukwa note on molecular Genetics, unpublished). This is the site where RNA polymerases binds and catalyses gene transcription. (Hoffbrand 2011) capping of the transcripted RNA involves addition of a structure at the 5’ and which contains seven methyl-guanasine groups which are very essential for attachment of the MRNA to ribosomes.
Initiation site also called transcription initiation sequence (TIS) comprises of nucleotides that forms the starting region of the MRNA.
Beta-thalassaemia (β+) can occur due to reduction of transcription or translation of the mRNA as a result of lesion in promoter, CAP or initiation region (Hoffbrand and Moss, 2011).
Mutation at poly A region: - After transcription, the newly formed mRNA is also poly adenylated at the 31 end, this ensures stabilization of the MRNA.
Mutation on the poly A addition signal results in failure of poly A addition and an unstable MRNA (Hoffbrand & Moss 2011) which may give rise to absence synthesis of beta globin chain thus β-thalassaemia result. Deletion of viable extent of the HBB gene cluster result to complex β-thalassaemia (delta-beta and gamma-delta-beta-thalassaemia). This is possible because, the ∂, Y (Gy and Ay) and β gene are located together on chromosome 11 gene cluster region.
However, studies have shown that β-thalassaemic mutations are relatively population specific i.e. each ethnic group has its own set of common mutant (Furum H et al., 1999 Setianuigsih et al., 1999, Colah et al., 2009, Colah et al., 2011)