Globin Flashcards
Describe the globin protein
Globin is a tetrameric protein formed from 2 alpha and 2 beta chains. Adult form is HbA. There are 5 other forms that the body forms throughout embryogenesis and childhood.
Hb Gower 1 = 2 zita and 2 epsilon Hb Gower 2 = 2 alpha and 2 epsilon Hb Portland = 2 zita and 2 gamma Foetal Hb = 2 alpha and 2 gamma Adult HbA and HbA2
Describe the alpha and beta globin cluster
The alpha globin gene cluster contains the zita gene, 2 alpha genes and then some pseudogenes.
The beta globin gene cluster contains epsilon, 2 gamma, a pseudogene, delta (same as beta) then the beta gene. These happen to be in chronological order.
Where is globin produced from?
Different organs at different times - starts from the yolk sac, then liver, spleenand bone marrow.
Switches occur at 5-6 weeks and just prior to birth. Switch occurs in the beta cluster only.
How do mutations effect globin production at different ages?
Alpha is expressed throughout the life cycle and so mutations will effect both the embryo and adult. Beta is only present in the adult and so mutations only effect the adult.
What is sickle cell anameia and how is it treated?
Symptoms: sickled RBC, unequal finger lengths and bony proportions of hand bones.
This occurs due to a mutations in the beta globin gene. The resulting HbS blocks blood vessels leading to ulcers, pain, stroke and heart failure.
There is only one mutation causing HbS and it can be detected by loss of an MstII restriction site or ASO dot blotting using (allele specific oligonucleotides).
Treatment: folate, antibioitcs, analgesia during crisis. Removal of gall bladder and kidney transplants are sometimes required. Bone marrow transplant can alleviate symptoms.
HbS allele is common in some parts due to the resistance it confers for malaria.
What other Hb variants are there?
HbC = crystallizes and becomes rigid resulting in haemolysis - autosomal recessive
HbE = abnormal Hb and decreased synthesis resulting in thalassaemia - autosomal recessive
Hb Kempsey = Hb stays in a high O2 affinity state - autosomal dominant
All caused by a miss sense SNP
HbS = Glu to Val
What are thalassaemias?
Reduced synthesis of alpha or beta globin genes. Excess globins precipitate resulting in damaged RBC and cause anaemia, iron overload and abnormal expansion of bone marrow.
Discuss apha thalassaemias?
Most often cause by deletions of alphaglobin genes. This results in gamma and beta tetramers known as Hbbart’s and HbH respectively. Neither function properly at releasing oxygen.
If all alpha genes are deleted this results in a severe form of the disease where only Hbbart’s is made. This results in fluid accumulation termed hydrops fetalis and hypoxia and is lethal.
If 3 genes are deleted this results in HbH disease (Beta tetramers). This causes bone deformaties and enlarged spleen but can be treated.
Can also get non deletion alpha-thal due to unstable proteins called Hb constant spring - this results in 3% of normal alpha production
Are the 2 alpha genes on each chromsomes exactly the same?
No - the second genes has a much more efficient promtoer producing 2-3x more protein. Duplications of alpha genes are fine but deletions cause disease. There are 8 different deletions events that are known and each has distinct geographical distributions. Deletions and duplications occur as a result of missalignment during recombination.
Homozygous alpha thal is generally restricted to south east asia. There are two possible heterozygous states - aa/– or a-/a-. The most common haplotype is a-.
Discuss beta thalassaemia
Symptoms = anaemia, enlarged spleen, delayed puberty, characteristic bone features of face and iron overload.
This only manifests after birth. Alpha tetramers forms which increased RBC degredation. Hets are clinically well with a slight anaemia and increased HbA2 (delta takes over).
Homozygotes are either B0 - no production or B+thal = some production. There are lots of examples of point mutations causing Beta thal meaning that most homozygotes are actually compound heterozygotes.
Give some examples of mutations types for beta thal
Splicing variant - change in acceptor site or alternate splce variants which cause B+ type.
HbE mutation resulting in AA change in globin chain
Beta globin deletions are rare. Gross deletion have been documented and may effect other regions of the cluster or cause ectopic expression of gamma globin - hereditary persistence of foetal haemoglobin - not always deletions - mutations are promoter of gamma globin also.
Hb Leopore - Delta and beta diverged 40mya but only 10 different aa. HbL occurs when a deletion causes compounds gene between the two but with delta promoter so low expression = Beta thal. 3 different variant identified.
Describe nonsense mediated decay in relation to beta globin gene
Ribosome normally scrapes off exon junction complexes that have formed. If the stop codon for translation is moved or too far from end of second exon then this might not happen and result in NMD.
There is an example of recessive Beta thal due to a mutation causing an early stop codon. NMD resultin little or no protein production or abnormal truncated protein production.
Are there any examples of dominant variants for Beta thal.
Nonsense (stop) variant found in exon 3 but does not cause NMD, instead it results in a short protein. This protein associates with alpha thal but can’t form tetramers. Instead inclusion bodies form and prevent red cell maturation. As a result hets have a mild phenotype.
