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Molecular medicine ILA
Dr J Wright Consultant Haematologist
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Homozygous sickle cell disease Medical issues include
GL 50 years Homozygous sickle cell disease Medical issues include Occasional bone pain Proteinuria and renal impairment Low oxygen saturations ?chronic lung problems Previous episodes of sickle chest syndrome
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Test results Haemoglobin 9.8g/dL White cell count 7.7x106
Platelets 348x109 Reticulocytes increased Bilirubin increased Electropheresis confirms sickle cell disease Sickle cell disease Normal adult Control S carrier
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Structure of haemoglobin
2 alpha chains 2 beta chains 4 molecules of haem Brief explanation of the structure of haemoglobin
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Fetal haemoglobin switches to adult haemoglobin in the 1st few months
HbA (adult) 2 a and 2 b chains HbA2:2 a and 2 d chains HbF: 2 a and 2 g chains normal individual: (F) A A2 Differences between adult and fetal Hb,explanation of why sickle is not apparent at birth
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The a and b globin gene clusters
Genes for a and b globin Haemoglobin S is a variant haemoglobin arising because of a point mutation in the b globin gene.
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Distribution of malaria
Explanation of why sickle exists. Very similar distribution to that of malaria
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Malaria & haemoglobinopathies
Falciparum ring forms within red blood cells Anopheles mosquito Heterozygote advantage for carriers in terms of resistance to falciparum malaria Sickle- An example of a balanced polymorphism
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Inheritance SS:SCD
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SICKLE CELL Thiamine for adenine in 6th codon of b globin gene results in single amino acid change of valine for glutamine
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Why does such a simple genetic lesion cause such a complex disease?
Electron micrograph of sickle red cell and normal red cell
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Polymerisation of Hb S Hb extraordinarily soluble
Valine for glutamine at b6 results in polymerisation Continuous polymerisation /depolymerisation Rate depends on Deoxygenation rate Hb concentration HbF
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Effect of polymerised HbS
Interaction with red cell membrane Red blood cell dehydration Externalisation of –ve charged phospholipids Decreased deformability Shortened life span
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Vaso-occlusion A multifactorial process
Red cell density Endothelial interaction Coagulation/platelet activation Role of leucocytes Vascular tone
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Large vessel occlusion
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….And small vessel AA SS
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Sickle cell: a multi-system disorder
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How can we treat sickle cell disease?
Transfusion Reduces sickle complications....BUT Iron overload Alloimmunisation Hydroxyurea Bone marrow transplant Gene therapy
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Bone marrow transplant
Toxic procedure High risk Significant procedural mortality Significant late effects Infertility Graft versus host disease
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Hydroxyurea decreases pain and may increase life expectancy
40 30 20 10 Placebo Hydroxyurea Cumulative Death Rate %
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Gene therapy for sickle cell disease?
A simple genetic disease Insertion of normal b globin gene Viral vector- risks!! Need to switch gene on- How? Only achieved in mice so far.
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