Lecture 6 - Haemoglobinopathies Flashcards
Decribe the structure of haemoglobin.
Haemoglobin consists of 33% of RBC weight with roughly 640 million Hb molecules per RBC. There are to a-globin chains and to B-globin chains. There is also a non-protein component haem containing Ge2+. The haemoglobin is folded in alpha helixes and regions of hydrophobicity which allow it to maintain its state.
* Adult haemoglobin = HbA
* Foetal = HbF
The oxygen dissociation curve explains how oxygen is unloaded and loaded onto haemoglobin.
The HbF form has a higher affinity for oxygen so the curve is to the left of the HbA curve. pH and other factors will also influence the curve.
Describe globin genes.
Chromosome 11 has B-chain like globin’s and chromosome 16 have a-chain like globin’s.
In the embryo the zeta chain substitutes for the alpha chain, and the gamma or epsilon chain substitute for the beta chain in the foetus.
After 3-6 months there is a major switch from gamma to beta chin synthesis (i.e. HbF - HbA)
If there are issues with the beta chain anaemia like symptoms will start to show at 3-6 months of life.
Zeta chain synthesis is replaced by alpha chain synthesis very early on in development so if there are any issues symptoms will appear much earlier
What are haemoglobinopathies.
Refers to abnormalities of haemoglobin
* Reduced rate of synthesis of normal a or b globins.
* Synthesis of abnormal haemoglobins.
What is a-thalassaemia?
a-thalassemia is reduced a-globin synthesis and is more common in the far East. Mutations normally in promotor region , or frame shift, resulting in no gene product
Trait is symptomless although there are increased RBC and decreased MCH and MCV
Where there is mutations in more than two of the alpha genes symptoms will occur e.g. Hb H disease.
In Hydrops fetalis there is no alpha globin synthesis leading to death in utero.
α-Thalassaemia Hb H disease
When 3 a-globin chains are deleted or dysfunctional.
* Unstable beta globin tetramers formed (B4) (lots of B not very much a) - precipitation of these tetramers in red blood cells. In fetal life Hb Barts (Y4) occurs.
* Methylene blue or brilliant cresyl bule staining reveals these golf ball cells
* Hypochromic, microcytic, target cells, poikilocytosis.
Diagnosis
High performance liquid chromatography (HPLC) - different haemoglobins migrate at different rates so you get different peaks. Useful in diagnosis of all haemoglobinopathies. Fast, automated and accurate
The other option is cellulose acetate electrophoresis
α-Thalassaemia Hydrops fetalis
When all 4 alpha globin genes are deleted. It is incompatible with life
What is B-thalassaemia?
There are over 400 genetic defects (usually point mutations targeting the gene itself instead of the promotor region) that cause beta-thalassemia major, intermedia and minor.
Absent beta chain or reduced production is not fatal as can still produce delta and gamma chain. There is a dysregulation of the ratio of alpha and beta chains. Excess alpha chains leads to a-globin precipitate in erythroblasts which leads to haemolysis and ineffective RBC production. The greater the excess of a chains the greater the anaemia.
Diagnosis may be via cellulose acetate electrophoresis
B-thalassaemia major
Severe anaemia at 3-6 months old as this is when the gamma chain is down regulated and the beta chain is upregulated.
Hepatosplenomegaly due to increased RBC destruction due to faulty RBCs
There is also expansion of bones and bone marrow as body tries to produce more RBCs to make up for the loss.
Treatment involves regular blood transfusions. Iron chelation therapy needed to reduce iron overload. Splenectomy may also help. Stem cell transplantation, gene therapy or drugs to stimulate HbF.
Lab tests see: absence or almost absent HbA and increase in HbF and HbA2
The blood smear will show hypochromic, microcytic, nucleated RBC as well as target cells.
B-thalassaemia intermedia
Moderate anaemia. Homozygous mild β+ thalassaemia or heterozygous very severe β defect or mixture of thalassaemias.
Presentation later than β – thalassaemia major (2-5 years).
Blood smear hypochromic, microcytic, RBC raised.
Hb reduced. Possibly bone deformity, hepatosplenomegaly
Regular transfusion not needed.
B-thalassaemia minor (trait)
Heterozygotes β+ or βo.
Usually asymptomatic
Blood smear: hypochromic, microcytic, RBC raised
What is sickle cell disease
Normal levels of B-chain however the B-chain is abnormal. There is an amino acid substitution at position 6 in beta chain - alteration of glutamic acid (acidic) to valine (hydrophobic).
This causes the haemoglobin chains to stick together forming insoluble crystals when oxygen concentration is low.
HbSS = homozygote = sickle cell anaemia
Severe anaemia
HbAS =Heterozygotes (sickle cell trait)
No anaemia. Normal blood smear.
Protection from malaria
Diagnosis - Sickle solubility test (cheap and quick) HbS is insoluble when deoxygenated in high concentrated phosphate buffer. Cellulose acetate electrophoresis can also be used.
Sickle cell anaemia blood film
Sickle cells (inflexible) block small blood vessels causing a painful ‘crisis’. Can lead to death of tissue (infarction). Vaso-occlusion in small bones (finger) = pain and affects growth. Brain = stroke.
