Haem 4 - Haemoglobin molecule and thalassaemia Flashcards
What the approximate number of red blood cells in our bodies?
3.505 x10^12/L
How many molecules of Hb are there in a RBC?
640 million molecules
Note: Hb is found exclusively in the RBC. They are very toxic if in the blood stream - can activate free radicles.
Numerical facts about Hb?
Normal concentration in adults: 120-165g/L
Approximately 90mg/kg produced and destroyed in the body everyday
Each gram of Hb contains 3.4mg of Fe
Describe the synthesis of Hb?
Synthesis occurs during development of RBC and begins in pro-erythroblast:
- 65% erythroblast stage
- 35% reticulocyte stage
Where is haem and globin synthesised?
haem - mitochondria
globin - ribosomes
Describe the structure and synthesis of haem
1) Also contained in other proteins e.g. myoglobin, cytochromes, peroxidases, catalases, tryptophan
2) Same in all types of Hb
3) Combination of protoporphyrin ring with central iron atom (ferroprotoporphyrin). Iron usually in ferrous form (Fe2+)
4) Able to combine reversibly with oxygen
5) Synthesised mainly in mitochondria which contain the enzyme ALAS. Regulation - ALAS is part of negative feedback mechanism which controls haem synthesis.
Describe the synthesis of globin
Various types which combine with haem to form different haemoglobin molecules
Eight functional globin chains, arranged in two clusters:
b- cluster (b, g, d and e globin genes - e globin genes are only important in embryogenesis) on 11p (short arm of chromosome 11)
a- cluster (a and z globin genes - zeta globin chains are only important in embryogenesis) on 16p (short arm of chromosome 16) 16. a1 and a2
See graphs which show the varying levels of globin
What is the normal proportions of Hb in adults?
See table
Hb A (adult) - 96-98%
HbA2 - 1.5-3.2%
Hb F (foetal) - 0.5-0.8%
What is the globin structure?
Primary:
1) α 141 amino acids
2) Non- α 146 amino acids
Secondary:
1) 75% α and b chains-helical arrangement
Tertiary:
1) Approximate sphere
2) Hydrophilic surface (charged polar side chains), hydrophobic core
3) Haem pocket
See diagram - oxygenated Hb adopts a more open state with the haem pocket holding the O2.
Draw the oxygen-haemoglobin dissociation curve
See graph
Sigmoid state - cooperativity
What it the P50?
Partial pressure of O2 at which Hb is half saturated with O2. About 26.6mmHg
What happens when the oxygen-haemoglobin dissociation curve shifts to the left?
Means the oxygen carrying capacity is greater - caused by increased pH, Hb F and decreases 2,3-DPG
Left shift (give up oxygen less readily):
- Low 2,3-DPG
- HbF
What happens when the oxygen-haemoglobin dissociation curve shifts to the right?
Means the oxygen carrying capacity is decreased - caused by decreased pH, Hb S and increased 2,3-DPG
Right shift (easy oxygen delivery): High 2,3-DPG - it binds more readily to deoxy Hb decreasing its affinity for oxygen. High H+ High CO2 HbS
Metabolising tissues produce the lower pH (CO2 produced) and 2,3-DPG causing the curve to shift to the right meaning oxygen is released more readily; at these sites of respiring tissue where they need oxygen. Opposite effect as well
What does the normal position of the oxygen-dissociation curve depend on?
1) Concentration of 2,3-DPG
2) H+ ion concentration (pH)
3) CO2 in red blood cells
4) Structure of Hb
What are thalassaemias?
Genetic disorders characterized by a defect of globin chain synthesis
Most common inherited single gene disorder worldwide - see map
How can you classify thalassaemia?
Globin chain affected and [clinical severity (minor (trait), intermedia or major)]
Clinical severity not really used anymore - normally classified as transfusion dependent or non-transfusion dependent
What is the definition of beta thalassaemia?
Deletion or mutation in b globin gene(s)
Resulting reduced or absent production of b globin chains
How is beta thalassaemia inherited?
It is inherited via the recessive mandelian inheritance. See inheritance tree.
The clinical classification of Minor, Intermedia and Major shows how the mutations vary in severity. This can be explained by the degree of suppression of globin chain synthesis, some mutations result in no globin production (b0) where as others have decreased levels of production (b+). Inheritance of 2 b0 genes will give rise to a Major where as inheritance of 2 b+ genes will give rise to an intermedia (a clinically milder form)
What is the laboratory diagnosis of beta thalassemia?
1) FBC- Microcytic Hypochromic indices (reduction in MCV and MCHC), increased RBCs relative to Hb
2) Film- Target cells, poikilocytosis but no anisocytosis
3) Hb EPS (electrophoresis) / HPLC
alpha - thal- Normal HbA2 and HbF, +/- HbH
beta - thal- Raised HbA2 and raised HbF
4) Globin Chain synthesis/ DNA studies - Genetic analysis for β-thalassaemia mutations and XmnI polymorphism (in β-thalassaemias) and α-thalassaemia genotype (in all cases)
For a-thalassaemia DNA analysis will be carried out.
See diagram for beta thalassemia trait graph
Describe beta thalassaemia major?
Carry 2 abnormal copies of the beta globin gene
Severe anaemia, incompatible with life without regular blood transfusions
Clinical presentation usually after 4-6 months of life
See pictures - see lecture
What are the clinical presentations of beta thalassaemia major? What clinical features are there?
Severe anaemia usually presenting after 4-6 months
Hepatosplenomegaly
Blood film shows gross hypochromia, poikilocytosis and many NRBCs
Bone marrow - erythroid hyperplasia
Extra-medullary haematopoiesis
Beta Thalassaemia Major presents at 6-12 months of age as the synthesis of g globin chains is reduced and the b globin chains would be expected to be increasing toward normal adult levels but in the case of beta thalassaemia major the beta genes are suppressed.
As a result of the gross anaemia the marrow becomes hyperplastic in an attempt to redress the anaemia. Should this persist the bone marrow expands giving rise to characteristic facial changes, frontal bossing and thickening of the maxillary bones
The patient will show splenomegaly as extramedullary haemopoesis (erythropoesis outside the bone marrow) develops.
Chronic fatigue Failure to thrive Jaundice Delay in growth and puberty Skeletal deformity Splenomegaly Iron overload
What are other complications of beta thalassaemia major?
Cholelithiasis and biliary sepsis - caused by chronic haemoloysis
Cardiac failure - iron overload
Endocrinopathies - iron overload
Liver failure - iron overload
What is the most common cause of death from beta thalassaemia?
Cardiac disease
How do you treat beta thalassaemia major treatment?
Regular blood transfusions - main treatments
Iron chelation therapy - main treatments
Splenectomy
Supportive medical care (for cardiac problems, ironover load etc)
Hormone therapy
Hydroxyurea to boost HbF
Bone marrow transplant
Usually the child will be identified before they were even born if they risked being beta thalassaemia.
What is required from blood transfusions?
Phenotyped red cells - aim to get similar RBC
Aim to keep the Hb concentration at the pre-transfusion Hb of 95-100g/L - this is enough to suppress extramedullary haematopoeisis
Regular transfusion 2-4 weekly
If high requirement, consider splenectomy
What infections can occur from beta thalassaemia?
Yersinia
Other Gram negative sepsis - they thrive off the iron
Prophylaxis in splenectomised patients – immunisation and antibiotics. Protected from mainly encapsulated organisms
Describe iron chelation therapy (reduce iron concentrations)?
Start after 10-2 blood transfusions or when the serum ferritin >1000 mcg/l
Audiology and ophthalmology screening prior to starting
Compare currently available iron chelating agents?
See notes/lecture
Deferasirox (Exjade), Desferrioxamine (Desferal), Deferiprone (Ferriprox).
How do you monitor iron overload?
Serum ferritin - >2500 associated with significantly increased complications (not accurate), Acute phase protein, Check 3 monthy if transfused otherwise annually
Liver biopsy - Rarely performed
T2* cardiac and hepatic MRI - <20ms – increased risk of impaired LF function, Check annually or 3-6monthly if cardiac dysfunction
Ferriscan - R2 MRI
1) Non-invasive quantitation of LIC (liver iron concentration)
2) Not affected by inflammation or cirrhosis (unlike standard MRI)
3)<3mg/g normal
4) >15mg/g associated with cardic disease
5) Check annually or 6monthly if result >20
Describe alpha thalassaemia
Deletion or mutation in a globin gene(s)
Reduced or absent production of a globin chains
Affects both foetus and adult
Excess b and g chains form tetramers of HbH and Hb Barts respectively
Severity depends on number of a globin genes affected
What is a thalassaemia carrier?
Also known as Thalassaemia minor / trait
Carry a single abnormal copy of the beta globin gene
Usually asymptomatic
Mild anaemia
