Haemoglobin molecules and thalassemia Flashcards
1
Q
Where are haem and globin synthesised?
A
Haem: synthesised in mitochondria
Globin: synthesised in ribosomes
2
Q
Facts about RBCs
A
- Carry oxygen from lungs to tissues
- Transfer CO2 from tissues to lungs
- 3.5-5 x 1012 /L
- Contain haemoglobin (Hb)
- Each RBC contains approx. 640 million molecules of Hb
- Mature RBCs do not have nucleus or mitochondria
3
Q
Facts about Hb
A
- Found exclusively in RBCs
- MW 64-64.5 kDa
- Normal concentration in adults:120-165g/L
- Approximately 90 mg/kg produced and destroyed in the body every day
- Each gram of Hb contains 3.4mg Fe
4
Q
Why is Hb packaged and not free in the blood?
A
- It is toxic by itself
- oxidative properties
- in certain diseases with heamolysis the Hb can cause damage.
5
Q
When in the cell stage is Hb synthesised?
A
Synthesis occurs during development of RBC and begins in pro-erythroblast:
65% erythroblast stage
35% reticulocyte stage
6
Q
What is HbA made up of?
A
2 alpha and 2 beta chains with a Haem group containing iron in the centre.
7
Q
Where can the haem component be found?
A
- Also contained in other proteins e.g. myoglobin, cytochromes, peroxidases, catalases, tryptophan
- Same in all types of Hb
8
Q
Structure of Haem
A
- Combination of protoporphyrin ring with central iron atom (ferroprotoporphyrin)
- Iron usually in ferrous form (Fe2+)
- able to combine reversibly with oxygen
9
Q
In what form is iron found in haem?
A
- usually in ferrous form (Fe2+)
10
Q
Regulation of haem synthesis
A
- haem exerts negative feedback on delta-ALA
- this is a regulatory step in the production of haem
11
Q
Synthesis of globin
A
- 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) on the short arm of chromosome 11
- a- cluster (a and z globin genes) on the short arm of chromosome 16
12
Q
Complete inability to produce alpha globes - consequences?
A
usually results in embryonic death.
13
Q
Globin gene clusters
A
- alpha gene cluster in chromosome 16 is composed of zeta gene as well as a1 and a2
- zeta genes are involved in the embryonic stage of development
- beta globin cluster on chromosome 11 - beta, delta and gamma.
14
Q
When do pathologies in the beta globin chains present?
A
- usually after brith
- later than alpha
- there is a switch from HbF to Hba
- thalassemia (beta) usually presents at about 3 months
15
Q
Normal adult Hb fractions
A
HbA: 96-98%
HbA2: 1.5 - 3.2 %
HbF:- 0.5 - 0.8%
16
Q
Globin structure
A
- primary: alpha 141 AA; non alpha 146 AA;
- secondary:75% α and b chains-helical arrangement
- tertiary: Approximate sphere,
Hydrophilic surface (charged polar side chains); hydrophobic core; Haem pocket;
17
Q
2,3 DPG
A
- causes oxygen unbinding, favours O2 disscociation
- 2,3 diphosphoglycerate
18
Q
Oxygen-Hb dissociation curve
A
- O2 carrying capacity of Hb at different pO2
- Sigmoid shape:
- Binding of one molecule facilitate the second molecule binding (cooperativity)
- P 50 (partial pressure of O2 at which Hb is half saturated with O2) 26.6mmHg
19
Q
p50
A
(partial pressure of O2 at which Hb is half saturated with O2) 26.6mmHg
20
Q
cooperative binding
A
- Binding of one molecule facilitate the second molecule binding (cooperative binding)
- due to changes in the structure of the molecule
21
Q
What shift the ODC to the left?
A
- high pH
- HbF
- low 2,3 DPG
=> give up oxygen less readily
22
Q
What shifts the ODC to the right?
A
- low pH
- HbS
- high 2,3 DPG
- high CO2
=> easy O2 delivery
23
Q
What does the normal position of the Hb-O2 dissociation curve depend on?
A
- Concentration of 2,3-DPG
- H+ ion concentration (pH)
- CO2 in red blood cells
- Structure of Hb
24
Q
Haemoglobinopathies
A
- Structural variants of haemoglobin
or - Defects in globin chain synthesis (thalassaemia)
25
Thalassemias
- Genetic disorders characterized by a defect of globin chain synthesis
- Most common inherited single gene disorder worldwide
26
What disease is Thalassemia linked to in a way?
- It is common in areas with malaria.
| - Haemoglobinopathies offer protection against Malaria
27
How can you classify thalassemia?
- Globin type affected (e.g. Beta-thalassemia)
- Clinical severity
- minor / “trait” (usually asymptomatic but able to pass on offspring)
- intermediate (wide spectrum)
- major (cannot survive without regular transfusions)
28
Beta thalassemia - what is the defect?
- Deletion or mutation in b globin gene(s)
| - Reduced or absent production of b globin chains
29
Where is beta thalassemia prevalent?
mainly Mediterranean countries (Greece, Cyprus, Southern Italy), Arabian peninsula, Iran, Indian subcontinent, Africa, Southern China, South-East Asia
30
Inheritance of thalassemia
- recessive
- autosomal
- mutations vary in phenotype and severity
=> classic mendelian
31
Lab diagnosis of Thalassemia
- FBC - Full blood count abnormalities -> Microcytic Hypochromic indices, Increased RBCs relative to Hb
- Film: target cells, poikilocytosis but no anisocytosis -> size uniform but different shapes + morphologies
- Hb Electrophoresis / HPLC -> a-thalassemia normal HbA2 and HbF +/- HbH; beta thalassemia: raised HbA2 and raised HbF
- globin chain synthesis / DNA studies
32
Thalassemia 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
= transfusion dependant Thalassemia
33
What does a blood film in beta thalassemia major look like?
The peripheral blood film will show extreme hypochromia, microcytosis and poikilocytosis.
Often Howell Jolly bodies and nucleated RBC’s will be present as a result of splenectophy and a hyperplastic bone marrow.
In Beta thalassaemia major two forms of inclusion bodies may be seen, Alpha globin precipitates and pappenheimer bodies. .
34
Clinical presentation of thalassemia major
- Severe anaemia usually presenting after 4 months
- Hepatosplenomegaly - due to extramedullary haematopoiesis (in the liver and the spleen, these organs go back to producing RBCs)
- Blood film shows gross hypochromia, poikilocytosis and many NRBCs
- Bone marrow - erythroid hyperplasia
- Extra-medullary haematopoiesis
35
Symptoms and signs of thalassemia major
- Chronic fatigue
- Failure to thrive
- Jaundice
- Delay in growth and puberty
- Skeletal deformity
- Splenomegaly
- Iron overload
36
Other complications of thalassemi major
- Cholelithiasis and biliary sepsis
- Cardiac failure
- Endocrinopathies
- Liver failure
=> these are associated with iron overload.
37
What are the major causes of death in thalassemia patients?
- cardiac disease (most common)
- infections (2nd most common)
- liver disease
- other causes
38
Treatment of Thalassemia major
- Regular blood transfusions
- Iron chelation therapy
- Splenectomy (in splenectomy and if transfusion requirement is high but also has complications such as thromboembolisms)
- Supportive medical care
- Hormone therapy
- Hydroxyurea to boost HbF
- Bone marrow transplant
39
Transfusions in beta-thalassemia
- Phenotyped red cells
- Aim for pre-transfusion Hb 95-100g/L
- If high requirement, consider splenectomy
- start ASAP
- 2-4x/week
40
How often are transfusions given in beta thalassemia?
- Regular transfusion 2-4 weekly
41
Iron Chelation therapy
- Start after 10-2 transfusions or when serum ferritin >1000 mcg/l
- Audiology and ophthalmology screening prior to starting
- increased survival!!
42
What are problems with compliance to chelation theerapy?
- s.c.
- regular (infusion over 12h)
- interferes with social life
-> now oral options available
43
What are the 3 common iron chelation drugs?
- DFO (s.c., i.v.)
- deferiprone (oral, 3x/d)
- deferasirox (oral, 1x/d)
44
Deferasirox (Exjade)
- Oral
- Dose 20-40mg/kg
- SE: rash, GI symptoms, hepatitis, renal impairment
- iron chelation
45
Desferrioxamine (Desferal)
- Long-established
- s.c. infusion 8-12 hours 5-7 days per week (or I.V. in cardiac iron overload)
- Dose 20-50 mg/kg/day
- SE: vertebral dysplasia, pseudo-rickets, genu valgum, retinopathy, high tone sensorineural loss, increased risk of Klebsiella and Yersinia infection
- Compliance
- Vitamin C
- iron chelation
46
Deferiprone (Ferriprox)
- Licensed 1999
- Oral
- Dose 5-100 mg/kg/day
- Effective in reducing myocardial iron
- SE: GI disturbance, hepatic impairment, neutropenia, agranulocytosis, arthropathy
- increased susceptibility to developing infection -> weekly blood counts recommended whilst on therapy
- also side effects: zinc deficeincy? rewatch
47
Deferiprone (Ferriprox)
- Licensed 1999
- Oral
- Dose 5-100 mg/kg/day
- Effective in reducing myocardial iron
- SE: GI disturbance, hepatic impairment, neutropenia, agranulocytosis, arthropathy
- increased susceptibility to developing infection -> weekly blood counts recommended whilst on therapy
- also side effects: zinc deficeincy, and atropathy?
48
Combined therapy (chelation)
- any 2 of these drugs can be used in combination
- different combinations are used in clinical practice
- advantage: you can limit the toxicity of the drugs by limiting the dose of the drugs in the combination
49
Comparison
see PP slide!
50
Monitoring iron overload
- Serum ferritin
>2500 associated with significantly increased complications
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
Non-invasive quantitation of LIC
Not affected by inflammation or cirrhosis
<3mg/g normal
>15mg/g associated with cardic disease
Check annually or 6monthly if result >20
```
51
Synthesis on Hb
- diagram on PP!
- Transferrin enters the cell -> Fe goes to Ferritin or enters the mitochondrion
- transferrin enters the circulation again
- Fe is used to make haem
- Aminoacids -> Ribosomes form a2b2 chains
- a2b2 chains combine with haem to make Hb
52
What infections are common in beta thalassemia?
- yersinia
- other gram-ve sepsis
- prophylaxis in splenoctomised patients - immunisation and antibiotics
- some of these organisms thrive on iron which makes thalassemia patients susceptible due to the iron rich environment
53
Alpha Thalassemia
- Deletion or mutation in a globin gene(s)
- Reduced or absent production of a globin chains
- Affects both foetus and adult
- Excess beta and gamma chains form tetramers of HbH and Hb Barts respectively
- Severity depends on number of alpha globin genes affected
54
Thalassemia carrier
- Also known as Thalassaemia minor / trait
- Carry a single abnormal copy of the beta globin gene
- Usually asymptomatic
- Mild anaemia
55
HbH disease
- has a haemolytic element to it with microcytosis, anisoocytosis poikilocytosis and “puddling” of haemoglobin within the RBC.
56
Problems Associated with Treatment in Developing Countries
- Lack of awareness of the problems
- Lack of experience of health care providers
- Availability of blood
- Cost and compliance with iron chelation therapy
- Availability of and very high cost of bone marrow transplant
57
Screening and prevention of thalassemia
- Counselling and health education for thalassaemics, family members and general public
- Extended family screening
- Pre-marital screening?
- Discourage marriage between relatives?
- Antenatal testing
- Pre-natal diagnosis (CVS)
