Haem 4 - The Hb molecule and Thalassaemia Flashcards
Describe Hb synthesis
Occurs during RBC development - begins in pro-erythroblast stage
- 65% in erythroblast stage (still contains nucleus and mitochondria at this point)
- 35% reticulocyte stage
Where is haem synthesised
In mitochondria
Where is globin synthesised (alpha and beta globin chains)
Ribosomes
What is the transporter the body for free iron?
Transferrin
Describe the structure and synthesis of haem
The same haem is used in all type of haemoglobin
Haem is a combination of a protoporphyrin ring with central iron atom - ferroprotoporyphyrin
Iron usually in Fe2+ form (ferrous form)
-Haem synthesised mainly in mitochondria (which contain delta-ALAS enzyme)
Haem is able to combine …….. with oxygen
Reversibly
Various forms of globin can combine with haem - forming different Hb molecules. There are 8 functional globin chains - how are they arranged?
8 functional globin chains in 2 clusters
- Beta cluster - Beta, gama, delta, epsilon globin genes - short arm of Chr 11
- Alpha cluster - alpha and zeta globin genes - short arm of Chr 16
When does zeta and epsilon production stop?
In foetal life - taken over by a-globin production.
Hence a-globin defects manifest earlier.
From birth, where is most of the alpha and beta globin made?
In the bone marrow - this means beta-thalassaemia usually develops after birth
Describe the tertiary globin structure
- Approximate sphere
- Hydrophilic surface, hydrophobic core (accounting for solubility properties)
- Haem pocket
Describe deoxyhaemoglobin
It has 2,3-DPG attached - stops O2 binding, has a less stable structure
Describe the primary and secondary globin structure
Primary structure = alpha chains have 141 amino acids
Secondary structure has 146 non amino acids
What is the p50 of the Hb-O2 dissociation curve?
26.6mmHg
What does the position of the Hb-O2 Dissociation curve depend on
- 2,3-DPG. If high - shifts curve right
- H+ conc - pH. High H+ = shifts curve right (decreased pH)
- CO2 conc in RBCs. High CO2 shifts right
- Hb structure. HbS shifts right, HbF shifts left
What are the 2 categories of haemoglobinopathies
- Hb structural variants
2. Defects in globin chain synthesis (thalassaemia)
What is the most common inherited single gene disorder worldwide
Thalassaemia
How can thalassaemias be classified
- Globin type affected - most commonly alpha and beta
2. Clinical severity - minor (trait), intermediate, major
Describe Beta-Thalassaemia
Autosomal recessive (Chr 11)
- Deletion/mutation in beta-globin chain –> reduced production of B-globin
- Mainly occurs in Mediterranean countries
- Carriers = asymptomatic (except microcytic hypo chromic indices)
- Variation in severity as varying degree of interference with globin chain synthesis (some mutations = no globin production, some mutations = decreased production)
- 2x beta(0) genes = major, 2x beta(1) genes = intermediate
How are thalassaemias diagnosed
Lab diagnoses:
- FBC - microcytic hypochromic indices + increased RBCs relative to Hb
- Film - increased no. of target cells, poikilocytosis, but no anisocytosis
Hb EPS/HPLC (electrophoresis):
alpha thalassaemia - normal HbA2 and HbF, +/- HbH, not definitive
beta thalassaemia - raised HbA2 and HbF - more straightforward/easy to diagnose
What is the gold standard for a-thalassaemia diagnosis
Globin chain synthesis/DNA studies
Genetic analysis for B-thalassaemia mutations + Xmnl polymorphism (in B-thalassaemias) _ a-thalassaemia genotype (in all cases)
Describe B-thalassaemia major
- 2 Abnormal copies of B-globin gene
- Clinical presentation = after 4-6 months of life
- Abnormally shaped RBC
- RBC inclusions - alpha chain precipitates
- Pappenheimer bodies = excess iron precipitating in granules due to ineffective erythropoiesis - seen using Perls stain
What is the clinical presentation of Alpha and beta thalassaemia
- Extra-medullary haematopoiesis (main cause of presentations)
- Severe anaemia (fatal) - presents after 4 months usually
- Hepatosplenomegaly - due to extramedullary haematopoiesis
- Blood fil shows gross hypochromia - poikilocytosis and many nucleated RBCs
- Bone marrow shows erythroid hyperplasia
- Bone marrow shows erythroid hyperplasia
What are some features of beta thalassaemia
- Chronic fatigue
- Skeletal deformity
- Jaundice
- Delay in growth an puberty
- Splenomegaly
- Iron overload
Other complications include - gallstones (cholelithiasis) & biliary sepsis (due to chronic haemolysis), cardiac failure - most deaths from b-thalassaemia (iron overload linked), endocrinopathies, liver failure (iron overload)
What are the features of beta-thalassaemia
- Microcytosis
- Hypochromia
(also some cells with basophilic stripping)
Which thalassaemia is lethal in utero
Alpha-thalassaemia major
What is the treatment for thalassaemia major
- Regular blood transfusions
2. Iron chelation therapy
Describe how transfusions are given to treat thalassaemia (major)
- RBCs are phenotypes/matched
- Try to get pre-transfusion conc of 95-100g/L –> this helps to reduce extra-medullary haematopoiesis
- Transfusions regularly every 2-4 weeks
- If a high requirement, consider splenectomy (+ if splenomegaly).
But splenectomy makes body more susceptible to infection - e.g. Yersinia (loves iron, thrives in this case), other gram negative sepsis, splenectomised patients should have immunisation + antibiotics
When is iron chelation therapy done?
After 10-12 transfusions or when serum ferritin > 1000mcg/l
AUDIOLOGY and OPHTHALMOLOGY screening done prior to starting (due to retinal side effects)
What are the types of iron chelation therapy
- Desferrioxamine - DFO (deferral). Sc infusion, 5-7 days a week. SEs include: vertebral dysplasia, pseudo-rickets, retinopathy. DON’T use in children. Issues with compliance.
- Deferiprone - oral dose. Most effective in reducing myocardial iron. SEs include: hepatic impairment, neutropenia, agranulocytosis. Patients need weekly blood tests to monitor WBCs
- Deferasirox - generally well tolerated. 1/day oral dose (20-40mg/kg). SEs include: renal impairment
How is iron overload monitored
- Measure serum ferritin. If over 2500mcg/L, less accurate (associated w greater complications). Ferritin is also an acute phase protein so may be increased during infection/inflammation. Check 3 monthly if transfused patient.
- T2 Cardiac and hepatic MRI. Cardiac MRI = relies on contractility of cardiac fibres. If heart relaxation time < 20ms –> stiff fibres –> iron overload —> increased risk of impaired LF function
- Liver ferriscan - R2 MRI. Has replaced liver biopsy. Non-invasive quantification of Liver Iron Conc. Unaffected by inflammation or cirrhosis –> more accurate estimation. LIC < 3mg/g normal, >15mg/g associated with cardiac disease. Checked annually or 6-monthly
What type of disorder is sickle-beta thalassaemia
Sickling disorder - NOT thalassaemia
Describe HbE-beta thalassaemia
HbE Common in south east Asia - leads to globin chain production.
It has clinically variable expression - but can be as severe as B-thalassaemia major
Describe Alpha-thalassaemia and who it affects
Deletion/mutation in alpha-globin chains –> reduced/absent production of a-globin chains.
Affects foetus and adult
Severity depends on number of a-globin chains deleted - if all 4 alpha globin chains deleted –> death of foetus
What do excess beta and gamma chains form?
Tetramers of HbH
Describe thalassaemia carrier/minor/trait
When person carries single abnormal copy (e.g. of beta-globin chain)
Usually asymptomatic + diagnosed with milid anaemia + microcytosis + hyperchromia
Describe Haemoglobin H (HbH) disease
3 a-globin chains affected - shows poikilocytosis
What are the problems associated with treatment in the developing world
- Blood availability
- Cost and compliance with iron chelation therapy
- Availability/cost of bone marrow transplant
