Haemoglobin Molecule and Thalassaemia Flashcards
What’s the normal concentration of haemoglobin?
120-165g/L
around 90mg/kg produced or destroyed per day
When does haemoglobin synthesis occur?
during development of RBCs
begins in pro-erythroblast stage
- 65% in erythroblast stage
- 35% in reticulocyte stage
Where are the components of haemoglobin made?
haem - mitochondria
globin - ribosomes
- Haem:
- Transferrin transports the ferrous to the RBC or the ferrous is liberated from the ferritin molecules.
- Glycine, B6 and Succinyl CoA create delta-ALA which then undergoes a few moderations outside the mitochondria and then passes back in as proto-porphyrin.
- Proto-porphyrin -> haem which binds with the globins. - Globin:
- Amino acids are used in ribosomes to create the globin chains. - Haemoglobin:
- Globins and haem associate
What is haem?
Haem is also contained in other proteins – e.g. myoglobin and cytochromes.
Haem is the SAME in all molecules (only globins change).
Haem is a combination of a proto-porphyrin ring and central ferrous – ferro-proto-porphyrin.
It is synthesised mainly in the mitochondria with the main enzyme being ALAS
What makes up the globin molecule?
There are 8 functional globin chains arranged in 2 clusters:
beta-cluster - beta, gamma, delta, epsilon - encoded on Chr11, p-arm (short arm)
alpha -cluster - alpha, zeta - encoded on Chr16, p-arm (short arm)
Describe the globin gene expression and switching
alpha- is made relatively early and stays high throughout.
beta - is equal and opposite to gamma and becomes dominant after birth.
gamma - is equal and opposite to beta and is dominant pre-natal.
delta- production begins mid-natal and remains low forever.
epsilon and zeta - is equal and opposite to alpha and levels drop ~0 after 8 weeks.
Describe the globin abundance in adults
HbA (a2b2) is the most common – 96-98%.
HbA2 (a2d2) is the second most common – 1.5-3.2%.
HbF (a2g2) is the least common – 0.5-0.8%.
Describe the structure of globin
Primary - a (141aa), non-a- globins (146aa).
Secondary – 75% of a and b chains show a helical arrangement.
Tertiary – approx. sphere with a hydrophilic surface and a hydrophobic core, contain a haem pocket.
What makes 2.3-DPG ?
muscle cells to increase dissociation of oxygen
What affects the position of the ODC?
What would cause a right shift?
2, 3-DPG conc., pH, CO2 conc., structure of Hb
Right shift – High H+, High 2, 3-DPG, High CO2, HbS.
What is haemoglobinopathis?
dude to structural variants of haemoglobin/ defects in globin chain synthesis (thalassaemia)
Most common inherited single gene disorder worldwide.
How can you classify thalassaemia?
Globin type affected
Clinical severity
Minor or “trait”
Intermedia
Major
- now classified as transfusion dependent or non tranfusion dependent
There are 4 alpha clusters in total (alpha1 and alpha2 on each chromosome 16) but only 2 beta clusters
What happens on beta - thalassamia?
autosomal recessive inheritance.
Deletion or mutation in b-globin chains – reduced or absent production of beta-globins.
Inheritance:
- When 2 beta trait have a child, there is a 25% chance of a beta-major offspring.
- Beta Thal Intermedia can also come about when one partner has a beta+ mutation which is less severe.
b0 = deletion of one beta globin-encoding gene.
b+ = mutation of one beta-globin encoding gene
How is b-thalassamia diagnosed in the laboratory?
FBC – microcytic Hypochromia and increased RBCs relative to Hb.
Film – Target cells, Poikilocytosis (shape change) but NO anisocytosis (unequal size).
Hb EPS (electrophysiology studies)/HPLC (high performance liquid chromatography):
- a-thal – normal HbA2 and HbF, ±HbH.
- b-thal – raised HbA2 and HbF.
Globin chain synthesis/DNA studies:
- Genetic analysis for b-thal mutations and Xmn1 polymorphisms (b-thal) and a-thal genotypes (in all cases)
What is the beta-thalassaemia trait?
is a carrier trait and often asymptomatic.
Diagnosis usually made by blood film showing hypochromic microcytic blood cells and raised HbA2 and HbF.
What is (beta-) thalassaemia major?
What would you see on a blood film?
Carry 2 abnormal copies of the beta-globin gene.
Results in severe anaemia and requires regular blood transfusions.
Clinical representation after 4-6 months.
Blood film:
- Anaemia, irregularly contracted cells, hypochromic cells, a-chain precipitates, nucleated RBCs, iron inclusions (Pappenheimer bodies).
What are the clinical features and complications of b- thalassaemia?
Clinical representation:
- Severe anaemia presenting after 4 months, hepatosplenomegaly, film (Hypochromia, Poikilocytosis, NRBCs (nucleated RBC), bone marrow (erythroid hyperplasia), extra-medullary haematopoiesis.
- Can show frontal bossing and maxilla bossing in the face due to extra-medullary haematopoiesis.
Clinical features: Chronic fatigue failure to thrive jaundice late puberty skeletal deformity splenomegaly iron overload
Complications: Cholelithiasis biliary sepsis cardiac/liver failure Endocrinopathies
- Most of the clinical complications of beta-thal come from the iron overload which is achieved in two ways:
- Non-transfusion dependant iron overload – ineffective erythropoiesis so iron excess is not utilised.
- Transfusion iron overload – many transfusions lead to iron overload.
- Largest cause of death in patients with beta-thal-major is cardiac failure.
What is the treatment for thalassaemia major?
Regular blood transfusions
- Require 2-4 weekly transfusions but if a high requirement is needed, consider splenectomy.
Iron chelation therapy (removal of iron)
- Started after 10-12 transfusions or when serum ferritin >1000mcg/L.
- Audiology and ophthalmology screening is needed before starting.
- 3 forms of iron chelating drugs:
> DFO, Deferiprone, Deferasirox.
> Deferasirox has limited clinical experience
Other – splenectomy, supportive medical care, hormone therapy, hydroxyurea (boost HbF), bone marrow transplant.
Prophylaxis is required in splenectomy patients – immunisation and AB.
- Infection:
- Patients that have a high iron content are more prone to Yersinia infection and other gram- infections (these bacteria have an iron-loving nature)
How can you monitor iron overload?
Serum levels of ferritin - >2500 associated with increased complications, and check 3-monthly if transfused otherwise annually.
Liver biopsy – rarely performed.
MRI T2 cardiac and hepatic - <20ms – increased risk of impaired LF function, check annually or 3-6-monthly if cardiac dysfunction.
Ferriscan (R2MRI) – non-invasive, <3mg/g is normal, >15mg/g associated with cardiac disease
What is alpha - thalassamia?
Due to a deletion or mutation in alpha globin genes – reduced or absent alpha globins
Affects both foetus and adult (alpha is in ALL globin variants)
Severity depends upon number of chains affected
Excess beta and gamma chains will form tetramers of HbH (beta excess) and HbBarts (gamma excess)
What are the problems with treatment?
*in other countries
Lack of awareness and experience of the problems
availability of blood
cost and compliance with iron chelation
availability of and cost of bone marrow transplants
How can you screen for and prevent thalassaemia?
counselling and health education for thalassamia, family members and general public
extended family screening
pre-marital screening?
discourage marriage between relatives?
antenatal testing
pre-natal diagnosis (CVS)
