Haemoglobin and Thalassaemia Flashcards
characteristics of mature RBC
no nucleus
no mitochondria
where and how much of Hb synthesis occurs in the RBC pro-erythroblast stage?
65% in erythroblast stage
35% in reticulocyte stage
main two components of Hb and where are they produced?
1) haem made in mitochondria
2) globin made in ribosomes
where is the source of iron from, to produce Hb?
liberated from ferritin molecules then transported by transferrin transporters to be endocytosed into the RBC
how is haem synthesis regulated?
delta-ALA enzyme which makes the regulatory step
when excess haem is made, there is negative feedback on ALA
how is delta-ALA formed?
glycine
B6
succinyl CoA
how does haem bind to globin?
ALA undergoes modifications outside the mitochondria before passing back in as proto-porphyrin, which causes the binding
where in the cell is globin created?
in the cytosol using amino acids
ribosomes
which other proteins contain haem?
myoglobin and cytochromes
its the identical molecule in different variants, only the globin differs
what are the two main components of haem?
proto-porphyrin ring and central ferrous
what globin chains make the alpha cluster?
alpha (adult variety)
zeta (embryonic variety)
hence alpha thal. can be in utero or adult
what globin chains make the beta cluster? [4]
beta (adult)
delta (adult)
gamma (foetal)
epsilon (embryonic)
which chromosome codes for the alpha cluster?
chromosome 16
p (short) arm
- alpha and zeta globins
alpha genes are duplicated so there are two functional alpha genes within the cluster
which chromosome codes for the beta cluster?
chromosome 11
p (short) arm
relative time period of alpha production
early
stays high throughout into postnatal years
relative time period of beta production
becomes dominant after birth
equally opposite to gamma, starting low
relative time period of gamma production
is dominant pre-natally
equally opposite to beta, starting high
relative time period of delta production
production begins mid-natal
remains low throughout
relative time period of epsilon and zeta production
equally opposite to alpha but drops off ~0 after 8 weeks, pre-natally
what are the variants of Hb
- HbA (2 alpha, 2 beta) MAJOR (>95%)
- HbA2 (2 alpha, 2 delta)
(3. 5%) - HbF (2 alpha, 2 gamma)
trace amount
there are 6 common variants but 3 of them are transient embryonic Hbs
relative abundances of the different Hb
HbA: 96-98%
HbA2: 1.5-3.2%
HbF: 0.5-0.8%
primary structure of globin
alpha (142aa)
non alpha globins (146aa)
secondary structure of globin
75% of alpha and beta chains show a helical arrangement.
tertiary structure of globin
approx. spherical with a hydrophilic surface and a hydrophobic core
contains a haem pocket.
what is a key difference in oxygenated and deoxygenated Hb
2,3-DPG found in deoxygenated Hb
cooperativity of Hb
Hb has the highest affinity for oxygen when binding is loose
once the first molecule binds, oxygen will bind more readily from then on hence sigmoid shape
what factors that affect the ODC?
2,3-DPG
pH (H+)
CO2 conc
- these three stabilise the Tight configuration meaning there is a low affinity for oxygen (cause right shift if these are high)
- Co2 effect on Hb affinity for oxygen is the Bohr effect
structure of Hb
what causes a right shift of the ODC
at metabolically active tissues:
- high H+ conc(low pH)
- high CO2 (acidity)
- high 2,3-DPG
therefore right shift to release more oxygen (low affinity of oxygen)
Tight configuration
what is a haemoglobinopathy?
genetic disorder characterised by defect in globin chain synthesis
most common inherited single gene disorder worldwide
classifications of thalassaemia?
- minor/trait
- intermedia
- major
what occurs in beta thalassaemia?
there is a deletion or mutation in beta globin chain gene
therefore there is a reduction/absence
inheritance of beta thalassaemia?
autosomal recessive inheritance
there is there inheritance of a deletion or mutation
mutation leads to intermedia (reduced production)
deletion leads to major (no production)
what are the 4 main lab diagnostic methods for thalassaemias?
- full blood count
- blood film
- high performance liquid chromatography
- globin chain synthesis/ DNA studies
what do you expect to see in a FBC for thalassaemia?
microcytic hypochromia
increased RBC relative to Hb
what do you expect to see in a blood film for thalassaemia?
target cells
poikilocytosis (shape change)
NO anisocytosis
what happens to the Hb variants numbers in alpha thalassaemia?
normal HbA2
normal HbF
reduced alpha globin chains
excess beta globin chains–> become HbH (unstable tetramer)
therefore harder to diagnosis
beta thal is easier to diagnose (simple rise in the other 2 variants)
what happens to the Hb variants numbers in beta thalassaemia?
raised HbA2
raised HbF
as compensation to low HbA
therefore easier to diagnose
what polymorphisms are looked for in the genetic analysis study for beta thal.?
Xmn1
symptoms in beta thal. trait
asymptomatic
usually diagnosed by blood film with hypo chromic blood cells, raised HbA2 and HbF
Beta Thal major
Carry 2 abnormal copies of the beta-globin gene.
Results in severe anaemia and requires regular blood transfusions.
Clinical representation after 4-6 months.
what do you see in a beta thal. major blood film?
anaemia irregularly contracted cells hypo chromic cells alpha chain precipitates NRBCs (nucleated) iron inclusions (Pappenheimer bodies )
clinical presentation of beta thal. major
- Severe anaemia presenting after 4 months
- hepatosplenomegaly
- film (Hypochromia, Poikilocytosis, NRBCs (nucleated RBC)
- bone marrow (erythroid hyperplasia)
- extra-medullary haematopoiesis and therefore frontal and maxilla bossing
clinical features of beta thal. major
Chronic fatigue failure to thrive jaundice late puberty skeletal deformity splenomegaly iron overload.
complications of beta thal. major (BTM)
Cholelithiasis (gallstones)
biliary sepsis
cardiac/liver failure
Endocrinopathies.
what causes most of the clinical complications of BTM?
iron overload
in what ways does iron overload cause complications?
non-transfusional dependent iron overload: ineffective erythropoiesis so iron excess is not utilised
transfusion iron overload: many transfusions leads to iron overload.
what is the largest cause of death in patients with BTM
cardiac failure
treatment for BTM
- regular blood transfusion (2-4 weekly, may need splenectomy if high requirement)
- iron chelation therapy
other
- splenectomy
- supportive medical care
- hormone therapy
- hydroxyurea (boost HbF, replacing low beta with gamma)
- bone marrow transplant.
what sort of infection are patients with BTM prone to whilst being treating?
Yersinia
gram neg infections
they prefer iron rich environments
iron chelation therapy
Started after 10-12 transfusions or when serum ferritin >1000mcg/L.
Audiology and ophthalmology screening is needed before starting.
3 iron chelating drugs
DFO
Deferiprone
Deferasirox
4 methods of monitoring iron overload in BTM
- serum levels of ferritin
- liver biopsy (rarely performed)
- MRI T2 cardiac and hepatic
- ferriscan (R2MRI)
examples of co-inherited Beta thal.
o Sickle Beta Thalassaemia.
o HbE Beta Thalassaemia – very common in SE Asia and can be as severe as beta-thal major
alpha thalassaemia
o Due to a deletion or mutation in alpha globin genes – reduced or absent alpha globins.
o Affects both foetus and adult (alpha is in ALL globin variants).
o Severity depends upon number of chains affected.
o Excess beta and gamma chains will form tetramers of HbH (beta excess) and HbBarts (gamma excess).
problems with treatment of Thal
- 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.
describe haemoglobin structure
- 2 pairs of globin chains
- 4 haem groups
- each haem group with protoporphyrin ring with an iron atom at centre
- single haem per single globin chain
when is HbF found?
predominant in foetal life and found in large amounts at birth
what is thalassemia?
group of disorder where there is underproduction of globin chains of adult haemoglobin
alpha and beta
expanded in selection (possible due to malaria)
what are the mechanisms of globin chain underproduction?
1) deletion or all or part of the gene
2) genetic mutation affecting transcription, mRNA processing, translation or stability of final product
in which haemoglobin are alpha chains found?
HbA (2alpha 2beta)
HbF (2alpha 2gamma) therefore alpha thal may be in utero
how does deletion cause alpha thal.? (>80% of cases of thal)
deletion of one or more alpha genes
as each alpha cluster (one on each chromosome) has two alpha genes, there are 4 syndromes possible, each with an increasing degree of anaemia associated
what are the 4 syndromes of alpha thal? what are the defects in each
1) alpha+ trait= one locus fails
2) alpha0 trait= two loci on the same chromosome fail
3) HbH disease= three loci final
4) Hb Bart’s hydrops fetalis= four loci defective–> death of the foetus in utero
alpha+ trait common in Africa
alpha0 trait common in south east Asia
what causes ineffective erythropoiesis?
beta thalassaemia:
missing beta chains means alpha chains can accumulate and precipate in the bone marrow to cause cell death
any cell with beta chains is removed by the spleen which then enlarges and results in anaemia that increases erythropoietin production causing expansion of bone marrow
patient with beta thal. major
- has profound anaemia
- needs regular transfusions
- presents at first year of life
- general malaise, fails to thrive
patient with beta thal. intermedia
has anaemia but does not need blood transfusion
what risk is involved with blood transfusion in beta thal major?
iron overload due to blood borne viruses like HBV, HCV and HIV
- this is the accumulation of iron in the liver, heart and endocrine glands which then become damaged progressively
- the iron is hard to remove
what drug is used to threat iron overload due to transfusions in BTM?
desferrioxamine: iron chelating agent
improves outcome but uncomfortable
bone marrow transplantation is a possible cure for under 16
beta thalassemia trait genetics aka beta thala. minor
heterozygotes:
- clinically silent
- abnormal red cells
what are the abnormal cells seen in beta thal minor?
- smaller RBCS (microcytosis)
- reduced MCH
- normal MCHC
- raised RBC count
- HBA2 may be raised
this is important to know so it is not confused with iron deficiency (they may need DNA analysis then)
why is it important to distinguish iron deficiency and beta thal minor?
avoid being put on long term iron and becoming iron overloaded
