4 - Haemoglobin molecule and thalassaemia Flashcards
Why are red blood cells unable to divide?
they have no nucleus or mitochondria
In what stage of development of an erythrocyte does Hb synthesis begin in?
pro-erythroblast
During what stages of development of RBC does haemoglobin synthesis occur?
65% erythroblast stage
35% reticulocyte stage
Where are the components of haemoglobin produced in a cell?
haem - synthesised in the mitochondria
global - synthesised in the cytoplasmic ribosomes
What is the global chain composition of HbA?
2 alpha and 2 beta globin chains *each with a haem molecule in the centre
In what form is iron when oxygen is unbound?
ferrous form (Fe2+)
Why is the enzyme ALAS in haem synthesis important?
it regulates negative feedback and hence haem synthesis
What are the 2 clusters that the 8 functional global chains are arranged into? And on what chromosomes are they located?
β cluster (b, g, d and e global chains) on the short arm of chromosome 11
α cluster (a1, a2 and z global chains) on the short arm of chromosome 16
Which global chains are present in embryonic haemoglobin?
e and z
What is the site of production of red blood cells in the very early embryo?
yolk sac
What is the site of production of red blood cells in the later stages of foetal life?
liver and spleen
NOTE: the switch to bone marrow occurs shortly after birth
For what duration are the e and z chains produced in embryonic life?
After this, what global chains are produced?
6-8 weeks
after this, 2 gamma and 2 alpha chains constitute fetal hemoglobin (HbF) which is normally replaced by adult hemoglobin (HbA) in the year following birth.
How long does gamma globin chain production persist into life? What are these chains then replaced by?
3-6 months
they are then replaced by beta chains
What are the manifestations of problems with different globin chains?
if there is a problem with alpha globin production, this manifests early on during embryonic life (very little that can be done for the baby is this happens)
beta thalassaemia will manifest after birth (due to the timing of the gamma-beta switch) (can treat the baby at the point - high chance of survival)
List the normal adult haemoglobin, their global chain composition and proportions in an adult
HbA - 2 alpha and 2 beta chains - 96-98%
HbA2 - 2 alpha and 2 delta chains - 1.5-3.2%
HbF - 2 alpha and 2 gamma chains - 0.5-0.8%
Describe the secondary structure of globin chains
75% alpha and beta chains form a helical arrangement
Describe the tertiary structure of globin chains
almost spherical in shape
hydrophilic surface
hydrophobic core
haem pocket
Describe the changes in confirmation of haemoglobin as oxygen binds
oxyhemoglobin has a more open configuration
when fully oxygenated with four oxygen molecules, the haemoglobin achieves its ‘R’ or ‘relaxed’ quaternary structure.
deoxyhaemoglobin is a tighter ‘tense’ structure
has 2,3-DPG bound
crevices are small, making it difficult for oxygen to gain access to the haem
What is the P50 of haemoglobin?
26.6 mmHg (4.5 kPa)
What factors does the normal position of the oxygen dissociation sigmoid curve?
- concentration of 2,3 DPG
- pH / H+ ion concentration
- conc of CO2 in red blood cells
- structure of Hb
What is the PO2 in arterial blood?
14 kPa
What is the PO2 in venous blood?
5.5 kPa
What does it mean for the oxygen dissociation curve to shift to the right? What factors can cause this to happen?
easier oxygen delivery - dissociates more readily in tissues
- high 2,3 DPG
- high H+
- high CO2
- HbS
i. e. conditions encountered in metabolising tissues
What does it mean for the oxygen dissociation curve to shift to the left? What factors can cause this to happen?
increased oxygen carrying capacity - gives up oxygen less readily
- low 2,3 DPG
- high pH
- HbF
What are haemoglobinopathies?
structural variants of haemoglobin
or
defects in globin chain synthesis (thalassaemia)
How can thalassaemia be classified?
by considering the globin chain affected
by looking at clinical severity
Where are the alpha genes found?
on chromosome 16
only 3 out of 7 genes are expressed
Where are the beta genes found?
on chromosome 11
Define beta thalassaemia
deletion or mutation in beta globin gene
results in reduced or absent production of beta globin chains
What are the 2 forms of inclusion bodies in beta thalassaemia major?
alpha globin precipitates
pappenheimer bodies
What kind of inheritance pattern does beta thalassaemia have?
autosomal recessive
Explain how the mutations of the beta globin gene lead to the different classifications of beta thalassaemia (minor, intermedia and major)
depends on the degree of suppression of globin chain synthesis
some mutations result in no globin production (β0)
some mutations result in decreased levels of production (β+)
inheritance of 2 β0 genes will give rise to Major, while inheritance of 2 β+ genes will give rise to Intermedia
What are the main features of thalassaemia in a full blood count?
MICROCYTIC HYPOCHROMIC INDICES
there will also be increased RBC count when compared to Hb
How can beta thalassaemia be identified demo a peripheral film?
HbA2 is raised in relation tot he severity of the mutation and raised HbF
What is the diagnostic for alpha thalassaemia?
no simple diagnostic process
have to do a DNA analysis and look at the DNA of the globin genes - GOLD STANDARD
NOTE: a presumptive diagnosis can be made if microcytosis and hypochromia is seen in the absence of iron deficiency
What is the diagnostic for beta thalassaemia?
electrophoresis
How is the beta thalassaemia trait seen on a blood film?
microcytosis, hypochromia and occasional cells showing basophilic stippling
some degree of poikilocytosis and target cells can be seen
What can be seen on a HPLC plot for beta thalassaemia trait?
How is this differentiated from beta thalassaemia major?
predominant Hb = HbA
BUT increased levels of HbA2 and HbF
the patient is still able to produce HbA SO NOT BETA THALASSAEMIA MAJOR
What is beta thalassaemia major?
How is it treated?
carrying 2 abnormal copies of the beta globin gene (this is due to mutations or deletions) so no HbA can be made
leads to severe anaemia, requires regular blood transfusions
When does beta thalassaemia major usually present?
usually 4-6 months of life
How does beta thalassaemia major present on a peripheral blood film?
extreme hypochromia, microcytosis and poikilocytosis
often Howell Jolly bodies and nucleated RBCs will be present
What is a major side effect of the treatment of beta thalassaemia major? How is this treated?
regular transfusions means that patients become iron overloaded, unless they are treated with iron chelators
What are pappenheimer bodies?
iron deposits
What 2 forms of inclusion bodies may be seen in beta thalassaemia major?
- alpha globin precipitates
- pappenheimer bodies
How does thalassaemia major present clinically?
- Severe anaemia usually presenting after 4 months (only applies to beta thalassaemia)
- Hepatosplenomegaly
- Blood film shows gross hypochromia, poikilocytosis and many nucleated RBCs
- Bone marrow will show erythroid hyperplasia
- Prominence of maxilla bones and separation of the teeth, due to extra-medullary haematopoiesis
What are the clinical features of beta thalassaemia?
- Chronic fatigue
- Failure to thrive
- Jaundice
- Delay in growth and puberty
- Skeletal deformity
- Splenomegaly
- Iron overload (due to ineffective erythropoiesis and Fe overload due to transfusion therapy)
How can beta thalassaemia lead to death?
cardiac disease (die to iron overload)
infections
liver disease and other causes.
Give some treatments for thalassaemia major?
- *Regular blood transfusions
- *Iron chelation therapy
- Splenectomy
- Supportive medical care
- Hormone therapy
- Hydroxyurea to boost HbF
- *Bone marrow transplant – CURATIVE
Why is the management of infection so important in thalassaemia patients?
they are more susceptible to:
- yersinia (siderophilic bacterium) - thrive in patients with Fe overload
- gram negative sepsis
When is iron chelation therapy started when treating with blood transfusions?
after 10-12 transfusions or when serum ferritin is >1000 mcg/l
Name the different type of iron chelation therapies available and how they are administered
DFO: SC or IV administration taken 5 days a week Deferiprone: orally 3 times a day MOST EFFECTIVE IN REDUCING FE IN CARDIAC IRON OVERLOAD Deferasirox: orally, once a day latest therapy
Give the side effects of each of the types of iron chelation therapy
Deferasirox: RENAL IMPAIRMENT, rash, GI symptoms, hepatitis,
DFO: vertebral dysplasia, pseudo-rickets, gene valgum, retinopathy, high tone sensoneural loss
increased risk of Klebsella and Yersinia infection
Deferiprone: GI disturbance, hepatic impairment, neutropenia, agranulocytosis, arthropathy
When is combination therapy for iron chelation most useful?
for those who have both hepatic and cardiac overload
What are the methods for monitoring iron overload?
- serum ferritin
- liver biopsy - rare
- T2 cardiac and hepatic MRI
- R2 MRI —> better technique
Describe what a ferriscan is and why it is a good method to measure iron overload
non invasive technique to qualify liver iron concentration
- not affected by inflammation or cirrhosis
What is sickle beta thalassaemia?
coinheritance of the sickle beta globin molecule with beta thalassaemia
a sickling disorder (rather than a thalassaemia)
How does sickle beta thalassaemia present on a blood film?
shows features of both sickle cell and beta thalassaemia:
sickled cells, target cells, microcytosis, hypochromia
What is alpha thalassaemia?
What is its cause?
What impacts does this have?
- Deletion or mutation in the alpha globin chain gene
- This leads to reduced or absent production of alpha globin chains
- Alpha globin production starts early in embryogenesis, so α-thalassaemia affects both foetus and adult
- Severity depends on number of a globin genes affected – there are 4 alpha globin genes in total
thalassamia carrier/thalassmaemia minor trait
What genes have to be present to have the trait?
How does it present?
- Carry a single abnormal copy of the beta globin gene (w.r.t. beta thalassaemia)
- Carry either one or two abnormal copies of the alpha globin gene (w.r.t. alpha thalassaemia)
- Usually asymptomatic, usually diagnosed on the basis of mild anaemia
What are the 4 types of alpha thalassaemia?
- Hb Bart syndrome (the more severe form) - loss of 4 alpha-globin alleles
- HbH disease - loss of 3 alpha-globin alleles
- thalassaemia trait - loss of 2 alpha-globin alleles
- thalassaemia silent carriers - loss of 1 alpha-globin alleles
What does the peripheral blood film of HbH disease present like?
(has a haemolytic element to it with) microcytosis, anisoocytosis, poikilocytosis and “puddling” of haemoglobin within the RBC.
How is HbH shown on electrophoresis?
shows a fast band, which is probably more easily seen shortly after the strip has started to run.
