The Haemoglobin Molecule and Thalassemia

Question 1

State the 3 different haemoglobins that are present in the human body.

Answer 1

HbA: alpha + beta = 95%
HbA2: alpha + delta = 1-3.5%
HbF: alpha + gamma = trace

Question 2

Describe how affinity of haemoglobin changes with oxygen binding and how this helps its role of oxygen transport.

Answer 2

The more O2 binds, the greater the affinity of the Hb for O2.
Beneficial because if deoxyHb has low affinity for O2 (as no O2 is already bound), it will only pick up O2 if O2 sat. is very high (i.e. lungs) so won’t take up O2 in metabolically active tissues where O2 sat. is low + where tissues need O2.
Similarly, oxyHb has a high affinity for O2 so will only give up O2 in environments where the O2 sat. is very low (i.e. respiring tissues that need O2)

Question 3

State 2 factors that can shift the oxygen dissociation curve to the left and what this means

Answer 3

Gives up O2 less readily
Low 2,3-DPG
Increased pH

Question 4

State 3 factors that can shift the oxygen dissociation curve to the right and what this means

Answer 4

Gives up O2 more readily
High 2,3-DPG
High [H+]
High CO2

Question 5

What effect do HbS and HbF have on the oxygen dissociation curve?

Answer 5

HbS has a lower affinity for O2 than HbA so it shifts the ODC right
HbF has a higher affinity for O2 than HbA so it shifts the ODC left

Question 6

Which globin chains are present in early embryonic life but are switched off after about 3 months gestation?

Answer 6

Zeta

Epsilon

Question 7

Which globins are present in foetal haemoglobin?

Answer 7

Alpha

Gamma

Question 8

When are the genes coding for the globin in foetal haemoglobin switched off?

Answer 8

Decreased towards birth + in 1st year after birth.

After 1 year of life, normal adult pattern of Hb synthesis is established.

Question 9

On which chromosomes are the 2 globin gene clusters and which genes are present in each cluster?

Answer 9

Chromosome 16: ALPHA cluster 
2 alpha genes  
Zeta gene  
Chromosome 11: BETA cluster 
Beta gene  
Gamma gene 
Delta gene  
Epsilon gene

Question 10

What is thalassemia?

Answer 10

Genetic disorders characterised by a defect in globin chain synthesis (alpha or beta)

Question 11

What are the 2 clinical variations of thalassemia?

Answer 11

Trait (minor)= carrier state, usually asymptomatic
(Intermedia= spectrum of phenotypes) 
Transfusion dependent (Major)= fatal without transfusion

Question 12

What is the outcome of alpha thalassemia major?

Answer 12

Fatal in utero because alpha globin is needed to make HbF (alpha + gamma)

Question 13

What is the outcome of beta thalassemia major?

Answer 13

Diagnosed + treated in early infancy with regular transfusions

Question 14

What is the name given to the loss of function of 3 alpha globin genes?

Answer 14

Haemoglobin H

Need life-long transfusions

Question 15

What is the name given to the loss of function of 4 alpha globin genes?

Answer 15

Haemoglobin barts

Fatal in utero because alpha globin is needed to make HbF

Question 16

What is beta thalassemia major? Describe how the disease progresses.

Answer 16

Severe defect in both beta globin chains
The foetus will have no problem in utero because they have normal functioning HbF (doesn’t need beta globin)
At 2-3 month after birth, there is transition from HbF to HbA
Here, baby becomes profoundly anaemic.
Need life-long transfusions from this point.

Question 17

State the classic features of an FBC of a patient with thalassemia

Answer 17

Microcytic hypochromic blood picture in absence of iron deficiency
Increased RBCs relative to Hb

Question 18

What can be used to distinguish between alpha thalassemia trait and beta thalassemia trait?

Answer 18

Haemoglobin electrophoresis can be used to measure relative proportions of HbA2
Beta = raised HbA2 (> 3.5%) + raised HbF
Alpha = normal HbA2 + normal HbF

Question 19

Why does the distribution of thalassaemia correlate with M. falciparum distribution?

Answer 19

Thalassaemias offer protection against malaria

Make it harder for parasite to enter RBC

Question 20

What is beta + thalassemia?

Answer 20

Reduction in beta globin output but there is still some residual beta globin gene expression

Question 21

What is beta 0 thalassemia?

Answer 21

NO output of beta globin

Question 22

What are the clinical presentations of beta thalassemia major that are present in the first year of life?

Answer 22

Profound anaemia
Hepatosplenomegaly (due to extra medullary haematopoiesis)
Blood film shows gross hypochromia, poikilocytosis + NRBC’s
Erythroid hyperplasia (BM trying to increase output)

Question 23

List 7 clinical features of beta thalassemia.

Answer 23

Chronic fatigue
Failure to thrive
Jaundice
Delay in growth
Skeletal deformity
Splenomegaly
Iron overload

Question 24

List 4 complications that arise in beta thalassemia major due to iron overload

Answer 24

Liver failure
Cardiac failure
Endocrinopathies
Cholelithiasis + biliary sepsis

Question 25

Name 3 drugs that can be used as iron chelators. List any negative aspects of these drugs.

Answer 25

Desferrioxamine (DFO) Must be given via subcutaneous infusion + Expensive Deferiprone: Urinary excretion Deferasirox: Faecal excretion

Question 26

What are the 4 methods of monitoring iron overload?

Answer 26

Serum ferritin Liver biopsy T2 cardiac + hepatic MRI Ferriscan- R2 MRI

Question 27

What are 4 good aspects of stem cell transplantation?

Answer 27

No transfusions No chelation Growth is normal Curative treatment

Question 28

What are 4 bad aspects of stem cell transplantation?

Answer 28

Transplant associated mortality is high over the age of 17 (30%) Relatively few transplants done in adults Infertility due to stem cell transplant If the patient is iron overloaded at the time of transplantation there is a massively increased risk

Question 29

When is haemoglobin synthesised?

Answer 29

65% erythroblast stage | 35% reticulocyte stage

Question 30

Where are the 2 components of Hb synthesised?

Answer 30

``` Haem= Mitochondria Globin= Ribosomes ```

Question 31

Which enzyme is involved in the manufacture of Haem? How?

Answer 31

Delta-ALA | When excess haem is produced, it exerts a negative feedback on this enzyme + synthesis of haem is reduced

Question 32

What is haem composed of?

Answer 32

Protoporphyrin ring with central iron atom

Question 33

What technique can be used to separate different Haemaglobins based on their charge and molecular mass?

Answer 33

High performance liquid chromatography (HPLC)

Question 34

Describe the primary, secondary and tertiary structure of globin

Answer 34

Primary: alpha= 141 AA's, non-alpha= 146 AA's Secondary: 75% A + B chains helical arrangement Tertiary: ~Sphere, hydrophilic surface, haem pocket

Question 35

Describe the shape of the oxygen-haemoglobin dissociation curve. What is p50?

Answer 35

Sigmoid shape | P50= pO2 at which half Hb is saturated with O2 ~26.6mmHg

Question 36

What are the 2 types of haemoglobinopathy?

Answer 36

Structural variants of Hb being produced e.g. HbS | Defects in globin chain synthesis (Thalassaemia)

Question 37

What causes B thalassaemia?

Answer 37

Point mutation in B globin gene | Results in reduced or absent production of B globin chains

Question 38

Describe 3 features on a blood film seen in thalassaemia

Answer 38

Hypochromia Target cells Poikilocytosis

Question 39

What is the only reliable way of diagnosing alpha thalassaemias?

Answer 39

DNA analysis

Question 40

List 7 treatment options for thalassaemia major

Answer 40

``` Regular blood transfusions Iron chelation therapy Splenectomy Supportive medical care Hormone therapy Hydroxyurea to boost HbF BM transplant ```

Question 41

What is required in splenectomised patients? Why?

Answer 41

Prophylaxis (immunisation + antibiotics) | More susceptible to infection, particularly from encapsulated bacteria

Question 42

List 4 side effects of deferasirox

Answer 42

Rash GI symptoms Hepatitis Renal impairment

Question 43

List 4 side effects of Desferrioxamine

Answer 43

Vertebral dysplasia Pseudo-rickets Retinopathy High tone sensorineural loss

Question 44

List 4 side effects of Deferiprone

Answer 44

GI disturbance Hepatic impairment Neutropenia Agranulocytosis

Question 45

What is the best way of prescribing iron chelators?

Answer 45

Combination therapy- reducing dose of individual drugs, thus reducing side effects