The Haemoglobin Molecule and Thalassemia

Question 1

What are the different types of globin proteins?

Answer 1

Alpha

Beta

Gamma

Delta (and embryonic)

Question 2

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

Answer 2

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

Question 3

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

Answer 3

The more oxygen binds, the greater the affinity of the haemoglobin for oxygen. This is good because if deoxyhaemoglobin has a low affinity for oxygen(as no oxygen is already bound), it will only pick up oxygen if the oxygen saturation is very high (i.e. in the lungs) so it will not take up oxygen in the metabolically active tissues where the oxygen saturation is low and where the tissues need oxygen. Similarly, oxyhaemoglobin has a high affinity for oxygen so it will only give up oxygen in environments where the oxygen saturation is very low (i.e. respiring tissues that need oxygen)

Question 4

What effect does 2,3-DPG have on oxygen delivery?

Answer 4

It facilitates oxygen delivery by making the haemoglobin molecule less flexible and pushing out the oxygen.

Question 5

State 3 factors that can shift the oxygen dissociation curve to the right.

Answer 5

Increase in 2,3-DPG Increase in [H+] CO2

Question 6

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

Answer 6

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

Question 7

What is special about alpha globin genes?

Answer 7

There are TWO alpha globin genes from each parent so there are FOUR alpha globin genes in total.

Question 8

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

Answer 8

Zeta and epsilon

Question 9

Which globins are present in foetal haemoglobin?

Answer 9

Alpha Gamma

Question 10

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

Answer 10

It is decreased towards birth and in the first year after birth. After 1 year of life, the normal adult pattern of haemoglobin synthesis would have been established.

Question 11

Which globin chains are present in HbA2?

Answer 11

Alpha Delta

Question 12

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

Answer 12

Chromosome 16 – ALPHA cluster  TWO alpha genes  Zeta gene Chromosome 11 – BETA cluster  Beta gene  Gamma gene  Delta gene  Epsilon gene

Question 13

What is thalassemia?

Answer 13

Disorders in which there is a reduced production of one of the two types of globin chains in haemoglobin leading to imbalanced globin chain synthesis

Question 14

What are the two clinical variations of thalassemia?

Answer 14

Thalassemia trait = common variant with little clinical significance Transfusion dependent – Thalassemia Major = fatal without transfusion

Question 15

What is the outcome of alpha thalassemia major?

Answer 15

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

Question 16

What is the outcome of beta thalassemia major?

Answer 16

Diagnosed and treated in early infancy with regular transfusions

Question 17

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

Answer 17

Haemoglobin H Need life-long transfusions

Question 18

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

Answer 18

Haemoglobin barts Fatal in utero because alpha globin is needed to make HbF

Question 19

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

Answer 19

Severe defect in both beta globin chains The foetus will have no problem in utero because they have normal functioning HbF (which doesn’t need beta globin) At around 2-3 month after birth, you get a transition from HbF to HbA At this time the baby will become profoundly anaemic. They will need life-long transfusions from this point onwards.

Question 20

State some features of thalassemia trait.

Answer 20

This the carrier state of thalassemia. They may be mildly anaemic but they can also be normal. Usually have a LOW MCH and LOW MCV

Question 21

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

Answer 21

Haemoglobin electrophoresis can be used to measure the relative proportions of HbA2 Beta Thalassemia = raised HbA2 (> 3.5%) Alpha Thalassemia = normal/low HbA2

Question 22

What types of mutation cause alpha thalassemia and beta thalassemia?

Answer 22

Alpha thalassemia – deletion Beta thalassemia – point mutation

Question 23

What is alpha + thalassemia?

Answer 23

The situation in which one of the two globin genes on a chromosome are deleted (this can happen on one or both chromosomes) Someone who is heterozygous for alpha+ thalassemia will still have 3 functioning alpha globin genes so they will only be mildly anaemic

Question 24

What is alpha 0 thalassemia?

Answer 24

The situation in which both of the two globin genes on a chromosome are deleted (this can happen in one or both chromosomes) In the heterozygous state there are still 2 functioning alpha globin genes so they will also only experience mild anaemia

Question 25

How can you distinguish between alpha + and alpha 0 thalassemia?

Answer 25

Most people with alpha 0 thalassemia have a MCH < 25 pg

Question 26

What could be the potentially devastating consequences for someone with mild anaemia caused by alpha 0 thalassemia?

Answer 26

Someone with alpha 0 thalassemia may not experience any symptoms themselves, but if they try and have a child with someone who also has alpha 0 thalassemia then there is a chance that their child may not have any functioning alpha genes (haemoglobin barts).

Question 27

Which ethnic groups have a high prevalence of thalassemia?

Answer 27

Chinese South-east Asian Greek Turkish Cypriot

Question 28

What is beta + thalassemia?

Answer 28

There is a reduction in beta globin output but there is still some residual beta globin gene expression

Question 29

What is beta 0 thalassemia?

Answer 29

There is NO output of beta globin

Question 30

What is the main cause of the pathophysiology of beta thalassemia?

Answer 30

The surplus of alpha globin chains form tetramers. These alpha globin tetramers precipitate in the bone marrow, which leads to ineffective erythropoiesis and haemolysis in the peripheralcirculation.

Question 31

What are the features of beta thalassemia trait?

Answer 31

Haemoglobin may be normal LOW MCV LOW MCH HIGH RBC HbA2 INCREASED

Question 32

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

Answer 32

Profound anaemia Failure to thrive Malaise Splenomegaly

Question 33

Describe the pathogenesis of beta thalassemia.

Answer 33

There is death of red cells in the marrow because of ineffective erythropoiesis The removal of red cells by the spleen causes:  Large spleen  Anaemia  Increased EPO  Expansion of bone marrow

Question 34

What are the consequences of regular blood transfusions used to treat beta thalassemia major?

Answer 34

Iron Overload  Iron can accumulate in the liver (cirrhosis), heart (cardiac failure), endocrine organs (hypopituitarism, hypothyroidism, diabetes) etc. Potential viral transmission

Question 35

Name three drugs that can be used to treat iron overload. List anynegative aspects of these drugs.

Answer 35

Desferrioxamine (DFO)  Not orally active – must be given via subcutaneous infusion  Expensive Deferiprone  Urinary excretion  Risk of agranulocytosis (which can lead to life-threatening sepsis)  Patients need to be monitored closely for neutropenia Deferasirox  Faecal excretion

Question 36

What are the key indicators of iron overload?

Answer 36

Serum ferritin Liver iron

Question 37

What are the good aspects of stem cell transplantation?

Answer 37

No transfusions No chelation Growth is normal Curative treatment

Question 38

What are the bad aspects of stem cell transplantation?

Answer 38

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