The Haemoglobin Molecule and Thalassemia

Question 1

What are the different types of globin proteins?

Answer 1

Alpha, Beta, Gamma, Delta (and embryonic- epsilon and zeta)

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 (hypercapnia)
Increase temperature

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

How is thalassemia classified in terms of clinical severity?

Answer 14

Minor: Thalassemia trait = common variant with little clinical significance

Intermediate: Thalassemia intermedia

Major: 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

Clinical features of thalassemia major?

Answer 16

a. Severe anaemia presenting at 4 months leading to Chronic fatigue
b. Failure to thrive
c. Jaundice
d. Hyperactive bone marrow
e. Delay in growth and puberty
f. Extramedullary haematopoiesis
g. Skeletal deformity (bone marrow expansion due to extramed haematopoiesis causing the distinctive facial features)
h. Hepatosplenomegaly
i. Iron overload (patients with thalassaemia absorb more iron in GI tract)

Question 17

Which 2 types of abnormal haemoglobin can result due to Alpha thalassaemia

Answer 17

Haemoglobin H and Hb barts

Lack of Alpha chain forms tetramers of Beta chains instead of 2 alpha and beta= HbH

Same but tetramer of gamma chain= Hb barts

Question 18

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

Answer 18

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 19

State some features of thalassemia trait.

Answer 19

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 20

Summarise the laboratory diagnosis of Thalassemia

Answer 20

Start simple:
-	FBC 
-	Film 
Complex tests:
-	Hb electrophoresis/ HPLC 
-	DNA studies eg PCR to look at genes for globin chains

Question 21

What types of mutation cause alpha thalassemia and beta thalassemia?

Answer 21

both can be caused by deletion or point mutation in the respective alpha/beta globin gene

Question 22

Which ethnic groups have a high prevalence of thalassemia?

Answer 22

Chinese 
South-east Asian 
Greek 
Turkish 
Cypriot

Question 23

Laboratory diagnosis: What features would you see when looking at FBC of thalassemia?

Answer 23

• Poikilocytosis (no anisocytosis)
• Target cells
• Hypochromasia
• Nucleated RBC

Question 24

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

Answer 24

Iron overload: Chronic transfusion has been associated with iron accumulation. There’s not enough transferrin to bind to all the iron, the unbound iron can catalyse production of reactive oxygen species which can damage the liver (cirrhosis), heart (cardiac failure), endocrine organs (hypopituitarism, hypothyroidism, diabetes) etc.
Also increases chance of infection.

Question 25

Name three iron chelator drugs that can be used to treat iron overload.

Answer 25

Desferrioxamine (DFO/desferal)
Deferiprone
Deferasirox

Question 26

What are the key indicators when monitoring the iron overload in the management of thalassemia patients

Answer 26

1. Serum ferritin (but not that accurate as it is also an ACP that rises in inflammation etc)
2. Cardiac and hepatic MRI to measure iron levels in heart and liver

Liver biopsy used to be done but not anymore

Question 27

Laboratory diagnosis: What features would you see when looking at Blood film of thalassemia?

Answer 27

target cells
poikilocytosis (different shape)
NO anisocytosis(different size)
Microcytic and hypochromic (relate this to the FBC card earlier)

Question 28

Laboratory diagnosis:

What is the most definitive test for thalassemia

Answer 28

DNA studies - PCR, sanger sequencing etc

Most alpha thalassemias are diagnosed this way because they are quite hard to diagnose (HPLC gives normal HbA2 and HbF)

Question 29

Complications of thalassemia

Answer 29

Cholelithiasis (gallstones)
Biliary sepsis
Cardiac failure
endocrinopathies
liver failure

Question 30

Treatments for thalassemia major (minor usually asymtomatic)

Answer 30

Main treatments:

• Regular blood transfusions
• Iron chelation therapy (remove XS iron)

Other treatments:

• Splenectomy (patients may have splenomegaly due to extramedullary hematopoiesis)
• Supportive medical care for cardiac, liver problems
• Hormone therapy, iron overload can cause hypogonadism
• Hydroxyurea to boost HbF production
• Bone marrow transplant

Question 31

Why are thalassemia patients prone to infection? What can be done to reduce the chances?

Answer 31

They tend to have iron overload and there are pathogens that thrive on iron especially gram - bacteria like YERSINIA

Also, thalassemia patients could have splenectomy due to splenomegaly via extramedullar haematopoiesis which is an important immune organ.

to prevent this, immunisation in splenectomised patients is good. (prophylaxis)

Question 32

Main concern for management of thalassemia patients

Answer 32

infection and monitor iron overload

Question 33

What are:
Sickle b thalassemia
HbE b thalassemia

Answer 33

Sickle b thal:
inherits one sickle allele from one parent and b thal allele from the other parent
NB sickle allele is also a mutation on the beta globin gene. So this disease is basically inheriting two different faulty beta globin alleles from the 2 parents

HbE b thal:

• Single point Mutation in beta chain, forming HbE which is an abnormal haemoglobin
• Reduced synthesis of functional beta chain leads to beta thalassemia