The Haemoglobin Molecule and Thalassaemia

Question 1

What is the role of red blood cells, what is it’s normal concentration and what does it contain

Answer 1

Carrys oxygen from lungs to tissue and CO2 from tissue to lungs
3.5-5 x 10^12/L
Contains haemoglobin
No nucleus

Question 2

Where is haemoglobin found, what is its normal concentration and how much is produced/destroyed every day

Answer 2

Found only in RBCs
normal conc. = 120-165g/L
Approx 90mg/kg produced and destroyed in the body every day

Question 3

Describe the structure of haemoglobin

Answer 3

4 polypeptide subunits of 2 alpha chains and 2 beta chains with a prosthetic haem group

4 haem+4 globin chains = 1 Hb molecule

Question 4

Which organelles are involved in haemoglobin synthesis

Answer 4

haem - mitochondria

globin - ribosomes

Question 5

Describe the structure of haem

Answer 5

Same in all types of Hb
Combination of protoporphyrin ring with central iron atom, usually in the ferrous form (Fe2+)
Able to reversibly bind to oxygen

Question 6

Describe the synthesis of haem

Answer 6

Mainly in the mitochondria which contains the ALAS enzyme
more haem made = -ive feedback on enzyme activity
Iron is brought to the mitochondria (via transferrin) for haem synthesis

Question 7

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

Answer 7

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 8

Describe the global gene clusters

Answer 8

Eight functional globin chains, arranged in two clusters:

beta cluster (b, g, d and e globin genes) on the short arm of chromosome 11

alpha cluster (a x2 and z globin genes) on the short arm of chromosome 16

Question 9

Describe the changes in location of RBC production in the foetus

Answer 9

Main site of RBC production at first is the yolk sac, then liver, then spleen
After birth, main production is the bone marrow

Question 10

Describe the alteration of gene expression and globin production in the foetus

Answer 10

1. zeta and epsilon globin chains immediately
2. switch to alpha and gamma (3-6 months of life)
3. Then alpha and beta
   Little delta globin synthesis

Question 11

How many alpha genes are there in total

Answer 11

There are 2 alpha globin genes from each parent so there are 4 alpha globin genes in total.

Question 12

What makes up Hb A and what is the normal percentage

Answer 12

alpha2,beta2

96-98%

Question 13

What makes up Hb A2 and what is the normal percentage

Answer 13

alpha2,delta2

1.5-3.2%

Question 14

What makes up Hb F and what is the normal percentage

Answer 14

alpha2,g2

0.5-0.8%

Question 15

Describe the secondary structure of globin

Answer 15

75% alpha and beta chains in helical arrangement
Structure varies with the no. of O2 molecules bound
With no or few O2 bound, Hb is in the ‘tight’ configuration and is stabilised by 2,3-BPG.

Question 16

Describe the tertiary structure of globin

Answer 16

approx sphere
Hydrophilic surface (charged polar side chains) and a hydrophobic core
Haem pocket

Question 17

Describe the oxygen-haemoglobin dissociation curve

Answer 17

Sigmoid shape

Binding of one molecule facilitate the second molecule binding (cooperativity)

Question 18

What is P50 on the oxygen dissociation curve

Answer 18

Partial pressure of O2 at which Hb is half saturated with O2

26.6mmHg

Question 19

What is the advantage of the cooperative binding of haemoglobin

Answer 19

Deoxyhaemoglobin (tight form) has a very low affinity for O2, so it will only pick it up when the pO2 is very high, like in the lungs, and it won’t take O2 away from metabolically active tissues that need it.

Question 20

What does position of the Hb-oxygen dissociation curve depend on

Answer 20

Concentration of 2,3-DPG
H+ ion concentration (pH)
CO2 in red blood cells
Structure of Hb

Question 21

What causes right shift on the oxygen-dissociation curve

Answer 21

High 2,3-DPG
High H+
High CO2
HbS

Question 22

What causes left shift on the oxygen-dissociation curve

Answer 22

Low 2,3-DPG
Low H+
Low CO2
HbF

Question 23

What are haemoglobinopathies

Answer 23

Structural variants of haemoglobin
or
Defects in globin chain synthesis (thalassaemia)

Question 24

Define thalassaemia

Answer 24

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

Question 25

What are the two clinical variations of thalassemia

Answer 25

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

Question 26

What is the most common type of mutation in alpha and beta thalassaemia

Answer 26

alpha - deletion | Beta - point mutation

Question 27

Describe beta thalassaemia major

Answer 27

Severe defect in both β chains (beta0)

Question 28

What are the clinical features of beta thalassaemia major in the first year of life and why may it be a problem in the first 2-3 months

Answer 28

Profound anemia Failure to thrive Malaise (general discomfort feeling) Splenomegaly Problem as there is a switch from HbF to HbA

Question 29

Why does beta thalassaemia major cause splenomegaly

Answer 29

α chains form tetramers (α4), which precipitate in bone marrow causing ineffective erythropoiesis If they do enter the circulation, the spleen removes them -> splenomegaly

Question 30

Describe the inheritance of beta thalassaemia

Answer 30

Autosomal recessive B0 = thal. major (both genes) B+ = only one gene damaged Carriers are aymsymptomatic but to have microcytic hypo chromic indicies

Question 31

What investigations should be done for thalassaemia

Answer 31

FBC Rilm Hb EPS/HPLC Globin chains synthesis/ DNA studies

Question 32

What will show on the film for a beta thalassaemia major

Answer 32

``` Extreme hypochromia Microcytosis Poikilocytosis Tear drop cells + elliptical Howell jolly bodies ```

Question 33

Which are the 2 major inclusion bodies seen in beta thalassaemia

Answer 33

``` Alpha globin precipitates Pappenheimer bodies (Perls stain - inky blue granules) ```

Question 34

What are the clinical features of beta thalassaemia major

Answer 34

``` Splenomegaly Skeletal deformity (increased BM) Chronic fatigue Failure to thrive Jaundice Iron overload ```

Question 35

What is the life expectancy of un-treated beta thalassaemia patients

Answer 35

Death at 7 years without transfusion

Question 36

What is the life expectancy of treated (transfusion) beta thalassaemia patients and why

Answer 36

25 Iron overload -> liver cirrhosis, heart failure, diabetes, endocrine failure potential viral transmission

Question 37

What is the treatment for thalassaemia major

Answer 37

``` Regular blood transfusions Iron chelation therapy (Splenectomy Supportive medical care Hormone therapy Hydroxyurea to boost HbF Bone marrow transplant) ```

Question 38

What are the types of iron chelation therapies

Answer 38

Deferasirox (Exjade) Desferrioxamine (deferral) Deferiprone (ferriprox)

Question 39

What is the purpose of iron chelation therapy

Answer 39

Remove iron to prevent overload

Question 40

Why is desferrioxamine not used as much as other iron chelation therapies

Answer 40

SC injected (not orally active) Expensive Compliance issues Increased risk of Klebsiella and Yersinia infection

Question 41

Describe Deferasirox (Exjade) (admin, dose)

Answer 41

Oral | Dose 20-40mg/kg

Question 42

What are the unwanted effects of defarasirox

Answer 42

rash GI symptoms hepatitis renal impairment

Question 43

Describe deferiprone (admin and dose)

Answer 43

Oral | Dose 5-100 mg/kg/day

Question 44

What is used to monitor iron overload

Answer 44

Serum ferritin Liver iron Hepatic and cardiac MRI

Question 45

Describe HbE beta thalassaemia

Answer 45

Very common combo in south east Asia | Variable in expression - may be as severe as beta thalassaemia major

Question 46

Describe beta thalassaemia trait

Answer 46

Little drop in Hb May cause mild anaemia high RBC, low MCV and MCH. In β-thalassaemia trait, haemoglobin electrophoresis will show a greater proportion of HbA2 (d chains not β).

Question 47

Describe alpha thalassamia

Answer 47

Varying prognoses depending on how many chains are affected 2 or 3 - mild anaemia 1 - HnH formation + severe anaemia 0 - haemoglobin Barts - fatal in utero

Question 48

What is the chance of HbBarts with 2 parents with only 2 alpha chains

Answer 48

25%