Haemoglobin and thalassaemia

Question 1

What are the key components of the oxygen dissociation curve?

Answer 1

O2 carrying capacity of Hb at different pO2
x = partial pressure of oxygen
y = HbO2%

Sigmoid shape:
Binding of one molecule facilitates binding of 2nd molecule
P50 = 26.6mmHg

Question 2

What factors affect the oxygen dissociation curve?

Answer 2

[2,3-DPG]
[H+] - pH
[CO2] in red cells - linked to pH
Structure of Hb

Question 3

How is a haemoglobin molecule structured?

Answer 3

Haem
4 globin - 2 alpha chains, 2 beta chains
Each chain protects a haem molecule at its centre

Question 4

What is the function of haemoglobin?

Answer 4

Haem binds reversibly with oxygen

O2 transport

Question 5

What are the normal variants of haemoglobin?

Answer 5

HbA - a2b2
HbA2 - a2d2
HbF - a2g2

Question 6

Explain the relationship between globin genes and different types of haemoglobinopathy

Answer 6

8 functional globin chains, arranged in 2 clusters:
b- cluster (b, g, d, e globin genes) on short arm of chr 11
a - cluster (a, z globin genes) on short arm chr 16

Defects in globin chain synthesis = thalassaemia

Question 7

Explain the genetic basis of thalassaemia

Answer 7

Beta thalassaemia:
Deletion/mutation in b globin gene(s)

Thalassaemia major:
2 abnormal copies of b globin gene

Question 8

Explain the pathophysiology of thalassaemia

Answer 8

Deletion/mutation in b globin gene(s)

Reduced/absent b globin chain production

Question 9

What are the clinical features of a/b thalassaemia major?

Answer 9

Both:
Hepatosplenomegaly
Erythroid hyperplasia in bone marrow
Extra-medullary haematopoiesis

Alpha:
Lethal in utero if all 4 alpha globin genes mutated
Survivors usually have 3 alpha globin gene mutations

Beta:
Chronic fatigue
Failure to thrive, delay in growth and puberty, skeletal deformity
Splenomegaly
Iron overload
Jaundice
Severe anaemia usually presents after 4 months

Question 10

What are the haematological features of beta thalassaemia?

Answer 10

Marked hypochromia
Poikilocytosis
Basophilic stipling
Nucleated RBCs
Alpha chain precipitates
Iron inclusions
Pappenheimer bodies (Perls stain)

Question 11

How is thalassaemia diagnosed?

Answer 11

Laboratory diagnosis

FBC: microcytic hypochromic indices, increased RBCs relative to Hb

Film: target cells, poikilocytosis but no anisocytosis

Hb EPS/HPLC:
a-thal - normal HbA2 and HbF, +/- HbH
b-thal - raised HbA2 and HbF

Globin chain synthesis/DNA studies

Genetic analysis for B-thal mutations and xmni polymorphism (in b-thal) and a-thal genotype (in all cases)

Question 12

How is thalassaemia managed?

Answer 12

1. Regular blood transfusions
2. Iron chelation therapy
3. Splenectomy if transfusion requirement is high
4. Supportive medical care
5. Hormone therapy
6. Hydroxyurea to boost HbF
7. BM transplant

Question 13

Explain the structure of globin

Answer 13

Primary
alpha - 141aa
non-a - 146aa

Secondary
75% alpha and beta chains - helical arrangement

Tertiary
Approx sphere
Hydrophilic surface (charged polar side chains)
Hydrophobic core
Haem pocket

Question 14

What is P50

Answer 14

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

On normal ODC = 26.6 mmHg

Question 15

Explain cooperativity with regards to oxygen and haemoglobin

Answer 15

Binding of one molecule of oxygen facilitates the binding of a second molecule

Question 16

What can cause the ODC to shift right?

Answer 16

High [2,3-DPG]
high [H+]
High [CO2]
HbS

All consequences of metabolising tissues

Question 17

What can cause the ODC to shift left?

Answer 17

Low [2,3-DPG]

HbF

Question 18

What are the effects of the ODC shifting right?

Answer 18

Easy oxygen delivery

Reduced oxygen affinity

Question 19

What are the effects of the ODC shifting left?

Answer 19

Give up oxygen less readily

Increased oxygen affinity

Question 20

Explain thalassaemia intermedia

Answer 20

Present beta globin gene
Produces reduced no of globin chains
Not totally dysfunctional
Varying clinical phenotypes
May require episodic blood transfusions

Question 21

When does thalassaemia major usually present

Answer 21

After 4-6 months of life

When genes switch from gamma to beta

Question 22

What are the clinical features of beta thalassaemia?

Answer 22

Hepatosplenomegaly
Erythroid hyperplasia in bone marrow
Extra-medullary haematopoiesis
Chronic fatigue
Failure to thrive
Jaundice
Delay in growth and puberty
Skeletal deformity
Splenomegaly
Iron overload

Question 23

What are some complications of beta thalassaemia?

Answer 23

Cholelithiasis
Biliary sepsis
Cardiac failure
Endocrinopathies
Liver failure

Question 24

What’s the most common cause of death in individuals with beta thalassaemia?

Answer 24

Cardiac death due to iron overload