Case 2: Sickle cell Anaemia

Question 1

What chains make up normal Hb?

Answer 1

2 α chains and 2 β chains

Question 2

What kind of mutation causes sickle cell anaemia?

Answer 2

A point (substitution) mutation

Question 3

What amino acid is swapped out?

Answer 3

Glutamate, a hydrophillic, negatively charged amino acid, on a β chain.

Question 4

What amino acid replaces glutamate?

Answer 4

Valine 6, a hydrophobic, non-polar amino acid

Question 5

What causes the HbS polymer?

Answer 5

The mutant hydrophobic valine (valine 6) along with alanine and leucine, reside on the exterior of the haemoglobin molecules. They can make hydrophobic interactions with each other, forming a hydrophobic socket (they clump together) rather than making hydrogen bonds with water, which gives rise to the insoluble polymer HbS.

Question 6

What type of genetic disorder is Sickle cell anaemia?

Answer 6

Autosomal recessive (both copies need to be lost in order for the person to have the disorder).

Question 7

Why don’t the other hydrophobic amino acids stick together?

Answer 7

They don’t form the knob + socket structure due to steric hindrance (a ridge prevents them for interacting).

Question 8

What causes the RBCs to gain a sickle shape?

Answer 8

The rigid HbS polymer fibres distort the shape of RBCs.

Question 9

Why do carriers (heterozygotes) also have sickled RBCs in their blood films?

Answer 9

If a person has one copy of the sickle cell allele, half of their red blood cells will be misshapen (the HbS allele is codominant for the phenotype).

Question 10

Why do sickle cells function normally when oxygenated?

Answer 10

When oxygen binds to Hb, it causes a conformational change which changes the position of amino acids (there is no longer a hydrophobic pocket). This is positive co-operativity.

Question 11

What tends to cause sickling?

Answer 11

• Acidosis (e.g. build up of lactic acid)

- Low flow of blood

Question 12

What can be seen in the blood film of a sufferer of sickle cell anaemia?

Answer 12

• High concentration of reticulocytes (large, immature RBCs)
• Target cells (abnormal RBCs that have Hb in their centre of pallor)
• Sickled cells
• Normal RBCs (oxygenated sickled cells)
• Howell jolly bodies

Question 13

What can be seen in the blood film of a carrier of the sickle cell trait?

Answer 13

• Normal RBCs
• Reticulocytes
• Howell jolly bodies (RBCs with nuclear fragments)

Question 14

Why are reticulocytes and target cells found in sickle cell blood films?

Answer 14

• The number of immature RBCs is very high because the bone marrow has to produce more RBCs to compensate for the defective sickled cells.
• The spleen also becomes overworked so it cannot remove the defective cells.

Question 15

Why are Howell-jolly bodies found in the blood film?

Answer 15

The spleen becomes overworked and is unable to remove all defective cells.

Question 16

What are the symptoms of SCD?

Answer 16

• Vision loss
• Stroke
• Bone crises
• Splenomegaly

Question 17

Why can SCD cause vision loss?

Answer 17

• Sickle cells can aggregate and occlude the small blood vessels which can cause damage to retina tissues (ischaemia).
• Ischaemia can also result in the release of chemokines which stimulate the growth of new blood vessels (angiogenesis forming collaterals) in an effort to maintain oxygen supply. These collaterals can grow across the eye/ burst as they are very fragile.

Question 18

What is the cause of bone crises?

Answer 18

There is reduced blood flow to the bone marrow due to sickling within blood vessels (microvascular occlusion) which means cells become deprived of oxygen. Therefore, osteonecrosis/ infarction may occur.

Question 19

What is splenomegaly?

Answer 19

An enlarged spleen due to microfarctions caused by sickle cell occlusions.

Question 20

What are the different types of stroke?

Answer 20

• Silent stroke
• Ischaemic stroke
• Haemorrhagic stroke

Question 21

What is the cause of ischaemic strokes?

Answer 21

Cerebral arteries feeding the brain with blood can become blocked due to the sickling leading to deprivation of brain tissue.

Question 22

What is a silent stroke?

Answer 22

Asymptomatic strokes caused by the occlusion (caused by the sickling) of one of the smaller arteries.

Question 23

What can cause haemorragic strokes?

Answer 23

When the fragile collaterals produced by angiogenesis rupture, it can bleed into the surrounding brain, damaging brain cells.

Question 24

What is the difference between acute and chronic pain? Use SCD as an example.

Answer 24

Acute pain is pain that goes away quickly like pain from crises/ bone infections. Chronic pain is ongoing and can worsen over time. This may be caused by the loss of organ function or joint replacements.

Question 25

What can trigger bone crises?

Answer 25

• Dehydration
• Cold weather
• Stress
• Exercise

Question 26

How can dehydration cause a crisis?

Answer 26

It can cause coagulation as there’s less fluid in the blood. This means it’d be easier for sickled cells to aggregate and occlude blood vessels.

Question 27

How can excessive stress/exercise cause a crisis?

Answer 27

• Exercise increases deoxygenation of Hb, triggering sickling.
• Stress/exercise can raise blood pressure which causes vasoconstriction. If blood vessels become smaller, it can make it harder for sickled cells to fit in vessels. Therefore, it is more likely that the sickle cells will aggregate and occlude vessels.
• Cortisol levels increase which can suppress the immune system. Blood infections are therefore more likely.

Question 28

Why does cold weather increase the chances of having a crisis?

Answer 28

• Vasoconstriction
• Sickled cells cannot pass through the smaller vessels
• Occlusion

Question 29

What are the different techniques used to diagnose Sickle cell disease?

Answer 29

• Electrophoresis
• Sickle solubility test
• Screening for expecting carrier parents

Question 30

Which sample runs furthest towards the + electrode, HbA or HbS?

Answer 30

HbA because the mutated valine in HbS has no charge so HbS has an overall, less negative charge.

Question 31

Describe the solubility test

Answer 31

1) Take blood sample
2) Add Sodium Dithionite to the blood, a haemolysis buffer. It causes the Hb to become deoxygenated.
3) HbA dissolves readily in the solution but the solution becomes turbid in the presence of HbS (it is an insoluble precipitate).
This test does not differentiate between the sickle cell trait (HbAS) and HbSS.

Question 32

What can be used to alleviate bone crises?

Answer 32

• Analgesia (ibuprofen and opioids like morphine)
• Heat packs
• Hydration
• Antibiotics + folic acid

Question 33

What are the long term/more serious treatments for severe SCD?

Answer 33

• Blood transfusions
• Bone marrow transplants
• Hydroxycarbamide/ hydroxyurea
• Gene therapy
• Psychological help from therapists (for the pain)

Question 34

What is the purpose of blood transfusions and what are the cons?

Answer 34

:) These supply functioning erythrocytes

:( Multiple transfusions may result in hemosiderosis (iron overload) causing damage to the liver, heart and pancreas.

Question 35

What is the purpose of hydroxycarbamide and what are the cons?

Answer 35

:) It stimulates the production of foetal Hb which increases oxygen supply to tissues.
:( It suppresses marrow production of reticulocytes + neutrophils which may increase the risk of infection.

Question 36

What is the purpose of bone marrow transplants and what are the cons?

Answer 36

:) Transplant removes mutated haemopoietic stem cells and replaces then with functional HSC that can generate normal RBCs.

Question 37

Describe ICE for Murewa Abibi

Answer 37

I: “ I’m having a bone crisis. My knees and ankles hurt so I want to be prescribed morphine.”

C: “Ibuprofen is insufficient to alleviate pain. I’m unable to do ballet, work part-time and go out with friends. My education is also being affected. “

E: Prescription of morphine + an empathic doctor willing to discuss concerns