Sickkle Cell Anaemia

Question 1

What is the structure of HbA?

Answer 1

2 alpha chains + 2 beta chains —> arranges tetrahedrally

4 haem groups

Question 2

What happens to haemoglobin once O2 bind to it?

Answer 2

Conformational change —> changes shape —> increases binding capacity to O2

Question 3

What causes HbS?

Answer 3

Autosomal recessive mutation in the beta globin (HBB) gene

—> non-conservative missense mutation
—> 6th amino acid (glutamate) swapped w/ valine —> hydrophobic —> promotes binding of Hb molecules —> sickle shape

Question 4

What happens to HbS when it is bound to O2?

Answer 4

Nothing, carries O2 as normal

Question 5

What happens to HbS when it is deoxygenated?

Answer 5

Beta globin chains change shape —> Hb clumps together —> chain —> bends into a sickle shape

Question 6

What factors can escalate the condition?

Answer 6

Acidosis

Low O2 conc.

Vessels w/ small radius

Question 7

Hoe does acidosis escalate the condition?

Answer 7

Blood pH decreases —> increases sickling

E.g from exercise —> lactic acid build up in tissue

Question 8

How does low O2 conc. escalate the condition?

Answer 8

When Hb deoxygenated —> changes shape —> clumps together —> sickle shape

Question 9

How do vessels w/ small radius escalate the condition?

Answer 9

Sickle cells not as flexible as normal RBCs

Build up of sickle cells in blood vessels —> blockage of blood flow —> can’t deliver blood to tissues —> no O2 —> infarction

Question 10

What is electrophoresis?

Answer 10

Technique (using electricity) to separate molecules based on their charge and mass

Question 11

What are the charges of glutamate and valine?

Answer 11

Glutamate = negative

Valine = neutral

Question 12

What is the charge of HbS compared to HbA?

Answer 12

HbS is less negative than HbA

—> valine is neutral

Question 13

What would you see in electrophoresis of sickle cell anaemia, sickle cell trait and normal?

Answer 13

HbA moves furthest towards the positive

HbS moves less towards the positive

Sickle cell trait —> two bands (HbA/HbS) —> autosomal recessive

Question 14

What cells would you find in a blood film analysis?

Answer 14

Reticulocyte

Target cells

Normal RBCs

Sickle cells

Question 15

Why would you find a higher than normal reticulocyte count in a blood film analysis?

Answer 15

Defective cells can’t carry O2 as well

Bone marrow has to produce more RBCs

Reticulocytes are immature but can still carry O2 —> not as well

Question 16

Why is it bad having a higher than normal count of reticulocytes?

Answer 16

Don’t have flexibility of RBCs + bigger than normal RBCs —> can get stuck or broken

Question 17

Why are there target cells in the blood film?

Answer 17

Have layer of decreased density in middle —> Hb spread out to sides due to binding of Hb

Question 18

What are some symptoms of sickle cell?

Answer 18

Vision loss

Stroke

Bone crisis

Splenomegaly

Question 19

Why does sickle cell cause vision loss?

Answer 19

Sickle cells accumulate in microvasculature of the retina

Pressure increases

Blood vessels become damaged

Ischaemia causes chemokine release —> tells eyes to produce more blood vessels

Angiogenesis causes collateral formation

Collaterals are easily damaged and can grow across areas of vision

Question 20

What 2 type of strokes are caused?

Answer 20

Ischaemic

Haemorrhagic

Question 21

How does an ischaemic stroke occur?

Answer 21

Sickle cells block blood vessels

Lack of blood supply leading to ischaemia and infarction

Silent strokes are asymptomatic

Question 22

How does an haemorrhagic stroke occur?

Answer 22

Occurs due to angiogenesis

Collaterals are easily broken

Causing blood to leak out into tissues

Question 23

Why does sickle cell cause bone crisis?

Answer 23

Reduced blood flow to bone marrow due to microvascular occlusion

Prolonged ischaemia leads ro infarction

Ischaemia exacerbates sickling

Question 24

How does sickle cell cause splenomagaly?

Answer 24

Spleen has a very good blood supply —> lots of blood coming in but tiny vessels —> lots of blockages —> lots of blood in spleen

—> enlarged spleen

Question 25

What can we use to diagnose sickle cell?

Answer 25

Electrophoresis

Screening —> pregnant women, genetic counselling

Sickle solubility test

Question 26

What is the sickle cell solubility test?

Answer 26

Take blood

Add sodium dithionite as a reducing agent

Hb released

HbA dissolves easily in blood plasma

HbS less soluble causing solution to become turbid (cloudy)

Question 27

How can we manage sickle cell?

Answer 27

Blood transfusions

Managing crisis

Carboxycarbamide

Bone marrow transplant

Antibiotics

Gene therapy

Question 28

How do blood transfusions work?

Answer 28

Replacing defective blood cells w/ healthy blood cells

—> hemosiderosis —> iron overload —> toxic

Question 29

How can you manage a crisis?

Answer 29

Painkillers, opioids

Hydration

Keeping warm

Avoiding high altitudes and low O2 levels

—> opioids are highly addictive

Question 30

What is hydroxycarbamide?

Answer 30

Stimulates synthesis of HbF —> higher affinity for O2 than HbA

Supresses reticulocyte and neutrophil production

—> increased risk of infection —> lower neutrophil count

Question 31

How do bone marrow transplants work?

Answer 31

Replacing mutated Haemopoietic stem cells w/ HSC that are not mutated

—> very invasive
—> only available to young patients
—> matched donor needs to be found

Question 32

What are the 2 ways of using CRISPR -Cas9 gene editing?

Answer 32

1. Inject Cas9 enzyme into target —> enzyme repairs faulty beta-globin chain —> swaps T w/ A —> gene repaired
2. Cas9 block/inhibits gene that blocks HbF production —> greater production of HbF —> higher affinity to O2