Sickle Cell Disease

Question 1

What mutation is responsible for sickle haemoglobin?

Answer 1

Missense mutation at codon 6 of the beta globin gene
=> glutamic acid replaced by valine
=> In HbS there are TWO normal alpha chains and TWO variant beta chains

Question 2

The distribution of the mutant beta globin gene corresponds to areas in which what deadly disease has been endemic?

Answer 2

Plasmodium falciparum (malaria)

Note: Up to 25% Africans (sub-Saharan) and 10% Caribbeans carry sickle gene

Question 3

Describe the difference in character between the amino acids thatchange in this mutation.

Answer 3

Glutamic acid = polar + soluble (negatively charged)

Valine = non-polar + insoluble

Question 4

How does the HbS cause the red cell to sickle?

Answer 4

• The effects of valine substitution are only seen in the deoxyhaemoglobin S conformation, which becomes insoluble.
• deoxyHbS polymerises to form fibres called tactoids, stabilised by intertetrameric contacts.
• This polymerisation causes the distortion/sickling of the cells

Question 5

State the 3 stages associated with the sickling of red cells

Answer 5

1. Distortion (and increased rigidity):
   - Polymerisation initially reversible with formation of oxyhaemoglobinS
   - Subsequently irreversible
2. Dehydration
3. Increased adherence to the vascular endothelium

Question 6

Originally, it was not understood why sickling causes such profound clinical problems because it appeared that the normal transit time of red blood cells is sufficient for the red cells tobecome reoxygenated and for the polymers to be broken downbefore much sickling takes place. What key feature of sickle cells explains their ability to cause such problems?

Answer 6

The sickle cells are more adherent to the vascular endothelium so they stick to the vessel walls and increase their transit time
This allows more time for the polymerisation to occur

Question 7

What is the difference between sickle cell disease and sickle cell anaemia?

Answer 7

Sickle cell disease = generic term that encompasses all disease syndromes due to sickling

• Compound heterozygous states e.g. SC, Sb thalassaemia
• Sickle cell anaemia = homozygous (SS)

Question 8

What effect does sickling have on the lifespan of red blood cells?

Answer 8

As the cells are distorted, the body more avidly removes them (increased haemolysis)
They have a lifespan of around 20 days

Question 9

What are the consequences of increased haemolysis?

Answer 9

Anaemia
Gallstones
Aplastic Crisis

Question 10

Other than the increased break down of red blood cells, what else is partly responsible for anaemia in sickle cell patients?

Answer 10

There is reduced erythropoietic drive as HbS has a low affinity for oxygen so it delivers the oxygen more effectively to tissues
So hypoxia doesn’t stimulate EPO release from the kidneys as much

Question 11

Why does increased haemolysis cause gallstones? By 25 years, prevalence
of gallstones in sickle cell anaemia patents = ?

Answer 11

Increased haemolysis means increased release of bilirubin and other red cell breakdown products
These get excreted through the biliary tract and carry a risk of causing gallstones

= 50%

Question 12

What is Aplastic Crisis caused by? How can sickle cell disease lead to it?

Answer 12

Aplastic crisis is caused by Parvovirus B19 infection (a common respiratory virus)
The virus infects developing red cells in the bone marrow and blocks their production
This doesn’t have much effect on normal people with a 120-day red cell lifespan
But because the lifespan of red cells in sickle cell disease is so low, a parvovirus infection could cause a steep drop in haemoglobin (anaemia)

Question 13

What genetic modifier can increase the risk of getting gallstones in patients with hereditary haemolytic anaemia (like sickle cell disease)?

Answer 13

Coinheritance of Gilbert’s Syndrome

Question 14

What is this syndrome caused by?

Answer 14

Reduced activity of UGT (UGT 1A1)
Caused by an extra TA dinucleotide in the promoter on each chromosome (there are normally 6 TA repeats)
It increases the risk of gallstones 3-5 fold

Question 15

Why does blockage to microvascular circulation cause infarction? What is it associated with?

Answer 15

• Distorted sticky red cells can clog up blood vessels and lead to tissue damage and necrosis.
• It is associated with severe pain and loss of function

Question 16

Blockage of the microvascular circulation is a major problem in sickle cell disease. State three tissues that are commonly infarcted and the consequences of infarction in these tissues.

Answer 16

1. Spleen:
   - Leads to hyposplenism which causes increased risk of life-threatening infection by capsulated bacteria (mainly pneumococcal)
2. Bones and joints:
   - dactylitis
   - avascular necrosis (especially of the femoral head due to blood supply cut off to bone)
   - osteomyelitis (due to bone infection)
3. Skin:
   - ulceration

Question 17

How can sickle cell disease affect the lungs?

Answer 17

Acute chest syndrome (vaso-occlusive crisis of the pulmonary vasculature)

• New pulmonary infiltrate on chest X-ray with symptoms of Fever, Cough, Chest pain, Tachypnoea
• most common cause of death in adults with SCD

Chronic damage
Pulmonary hypertension

Question 18

Describe the likely mechanism by which sickle cell disease gives rise to pulmonary hypertension

Answer 18

• Pulmonary hypertension correlates with the severity of haemolysis
• The likely mechanism is that the free plasma haemoglobin resulting from intravascular haemolysis scavenges NO and causes vasoconstriction

Question 19

How can sickle cell disease affect the urinary tract?

Answer 19

• Haematuria (due to papillary necrosis)
• Hyposthenuria (inability to control urine concentration – can lead to dehydration)
• Renal failure
• Priapism (painful erections)

Question 20

How can sickle cell disease affect the brain?

Answer 20

Stroke

Cognitive impairment

Question 21

How can sickle cell disease affect the eyes?

Answer 21

Proliferative retinopathy

Question 22

What are the early presentations of sickle cell disease?

Answer 22

• Painful dactylitis (most common in children)
• Splenic sequestration (accumulation of red cells in the spleen leading to a drop in blood count)
• Infection (S. pneumoniae)

Question 23

When do the symptoms roughly start? What does their onset coincide with?

Answer 23

Symptoms rare before 3-6 months

Onset coincides with switch from fetal to adult Hb synthesis

Question 24

What are the observations in the case for a sickle emergency?

Answer 24

• Septic shock (v. low BP)
• Neurological signs or symptoms
• SpO2 <92% on air
• Symptoms/signs of anaemia
• Priapism >4 hours

Question 25

What are painful crises and what could they be triggered by?

Answer 25

They are episodes of extreme pain caused by blood vessels becoming occluded. 
They can be triggered by: 
- Dehydration 
- Exertion 
- Hypoxia 
- Infection 
- Psychological stress

Question 26

State some general measures taken for patients with sickle cell disease.

Answer 26

• Folic acid
• Vaccines
• Penicillin
• Monitor spleen size
• Blood transfusions for acute anaemia events, acute chest syndrome and stoke

Question 27

Describe how to manage a painful sickle cell crisis.

Answer 27

• Pain management (opioids)
• Hydration
• Keep warm
• Oxygen if hypoxic
• Exclude infection by examining blood and urine cultures, and CXR

Question 28

What SCD complications are transfusions used to prevent?

Answer 28

Stroke

Acute chest syndrome

Question 29

In which group of patients is haematopoietic stem cell transplantation effective and just how effective is it?

Answer 29

Young patients (< 16 yrs with severe disease)

Curative in 85-90% with Survival 90-95%; But relatively few children will find perfectly matched donors

Question 30

Name a drug that is used to induce HbF in sickle cell patients and explain the principle behind this treatment. What chemical also has a similar effect?

Answer 30

Hydroxyurea/Hydroxycarbamide

• It is a ribonucleotide reductase inhibitor - cytotoxic
• It induces the production of red cells in the bone marrow that mainly contain HbF
• So, over time, there will be an increase in the number of red cells that are unable to sickle
• It significantly reduces the frequency of crises

Butyrate also has a similar effect

Question 31

State some laboratory features of sickle cell disease.

Answer 31

Hb LOW (typically 6-8 g/dL)

Reticulocytes HIGH (except in aplastic crisis)

Blood film:

• Sickle cells
• Boat cells
• Target cells
• Howell-Jolly bodies

Question 32

Describe two methods of diagnosing sickle cell disease.

Answer 32

1. Solubility Test:
   - Blood sample is mixed with a reducing agent
   - Oxyhaemoglobin is converted to deoxyhaemoglobin
   - Solubility decreases (due to the presence of deoxyHbS) => the solution becomes TURBID
   - Note that this does NOT distinguish between HbS and sickle cell trait (HbAS)
2. Electrophoresis/HPLC (confirms diagnosis):
   - takes advantage of the difference in charge between HbS, HbAS and HbA

Question 33

Describe the features of a patient with sickle cell trait (HbAS)

Answer 33

• Normal life expectancy
• Normal blood count
• Usually asymptomatic
• Rarely painless haematuria (due to papillary necrosis)
• Under conditions of hypoxia (e.g. anaesthesia, high altitude, extreme exertion) they may have some complications due to sickling of red cells

Question 34

How does the drug Crizanlizumab help to lower crisis rate?

Answer 34

It inhibits P-selectin, which is one of adhesion molecules on RBC