Week 2 - Sickle Cell Flashcards
Familiarise yourself with the scenario
Daniel is an only child of non-consanguineous Afro-Caribbean parents. He is a 2.5 years old male infant who is brought into the Accident and Emergency department by his parents because of a 6 day history of fever, cough, breathlessness, pallor and weakness. His parents report no previous serious medical history and they also report no major family medical history of which they are aware. On examination he has a temperature of 39.5oC measured in the ear (normal 35.8-38oC) he looks pale and his pulse rate is 110 beats per minute, respiratory rate of 32 breaths per min, and his blood pressure was recorded as 90/60mmHg. On further examination there was an enlarged abdomen caused by hepatosplenomegaly. On auscultation of the lung fields rales (crackles) and sonorous wheeze could be heard.
Initial laboratory investigations reported: haemoglobin of 4.5 g/dL (normal 10.5-13.5g/dL)
white cell count of 19 x109/L (normal range 5-17 x109/L (55% neutrophils, 45% lymphocyte),
platelet count of 70 x109/L (normal range 150-400 x109/L)
and an erythrocyte sedimentation rate of 45 mm/hour (normal range 0 – 15 mm/hr).
Renal function tests and urinalysis were normal. A chest radiograph revealed bilateral haziness. Daniel was admitted and the next day dactylitis was observed in both hands.
Further investigations including Hb electrophoresis, showed that Daniel had Hb S of 80%, Hb F: 18%, and Hb A2: 2%. Finally in peripheral blood smear in low oxygen showed sickling of RBC plus a high proportion of nucleated red blood cells, Daniel was diagnosed with sickle cell anaemia and acute splenic sequestration crisis which was associated with acute chest syndrome. He was given wide spectrum antibiotic treatment (cefotaxim and erythromycin) and also had a blood transfusion exchange. The patient was discharged after 8 days as he stable. He and his parents were referred to the haematology and genetic counselling departments.
Explain why Daniel does not develop symptoms until after the 6 months of life
2 marks
Babies still have fetal Hb and t is not until this starts to decline after birth
And is replaced by the adult but mutated Hb that RBC will start to sickle
What is the inheritance pattern of sickle cell disease?
Autosomal recessive inheritance
How can you use a blood smear to diagnose sickle cell anaemia?
You can see under a microscope that the cells look sickled or not
They are only sickled in deoxygenated conditions so you have to create deoxygenated and non-coagulating conditions (avoiding the clotting cascade)
How does hydroxyurea work as a treatment for patients with sickle cell disease?
It works in 2 ways
1) Through the Red Blood Cells.
Red Blood Cells contain haemoglobin carrying oxygen to the body.
People will sickle cell disease have abnormal haemoglobin called sickle haemoglobin
Sickle haemoglobin causes RBC to be me stiff and sticky
The sickle cell can then cause a block in the flow of blood to important organs
Hydroxyurea helps the body make another haemoglobin called fetal haemoglobin which is usually only produced by babies before birth. With higher levels of Hb F, RBC are less likely to sickle and cause problems
2) Hydroxyurea LOWERS the number of neutrophils (infection - getting white blood cells). Increased neutrophil count makes the blood thicker and stickier which increases the chance of sickled cells to get stuck in the veins and cause a blockage
How does the amino acid change, change the structure of the haemoglobin in sickle cell?
Glutamic acid to Valine
Hydrophobic region is exposed in beta subunit
Abnormal protein made
Causes crystallisation into a fibre and reduces the oxygen carrying capacity of RBC
When the RBC come into a low oxygen conc environment - causes sickling
How is electrophoresis used to diagnose sickle cell?
Use an acetate gel, load the blood samples
Differentiate between homozygous and heterozygous states
Loss of charged glutamic amino acid and changes the speed that the sample moves
Haemoglobin has a net negative charge so when you have glutamic acid (negative) and valine (no charge) haemoglobin becomes less negative and travels further down the gel
If positive end on the other side
Normal one travels the most as it is more negatively charged
In the trait you see one normal (Hb-A) and one sickle cell anaemia (Hb-S)
If negative end on other side, Hb-S moves the most as it is less negatively charged
Trait is in the middle
Hydroxyurea is a drug for sickle cell and it is a myosuppression drug, what does this mean?
It kills erythrocyte progenitors
Highly toxic
There is a sharp shock where the erythrocyte progenitor are killed
The drug is then withdrawn
The erythrocyte progenitors grow again and then you get an increase in fetal haemoglobin
Daniel has sickle cell
If he was to marry a woman who did not carry the mutated allele, what would the status of any children be and justify your answer
2 marks
All would be carriers of the mutated allele as Daniel is homozygous and would pass on a mutated copy to all his children (1 mark)
But the wife would pass on the normal allele to all children (1 mark)
Daniel’s blood test had
Erythrocyte sedimentation rate of 45mm/hr (normal range 0-15mm/hr)
What does this indicate?
Erythrocyte sedimentation rate basically puts blood and lets it drop down in a period of one hour
(Blood sediments - settles to the bottom of liquid)
And if the blood is heavier because of the proteins inside of it, it drops quicker
These proteins are indicative of an infection, inflammation and anaemia
Daniel’s blood test had:
Haemoglobin of 4.5 g/dL (normal range 10.5-13.5g/dL)
Platelet count of 70 x10^9/L (normal range 150-400 x 10^9/L)
What does this indicate?
Low haemoglobin levels and RBC
This is because the RBC do not last as long because the sickle haemoglobin damages them
They have fewer red blood cells than normal
Daniel’s blood test had
White cell count of 19 x 10^9/L (normal range 5-17 x 10^9/L)
55% neutrophils, 45% lymphocyte
What does this indicate?
Indicative of an infection
What are some advice for coping with a child with a chronic illness?
- Include all family members in decision makings
o Empowering patients, feel in control, makes situation easier and more bearable and tolerable
§ Don’t want to feel like our choice is being taken away
§ Can lead depression and making the condition worse
Impact on families, wider families and the children.
What are the different subunits of of fetal haemoglobin?
2 alpha
2 gamma
What are the different transitions between fetal and adult haemoglobin?
Alpha chains increases at the yolk sac and then remains at a high level
At birth we start to see a decrease in gamma chains and increase in the beta chains
At 25 weeks after birth we see a large decrease in gamma and we see large increase in beta chains
What are the different treatment options for sickle cell anaemia?
Bone marrow transplant (can cure 85% of children)
Hydroxyurea Antibiotics Pain relievers Blood transfusions Nitric oxide
SEG101 - Crizanlizumab
Anti P selectin monoclonal antibody-adhesion molecule
What are the different ways you can diagnose sickle cell anaemia?
Electrophoresis
Solubility Test
Blood Smear
What are the positives of the solubility test to diagnose sickle cell anaemia?
Cheap (low cost paper based)
Don’t require specialist equipment
100% sensitive
100% specific
What are the properties of fetal haemoglobin?
Fetal haemoglobin has a higher affinity for oxygen than an adult
Can carry up to 30% more oxygen
This is important as the maternal blood’s oxygen is then readily transferred to the fetal blood across the placenta
What are the stats with bone marrow transplants to treat sickle cell anaemia?
85% succes rate in children with sickle anaemia
5-10% fatality risk
What are the symptoms of a sickle cell crisis?
Sudden crisis that are periodic, very painful (worse than cancer pain)
You might have pain in your back, knees, legs, arms, chest or stomach
Pain may last for several hours
What are the triplet codes for the stop codons?
UAA
UAG
UGA
What does a non-conservative missense mutation mean?
Change in amino acid that does not have similar properties to the original one
What does it mean for the genetic code to be degenerate? What are the exceptions?
Multiple triplet codes make the same amino acids
Start codon methionine and tryptophan
What is dactylitis and why is it a common symptom of sickle cell?
Dactylitis is severe pain that affects the bones of the hands, the feet, or both. It’s often the first symptom of sickle cell disease in babies.
Dactylitis is caused by blocked blood circulation.
What is hydroxyurea?
It is a medication given to patients with sickle cell anaemia
Recommended to people who have had two or more episodes of sickle cell crisis or acute chest syndrome in their life
What is the amino acid change in sickle cell?
Glutamic acid to valine
What is the methodology of using a blood smear to diagnose sickle cell anaemia?
You take a blood sample
Add EDTA to stop the blood from clotting by removing the divalent cations (stopping the clotting cascade)
Add 2% sodiu metabisulphite to create deoxygenated conditions
(Cells don’t sickle in oxygenated conditions)
What is the parasite that causes malaria?
Plasmodium falciparum
What is the triplet code of the start codon?
AUG
Methionine
What subunits of haemoglobin are encoded on chromosome 11?
Beta
Gamma
Epsilon
Zeta
What type of genetic mutation causes the sickle cell mutation and explain how this causes a change in the amino acid sequence?
3 marks
Point mutation a single nucleotide change (A to T) (1 mark)
This changes the three amino acid codon sequence so that the amino acid coded changes (1 mark)
from glutamic acid to valine. (1 mark).
What was the previous life expectancy of sickle cell anaemia? What is the life expectancy now?
20-40 years
Now its basically normal with treatments available
Which chromosome are beta subunits of haemoglobin encoded on?
11
Which chromosome is the alpha subunit for haemoglobin encoded on?
16
Why do you get hepatosplenomegaly with sickle cell?
Pooling of sickling cells, accumulating in the spleen and liver
Cells start to die within these organs causing splenic atrophy and portal hypertension
Why is Hb electrophoresis used to diagnose sickle cell disease?
2 marks
The change in the amino acid changes the electrical charge to be less negative (1 mark).
This means that the Hb travels at a different speed to normal as travels slower than normal. (1 mark)
What are the stats of sickle cell trait protecting children in Western Kenya?
60% protection against mortality in
2-16 months
What is the mechanism protection by sickle cell trait against P. falciparum?
When the parasite bites the RBC, there is an increased O2 consumption, the RBCs also stick to the endothelium.
This is because the parasite puts protein knobs called PfEMP1 on the surface of the RBC. These protein knobs will interact with other cell adhesion molecules like ICAM and makes the RBC sticky.
Increased O2 causes RBC to sickle if you have HbS and the sickle cell trait
There is less adherence to the endothelium due to the sickling, so the RBCs are removed by macrophages
When the RBCs die, the parasites die with it too, so less parasite growth and maturation
In the trait the protein knobs there are fewer knobs that are not evenly distributed and are concentrated on certain bits, this reduces adhesion and therefore if you have sickle cell trait, the RBC are less sticky and detach and therefore die while the normal RBC can carry enough oxygen.
A mutation in HbS (haemoglobin gene)
I) is what inheritance?
II) Causes what disease if homozygous?
III) Provides what advantage if heterozygous?
I) Autosomal recessive
II) Sickle cell anaemia
III) Resistance to malaria
A mutation in CFTR (gene encoding the CFTR sodium channel)
I) is what inheritance?
II) Causes what disease if homozygous?
III) Provides what advantage if heterozygous?
I) Autosomal recessive
II) Cystic fibrosis
III) Resistance to tuberculosis? Cholera?
