Daniel is a 2.5 year old baby, born to parents of Afro-Caribbean origin, who was diagnosed with sickle cell anaemia. He was admitted into hospital due to a six day history of fever, breathlessness and dactylitis – symptoms associated with the disease. Laboratory results showed he had a low concentration of haemoglobin and a high white blood cell count. He was given antibiotics and a blood transfusion as means of treatment and his parents were referred to the genetic counselling department.
This write-up aims to cover what sickle cell anaemia is, its pattern of inheritance, symptoms, diagnosis and treatment. It will also touch upon topics such as support strategies and the switching of haemoglobin at birth.
Sickle cell anaemia is an inherited autosomal recessive condition that is caused by a mutated beta globin gene in haemoglobin, thus it affects the red blood cells (RBCs). As the disease is recessive, you will need to inherit two copies of the mutated gene (HbS, instead of HbA) from both parents in order to express phenotype. If you inherit one mutant gene, it means you have the sickle cell trait, a heterozygous genotype, and are a carrier (GJ, et al., 2001). This explains why Daniel’s parents reported no major family medical history of similar symptoms. His parents had a 25% chance that their child would be born with sickle cell anaemia, as can be seen by figure 1.
The mutation for sickle cell anaemia is most prevalent amongst people in malaria-stricken areas, where it affects approximately 7% of the population (Erekberg ; Palmer, 2018). It provides protection against the malaria parasite which reproduces in the RBC, as the sickle red blood cell will be fragile and more prone to rupture (Jr, n.d.). As Daniel is a child of non-consanguineous Afro-Caribbean parents, this could be a contributing factor as to why he inherited the condition.
A non-conservative missense point mutation in the beta haemoglobin gene on the 11th chromosome, changes the thymine to an adenine, so subsequently adenine to uracil in mRNA. This results in transcription of a valine amino acid instead of glutamic acid, changing the primary structure and so the function of the haemoglobin protein (Figures 2 and 3). Although the genetic code is degenerate, as this is a missense mutation, a different amino acid is coded for so properties do not resemble the original. This leads to a distorted crescent shaped RBC with different properties of the haemoglobin (HbS).
RBCs are usually disc-shaped and flexible so can move easily through blood vessels. However due to the tertiary structure changing, in sickle cell they are rigid, sticky and crescent-shaped. Glutamic acid, which the amino acid usually coded for is charged, polar and tucks into haemoglobin. The valine in sickle cell is hydrophobic, non-polar and sticking out of the haemoglobin, resulting in the abnormal shape and clumping of long non-liquid fibres (Figures 4 and 5) (Al-Karadaghi, n.d.).
Sickle cells block the arteries due to their shape, so stop blood flow to certain areas in the body, which is the primary reason for the majority of the symptoms showed by patients. By obstructing blood vessels, they cause organ damage as a result of oxygen deprivation. This can lead to vision problems as the tiny blood vessels supplying your eye may be blocked, damaging the retina and could potentially lead to blindness or clog blood vessels in the brain, causing a stroke.
The sickle cells can stick to walls of the blood vessels in the spleen, causing it to swell with blood and become extremely painful. This is called splenic sequestration and can be removed by surgery for children (GJ, et al., 2001). This can be accompanied with enlargement of the liver, known as hepatosplenomegaly, which results in an enlarged abdomen, a symptom Daniel expressed.
As the liver and spleen are both organs that detoxify the blood and work as part of the lymphatics in the immune response, swelling of these organs can lead to reduced function (Franks ; Falck, 2017). This means affected individuals are more vulnerable to infection, as antibodies are not created against bacteria such as pneumococcus, so pathogens are not dealt with effectively. This could explain why Daniel’s white blood cell count was higher than normal (19 x109/L instead of 5-17 x109/L) to fight infection.
Daniel was also diagnosed with acute splenic sequestration crisis, which is associated with acute chest syndrome. This is caused by inflammation and blockages of the blood vessels in the lungs and the symptoms include difficulty breathing, and fever, some of which Daniel suffered from. He showed signs of a fever (39.5 when normal should be 35.8-38), cough, breathlessness (pulse rate of 110 beats per min, respiratory rate of 32 breaths per min and blood pressure is 90/60mmHg), pallor and weakness (Miller, 2018).
The symptoms of paleness and a fast heartbeat could also be a result of aplastic crisis, where the body is not making enough red blood cells, also leading to a person feeling tired. This is explained by Daniel’s low haemoglobin level (4.5 g/dL instead of normal range 10.5-13.5g/dL), which is due to sickle cells dying after 10-20 days, instead of the 90-120 days (NIH, n.d.).
Dactylitis was another symptom Daniel displayed, which is swelling of the fingers and toes. This is caused by continued insufficient blood circulation to the bones in the fingers and is usually accompanied with joint inflammation (Jr, n.d.).
There are many different diagnosis investigations that can be carried out to confirm sickle cell anaemia, including haemoglobin electrophoresis, peripheral blood smears and a solubility test, as mentioned in the scenario. Screening and frequent blood test can also be carried out at any time to check for sickle cell or the risk of the disease.
The primary form of diagnosis is electrophoresis as it excludes other conditions. It measures the levels of the different types of haemoglobin in the blood, compared to normal haemoglobin. An electric current is applied through the sample; the different types of haemoglobin will travel and separate characteristically into different bands along the gel, depending on its shape, charge on the protein surface and the amount present. This can be used to differentiate between homozygous and heterozygous states for sickle cell. The investigation into Daniel’s results suggested he had sickle cell as he had Hb S of 80%, Hb F: 18%, and Hb A2: 2%, when in normal blood HbS should not be present (Carr, 2014).
The change in the amino acid means the negatively charged glutamic acid is no longer present, but a neural so less negative valine replaces it. This means the sickle cell haemoglobin will travel slower and less distance in the gel than the normal haemoglobin, as it will be repelled less by the electric current being passed through it (Figure 6 and 7) (Carr, 2014).
A solubility test can be used to assess the presence of HbS, but it does not tell us if the genotype is homozygous or heterozygous. It works by seeing how well haemoglobin dissolves in blood plasma. Blood is drawn, a buffer is added along with a reducing agent such as sodium dithionite, which breaks open the RBCs, releasing the haemoglobin. Haemoglobin A, the normal haemoglobin, dissolves easily in to the liquid, resulting in a clear red plasma. On the other hand, haemoglobin S does not dissolve well and forms small crystals that make the plasma cloudy, as can be seen in figure 8. Any cloudiness in the test tube is indicative of either the trait or disease (Chaudhary, 2017).
A blood smear can also be used to look for abnormalities in blood cells. It tells you about the shape and number of each type of cell to diagnose conditions. The results are mainly evident in low oxygen, where the cells sickle and there is a high proportion of nucleated RBCs, as seen in Daniel’s case (Sampson ; Krause, 2017).
Usually people with sickle cell died between 20-40 years, however with better treatment, they can live longer. Basic treatment to relieve pain include keeping warm, drinking fluids, avoiding exercise, alcohol and smoking, as well as taking painkillers such as ibuprofen. Antibiotics such as penicillin may be prescribed to prevent infections such as pneumonia and acute chest syndrome, as the spleen would not be able to help mount an adequate immune response. Daniel was prescribed antibiotics such as cefotaxime and erythromycin, which protects the body from a wide range of bacterial infections.
Treatment options also include nitric oxide gas, which keeps the blood vessels open, preventing the sticky sickle cells attaching the walls (Chaudhary, 2017). Crizanlizumab, a monoclonal antibody may also be prescribed. It works by binding to P selectin on the endothelial cells that line the blood vessels and platelets, which usually regulates the flow and adhesion of WBCs to vessel walls during an infection. This immune response can be caused by sickle cells blocking the blood flow. Crizanlizumab binds to the P selectin to prevent the adhesion molecule starting the blockage, maintaining blood flow (News, n.d.) Hydroxyurea is an oral medication that could also be prescribed to suffers as a protection against the effects of HbS. It promotes the production of foetal haemoglobin and reduces inflammation and frequency of painful episodes but can also cause WBCs and platelet numbers to drop. (Agrawal, et al., 2013)
The only way to potentially cure sickle cell would be to have a bone marrow or stem cell transplant. A HbS producing bone marrow is replaced with one that produces normal HbA RBCs. Healthy stem cells can be intravenously injected into the bloodstream and migrate towards the bone marrow. However, a matching donor must be available or it can lead to rejection, infection or an immune response.
Daniel was also treated with a blood transfusion. This increases the number of normal RBCs in circulation, increasing the life span of the blood cells and reduces risk of blockages ensuring oxygen is delivered to the body.
Both parents of a new-born and prospective parents with the disease or trait should be offered genetic counselling, as Daniel’s parents were. This will help them make well informed decisions and have a better understanding of the genetic basis of the disease, related health problems, face the possibility of early death and future family planning (if they carry the trait).
For parents who have a child with the disease, lifestyle changes and a care plan can be implemented to regulate severe symptoms and complications associated with sickle cell anaemia. For example, as splenic sequestration crisis can arise, guardians should “palpate a child’s spleen” daily to check if it has increased in size (NIH, n.d.). They should have access to support services, regular visits and screening tests for evaluation from medical staff to help them recognise and reduce the pain.
Other suggested activities to learn about the disease include joining support groups with parents who also have children suffering from the disease as a way to learn and gain advice from their experiences. They should also maintain good communication within the family as well as with the health care professionals to provide the utmost care especially during times of emergency .This will allow them to support and comfort each other, as well as the child as having the condition not only affect the child but also the extended family.
Haemoglobin is a protein in the RBCs that binds to oxygen and transports it around the body. It is made of four iron containing haem prosthetic groups and is surrounded by a protein called globin, depending on the type of haemoglobin (Kumar, n.d.). The globin in foetal haemoglobin is made of two alpha (chromosome 16) and two gamma globin chains, whereas adult haemoglobin is made of two alpha and two beta globin (chromosome 11).
Infants who are affected with sickle cell usually develop symptoms after a couple of months, because foetal haemoglobin is still present in their blood, so the effect of the sickled cells is little (Figures 9 and 10). After 6 months, the sickled red blood cells are more prominent and foetal haemoglobin declines, causing the symptoms to manifest, as can be seen in Daniel’s case (Jr, n.d.). Before birth and for the first few month, there is only foetal haemoglobin; it does not have the beta subunit, which is affected in sickle cell mutation, so a young baby would not show symptoms.
The locus control region regulates the switching off of haemoglobin genotypes during development, by opening and remoulding the chromatin structure (bot, 2016). It could lead to deletion which would silence certain globin genes, such as beta or demethylate CpG sites in silenced gamma globin to increase foetal haemoglobin production.
The oxygen dissociation curve is a graph that shows the relationship between the percentage oxygen saturation and partial pressure of oxygen (Figure 11). It is a sigmoidal graph, illustrating how it is easier for subsequent oxygens to bind to haemoglobin after one has bound (co-operative binding). At high partial pressure, the haemoglobin has high affinity for oxygen, so binds to form oxyhaemoglobin and lower affinity when partial pressure is low, promoting oxygen unloading.
The foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, due to the different subunits. It carries up to 30% more oxygen, because the foetus gains its oxygen across the mother’s placenta where partial pressure of oxygen is low, so the dissociation curve is shifted to the left (Figure 12). The number of foetal haemoglobin falls after birth as oxygen is drawn from the lungs instead, where there is a high partial pressure of oxygen.
If haemoglobin didn’t change, we would have a poor supply of oxygen to the cells, because foetal haemoglobin would be less able to dissociate than normal haemoglobin within the body and to the tissues. As a result, oxygen transfer would be much slower due to the high affinity binding to the oxygen, as it is not released a easily.
In conclusion, sickle cell anaemia is an inherited condition caused by a mutation in the gene for beta haemoglobin in HbA, leading to the formation of sickle RBCs, HbS. These can cause disastrous effects by blocking blood vessel, depriving organs of oxygen and leading to organ failure or damage. Symptoms do not usually manifest until after the first six months when foetal haemoglobin declines, but adult haemoglobin, where the mutation occurs, is expressed. Sickle cell diagnosis is primarily confirmed by carrying out haemoglobin electrophoresis, but can be detected by a solubility or blood smear test. The symptoms can be relieved by a number of medications, such as antibiotics or hydroxyurea or completed treated by a bone marrow or stem cell transplant.