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World Sickle Cell Day 2023: Sickle cell disease is a group of inherited red blood cell disorders characterised by abnormal haemoglobin, because of which red blood cells come hard and sticky, and appear like a sickle. Red blood cells must be round, and therefore, sickle-celled ones die early, resulting in a constant shortage of red blood cells in the bodies of those with a sickle cell disease.
When sickle cells travel through small blood vessels, they get stuck and clog the blood flow, resulting in pain and serious health problems such as infection, stroke and acute chest syndrome.
Causes of different types of sickle cell disease
Sickle cell disease can be of different types, depending on the genes a person inherits from their parents. When a person inherits genes that code for abnormal haemoglobin, they can suffer from sickle cell disease.
The most common types of sickle cell disease are HbSS, HbSC and HbS beta thalassaemia. HbSD, HbSE and HbSO are rare forms of sickle cell disease.
Sickle cell disease causes severe symptoms when a person inherits the “S” gene, which codes haemoglobin S, from both the parents, or the “S” gene from one parent and a gene for beta thalassaemia from the other parent.
When a person inherits an “S” gene from one parent and a “C” gene, which codes for a different type of abnormal haemoglobin called haemoglobin C, from the other parent, they have a milder form of sickle cell disease.
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HbSS is the form of sickle cell disease in which a person inherits the gene coding for haemoglobin S from each parent, as a result of which the person’s body only codes for haemoglobin S. This causes the red blood cells to become rigid and sickle-shaped. HbSS disease is also known as sickle cell anaemia, and is the most severe form of sickle cell disease.
HbSC sickle cell disease is the one in which a person inherits the gene coding for haemoglobin S from one parent, and the gene for haemoglobin C from the other parent.
In the case of HbS beta thalassaemia, a person inherits the gene coding for haemoglobin S from one parent, and a gene for beta thalassaemia from the other parent. Thalassaemia is an inherited blood disorder in which the body does not make enough haemoglobin, because of which there is a lack of red blood cells.
Haemoglobin is made of two different parts, called alpha and beta, and the type of thalassaemia a person suffers from is named based on the part of haemoglobin that is not being made in their body. This means that in a person with beta thalassaemia, the beta part of haemoglobin is not made, as a result of which there are not enough building blocks to make normal amounts of haemoglobin.
The words trait, minor, intermedia and major are used to describe how severe the thalassaemia is. A person with thalassaemia may not have any symptoms, or may suffer from mild anaemia. Meanwhile, a person with thalassaemia major usually has severe symptoms.
If a person receives a beta thalassaemia trait from each parent, they will have beta thalassaemia major. If a person receives a beta thalassaemia trait from one parent, and normal parts from the other parent, they have beta thalassaemia major. It is caused by the beta-globin gene, and there is a total absence of beta chains.
In the case of beta thalassaemia minor, only one gene for beta thalassaemia is inherited, because of which reduced amounts of beta chains are produced.
Beta thalassaemia major is also known as beta-zero thalassaemia.
Beta thalassaemia minor is also called beta-plus thalassaemia.
Therefore, HbS beta thalassaemia is of two types: HbS beta⁰ and HbS beta+.
A person suffering from HbS beta⁰ thalassaemia has a severe form of sickle cell disease, because that person not only codes for S haemoglobin, but also does not produce beta chains. Meanwhile, a person suffering from HbS beta plus thalassaemia usually has a milder form of sickle cell disease.
ABP Live spoke to Dr Monisha Harimadhavan from Amrita Hospital, Kochi, and Dr Nitin Agarwal from DKMS BMST Foundation India, and asked them about sickle cell disease and its treatment.
Dr Harimadhavan is a consultant at the Department of Haematology and Bone Marrow Transplantation, Amrita Hospital, Kochi, and Dr Agarwal is the HoD of Donor Request Management, DKMS BMST Foundation India.
“Sickle cell disease can be asymptomatic; symptomatic, as in the case of sickle cell trait; and severe, as in the case of sickle cell anaemia. The most common symptoms are severe episodes of pain, which can occur in different body parts, jaundice due to increased destruction of the red blood cells, and anaemia due to decreased haemoglobin, resulting in fatigue and breathlessness. There is an increased requirement for regular blood transfusions, and an increased susceptibility to infections and organ damage. Sickle cell disease can also coexist with other genetic haemoglobin disorders such as thalassaemia, HbE disease, HbD disease, HbC disease. The presence of sickle cell gene with HbE decreases the severity of sickle cell disease, whereas severity is increased with HbD or HbC. In patients who inherit both thalassaemia and sickle cell mutations, the symptoms depend on the severity of the thalassaemia mutation,” Dr Harimadhavan told ABP Live.
HbE disease, which stands for Haemoglobin E disease, is a mild inherited blood disorder characterised by an abnormal form of haemoglobin called haemoglobin E, which is an extremely common structural haemoglobin variant that occurs at high frequencies in many Asian countries. The condition usually does not cause any symptoms, but can result in very mild anaemia.
HbD disease, which stands for haemoglobin D disease, occurs due to a mutation of beta chain or alpha gene, and is characterised by mild haemolytic anaemia. In the case of HbD, glutamate is replaced by glutamine at codon 121 of the beta chain. A codon is a sequence of three consecutive nucleotides in a DNA or RNA molecule that codes for a specific amino acid.
Haemoglobin C or HbC is caused due to a mutation in the beta-globin chain in which glutamate is replaced by lysine in the sixth position of the beta-globin chain.
When a person inherits a gene for haemoglobin S from one parent and a normal gene, which codes for haemoglobin A, from the other parent, they suffer from sickle cell trait. They mostly do not show any signs of the disease, but may suffer from health problems if they are dehydrated or exercise strenuously.
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Treatments for sickle cell disease
Several medications are available for sickle cell disease. The US Food and Drug Administration has approved four medications for treating sickle cell disease: hydroxyurea, L-glutamine, voxelotor, and crizanlizumab.
Hydroxyurea is used to treat people with sickle cell disease aged two years and older, L-glutamine is used to treat patients aged five years and older, voxelotor is used to treat patients with sickle cell disease aged four years and older, and crizanlizumab is used to treat those aged 16 years and above, according to the Centers for Disease Control and Prevention (CDC).
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Sickle cell anaemia can be completely cured only through haematopoietic stem cell transplantation. Haematopoietic stem cell transplant, or blood stem cell transplant is the only stem-cell based treatment routinely reviewed and approved by the FDA, and is used to treat patients with cancers and disorders affecting the blood and immune system.
Stem cell transplant is used to treat conditions in which the bone marrow is damaged and is no longer able to produce healthy blood cells, and can also be used to replace blood cells that have been damaged as a result of intensive cancer treatment, according to the National Health Service (NHS). Stem cells can either come from the patient’s own body or from a donor.
“The success rate of stem cell transplants for blood disorders like thalassaemia, aplastic anaemia and sickle cell anaemia is more than 80 per cent. There are rarely any cases in which the body rejects the newly injected blood cells following stem cell transplantation,” Dr Agarwal said.
He explained that dysfunctional cells in the patient are destroyed using chemotherapy or some other method, before stem cell transplantation is performed. This process is called conditioning. They are given immunosuppressive drugs for a period of three to six months after stem cell transfusion.
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This is counterintuitive because on the one hand, the patient’s immune system becomes weak due to conditioning, and on the other hand, they are given immunosuppressants to ensure that the body does not reject the newly introduced stem cells.
Children who have minimal organ damage from severe sickle cell disease can undergo stem cell transplants.
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