Sickle Cell Disease (SCD) is a hereditary homozygous disorder of Red Blood Cells (RBCs), termed Erythrocytes, caused by the receiving of two faulty alleles from both parents making it an autosomal recessive condition. It is caused by the substitution of a single nucleotide on codon 6, Adenine (A) to Thymine (T), termed a single nucleotide polymorphism in which Glutamic acid (GAG) becomes Valine acid (GTG) on chromosome 11; leading to the polymerisation of HbS and sickling of erythrocytes Mnika, K et al (2016)). This alteration occurs in β-Globin genes (HBB) and leads to Haemoglobin S (HbS) (De Witt, Ma et al (2016)). Polymerisation of RBCs leads to hemolysis, the rupture and break down of RBC, and vaso-occlusion, blockage of blood vessels by sickled RBC which causes severe pain and is the most common and noticeable symptom of SCD (Inati, A (2009)). Some other symptoms include: organ dysfunction, stroke, various types of pulmonary disease, pulmonary artery hypertension, nephropathy (Abraham, A (2016)). SCD cases are found worldwide but are found in high frequency and concentrated in Africa, having the highest prevalence among Africans, black Americans and Mediterranean’s; but since mass population migration, it is now one of the most prevalent genetic conditions worldwide (Vichinsky, EP (2011)). Despite its long history, treatment is limited with allogenic haemopoietid stem-cell transplant existing as the only known cure; which however remains inaccessible to many countries especially those in Africa (Lettre, G. Bauer, DE (2016)). SCD has been estimated to affect around 1/2500 births and currently affects 100,000 people in the USA and is becoming increasingly more common. (Strouse, J. (2016)). This essay shall be discussing the current managements methods being used that aim to prevent and treat SCD along with the difficulties presented by the increasing age-associated chronic complications of this disease.
Those with SCD will suffer throughout their lives and experience a range of symptoms with high rates of morbidity and mortality. Additional symptoms to the ones named above are: recurrent pain, chronic anaemia, acute chest pain, jaundice, organ failure and stroke. In addition, approximately one to two thirds of children with SCD have been reported to have psychological symptoms and experience psychosocial challenges (Aimee, K (2015)). Managing those with SCD is therefore done within a hospital and at home. One of the commonest management methods for treating SCD is the use of a drug called hydroxyurea (HU). HU is currently the only approved drug for SCD (Lanzkron, S (2008)). HU alters enzyme ribonucleotide reductase causing S-phase cell arrest which therefore results in more HbF production (Frank, A. Ferrone (2016) a). The mechanism of action of the drug is suggested to act as a free radical within cells, where is becomes nitroxide, and alters tyrosyl free radicals and the active sites of M2 proteins units found on the enzymes- ribonucleotide reductases, ultimately leading to their inactivation (Yarbro, JW (1992)). Theses enzymes are crucial in DNA synthesis, so their inactivation causes DNA synthesis inhibition which is why there is S-phase cell arrest. HU acts on the bone marrow cells and increases the synthesis of HbF as it is more cytotoxic to rapidly dividing late erythroid precursor cells, so will destroy them allowing for the imitation of regeneration (Segal JB, et al (2008) & Yarbro, JW (1992)). HU increases the chances of erythrocyte progenitors becoming F-cells. Le, Phu Quoc et al (2015) carried out a study on 469 patients between 2008 and 2012 and the effects some disease modifying treatments (DMT). At the time the global mortality rate was 25/100 patient years (PY). In the trial, 185 patients were given hydroxyurea, 90 had hematopoietic stem cell transplants, 24 were chronically transfused and 170 had no DMT. There mortality rates for those who received hydroxyurea, transplant and no DMT were: 0.14, 0.36 and 036 per 100 PY respectively. This showed that hydroxyurea group benefited more that they experienced an improvement compared to the global mortality rate. Another study carried out by Keikhaei, B et al (2016) also supports the evidence that hydroxyurea has shown to significantly improve the survival of SCD sufferers. They carried out a trial on 48 SCD patients and administered 10mg/kg/day of hydroxyurea for one year. After the year the patients showed significant improvements in their symptoms compared to the previous year. Across the group there were decreased transfusions, hospitalisations. There was also a decrease in spleen size opposing the typical atrophy found in spleens of SCD patients. Asplenias and autoinfarction cause enlargement of spleens during SCD where this where the spleen is gradually replaced by patches of fibrosis, calcium depositions (Gardner, C.S., Boll, D.T., Bhosale, P. et al. AbdomRadiol (2016)). In addition, results showed an increased number of RBC indices and haemoglobin (Hb) and an increased number of HbF. The trail also indicated that the patients suffered no noticeable side effects from the hydroxyurea, making it a promising option a DMT. The drug, Hydroxyurea, is also thought to increase the production of fetal haemoglobin, but has shown greater effects onaltering polymerisation that occurs in SCD (Frank, A. Ferrone (2016) b).
Treatment for SCD is said best through the prevention of polymerisation and reversal of its process. During polymerisation, long bundles form and twist in a rope-like fashion and grow to such an extent that they distort the cells shape. The bundles grow from a double nucleation mechanism spreading out in multiple directions forming a star-shaped RBC. This polymerisation occurs in HbS. Fetal haemoglobin which isfound in all infants at levels of around 98% at birth and gradually decreasing by 5% per week until it has nearly all gone by 6 months (Edoh, D (2006)); this is why most children show symptoms of SCD at around 6 months of age as polymerisation cant occur in HbF. Coleman E and Inusa B (2016) suggested that polymerisation of HbS is what causes the characteristic sickling of RBCs in SCD, due to the abnormal Hb chains; caused by the nucleotide substitution of A-T. They suggested the sickling occurs in deoxygenated states. We can decrease polymerisation by increasing the Fetal Haemoglobin levels in the body. This is because Fetal haemoglobin has a much higher affinity for oxygen so a deoxygenated state will not be achieved and therefore no polymerisation will occur. Increasing the levels of Fetal haemoglobin can be done pharmacologically or use of gene therapy and genetic engineering techniques. Pharmacologically, there are two main methods of treatment: first method involves modifying DNA by promotion of globin gene expression, second involves synthesising an environment within the body that favours production of F-cells in bone marrow. A drug used for the first method is 5-azacytidine, a cytosine analog (Coleman E and Inusa B (2016)). Its properties include being resistant to methylation. It adds itself to DNA and through gene expression caused by hypomethylation and increases expression of Gamma-globin gene leading to increased levels of HbF. HbF is comprised of 2 alpha chains and 2 gamma chains (2α2γ) compared to the composition of HbA being 2 alpha, 2 beta (2α2β). Therefore HbF cannot undergo polymerisation or a nucleotide substitution as there is no Beta gene (Coleman E and Inusa B (2016)). 5-azacytidine is however toxic and therefore its use as a treatment option is limited. Its use in treatment may be more relation to its cytoxicity. Because it is cytotoxic it puts stress on bone marrow causing production of progenitor cells that are more likely to produce HbF when they differentiate (Coleman E and Inusa B (2016)). Further trials were carried out using 5-azacytidine and a new analog, 5-aza-2’-deoxycitide, was created. When investigated it was found to be less toxic and more incorporated into DNA and RNA that 5-azacytodine. It showed to act in the same way as 5-azacytodine by increasing gene expression of gamma-globin and therefore increase HbF levels (Coleman E and Inusa B (2016) extracted from Raphael R, Vichinshky (2005))). In conclusion this new analog could become a more viable, promising treatment option for SCD in the future.
Hb modifying drugs have recently been studied to explore their effects on polymerisations and anti-sickling abilities. One such drug called GBT440 has recently been used to treat SCD by increasing the affinity of oxygen for Hb and thus stopping in vitro polymerisation of HbS and HbA mixtures. Furthermore the drug has shown to prevent sickling of RBC under conditions such as strenuous exercise with hypoxia, dehydration and acidosis; conditions that increase oxygen demand and dehydration which increase HbS polymerisation (Dafu, K et al (2016)). Overall the drug has shown it could protect SCD patients from sickling related complications during conditions that favour HbS polymerisation. However other Hb modifying drugs have not shown the same promising results as GBT440. 18 new compounds, called the KAUS II series where created and evaluated for their anti-sickling ability. These compounds were created on old reports that non-covalent Hb binding characteristics of substitutedarlyoxyalkanoic acids had proved to have anti-sickling abilities. Their findings however contradicted this theory and showed no signs on anti-sickling properties or no effects of increase Hb affinity (Omar, AM et al (2016)).
The second method for preventing and treating polymerisation is via gene therapy which is increasingly becoming a more useful, well understood and effective option. Because SCD is a result of a point mutation on codon 6, changing of Adenine to Thymine, the reverse of this will reform the correct codon and prevent polymerisation of the RBC. In 2015, two patients with SCD were each given busulfan chemotherapy conditioning which was then followed up with autologous bone marrow cells that were genetically altered by lentiviral vector that transferred an engineered β-globin gene in their codon (BA(T87Q)) (Malik, P (2016) & Cavazzana, M (2015)). This gene has shown to have anti-sickling properties similar to γ-globin gene. A further successful trial was then carried out on a 13year old patient with SCD who received BA(T87Q) lentivirally transfused autologous bone marrow after receiving busulfan at myelablative doses. Prior to this gene therapy, the 13 year old was being treatedwith hydroxyurea and blood transfusions, both of which were fairly unsuccessful (M.R Cavazzana, J et al (2015)). After three months the patient stopped having blood transfusions and after 9 months the patient stopped suffering from severe symptoms of SCD. This method of gene therapy shows promising signs as a future treatment and prevention method. It however is a relatively new method and requires a lot of money, effort and fine-tuning in order to work as a consistent and effective option. A lot of monitoring and fine-tuning is required in order for the therapy methodto not disturb normal gene functions. Genetic engineering or genome-editing methods are used alongside gene therapy to help treat SCD. They are novel approaches and are currently being heavily investigated as potential future curative treatments for SCD. There are three well known constructible nucleases that are currently used in genome-editing of SCD: Zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (Tasan, I (2016)). These nucleases can created specific mutations within a wide range of cell types. A trial carried out and reported by Yuduan Ding et al (2016) showed that CRISPR-associated protein 9 was able to delete 13kb of β-Globin locus in hematopeioetic stem and progenitor cells and from this were able to replicate the Sicilian HPFP mutation. This deletion saw an increase in γ-Globin gene expression and proved to be a promising approach towards future, safer autologous transplantations for patients with SCD.
The aims of treating and managing chronic complications associated with SCD are mainly focused on targeting: vaso-occlusive crisis, chronic pain syndromes, chronic haemolytic anaemia, organ damage syndromes, prevention of stroke and treatment of pulmonary hypertension. There are many organ complications as a result of haemolytic anaemia and vaso-occlusive crisis. Cardiopulmonary organ dysfunctions, and other organ damages culminate to cause pulmonary hypertension, heart disease and will ultimately results in early death. (Gladwin, MT (2016)).
Vaso-occlusion however is said to be the hallmark of SCD. This is due to the shape of the RBC from HbS polymerisation and is the most common complication in SCD. Studies carried out on SCD mice has shown that αMβ2 integrin (Mac-1) mediates heterotypic interactions between RBC and Leukocytes promoting vaso-occlusive episodes (Chen, G et al (2016)). A report showed a16 year old patient with SCD was suffering from several vaso-occlusive episodes before having a kidney transplant and uninterrupted immunosuppressive therapy administered after the surgery (Chies, JA et al (2005)). The patient experienced no further vaso-occlusive crisis suggesting the immunosuppressive therapy benefited and aided the kidney graft by controlling the chronic inflammatory responses associated with SCD. It is important to reduce vaso-occlusive episodes as it can cause irreversible organ damages, high morbidity and early mortality.
A Study carried out by Silva, IV et al (2015) in the Paediatric Haematology Unit of a tertiary hospital in Portugal, observed chronic complications of 44 patients between the years 2004 and 2013, of which 55% were female and 98% were black. They found chronic complications in 80% of the 44 patients. They found the most common complication were: dilatation of the left ventricle (47.7%), proceeded by respiratory function disturbs (43.2%), microlithiasis or choloelithiasis (40.9%) and further complications of descending percentage. Silvia IV et al found that there is a correlation with increased leukocytes and dactylitis (inflammation of a whole digit) in first year of life and chronic complications and hospitalisations, making it a useful predictor of SCD severity. SCD has shown to cause chronic vasculopathy. Ranque, B et al (2016) assessed arterial stiffness in SCD and whether there is a relation with arterial stiffness and SCD-related vascular complications. Their study included looking at 3627 adult SCD patients and 943 healthy controls in Cameroon, Ivory Coast, Gabon, Mali and Senegal. They went under many tests including a measure of carotid-femoral pulse wave velocity (cf-PWV) and augmentation index (AL) at a steady state. They found that cf-PWV was lower in SCD patients than in controls. AL, increased more rapidly with age in SCD patients than in the controls. They concluded that AL and cf-PWV are heavily modified in SCD patients than in controls and also suggested that cf-PWV and AL are associated with several SCD clinical complications.
Pulmonary hypertension affects approximately 10% of adult SCD patients and elevates their risk of an early mortality (Gordeuk, VR et al (2016)). Management of this chronic conditions is suggested through using; anticoagulation for patients with thromboembolism; oxygen therapy for those suffering from a below average/low oxygen saturation; treating left ventricular failure if they have post-capillary pulmonary hypertension and using hydroxyurea/transfusions to raise HbF levels whilst reducing haemolysis and preventing vaso-occlusion crisis (Gordeuk, VR et al (2016)). These methods will lower pulmonary hypertension in SCD patients and prevent early deaths and reduce morbidity.
Management and treatment of chronic pain syndromes associated with SCD depend on the severity of pain. Generally analgesic drugs, nerve blockers, physiotherapy, orthopaedic intervention and surgery are used as options. With mild chronic pain, dilhrdrocodeine and co-proxamon, two tables at four-hourly intervals are given (DR Okpala, IE (2002)). If chronic pain is severe, opioid therapy is advised. In avascular necrosis of bones in joints, the joints can be lessened by nerve blocks. Physiotherapy is also used. Cognitive behaviour therapy is also used to help deal with the psychological problems associated with SCD (Iheanyi Okapala, Adel Tawil (2002)). In some cases especially those with avascular necrosis, orthopaedic surgery is considered the only effective treatment to fully realise the patient of pain. Arthritis, Osteopenia and osteoporosis, osteomyelitis, Dactylitis and osteonecrosis are all conditions that have strong associations with SCD (Da Silva Junior, GB et al (2012)). Treatment for osteonecrosis can be conservative or surgical.
In conclusion, there appears to be many viable and accessible methods of managing SCD. There is great focus on managing the chronic complications associated with SCD as directly addressing and prevent SCD polymerization is costly, inaccessible to many countries especially in high prevalence areas such as Africa, and requires much further fine-tuning. However there are promising signs for the future with regards to developing gene therapy and genetic engineering methods. Reversal of polymerization by changing the mutated nucleotide on the Beta-Globin gene appears to be a very promising future cure. The only current cure, hematopeioetic stem cell transplant is effective but inaccessible to many areas of the world and thus not the best option as a treatment method. Over the past few decades with the help of HU, birth screening, advances in genetic therapy and medications, there has been a decline in SCD cases. However due to the high prevalence of cases in low income countries it seems that the future to SCD would be to decrease the disparity between high income and low income countries; supported by (Chaturvedi S, DeBaun MR.(2016)). To conclude currently the best treatment and management option is the combination of managing chronic complications using pain relief, physiotherapy and further medications, alongside using hydroxyurea. This will relieve a patient of their chronic symptoms especially pain from vaso-occlusion whilst increasing their HbF levels which will stop further sickling of RBCs as deoxygenation states are not reached due to high affinity of oxygen HbF has. This conclusion is supported by studies from Frank, A Ferrone in 2016 and Phu Le et al in 2015 and Keikhaei B et al in 2016 used within my essay.
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