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Pros And Cons Of Gene Therapy Pdf

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Gene therapy is a medically-based practice that uses normalized genetics to replace genes which are either not present or abnormal for some individuals. Doctors would take the specific gene sequences that need adjustment, and then insert them into the cellular information of the patient in various ways. Most forms of gene therapy are still in the clinical research stage, but there have been stories of encouraging results.

Gene Therapy for Parkinson's Disease

Rachel Denyer, Michael R. Current pharmacological and surgical treatments for Parkinson's disease offer symptomatic improvements to those suffering from this incurable degenerative neurological disorder, but none of these has convincingly shown effects on disease progression. Novel approaches based on gene therapy have several potential advantages over conventional treatment modalities. These could be used to provide more consistent dopamine supplementation, potentially providing superior symptomatic relief with fewer side effects.

More radically, gene therapy could be used to correct the imbalances in basal ganglia circuitry associated with the symptoms of Parkinson's disease, or to preserve or restore dopaminergic neurons lost during the disease process itself. The latter neuroprotective approach is the most exciting, as it could theoretically be disease modifying rather than simply symptom alleviating. Gene therapy agents using these approaches are currently making the transition from the laboratory to the bedside.

This paper summarises the theoretical approaches to gene therapy for Parkinson's disease and the findings of clinical trials in this rapidly changing field. The specific incidence is dependent on the age structure of the population studied and is difficult to assess precisely but is around 17 per , according to a systematic review in this area [ 1 ].

PD is classically characterised by the loss of striatal dopaminergic neurons within the basal ganglia; however, the underlying pathophysiology is very complex.

These changes lead to disinhibition of subthalamic nucleus STN output, which in turn increases the activity of excitatory projections to the internal globus pallidus GPi and substantia nigra pars reticularis SNpr.

The net result is increased inhibitory outflow from the GPi and SNpr to other basal ganglia nuclei, thalamus, and cortex, leading to the typical motor features of PD [ 3 ]. Various therapeutic approaches that target the STN or GPi have been used to improve motor function in PD, including stereotactic lesioning [ 4 , 5 ], high frequency deep brain stimulation [ 6 , 7 ] and pharmacological silencing [ 8 ].

Dopamine replacement therapies, such as levodopa, were developed around fifty years ago and still constitute the mainstay of treatment for PD [ 9 ]. Patients generally respond very well to this strategy initially, to the extent that failure to respond to levodopa treatment should cause the physician to question the veracity of the diagnosis.

The Parkinson Study Group estimate that over half of patients with early PD receiving levodopa develop at least one of these side effects during the first two years of treatment [ 10 ].

The problems with long-term levodopa treatment have led to the search for new therapeutic strategies for PD. Pharmacological agents such as dopamine agonists can be used to delay the initiation of levodopa or as adjuvant therapies. In more advanced disease, continuous subcutaneous infusions of apomorphine or intraduodenally administered levodopa Duodopa can, to some extent, address the problem of fluctuations in clinical response and improve PD symptom control.

The pharmacological and surgical therapies described above aim to improve the symptoms of PD but none are proven to have a significant impact on the underlying disease process with respect to either slowing disease progression or restoring the affected dopaminergic neurons. Gene therapy has distinct potential advantages over conventional treatment modalities for PD as it could theoretically be used to preserve or restore dopaminergic neurons affected by PD through the action of neurotrophic factors [ 11 , 12 ] or alternatively increase the availability of enzymes required for dopamine synthesis [ 13 , 14 ].

Although the disease modifying properties of these therapies remain to be proven, they could potentially target the underlying pathophysiological imbalances and may result in much less fluctuation in response and a lower prevalence of dyskinesias than conventional pharmacotherapy for PD.

Alternative therapeutic approaches will be required to address these issues. The use of gene therapy to treat PD necessitates the use of a suitable method of delivery for the synthesised nucleic acid—viral or nonviral. The choice of vector greatly influences the technique used for delivery, as a peripherally administered vector must be able to cross the blood-brain barrier with an acceptable degree of tissue specificity. Alternatively, the surgical techniques used for deep brain stimulation can be harnessed to deliver the vector directly to a specific brain region.

Nonviral techniques are technically and conceptually more straightforward but are less well suited to treating a chronic neurodegenerative disorder such as PD, due to the short duration of gene expression that is typically achieved.

Low transfection rates mean that experiments using nonviral vectors have often used multiple dose regimens [ 17 ]. This poses particular problems for translation to human studies if repeated intracerebral injections, with their associated risks, are needed to achieve a meaningful clinical response. This approach may still prove effective, as seen in a recent study using the human glial cell-derived neurotrophic factor GDNF gene and a neurotensin polyplex nanoparticle vector in an animal model of PD, finding that a single intracerebral injection of the agent may prove sufficient to induce a biochemical and functional response [ 18 ].

Other nonviral vector studies in animal models of PD have incorporated region-specific ligands in order to maximise tissue specificity using intravenous vector administration [ 17 ]. For example, one group [ 19 ] has used Trojan horse liposomes and a monoclonal antibody to the transferrin receptor to facilitate transport across the blood brain barrier of a peripherally administered therapeutic plasmid containing DNA for GDNF.

They also incorporated the gene promoter for tyrosine hydroxylase TH , a key enzyme in the synthesis of dopamine, to restrict expression of the transgene to catecholaminergic neurons. Viral vectors, derived from either DNA or RNA viral vectors, are generally considered to be a more practical approach, with the potential to cause long lasting gene expression via episome formation or DNA integration into the host genome.

A range of different types of viruses, each with different properties and advantages, have been exploited in the search for a suitable vector for gene therapy in PD. These are detailed below, with particular attention to adeno-associated viruses which comprise by far the largest category of vectors used in clinical trials to date.

Adeno-associated viruses are relatively simple 4. They comprise two genes encoding capsid cap and viral replication rep proteins and inverted terminal repeat sequences, but require additional genes from other viruses e. AAVs are well suited to gene therapy for PD as they are capable of inducing long-term gene expression, usually via episome formation [ 21 ]. AAVs are also able to integrate into a specific site at chromosome 19 of the human genome raising potential concerns regarding insertional mutagenesis, although the frequency with which integration occurs in vivo remains unclear [ 22 , 23 ].

More than AAV variants have been identified and they are classified into nine genetic clades with differing tissue tropisms [ 24 ]. AAVs 1—10 have been used for gene therapy vector production but AAVderived vectors are the best characterised and most frequently utilised serotype in PD gene therapy studies.

One advantage of AAV-2, when administered locally, is that it transduces only neurons within the central nervous system and is particularly efficient in brain regions known to be involved in the pathophysiology of PD, such as the globus pallidus and substantia nigra [ 22 ].

Recent research additionally suggests that AAV-1, -5, and -8 are also able to transfect basal ganglia neurons in a highly efficient and specific manner in nonhuman primates and therefore these serotypes could be used in future gene therapy trials [ 26 ].

In addition, a recent study investigating the use of erythropoietin as a therapeutic agent for PD successfully delivered the gene to striatal neurons using AAV-9 in a rodent model discussed below [ 27 ]. In contrast, the utility of AAV-5 as a viral vector appears to be unaffected by the humoral immune response [ 28 , 29 ]. It therefore seems likely that future human trials of gene therapy for PD will not be limited to the AAV-2 serotype.

There is also evidence for a cellular immune response, which may also have implications for the efficiency of transgene expression using AAV [ 30 ].

AAVs have a very attractive profile in terms of safety, in addition to their tropism for basal ganglia neurons. AAV is not associated with any human disease and the wild-type virus is replication defective [ 31 , 32 ].

Three-plasmid systems are now well established and routinely used to produce highly purified AAVs, further improving their safety [ 33 ] Figure 1. Lentiviruses are retroviruses that can efficiently infect dividing and nondividing cells [ 22 ]. This class includes the human immunodeficiency virus HIV , which has been studied extensively, and most lentiviral vectors are consequently based on HIV [ 34 ].

HIVderived vectors incorporate a transgene between the long terminal repeats LTRs required for integration into the host genome. By substituting env for gene encoding other viral glycoproteins, such as the vesicular stomatitis virus glycoprotein, the cellular tropism can be broadened or made more specific to neurons [ 35 , 36 ].

Specificity for neurons can be further improved by the use of specific promoters such as neuron-specific enolase or synapsin-1, while introduction of the human glial fibrillary acidic promoter increases specificity for glia [ 37 , 38 ].

The more recent two or three plasmid systems have increased the safety profile of lentiviral-based vectors but concerns remain regarding the possibility of recombination events producing a replication competent virus [ 39 ]. They therefore have higher capacity and have been shown to achieve transgene expression with reduced toxicity [ 44 ]. Some advantages of this group include the relative ease of scaling up production, in comparison to AAV, for example, and robust gene expression [ 35 , 47 ].

In addition to a long-lasting episomal latency, the viral vector is neurotropic—the wild-type virus is associated with encephalitis, as well as cold sores and corneal ulceration [ 48 ], and HSV-1 has been shown to infect neurons [ 49 ].

HSV-1 vectors can be subdivided into recombinant viral and amplicon vector systems. Recombinant viral systems retain most of the wild-type genome, using homologous recombination to insert the required foreign gene. This system allows for very large genes to be inserted if all the wild-type genes were removed [ 22 ]. The amplicon vector, in contrast, contains only a cis acting viral origin of replication and packaging signal, with the genes required for replication and virus production are supplied in trans by a separate helper virus.

Animal studies suggest that long lasting transgene expression in neurons can be achieved using HSV-1 amplicons and the use of specific promoters, for example, those for tyrosine hydroxylase, can increase transduction specificity [ 50 — 52 ].

There remain some concerns over safety and toxicity of HSV-derived vectors, but research to address these issues is ongoing [ 35 , 53 ]. Several complex and interrelated issues need to be addressed in attempting to bring gene therapy for PD from the laboratory to the bedside.

The most fundamental of these issues is the selection of a suitable therapeutic target; PD has a complex pathophysiology that is by no means fully understood and involves multiple brain structures and signalling pathways.

There are three broad approaches to selection of a therapeutic target. The first and most straightforward of these is to increase dopamine levels in the basal ganglia by the introduction of transgenes encoding enzymes or cell signalling proteins involved in dopamine production or regulation. The second approach aims to modulate basal ganglia circuitry affected by PD, for example, by increasing levels of GABA to counteract the overactivity of the subthalamic nucleus observed in this condition.

Both of these approaches, if successful, are likely to result in symptomatic relief for patients rather than an alteration in disease progression. The final approach to choosing a therapeutic target aims to use neurotrophic factors, such as brain-derived neurotrophic factor BDNF [ 56 , 57 ], glial cell line-derived neurotrophic factor GDNF [ 11 , 58 ] or neurturin [ 59 ], to prevent the death of dopaminergic neurons.

This third approach could potentially be disease modifying, in addition to any symptomatic benefit obtained. An overview of therapeutic strategies used in clinical trials of gene therapy for PD is given in Table 1. Aromatic amino acid decarboxylase AADC is an enzyme responsible for the production of dopamine from endogenous or exogenous levodopa. Patients with PD require increasing doses of exogenous levodopa to control their symptoms as the disease progresses and it has been suggested that AADC activity may be reduced in PD.

Therefore increasing the activity of this enzyme using gene therapy may reduce both the symptoms of PD and the amount of levodopa required to control them, perhaps also alleviating the side effects of prolonged levodopa therapy [ 64 ]. The validity of this proposed therapeutic approach—targeting decarboxylase deficiency—is not absolutely established, as there is evidence suggesting that dopamine is efficiently decarboxylated in 5-hydroxytryptophan 5-HT immunoreactive neurons [ 65 ].

Similarly, it is quite commonly observed in clinical practice that patients with quite advanced PD still benefit from the use of low individual doses of oral levodopa, which would imply that decarboxylase deficiency is not a major issue. Nonetheless, several preclinical and clinical studies have been published, producing interesting results. Improvements in AADC activity, sustained transgene expression, lower levodopa requirements, and improved functional outcomes persist up to 8 years after viral injection [ 66 , 67 ].

Outcomes from these animals at eight years provide reassurance as to the safety and lasting efficacy of this approach in a primate model of PD [ 67 ]. There was also some improvement in 6-month UPDRS scores both on and off medication , and 3 participants were able to reduce their maintenance dose of levodopa, although there was no control group in this study and results must therefore be interpreted cautiously.

Worryingly, three of the 10 subjects studied in total developed intracerebral haemorrhages. While those conducting the study suggest these relate to the neurosurgical procedure used for vector delivery rather than the AAV-AADC itself, it nonetheless contributes to the perception of increased risk associated with this treatment modality when compared to conventional pharmacological therapies.

Two otherwise eligible participants were excluded from the study due to raised antibody titres to AAV, highlighting the concern felt regarding potential immune responses against AAV in this type of clinical trial. The in vivo chemical synthesis of dopamine begins with the conversion of L-tyrosine to levodopa by TH, and then the levodopa is converted to dopamine by AADC.

GCH-1 is a rate limiting enzyme in the synthesis of a cofactor for TH called tetrahydrobiopterin. In PD this synthetic process may be deficient at several different points and replacement of a single enzyme may not be sufficient to achieve a clinical response. This has led to attempts to intervene at multiple levels using a lentiviral vector containing genes encoding all three key enzymes required for dopamine synthesis [ 14 ].

An early study using a 6-hydroxydopamine 6-OHDA treated rat model of PD found that the single lentiviral vector was able to successfully transduce all three enzymes and this led to significant functional improvement in motor asymmetry [ 14 ].

A subsequent study in nonhuman primates with MPTP-induced parkinsonism found that the same tricistronic lentiviral vector restored extracellular dopamine levels within the striatum and also corrected functional motor deficits for the following 12 months without inducing dyskinesias [ 55 ]. Preliminary data from the manufacturer suggest that the safety profile and functional response are encouraging, though peer-reviewed outcome data are not yet available.

Glutamic acid decarboxylase GAD is the key enzyme involved in the synthesis of the inhibitory neurotransmitter GABA from excitatory glutamate. PD is associated with hyperactivity of the subthalamic nucleus as a consequence of reduced activity in inhibitory nigrostriatal projections [ 68 , 69 ]; therefore delivery of the gene encoding GAD could increase local GABA production within the subthalamic nucleus, restoring equilibrium between these pathways.

GAD exists in two genetically distinct isoforms, GAD65 and GAD67, with differing anatomical and subcellular distributions, as well as differing enzymatic properties [ 70 — 72 ]. Electrophysiological recordings were made during the experiment, with a stimulatory electrode placed in the subthalamic nucleus and microdialysis probes and recording electrodes in the SNpr. The animals showed significant improvements in several behavioural measures of dopaminergic deficit and locomotion, but also had increased survival of TH positive dopaminergic neurons, in comparison to controls injected with GFP or saline prior to lesioning.

The Future of Gene Therapy

Toggle navigation Clinical advice you can trust. Ironically, the gene mutations responsible for the suffering of patients with blinding diseases such as retinitis pigmentosa and Leber congenital amaurosis may also hold the key to their treatment. In this article, researchers on the forefront of gene therapy provide a glimpse of what gene therapy might look like five years from now, and what hurdles will need to be cleared before we can realize that vision. Though these approaches are being tried in different ailments, experts say that there are certain diseases that are likely candidates to have the first approved gene therapies in the United States in the next few years. Kirby professor of ophthalmology at the Perelman School of Medicine at the University of Pennsylvania, and one of the first researchers to use viral vector-based gene therapy to reverse blindness. However, I am excited about the data for the treatment of choroidal neovascular complications of age-related macular degeneration using sFlt-1, the receptor for VEGF.

Posted by Biolyse Nov 3, Gene Therapy 0. Over the years genetic disorders and gene-related illness have been responsible for high mortality rates and reduced quality of life. Some of the congenital abnormalities manifest quite early, and there are minimal hopes for survival in these children, this causes much pain to their families because management option is limited and there is very little at their disposal to modify such conditions. Scientists are developing a relatively new technique that will give hope to the hopeless and make life better. Genetic disorders can be due to misalignment, missing genes or excess of a gene.

Gene therapy for ischemic heart disease: review of clinical trials. Bruna Eibel I ; Clarissa G. Rodrigues II ; Imarilde I. Giusti I ; Ivo A. Nardi V ; Renato A.

Gene Therapy

Gene therapy is an experimental treatment using genetic material to treat or prevent certain diseases. While not yet widely available, gene therapy may one day help doctors treat diseases by directly replacing the disease-causing gene. Luxturna treats certain inherited retinal eye diseases. Gene therapy works by replacing or inactivating disease-causing genes. In some cases, gene therapy introduces new genes into the body to treat a specific disease.

20 Advantages and Disadvantages of Gene Therapy

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Can Adeno-Associated Viral Vectors Deliver Effectively Large Genes?

Gene therapy is an experimental technique in medical science that uses genes to either prevent or treat disease. There are several different approaches to gene therapy being researched today. One method involves the replacement of a mutated gene with a healthy copy of it.

Towards New Therapies for Parkinson's Disease. The use of levodopa L-3,4-dihyroxyphenylalanine as a symptomatic therapy was established nearly 50 years ago and continues to be an important approach in early and late disease Fahn Although the majority of patients initially respond well to dopaminergic therapies, many eventually develop fluctuations in their therapeutic response, often with associated dyskinesias Jenner These therapies are symptomatic, with no effect on the underlying pathogenic processes, particularly the progressive loss of dopaminergic neurons. Novel therapies to either compliment existing approaches, or potentially alter the course of disease, are clearly desirable. Disinhibited activity in the subthalamic nucleus STN correlates with increased activity in excitatory projections to the major nuclei of the basal ganglia — the internal globus pallidus GPi and substantia nigra pars reticularis SNr. More recently, these approaches and observations have prompted gene therapy trials to deliver therapeutic vectors into the striatum itself and these will be discussed later.


PDF | Gene therapy has become a significant issue in science-related news. advantages, disadvantages and prospects of gene therapy.


Gene Therapy Pros and Cons

The Cons of Gene Therapy

By: Jim Mumper, M. Genetic Testing. Genetic testing continues to increase in popularity. However, it is not for everyone. Results of genetic testing can often be uninformative and ultimately can cause more stress and anxiety over the possibility of a disease you may never get. Genetic testing should be encouraged only when there is effective therapy available to prevent or treat the condition tested for. There is little value in genetic tests that do not allow you to take action to reduce or change your risk for a particular disease.

Его доказательства, его программы всегда отличали кристальная ясность и законченность. Необходимость убрать пробелы показалась ей странной. Это была мелочь, но все же изъян, отсутствие чистоты - не этого она ожидала от Танкадо, наносящего свой коронный удар. - Тут что-то не так, - наконец сказала.  - Не думаю, что это ключ. Фонтейн глубоко вздохнул.

Я сам попытался отправить твой маячок, но ты использовала для него один из новейших гибридных языков, и мне не удалось привести его в действие. Он посылал какую-то тарабарщину. В конце концов пришлось смирить гордыню и вызвать тебя. Сьюзан это позабавило. Стратмор был блестящими программистом-криптографом, но его диапазон был ограничен работой с алгоритмами и тонкости этой не столь уж изощренной и устаревшей технологии программирования часто от него ускользали.

14 Advantages and Disadvantages of Gene Therapy

3 Comments

Leroy M. 13.03.2021 at 13:01

Rachel Denyer, Michael R.

TimothГ©e C. 15.03.2021 at 09:30

Citation: Ananth N. Gene Therapy–Potential, Pros, Cons And Ethics. Online J Health Allied. S cs. ; URL: icel3.org

Marine M. 22.03.2021 at 20:51

The novelty of gene therapy methods is one disadvantage. • Stimulation of immune response: The gene injected by a virus may cause immune responses due to.

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