Author: Matthew C. Canver
Institution: University of Pennsylvania
Date: January 2009
Gene therapy inserts genes into cells to reverse cellular dysfunction or create new cellular function. The advancements in the field of gene therapy have developed into two different strategies: ex vivo and in vivo gene delivery. Encouraging progress has been made in clinical use of gene-based therapy for a variety of diseases, most notable successes were apparent in the preliminary clinical trials for severe combined immunodeficiency (SCID-X1) disease adenosine deaminase-severe combined immunodeficiency (ADA-SCID), and more recently, Leber's congenital amaurosis (LCA). This article provides an overall review of the current status of the clinical applications of gene therapy and evaluates the clinical success of ex vivoand in vivo gene therapy strategies.
Introduction: The Emergence of Gene Therapy
Since Gregor Mendel's first description of genes from his pea plant experiments, science has made great strides in genetics. The 1970s and 1980s witnessed the emergence of gene therapy involving the placement of a functioning gene into cells to reverse cellular dysfunction or create new cellular function (Cotrim and Baum 2008). Entry of the gene into a cell is accomplished by the use of a viral vector such as adenovirus, adeno-associated virus, retrovirus, lentivirus, or through nonviral approaches (Edelstein et al. 2007). The discovery of recombinant deoxyribonucleic acid (rDNA) in 1972 (Cohen 1973) led to considerable progress in gene therapy research. In 1980, Dr. Martin Cline became the first person to conduct gene therapy experiments on humans. Cline's research was about β-thalassemia, an autosomal-recessive hematologic disease caused by a mutation on chromosome 11. The decreased production of β-globin chains leads to ineffective erythropoiesis and hemolysis and is responsible for causing anemia (Inati et al. 2006; Quek and Thein 2007; Smith and Byers 2002). Cline's landmark gene therapy achievements, though unsuccessful, were the target of intense criticism with allegations of his intentional efforts to perform human gene therapy trials abroad to circumvent the United States laws and regulations related to human clinical trials. Dubbed the Cline Affair, it was an ominous beginning for gene therapy researchers (Smith and Byers, 2002).
The first Recombinant DNA Advisory Committee (RAC)-approved gene transfer experiment occurred in 1989 when the gene coding for resistance to neomycin, an antibiotic, was transduced into human tumor-infiltrating lymphocytes (TIL) using a retrovirus to serve as a marker. The transduced TIL plus interleukin-2 were then infused into patients, which had been shown to induce regression of metastatic melanoma in 50% of patients. The procedure was determined to be safe based on all safety criteria (Rosenberg et al. 1990).
The first approved human gene therapy clinical trial took place the following year in 1990 to treat severe combined immunodeficiency (SCID) by transferring the adenosine deaminase (ADA) gene using a retroviral vector into T cells. No significant clinical benefits were observed from this study although the delivery appeared safe (Blaese et al. 1995; Trent and Alexander 2004). Since the first trial in 1990, the field of gene therapy has grown and made significant advances. This article summarizes the current status of gene therapy in the clinic and evaluates the success of the two dominant viral gene therapy strategies: ex vivo and in vivogene therapy.
Ex Vivo, In Vivo Gene Therapy and Viral Vectors for Gene Delivery Ex vivo gene therapy involves the harvesting of cells from a patient followed by subsequent viral transduction ex vivo in a laboratory setting by a virus carrying the therapeutic gene. The transduced cells are then returned to the patient. Conversely, in vivo gene therapy in involves the injection of a virus carrying the therapeutic gene directly into a patient's body. The four most common viral vectors used for both ex vivo and in vivo gene therapy are adenovirus, adeno-associated virus (AAV), lentivirus, and retrovirus. AAV has many unique properties that make it an ideal vector for gene delivery. Importantly, AAV vectors minimize potential immune response risks by containing no viral genes. AAV vectors are typically used in systems that require long-term gene expression. Since AAV vectors can exist for the entire lifespan of a given cell, they are commonly used in in vivo experiments requiring long-term expression, which lowers the number of treatment administrations. A disadvantage of AAV is its low carrying capacity, which is limited to approximately 4.5 kb per particle (Templeton 2008). In contrast, Adenovirus (Ad) are typically used in situations where the required level of expression is needed to persist for a shorter duration such as days to weeks. Ad illicits an immune response, which leads to the production of antibodies. These adaptations undertaken by the immune system to counteract Ad lead to the limited expression time. Also, Ad has the important property of being non-oncogenic (Templeton 2008). Lentivirus has several defining characteristics. Lentiviral vectors have a large carrying capacity of approximately 9,000 bp per particle and they only illicit a minimal immune response. Uniquely, lentiviral vectors have the ability to not only transduce, but also to permanently modify non-dividing cells (Templeton 2008). Retrovirus, the viral vector used in the first human gene therapy clinical trial, has the ability to cause sustained, long term expression. One disadvantage of retrovirus is the oncogenic properties of retrovirus caused by the random insertion of vectors near proto-oncogenes (Templeton 2008).
Status of Clinical Gene Therapy
Ex Vivo Gene Therapy Adenosine Deaminase-Severe Combined Immunodeficiency (ADA-SCID)
ADA-SCID results from an adenosine deaminase (ADA) deficiency, which leads to abnormal T, B, and natural kill cell development (Gaspar et al. 2006); the result is immune deficiency. Similar to SCID-X1, the condition is fatal within the first year of life due to infection (Gaspar et al. 2006). ADA is an enzyme in the purine salvage pathway, where purines are synthesized from intermediates from the degradation of DNA/RNA. Specifically, ADA catalyzes deanimation of adenosine to inosine and deoxyadenosine to deoxyinosine (Muul et al. 2003), which is important due to adenosine's known association with immune response inhibition (Vannoni et al. 2004). Also, accumulation of purine metabolites leads to organ/systemic toxicity (Aiuti et al. 2007). Mutation in the ADA gene leads to ADA-SCID (Muul, 2003). Enzyme replacement therapy with polyethylene glycol-conjugated bovine ADA (PEG-ADA) has been shown to provide transient immunity (Aiuti et al. 2007). In the clinical trial in 1990, two patients had autologous T cells infused (autologous peripheral blood lymphocytes (PBL gene therapy)), which had been transduced with a retroviral vector containing the ADA gene ex vivo (Blaese et al. 1995; Muul et al. 2003; Trent and Alexander 2004). The patients still show persistence of gene corrected T-cells 12 years after treatment (Aiuti et al. 2007; Muul et al. 2003). Patients have continued to receive weekly PEG-ADA injections; the treatment has been demonstrated to be a safe and efficacious treatment of ADA-SCID (Blaese et al. 1995; Muul et al. 2003). A discontinuation of PEG-ADA shows the preferential expansion of only ADA expressing T cells, which shows incomplete correction of the disorder (Aiuti et al. 2002). An alternative approach to ADA-SCID is HSC gene therapy. Patients are pre-conditioned with nonmyeloablative conditioning, which creates space in the bone marrow to deal with previous problems of low engraftment of corrected HSC (Aiuti et al. 2002). CD34+ bone marrow cells were collected and transduced using a retroviral vector containing the ADA gene. These cells were infused into patients, which resulted in long-term engraftment of HSC, which differentiated in both myeloid and lymphoid transduced cells; this resulted in long-term ADA activity without PEG-ADA. There were no adverse effects to the procedure (Aiuti et al. 2002).
Alzheimer's Disease (AD)
AD, the most common neurodegenerative disorder, involves the accumulation of β-amyloid peptide (Aβ) deposits (Turner et al. 2004). Aβ deposits are a major pathological feature of AD. The Aβ peptides exist either in their oligomeric form or in their plaque-associated version; both forms lead to neurodegeneration and AD. Aβ peptides are created from transmembrane β-amyloid precursor proteins (APPs) via proteolytic processing by β- and γ-secretases in endosomes (Rajendran et al. 2008). Patients exhibit loss of memory, orientation, and reasoning (Schindowski et al. 2008).The neurodegeneration of cholinergic neurons, which leads to the loss of acetylcholine, is observed in AD patients (Schindowski et al. 2008). Nerve growth factor (NGF) has been shown to prevent the death of cholinergic neurons (Aiuti et al. 2007). In a clinical trial, fibroblasts were obtained from patients and modified ex vivo using Moloney leukemia virus (MLV) retroviral vectors, so that they would express NGF. They were implanted into the Nucleus basalis of Meynert (NBM) region of the brain, a region containing degenerating cholinergic neurons. Five years post-treatment, patients still exhibit no adverse reactions to the procedure. There was improvement in the rate of cognitive decline (Aiuti et al. 2007; Tuszynski et al. 2005). A more recent trial utilized delivery of NGF via adeno-associated virus (AAV). The trial is currently ongoing and a phase II trial is planned (Aiuti et al. 2007).
Severe Combined Immunodeficiency (SCID-X1) Disease
The first documented success of gene therapy came ten years after the first approved clinical trial in 1990 with the treatment of SCID-X1 disease (Cavazzana-Calvo et al. 2000; Trent and Alexander 2004). SCID-X1 is an X-linked disorder that leads to an absence of any T and natural killer lymphocytes (B lymphocytes present in normal amounts), usually resulting in death within the first year of life due to infection (Cavazzana-Calvo et al. 2000; Hacein-Bey-Abina et al. 2002). SCID-X1, which accounts for 40-50% of all SCID cases, is caused by a deficiency of the cytokine receptor γ chain (γc), which is known to be a component of five cytokine receptors: interleukin-4, -7, -9, -15, and -21 receptors (Alexander et al. 2007). These receptors are responsible for the growth, survival, and differentiation of lymphoid progenitors. The deficiency in the number of γc cytokine receptors leads to stoppage of T and natural killer lymphocyte differentiation and immunodeficiency (Alexander et al. 2007; Cavazzana-Calvo et al. 2000). In a clinical trial, bone marrow was harvested from patients and then the CD34+ cells, hematopoietic progenitor cells (Kohn 2008), were isolated. These cells were transduced ex vivo for three days by a defective γc Moloney retrovirus (derived from a Moloney murine leukemia virus). The CD34+ cells were infused back into the patient. This process is referred to as hematopoietic stem cell (HSC) gene therapy (Alexander et al. 2007). Four of five injected patients showed enough T and natural killer lymphocytes in their blood within four months to provide protective immunity. Patients showed normal T lymphocyte proliferation to antigens for up to five years post-gene transfer with no adverse effects as a result of the procedure (Schwarzwaelder et al. 2007). The study concluded that ex vivo gene therapy can reverse immune definiciency in SCID-X1 patients (Hacein-Bey-Abina et al. 2002). Leukemia was observed in four patients in a French study (Alexander et al. 2007; Pike-Overzet et al. 2006), resulting in the death of one of these patients. It should be pointed out that none of the other groups carrying out these studies saw this effect; the leukemia seemed due to integration of the transgene at a proto-oncogene site (Pike-Overzet et al. 2006).
In Vivo Gene Therapy Cystic Fibrosis
Cystic Fibrosis is a recessive hereditary disorder that presents with progressive decline in pulmonary function (Harvey et al. 1999). It is caused by the mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which produces a glycoprotein that functions as a cyclic adenosine monophosphate (cAMP) activated chloride-channel (Accurso 2008; Moss et al. 2004). The channels are localized to the apical surface of epithelial cells with secretory and absorptive functions, particularly on airway surfaces (Accurso 2008; Harvery et al. 1999; Hyde et al. 2000). Lung disease and eventual mortality occur due to dehydration of airway surface liquid layers, which lead to impairment of mucociliary clearance, inflammation, infection, and structural injury (Accurso 2008). This disease has been at the forefront to gene therapy since it is loss of function caused by a single mutation, the CFTR mutation (Alexander et al. 2007). An early study delivered cationic liposomes complexed with human CFTR cDNA to the nasal epithelium of 15 patients. No adverse side effects were observed due to treatment; however, there was no therapeutic benefit (Caplen et al. 1995). Later studies utilized adenovirus carrying the human CFTR cDNA (Harvery et al. 1999). One study sprayed the virus into two areas of epithelial surfaces within the lungs. This resulted in expression of normal CFTR; however, the expression was transient for unknown reasons. Repetitive administration of the vector produced progressively less expression of CFTR, which is believed to be due to an increase in antivector immune responses. No adverse side effects were observed as a result of the procedure (Harvey et al. 1999). Other studies used adeno-associated virus (AAV) vectors, specifically AAV2. These studies have not been successful (Moss et al. 2004). The current studies are focused on repeat administration of nonviral vectors, which has already demonstrated proof-of principle in humans (Hyde et al. 2000).
Duchenne Muscular Dystrophy (DMD) and Becker Muscular Dystrophy (BMD)
DMD is an X-linked inherited disorder which leads to muscle fiber necrosis and the muscle weakness/degeneration. The disease results from a dystrophin deficiency due to a mutation or deletion in the dystrophin gene (Alexander et al. 2007; Romero et al. 2004; Takeshima et al. 2006). BMD, a milder allelic common form of muscular dystrophy, is caused by deletions that lead to the creation of truncated dystrophin (Romero et al. 2004). The cloning of the dystrophin gene opened the door for gene therapy; however, there are several challenges including (1) the large amount of skeletal muscle, (2) the involvement of myocardium, and (3) the large size of the dystrophin gene, which is 427 kDa, 79 exons, and four structural domains (Alexander et al. 2007; Chamberlain 2002; Rafael and Brown 2000). One study injected 9 DMD/BMD patients with the naked dystrophin plasmid in the radialis muscle. Patients were injected with one of three treatments: 200 µg once, 600 µg once, or 600 µg twice (two weeks apart). Patients were biopsied three weeks after injection. Plasmid DNA was found in six out of the nine injected patients.The first group and one patient from the second exhibited 0.8-8% of weak, complete sarcolemma labelling (dystrophin is localized underneath the sacrolemmal membrane) (Rafael and Brown 2000; Romero et al. 2004), while 3-26% of muscle fibers showed incomplete/partial labeling. The third group showed 2-5% complete sarcolemmal labeling and 6-7% showed partial labeling. There were no observed adverse effects to the treatment. The study concluded that the expression of dystrophin was low (Romero et al. 2004). Currently, a clinical trial is utilizing adeno-associated virus 1 (AAV1) to deliver a smaller version of the gene that is thought to maintain partial protein function (Alexander et al. 2007). The AAV vector provides superior transduction efficiency, but is a source for potential immune response (Walther and Stein 2000).
Hemophilia has two forms: Hemophilia A and Hemophilia B (Manno et al. 2006; Powell et al. 2003). Hemophilia A is an X-linked bleeding disorder caused by mutations in the genes encoding for the enzyme Factor VIII. Factor VIII is a cofactor for the production of thrombin, a coagulation protein (Manno et al. 2006). Hemophilia B is a sex-linked disorder that results from a deficiency of Factor IX (Xu, 2003). Patients with severe hemophilia exhibit frequent spontaneous bleeding into joints, soft tissues, and vital organs; mild hemophilia usually results in bleeding only after trauma or surgery (Roth, 2001). Many different industry-sponsored clinical trials have taken place for hemophilia A and B that used diverse vectors and target tissues to transfer Factors XIII and IX respectively (Manno et al. 2006; Powell et al. 2003; Roth et al. 2001). These trials produced only transient expression of Factor XIII/IX at therapeutic level; however, they produced important safety data (Aiuti et al. 2007). Current strategies include (1) a retroviral vector infused into neonates when hepatocytes are still dividing rapidly (Xu et al. 2003), (2) AAV delivery of Factor XIII/XI to skeletal muscle via intravascular delivery (Arruda et al. 2005), and (3) AAV delivery to liver via the hepatic artery or portal vein (Nathwani et al. 2007). One problem has been CD8+ T cells memory for AAV capsids, which causes a reduction in expression of Factor XIII/IX in humans when compared to animal models (Aiuti et al. 2007).
Huntington's Disease (HD)
HD is an autosomal dominant, monogenic neurodegenerative disorder (Bloch et al. 2004; Rodriguez-Lebron et al. 2005). HD patients present with motor abnormalities and cognitive impairments. HD is caused by a mutation in the huntingtin (Htt) gene. The mutation causes the Htt protein to express an expanded polyglutamine domain (pQ) in its N-terminus (CAG repeats); the expanded pQ domain is thought to cause HD. Patient death typically occurs 10-15 years once patients become symptomatic (Aiuti et al. 2007; Rodriguez-Lebron et al. 2005). RNA interference (RNAi) has shown promise in a mouse model; however, technical details on delivering RNAi to the brain are still being established (Harper et al. 2005). Neurotophic factors, which are proteins that function in the development and survival of neurons as well as maintence of mature neurons, have been delivered using adenovirus and lentivirus to treat HD in murine models (Mittoux et al. 2002; Zala et al. 2004); both specifically treated with Ciliary Neurotrophic Factor (CNTF). The only human clinical trial for HD treated patients with BHK cells encapsulated in polymer capsules over-expressing CNTF. Pores in the capsule allowed for release of CNTF, entry of oxygen and nutrients, but prevented penetration of large molecules in order to prevent immune response (Aiuti et al. 2007; Bloch et al. 2004). Only one capsule was implanted during the phase I trial designed for safety. This capsule was replaced every 6 months for two years. There was no clearly observed clinical benefit from the treatment; however, this may have been expected since 4 capsules were needed to correct the disorder in primates. Phase II studies to assess therapeutic potential are being planned (Aiuti et al. 2007).
Parkinson's Disease (PD)
PD, the second most common neurodegenerative disorder, is characterized by dopamine deficiency due to atrophy of the neuronal terminals that originate in the substantia nigra (area of midbrain) and extend to the striatum (Bankiewicz et al. 2006; Stowe et al. 2008). Dopamine, produced at neuronal terminals, normally functions as a neurotransmitter that controls signaling pathways that coordinate and control movement (Bankiewicz et al. 2006). PD patients suffer from bradykinesia, tremor, rigidity and postural instability (Stowe et al. 2008). Treatments with precursor L-dopa have only been shown to be effective in the early stages of PD before neuronal degeneration is too severe. There have been three different approaches taken to attack PD; all of the approaches have utilized adeno-associated virus (AAV) since it has been shown to be safe and effective in animal models (Alexander et al. 2007). This first trial used AAV to transfer the glutamic acid decarboxylase (GAD) gene to the subthalamic nucleus (STN). GAD is the rate-limiting enzyme in the production of γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the brain. In PD patients, the activity of the STN is increased. It has been shown that reduction in activity by GABA infusion can ameliorate the symptoms of PD (Kaplitt et al. 2007). A phase I clinical trial of GABA transfer by AAV to the STN leads to significant improvement on standard clinical ratings (Alexander et al. 2007; Kaplitt et al. 2007).
The second approach involved using AAV to transfer the neurturin (NTN) gene to putamenal cell bodies, which are targets for dopamine neuronal projections. Nigral neurons are particularly sensitive to glial-derived neurotrophic factor (GDNF); NTN is a naturally occurring analog of GDNF (Kordower et al. 2006). The aim is to prevent degeneration of nigral neurons and promote neuroplasticity (Alexander et al. 2007). The Phase I clinical trial utilizing this technique showed significant improvement in clinical rationings and was the first PD treatment to begin a Phase II study (Alexander et al. 2007). The third approach utilized AAV to transfer the gene for aromatic L-amino acid decarboxylase (AADC) into the striatum, which helps to increase the conversion of L-dopa into dopamine in both the striatum and nigra. AADC levels decline with disease progression. Increasing levels of AADC increase L-dopa sensitivity and thus increase the therapeutic window (Alexander et al. 2007). This has been demonstrated in animals (Bankiewicz et al. 2006) and is currently in a phase I clinical trial.
Leber's Congenital Amaurosis (LCA)
In May 2008, two research teams published the results of their clinical trials in the New England Journal of Medicine for Leber's Congenital Amaurosis (LCA) (Bainbridge et al. 2008; Maguire et al. 2008). Leber's Congenital Amaurosis is a group of early onset retinal degeneration diseases (Maguire, 2008). A specific form of LCA, LCA2, is caused by a mutation in the retinal pigment epithelium (RPE). The gene of interest, RPE65, encodes for a 65-kDa protein, an isomerohydrolase enzyme in RPE, which plays a significant role in the retinoid-visual cycle because an isomerohydrolase enzyme deficiency leads to insufficient levels of 11-cis retinal in the retina. The enzyme converts all-trans-retinyl esters to 11-cis retinal (Redmond et al. 1998). This compound is essential to generate rhodopsin in rod photoreceptors. Rhodopsin-mediated responses to light induce the electrophysiological/biochemical reactions leading to normal vision. The isomerohydrolase enzyme also plays a role in vitamin A metabolism in the retina. LCA2 caused by a RPE65mutation accounts for 7-16% of LCA cases (Lotery et al. 2000; Marlhens et al. 1997; Morimura et al. 1998). The eye is a convenient organ for gene therapy because it can be easily accessed surgically and provides limited risk of systemic exposure of the therapeutic agents (Alexander et al. 2007). The trials utilized an optimized adeno-associated virus (AAV) vector, AAV2, which carried the human RPE65 cDNA (Bainbridge et al. 2008; Maguire et al. 2008). One of the research teams, from the Children's Hospital of Philadelphia (CHOP), had improvement in subjective and objective tests of retinal function in all three injected patients (Maguire et al. 2008). The other research team, based in the United Kingdom, yielded improved subjective tests of visual function for one out of three injected patients (Bainbridge et al. 2008). Importantly, there were no signs of harmful humoral or cell-mediated immune responses to the vector (Maguire et al. 2008).
Cancer: Immunotherapy, Oncolytic Viruses, and Suicide Gene Therapy
Cancer is the target of 66.5% of all gene therapy clinical trials (Edelstein et al. 2007). Three strategies have been implemented for the diversity apparent in cancer: immunotherapy, oncolytic viruses, and suicide gene therapy. Immunotherapy aims to stimulate the adaptation of the immune system in vivo (Gattinoni et al. 2006). Oncolytic virus treatments aim to insert viruses, which multiply selectively in cancer cells to induce cancer cell death (Aiuti et al. 2007). Suicide gene therapy, or gene-directed enzyme pro-drug therapy, aims to transfer an enzyme to a tumor via a vector followed by administration of a pro-drug which is selectively activated in the tumor due to the presence of the enzyme. For example, the activated pro-drug can create toxic metabolites. This technique of making cancer cells more suspectible to toxins and/or drugs allows for cancer cell-specific targeting of chemotherapy treatments (Aiuti et al. 2007; Niculescu-Duvaz and Springer 2005).
The most notable clinical successes of gene therapy has been with clinical trials for severe combined immunodeficiency (SCID-X1) disease adenosine deaminase-severe combined immunodeficiency (ADA-SCID), and Leber's congenital amaurosis (LCA). Of these three, the treatments of SCID-X1 and ADA-SCID used ex vivo gene therapy while the LCA treatment utilized in vivo gene therapy. Based on the status of the clinical trials, it is not possible to determine which strategy has had more clinical success. Each strategy has had notable clinical success. Each strategy is implemented in particular situations since each strategy has its own advantages, disadvantages, and limitations. Ex vivo has several advantages including high transduction efficiency and the ability to evaluate transduction efficiency before implantation of cells back into the patient. Ex vivo gene therapy is limited by its requirement for mitotic cells, but it is usually associated with less immunogenic responses. In vivo gene therapy is the preferred strategy by most scientists. However, this has the major obstacle requiring highly targeted delivery so that only the desired cells and tissues receive the viral treatment. Imperfect targeted delivery usually involves an immune response, which poses many risks to patients.
Gene therapy became a topic of interest to the media in 1999 due to controversy surrounding the fatal outcome of a gene therapy treatment on an 18-year old man who was afflicted with an ornithine transcarbamylase (OTC) deficiency, deficiency of an enzyme that breaks down ammonia in the liver (Smith and Byers 2002). An OTC deficiency is the result of a defective gene located on the X chromosome resulting in insufficient production of OTC. The young man died 98 hours after gene transfer due to systemic inflammatory response syndrome. This patient was the eighteenth subject injected with the treatment and received the highest dose in this dose escalation study. The previous 17 subjects (including those who received the same dose) did not exhibit similar serious adverse events (Raper et al. 2003).
The unanticipated, early death of this patient was shocking news to the world because it was perceived as a direct consequence of the experimental gene therapy treatment. His was the first recognized human death due to gene-based treatment. Immediate halt of all gene therapy research work was necessary due to concern that this particular patient was unaware of the full spectrum of his gene-based treatment and he was inadequately informed about the potential adverse outcomes. Intense regulatory investigations have led to more stringent safeguards for human safety in clinical trials and further strengthened the concept of informed consent (Smith and Byers 2002; Zallen 2000). Investigators studying gene therapy were urged not to give false promises about the gene therapy applications and to disclose any potential conflicts of interest.
Conclusions and Future Directions
Despite the setbacks, the future of gene therapy remains encouraging. The successes of gene therapy in multiple paradigms (SCID-X1, ADA-SCID, and LCA) have demonstrated tangible clinical success for both ex vivo and in vivo gene therapy. Many scientists continue to believe that gene therapy will have a major role in the treatment of many forms of clinical disorders. The field has matured to include appropriate governmental regulations to protect the patients. As of 2007, there have been 1,340 completed or approved gene therapy clinical trials in 28 different countries worldwide, 400 since 2004. The United States is responsible for 64.2% of all gene therapy trials. Other major contributors include the United Kingdom at 11.1%, Germany at 5.5%, and Switzerland at 3.1% (Edelstein et al. 2007). It is likely that intense research supported by dedicated and focused team members of genetic scientists will yield to new or added gene-based therapeutic options for fatal or disabling forms of clinical diseases.
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