Abstract: Advances in understanding the molecular basis of obesity and obesity-associated diseases have made gene therapy a vital approach in coping with this world-wide epidemic. Gene therapy for obesity aims to increase or decrease gene product in favor of lipolysis and energy expenditure, leading toward fat reduction and loss of body weight. It involves successful delivery and expression of therapeutic genes in appropriate cells. The ultimate goal of gene therapy is to restore and maintain energy homeostasis. Here we summarize progress made in recent years in identifying genes responsible for obesity and present examples where the gene therapy approach has been applied to treating or preventing obesity. Discussion on advantages and limitations of gene therapy strategies employed is provided. The intent of this review is to inspire further studies toward the development of new strategies for successful treatment of obesity and obesity-associated diseases.
Obesity is a medical condition under which excess body fat accumulates in adipose tissue due to an imbalance in energy intake and expenditure. According to the National Health and Nutrition Examination Survey, more than one-third (~34.9%) of adults in the U.S. were obese in 2011-2012 (Ogden et al., 2013). Obesity is closely linked to a cluster of metabolic disorders, including diabetes, hyperlipidemia, nonalcoholic fatty liver disease, hypertension, and cardiovascular diseases, as well as a wide range of cancers including liver cancer, prostate cancer, breast cancer, and colon cancer (Park et al., 2010). Obesity associated medical costs in the U.S. are more than $150 billion each year (Hammond and Levine, 2010).
Obesity tends to run in families, suggesting a genetic cause. However, family members share not only genes, but also diet and lifestyle. Diet and lifestyle are two major environmental factors closely associated with the prevalence of obesity. High-fat diets (HFD), due to their auspicious taste, usually instigate more energy intake than the body requires, eventually causing obesity if accompanied by inadequate physical excise.
Biochemically, obesity is a physiological condition in which the fat storage exceeds the requirement limits. This is a result of the biochemical processes involving enzymes or genes responsible for maintaining metabolic homeostasis. If biochemical reactions in favor of fat synthesis and accumulation exceed those of the biochemical processes responsible for energy expenditure, obesity develops. As both lipid accumulation and consumption are functions of specific groups of proteins encoded by DNA sequences, genetic defects within the coding sequence or regulatory sequences for its expression constitute the bases for obesity. Current strategies in treating or preventing obesity focus on blocking lipid accumulation or/and increasing energy expenditure. The gene therapy-based approach focuses on transfer of gene coding or non-coding sequences to produce necessary and critical proteins to re-establish and maintain metabolic homeostasis.
Genes Known for Their Critical Role in Obesity
Metabolic homeostasis requires a coordinated and balanced activity from numerous genes including those coding for proteins/enzymes responsible for food intake, lipogenesis, lipolysis, glucose metabolism, and fat storage in adipose tissue. Genes that regulate the expression of these essential genes also play a critical role in determining the metabolic processes. Over or under expression of one or multiple genes critical for metabolic homeostasis can lead to obesity (Herrera and Lindgren, 2010) when energy is in excess. For example, mice deficient in leptin or its receptor in the brain are morbidly obese and have severe complications (Zhang et al., 1994). Genetic variants of pro-opiomelanocortin (POMC), pro-protein convertase subtilisin/kexin 1 (PCSK1), and melanocortin 4 receptor (MC4R) genes are a major cause of the 5% of severe obesity cases in the early stages of child development (Farooqi and O’Rahilly, 2005). Similarly, polygenic obesity exists primarily due to additive or synergistic effects of genes that are not sufficient individually for unbalancing the homeostasis (Perusse et al., 2005). Recent genome-wide association studies have identified many genes linking to etiology of obesity (Herrera and Lindgren, 2010), some of which are summarized in Table 1.
The most studied gene directly linked to obesity is leptin, which plays a key role in regulating energy intake, affecting appetite and hunger (Zhang et al., 1994). It is one of the most important adipose derived hormones. Leptin functions by binding to the leptin receptor in the brain to control the hunger sensation. Mice homozygous for the ob mutation (ob/ob) ate ravenously and developed severe obesity (Zhang et al., 1994). Similar to leptin, POMC plays an important role in regulating food intake by producing a cluster of key factors including the melanocortin peptides adrenocorticotrophin (ACTH), melanocyte-stimulating hormones (MSH) alpha, beta, and gamma, as well as the opioid-receptor ligand beta-endorphin (Krude et al., 1998). A genetic defect within the POMC gene has been linked to the development of severe early-onset obesity (Krude et al., 1998). Pro-protein convertase subtilisin/kexin 1 (PCSK1) is an essential enzyme required in post-translational processing of POMC, and a defect of the PCSK1 gene also results in obesity (Jackson et al., 1997). After being processed from POMC by PCSK1, MSH binds to its receptor, the MC4R, to exert its function in controlling food intake. A frame-shift mutation in MC4R is found to be associated with dominantly inherited human obesity (Vaisse et al., 1998). The above genes work collectively in regulating eating behavior, and a defect in either has resulted in severe obesity in humans.
The most critical genes in energy expenditure are those coding for uncoupling proteins (UCPs). UCPs are responsible for non-shivering thermogenesis which occurs primarily in brown adipose tissue (Nagai et al., 2011). UCP1 is a proton transporter located in the inner mitochondrial membrane and is abundant in brown adipose tissue. Accumulating studies demonstrate that UCP1 genetic polymorphism diminishes resting energy expenditure, which has an adverse effect on the regulation of energy balance (Nagai et al., 2011). Similar to UCP1, UCP2 and UCP3 are also involved in non-shivering thermogenesis, and, according to recent studies, variants of these proteins are linked to the prevalence of obesity (Qian et al., 2013). Non-shivering thermogenesis is regulated by the sympathetic nervous system and the adrenergic receptor signaling pathway. Therefore, adrenergic receptors such as β1-adrenergic receptor (ADRB1) and β3-adrenergic receptor (ADRB3) play critical roles in regulating energy expenditure, and genetic polymorphisms of these receptors have been linked to obesity and type 2 diabetes (Mattsson et al., 2011). Ueta et al. (2012) have shown that adrb1 gene knockout mice were more susceptible to obesity than the wild-type when fed an HFD, a condition possibly caused by impaired adaptive thermogenesis in brown adipose tissue.
The peroxisome proliferator-activated receptor-γ (PPAR-γ) is a master transcriptional factor for fat synthesis and accumulation that elicits expression of the set of genes crucial for lipogenesis and adipogenesis (Wang et al., 2013a). Adipose-specific disruption of PPAR-γ results in lipoatrophy which is accompanied by a cluster of severe metabolic complications (Wang et al., 2013a). Intriguingly, ectopic activation of PPAR-γ in brain or pancreas is linked to exacerbated obesity and glucose intolerance (Lu et al., 2011; Ohmura et al., 2010). As a target gene of PPAR-γ, fatty acid binding protein 4 (FABP4) serves as a cytoplasmic fatty acid chaperone, and genetic disruption of FABP4 leads to increased body weight and fat mass in mice on HFD (Yang et al., 2011). Similar to FABP4, the expression of perilipins, a group of proteins that protect lipid droplets from hydrolysis, is also regulated by PPAR-γ in adipose tissue (Arimura et al., 2004). Variants of these lipid droplet surrounding proteins are linked to obesity. For example, genetic deficiency of perilipin 1 causes mice to be resistant to HFD-induced obesity, which is likely induced by elevated basal lipolysis resulting from the loss of the protective function of perilipin on lipid droplets (Tansey et al., 2001). In addition to these chaperones and surrounding proteins, the enzymes for fat synthesis and hydrolysis such as stearoyl-CoA desaturase-1 (SCD-1) and adipose triglyceride lipase (ATGL) are also crucial for adipogenesis and obesity development (Haemmerle et al., 2006; Ntambi et al., 2002).
Additional genes found to be critical in the development of obesity include fat mass- and obesity-associated (FTO) gene (Frayling et al., 2007). FTO is primarily expressed in the brain, within a particular area of the hypothalamus. Genetic variants of the FTO gene are associated with body mass index and predispose to childhood and adult obesity (Frayling et al., 2007). In addition to FTO, the src homology 2B adaptor protein 1 (SH2B1) has also been linked to obesity (Ren et al., 2005). SH2B1 is a member of a family of adaptor proteins that influences a variety of signaling pathways mediated by Janus kinase (JAK) and receptor tyrosine kinases. Mice with a genetic disruption of SH2B1 develop obesity and metabolic syndrome, indicating that this gene is required in maintaining normal energy metabolism and body weight in mice (Ren et al., 2005). Additionally, recent genome-wide association studies reveal that an additional crucial gene, transmembrane protein 18 (TMEM18), is related to obesity (Almen et al., 2010). TMEM18 is active in nearly all tissue; however, its expression is especially important in the brain, specifically in the hypothalamus, suggesting that it may be involved in obesity development via regulating eating behavior and energy metabolism. Emerging evidence suggests that TMEM18 is one of the most conserved human obesity genes and its genetic variants are tightly associated with both adult and childhood obesity (Almen et al., 2010).
Examples of Gene Therapy-based Approaches for Improvement of Obesity and Obesity-associated Disorders
Genetic bases for obesity have prompted a significant effort in developing gene therapy for treating and preventing obesity. Many attempts have been made to provide copies of a functional gene to cells to compensate the loss of functional protein due to a hereditary defect. In contrast, approaches are also taken in an attempt to decrease the level of a gene product responsible for obesity. Table 2 provides a brief summary of a few examples that have emerged in the past few years.
A defect in ob gene coding for leptin has been linked to severe obesity (Zhang et al., 1994). Muzzin et al. (1996) demonstrated that recombinant adenovirus (Ad)-mediated leptin gene transfer in genetic ob/ob mice corrected obesity and diabetes. In addition to systemic administration, leptin gene delivery to the central nervous system has also been studied; however, it generated distinct effects in varying animal models. Using a single injection of recombinant adeno-associated virus (AAV) carrying the leptin gene into the third cerebroventricle of normal adult rats fed an HFD, Kalra and colleagues proved that central leptin gene transfer was able to generate long-term beneficial effects in maintaining body weight and improving systemic metabolism, possibly due to reduced food intake and a concomitant increase in energy expenditure (Kalra and Kalra, 2002). Using a similar approach, Wilsey et al. (2003) evaluated the therapeutic effect of AAV-mediated central delivery of leptin gene in HFD-induced obese rats. Although normal rats on regular chow responded robustly to this treatment showing significant weight loss, HFD-induced obese rats were unresponsive, likely caused by a reduction in the leptin receptor. Indeed, an additional study by Morton et al. (2003) demonstrated that central delivery of leptin receptor gene greatly reduced body weight of Koletsky rats, supporting the notion that leptin sensitivity in the brain is crucial in its therapeutic effect in obesity.
Glucagon-like peptide-1 (GLP-1) is an endogenous glucose-lowering peptide which plays important roles in maintaining postprandial glucose homeostasis and regulating appetite (Holst, 2007). Emerging studies proved that the action of GLP-1 is impaired in obese subjects, even in those showing normal glucose tolerance (Madsbad, 2014). In this context, using a helper-dependent adenoviral vector, Samson et al. (2008) evaluated the therapeutic effects of long-term expression of exendin-4, a polypeptide agonist of GLP-1 receptor, in C57BL/6 mice fed an HFD. Their data showed that transfer of the exendin-4 gene repressed body weight gain in the mice on HFD, consequently leading to improved glucose homeostasis, which was also partly mediated by elevated energy expenditure. In addition to this study, a more recent investigation by Di Pasquale et al. (2012) demonstrated that AAV-mediated exendin-4 expression in salivary glands suppressed body weight gain and alleviated insulin resistance in CD1 mice and Zucker fa/fa rats fed an HFD.
Brain-derived neurotrophic factor (BDNF) is a member of the “neurotrophin” family of growth factors which primarily acts on neuron differentiation, growth, and survival. Deficiency of BDNF causes severe hyperphagic obesity (Liao et al., 2012). Consistently, using intraperitoneal injection of BDNF recombinant protein in HFD-induced obese C57BL/6 mice and genetic obese KKAy mice, Nakagawa et al. (2003) demonstrated that exogenous administration of the BDNF protein was able to reduce cumulative food intake, consequently generating anti-obesity and anti-diabetic effects. Likewise, transfer of the BDNF gene is able to generate similar beneficial effects in metabolism. For example, using AAV-mediated BDNF gene transfer in the brain, Cao et al. (2009) developed a physiological auto-regulatory cassette which can produce a tightly controlled BDNF protein expression profile, mimicking the body’s endogenous physiological feedback mechanisms, and leading to resetting the hypothalamic set point to reverse obesity and to improve its related metabolic dysfunctions in genetic db/db mice.
Preclinical evaluation of fibroblast growth factor 21 (FGF21) gene-based gene therapy has been conducted. FGF21 is a secreted protein that plays important roles in regulating glucose and lipid metabolism (Kharitonenkov, 2009). Unlike many other members of the fibroblast growth factor family, FGF21 does not function in cell proliferation and differentiation. Mice lacking FGF21 show accelerated development of obesity when fed an HFD (Kharitonenkov, 2009). On the contrary, exogenous administration of FGF21 protein is effective in reducing body weight, indicating the potential application of FGF21 for treatment of obesity (Kharitonenkov, 2009). The pharmacokinetics of the FGF21 recombinant protein shows in mice a blood half-life of less than 2 hours (Xu et al., 2009). Therefore, multiple repeated injections are required to produce sustained benefits in treating obesity and diabetes. In this context, we evaluated the therapeutic effect of FGF21 gene transfer using a hydrodynamics-based procedure in HFD-induced obese C57BL/6 mice. Our results demonstrated that FGF21 gene transfer generated a sustained high level of blood FGF21, leading to a variety of beneficial effects including reduction of adiposity, alleviation of fatty liver, and improvement of glucose tolerance, which was correlated with altered expression of crucial genes involved in thermogenesis, adipogenesis, and chronic inflammation (Gao et al., 2014).
Adiponectin is an adipose-derived adipokine which is down-regulated in obesity (Fasshauer et al., 2004). Emerging evidence suggests that genetic disruption of the adiponectin gene accelerated HFD-induced obesity in C57BL/6 mice, which is associated with decreased myocardial autophagy (Guo et al., 2013). On the contrary, electroporation-mediated gene transfer of mouse adiponectin plasmid DNA into gastrocnemius muscle of C57BL/6 mice greatly suppressed diet-induced body weight gain (Kandasamy et al., 2012). As the adiponectin receptor is also down-regulated in obesity, we concurrently transferred genes of adiponectin and its receptor in HFD-fed AKR/J mice, and proved that this strategy was effective in blocking obesity and obesity-related insulin resistance (Ma and Liu, 2013). Adiponectin gene therapy is preferred over protein therapy due to the aggregate-prone property of recombinant adiponectin (Gao et al., 2013b; Li et al., 2012).
Irisin is a secreted protein serving as a messenger between muscle and adipose tissue (Bostrom et al., 2012). Overexpression of Irisin via Ad-mediated gene transfer in HFD-induced obese mice slightly reduces body weight and greatly improves glucose tolerance which is achieved primarily by browning white fat and elevated energy expenditure (Bostrom et al., 2012). A more recent study by Lee et al. (2014) demonstrated that cold exposure can also lead to an increase in circulating Irisin level, indicating that secretion of this protein could have evolved from shivering-related muscle contraction. Mechanistically, Irisin induces phosphorylation of the p38 mitogen-activated protein kinase and extracellular signal-related kinase signaling pathways, consequently upregulating UCP-1, decreasing body weight, and improving glucose homeostasis (Zhang et al., 2014).
Several lines of evidence suggest that chronic inflammation serves as a central player in obesity and obesity associated metabolic disorders. The chronic inflammation associated with obesity is primarily stemmed from adipose infiltrated macrophages. Importantly, suppression of chronic inflammation via depletion of adipose macrophages prevented obesity and improved systemic metabolism in C57BL/6 mice fed an HFD (Bu et al., 2013). To block diet induced and obesity associated inflammation, we assessed the effect of hydrodynamics-based transfer of IL10 gene in C57BL/6 mice fed an HFD. Consistent with previous studies, our results clearly show that IL-10 gene transfer significantly improved insulin resistance and glucose intolerance in HFD-fed C57BL/6 mice. Intriguingly, in addition to its benefits in glucose metabolism, sustained IL-10 overexpression greatly suppressed HFD-induced weight gain, which was associated with suppressed adipose macrophage infiltration and repressed chronic inflammation (Gao et al., 2013c).
One typical metabolic complication of obesity is insulin resistance, which subsequently results in dysfunction and apoptosis of pancreatic β cells. Increasing proliferation of pancreatic β cells remains a challenge for treatment of obesity-associated diabetes. A recent study by Yi et al. (2013) demonstrated that beta-trophin was effective in accelerating pancreatic β cell proliferation. Beta-trophin is a secreted protein primarily produced by the liver, serving as a messenger for cross-talk between liver and pancreas. Although a more recent study by Wang et al. (2013b) proved that beta-trophin was also involved in regulation of triglyceride metabolism and mice deficient of beta-trophin did not show impairment in glucose homeostasis, the in vivo data of Yi et al. (2013) elegantly demonstrated that overexpression of beta-trophin by a hydrodynamics-based procedure potently induced proliferation of pancreatic β cells, leading to elevation of circulating insulin and improvement of glucose tolerance in mice. These data strongly suggest that the beta-trophin gene can potentially be used as a therapeutic candidate in gene therapy for obesity-associated diabetes.
The siRNA-based gene silencing approach via gene transfer to decrease gene expression has been evaluated for its effectiveness in obesity treatment. For example, stress-activated c-Jun amino-terminal kinase (JNK) plays a pivotal role in obesity and diabetes. Elevated JNK activity was detected in insulin responsive tissues of both HFD-induced and genetic obese mice (Hirosumi et al., 2002). Genetic disruption of JNK1 protected mice from HFD-induced obesity and diabetes, indicating that JNK1 is a central mediator of obesity (Hirosumi et al., 2002). Additionally, in vivo activation of JNK in pancreatic β cells resulted in glucose intolerance in MKK7D mice (Lanuza-Masdeu et al., 2013). These studies indicated that the suppression of JNK expression may generate benefits in obesity and obesity-associated diabetes. Indeed, in a proof-of-concept study Wilcox et al. (2006) evaluated the strategy of JNK1 knockdown in both HFD-induced and genetic obese mice, and their data showed that Ad-mediated JNK1 shRNA potently reduced JNK1 expression in vivo by ~95%, consequently leading to a ~45% decrease in plasma insulin concentration, indicating that this approach was effective for treatment of obesity-associated hyperinsulinemia.
Hypoxia-inducible factor (HIF) is a transcriptional factor regulating cellular stress responses, and elevated expression of this transcriptional factor has been implicated in pathogenesis of a variety of diseases. Study by Krishnan et al. (2012) proved that obesity in both mice and human was associated with intra-adipose tissue hypoxia and activation of HIF. Mechanistically, HIF1α activation in white adipose tissue maintains obesity via negatively regulating the Sirt2-Pgc1α regulatory axis (Krishnan et al., 2012). In line with this study, adipose tissue specific disruption of HIF1 gene decreased adiposity and improved insulin resistance in C57BL/6 background mice fed an HFD (Jiang et al., 2011). Consistently, two separated investigations using HIF-1α antisense oligonucleotides consistently proved that in vivo knockdown of HIF1α reduced body weight in HFD-induced obese mice (Park et al., 2012; Shin et al., 2012).
Stearoyl-CoA desaturase 1 (SCD1) is a rate-limiting enzyme in the biosynthesis of monounsaturated fatty acids (Ntambi et al., 2002). Mice deficient in SCD1 have reduced body adiposity, increased insulin sensitivity, and are resistant to HFD-induced weight gain (Ntambi et al., 2002). Intriguingly, skin-specific deletion of the SCD-1 gene altered skin lipid composition and protected mice from HFD-induced obesity (Sampath et al., 2009), while liver-specific knockdown of SCD-1 reduced hepatic neutral lipids content without significant impact on body weight change (Xu et al., 2007). In an additional study, it was reported that hepatic SCD1 knockdown (by ~80%) reversed liver insulin resistance in HFD-fed rats, possibly mediated by increasing AKT phosphorylation and decreasing expression of G6P and PEPCK (Gutierrez-Juarez et al., 2006). Collectively, it appears that liver-specific knockdown of SCD-1 was able to improve hepatic lipid and glucose metabolism.
Macrophage-related pro-inflammatory cytokines play a central role in driving adipose chronic inflammation, which has been implicated in the development of a variety of obesity-associated metabolic disorders (Gao et al., 2013a). A recent study by Jourdan et al. (2013) proved that macrophage-specific knockdown of the Cannabinoid type 1 receptor prevented β cell failure and restored normoglycemia in the Zucker diabetic fatty rats. Likewise, adipose tissue specific suppression of macrophage-related chronic inflammation by delivering siRNA targeting TNFα or osteopontin markedly improved glucose homeostasis in genetic ob/ob mice (Aouadi et al., 2013). Although targeted delivery of genetic materials to adipose tissue remains a challenge in practice, results of these previous studies strongly suggest that targeting macrophage-related pro-inflammatory cytokines may be a promising strategy for treating obesity-associated metabolic disorders.
Perspective on Gene Therapy for Treatment and Prevention of Obesity
Obesity is one of the most pressing problems in the U.S. Family studies have shown that genetic factors play a significant role in the pathogenesis of obesity. Rare mutations in humans and model organisms have provided insights into multiple pathways that may lead to obesity. Studies of candidate genes indicate that some genes involved in pathways regulating energy expenditure and food intake play a critical role in the predisposition of obesity. In obesity cases where the genetic defect is clearly identified, transfer of copies of a functional gene to diseased cells will provide a cure similar to monogenic diseases applicable in gene therapy.
In addition to its genetic link, obesity is also considered a “modern” disease caused by changes in lifestyle and diet structure. Excess food intake, especially foods enriched with fat and high calories, and sedentary lifestyle are also considered a major cause of obesity. In principle, fat accumulation, a result of an increase in lipogenesis can be inhibited by decreasing the amount and activity of involved enzymes by overexpressing a transcription factor to selectively suppress transcription of genes responsible for lipogenesis and fat accumulation. Introduction of genes capable of enhancing energy expenditure may reduce fat accumulation and weight loss. In fact, recent animal studies have shown that gene transfer of BDNF (Cao et al., 2009), Irisin (Bostrom et al., 2012), or FGF21 (Gao et al., 2014) to obese mice was effective in reducing fat mass and alleviating insulin resistance and fatty liver, providing direct evidence in support of the gene therapy approach in re-establishing and maintaining the metabolic homeostasis. Although the potential of gene therapy in treating obesity appears evident, additional safety studies involving pharmacokinetics, biodistribution, and toxicity should be accurately evaluated in large animal models before clinical trials might proceed.
Although using gene therapy to treat obesity has been the major focus in the past, efforts have also been made in applying the gene therapy approach to obesity prevention. Progress has been made in recent years, demonstrating that gene transfer of exendin-4 (Di Pasquale et al., 2012; Samson et al., 2008), adiptonectin (Kandasamy et al., 2012; Ma and Liu, 2013), an anti-inflammatory cytokine such as IL-10 (Gao et al., 2013c), and the leptin gene (Kalra and Kalra, 2002) are highly effective in animal models in blocking diet-induced obesity. Our recent studies demonstrate that hydrodynamic delivery of the FGF21 gene enhances thermogenesis in adipose tissue and suppresses de novo lipogenesis in the liver, consequently leading to multiple beneficial metabolic effects in mice fed an HFD, including reduction in adiposity, alleviation of fatty liver, and improvement in glucose homeostasis (Gao et al., 2014). Future work in this area should focus on assessing long-term effects of gene transfer. Efforts in finding new genes that will block lipid absorption and lipogenesis, inflammation in adipose tissue, or enhance energy expenditure, could lead to more effective and better genes that would maintain metabolic homeostasis, even in the presence of excess energy. In our opinion, an ideal system would be a gene construct that carries the functional gene with its expression driven by regulatory elements sensitive to lipid concentration. At higher lipid concentrations, due to an excess food intake, transcription of the inserted function gene takes place, producing enzymes that enhance lipolysis and reduce lipid levels. When the lipid concentration returns to a normal range, gene expression is turned off. Research seeking energy sensitive elements for regulating gene expression is critical in making such a self-regulated genetic circuit that could serve as an extra chromosome gene expression system solely designed for burning off the extra energy to maintain energy homeostasis.
The potential and promise for gene therapy in treating obesity have grown exponentially. Effort made in the past has focused on seeking stable and regulated expression using three different methods of delivery: viral vectors, nonviral vectors, and physical methods. So far, viral vectors remain the most prevalent, despite the safety concerns associated with their in vivo applications. Synthetic vectors or physical methods, on the other hand, offer safer, though less efficient, alternatives to viral vectors. With the exception of hydrodynamic gene transfer, in vivo efficiency of nonviral approaches has not been fully demonstrated thus far. Current research is devoted to engineering more efficient, safer, and targeted systems for gene delivery and is aimed at overcoming challenges associated with current methods of gene delivery. A recent development of artificial endonucleases with tailored specificity has offered a new opportunity to avoid random insertion (Boch et al., 2009; Moscou and Bogdanove, 2009). This new system corrects mutated genes or introduces transgenes in a chosen locus. These targeted approaches markedly differ from current gene therapy strategies based on the random insertion of a complementing virus-borne transgene. As a result, they should bypass the odds of random insertion. With additional efforts it is highly possible to build custom-designed homing endonucleases that are more efficient without toxicity (Mussolino and Cathomen, 2012). Genome based editing represents total new revenue for gene therapy.
Maintaining sustained expression of the transgene is also a critical factor to consider since gene silencing is generally a major hurdle in gene therapy. Recent progress in promoter analysis and vector engineering provides hope in solving the problem of gene silencing (Gracey Maniar et al., 2013; Magnusson et al., 2011). In addition, selecting a proper therapeutic gene in obesity is equally crucial. Unlike cancer or single-gene deficiency-related genetic diseases, obesity is a chronic disease that is not lethal but may lead to medical complications. Careful assessment of the long term effects of gene transfer is absolutely necessary to ensure the safety of the expression of the therapeutic gene. Despite these challenges, it is predictable that the gene therapy based strategy in modulating metabolism and treating metabolic disorders will surely impact how we live a healthy life.
This work was supported in part by NIH grants RO1EB007357 and RO1HL098295. The authors would like to thank Ms. Ryan Fugett for proofreading and English editing of the manuscript.
The authors report no conflicts of interest.
Dexi Liu, Ph.D., Panoz Professor, Department of Pharmaceutical and Biomedical Sciences, College of Pharmacy, University of Georgia, 450 Pharmacy South, 250 W. Green St., Athens, GA 30602, USA.
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