Liver-directed Gene Therapy
CITATION
Kamimura K, Abe H, Suda T, Aoyagi Y, Liu D (2013) Liver-directed Gene Therapy. JSM Gastroenterol Hepatol 1(1): 1005.
INTRODUCTION
Liver is a vital organ in the body responsible for detoxification, protein synthesis and metabolism. Pathologically, the liver is involved in many metabolic and monogenic diseases [1]. The intrinsic anatomic properties of liver make it a preferred target for gene therapy of liver originated or monogenic diseases. Although none of the currently available methods of gene delivery is optimal for liver gene therapy, the concerted effort from researchers has provided a wide range of choices for gene transfer to the liver [2-4]. The objective of this mini-review is to provide a brief summary for various methods developed thus far that are applicable to liver gene therapy (Table 1).
| Method | Functional Component | Advantages | Disadvantage |
| Viral Vectors | |||
| Oncoretrovirus | RNA | High efficiency | Random integration, low titer |
| Lentivirus | RNA | High efficiency, sustained gene expression | Random integration, low titer |
| Foamy virus | RNA | High efficiency, sustained gene expression | Random integration, low titer |
| Adenovirus | Double stranded DNA | High efficiency, sustained gene expression, infect non-dividing cells | Host innate immune response |
| Adeno-associated virus | Single stranded DNA | No pathogenic, sustained gene expression, infect to non-dividing cells | Integration may occur, small capacity of transgene, low titer |
| Herpes simplex virus | Double stranded DNA | No pathogenic, sustained gene expression, infect to non-dividing cells | Integration may occur, small capacity of transgene, low titer |
| Nonviral Vectors | |||
| Lipids | Cationic lipids | No pathogenic, sustained gene expression, infect to non-dividing cells | Low efficiency in vivo, acute immune response |
| Polymers | Cationic polymers | Highly effective in vitro, ease to prepare | Toxic to cells, acute immune response |
| Proteins | Natural or chemically modified proteins in cationic nature | Highly effective in vitro, less toxic, can be target specific | Low activity in vivo |
| Peptides | Lysine or arginine residues in peptides | Highly effective in vitro, less toxic, can be target specific | Low activity in vivo |
| Physical Methods | |||
| Needle injection | Mechanic force | Simple | Low efficiency, expression limited to needle track |
| Gene gun | Pressure | Good efficiency | Limited to target area, need surgical procedure for internal organ |
| Electroporation | Electric pulse | High efficiency | Tissue damage, limited target area, need surgical procedure for internal organ |
| Sonoporation | Ultrasound | Site specific | Low efficiency, tissue damage |
| Hydrodynamic delivery | Hydrodynamic pressure | Simple, high efficiency, site specific | Need catheter insertion technique in large animals |
Table 1: Features of Liver-directed Gene Delivery Systems
Major advantages and disadvantages of each method are also provided for practical consideration
Virus-based gene-delivery system
Virus-based gene delivery system represents a group of artificially made, replication deficient viruses [5]. The most commonly used ones are adeno-associated viral vectors [6], lentiviral vectors [7], and adenoviral vectors [8]. Viral vectors under the development include foamy viral vectors [9], herpes simplex viral vectors [10], and oncoretroviral vectors [11]. Viral vector-mediated gene delivery to liver can be achieved via the hepatic artery [12,13], portal vein [14,15], or bile duct [14] or by direct injection to the liver [14]. Recent progress in a pilot phaseII trial revealed that the hepatic arterial injection of recombinant adenovirus p53 is safe and effective in unresectable hepatocellular carcinoma [15]. Adeno-associated virus 8 prefers hepatocytes [17] and has been used for liver-targeted gene therapy intended for treatment of the citrullinemia [18], hemophilia [19], alpha 1-antitrypsin deficiency [20] and viral hepatitis [21] diseases. Viral vectors are highly effective in gene delivery and have been used in approximately 67% clinical trials [22]. Viral vector based carcinogenesis and immunogenicity represent currently the major hurdle for viral vector-mediated gene therapy.
Nonviral gene-delivery system
Compared to viral vectors that employ their natural ability to transfer gene into cells, nonviral gene delivery systems use a physical force or cellular function of endocytosis to facilitate gene transfer to target cells. They are divided into two categories including nonviral vector-mediated gene delivery and physical methods.
Nonviral vectors are synthetic or natural compounds that are capable of forming complexes with plasmid DNA or gene coding fragments and facilitating intracellular gene transfer. Materials including lipids [23], polymers [23], proteins [24], and peptides [25] have been shown to be effective for gene delivery. Nonviral vectors have been evaluated for gene therapy of varieties of liver diseases including hepatic fibrosis, viral hepatitis, and liver cancer [26]. Taking advantage of membrane receptors on hepatic stellate cells, liver-targeted gene delivery for hepatic fibrosis has been attempted using mannose 6-phosphate/insulin-like growth factor-II receptor [27], integrins [28], high-affinity membrane receptor for retinol-binding protein [29], and galactosyl receptor [30] as the targets. Target specific gene delivery is a most desirable feature of any gene delivery systems. Clinically, 24% of gene therapy clinical trials have been conducted using nonviral vectors [22]. The major challenge for nonviral vector-mediated gene delivery is its relatively low efficiency.
Physical methods of gene delivery employ a physical force to overcome the membrane barrier of a cell. Compared to viral and nonviral vector-mediated gene delivery, physical approaches do not involve any substances that could be cytotoxic or immunogenic. Physical methods employed for gene delivery include needle injection, gene gun, electroporation, sonoporation, and hydrodynamic gene delivery [2]. Among these methods, hydrodynamic gene delivery has been the most efficient method for gene delivery to the liver, especially in small animals. This method has been used for functional analysis of therapeutic genes and regulatory elements in rodents since its establishment in 1999 [31,32]. Efforts have been made in developing a clinically applicable procedure for hydrodynamic gene delivery to the liver. For instance, Kamimura et al. examined a catheter insertion technique to hepatic lobular vein, which is a clinically well-established method, for site-specific, safe, and efficient gene delivery in large animals [33-35]. In combination with computer programming, engineering, and imaging technology, it is highly possible that an effective, simple, and safe hydrodynamic gene delivery to selected site of the liver will be achieved in near future. The remaining challenge for hydrodynamic gene delivery for gene therapy of liver diseases is to conduct safety and efficacy assessment in nonhuman primates to fine-tune different parameters in order to ensure clinical success in gene therapy for various liver diseases.
PERSPECTIVES
Despite the progress made in developing various methods for effective gene delivery, gene therapy for treatment of liver diseases remains in its infancy. This is primarily due to the fact that many of the liver diseases progress into a fibrotic stage with significant change of liver parenchyma, vasculature, and sinusoids. Consequently, efficient gene delivery by various highly effective methods established using health liver in animals cannot be achieved, resulting in insufficient production of gene product and failure to achieve a successful cure. Evidently, future studies need to take into the consideration of disease status when optimizing a method of gene delivery. There is no doubt, however, gene therapy will become one of the most effective treatments for liver diseases that are not curable with currently available modalities.
ACKNOWLEDGMENT
The authors would like to thank Enago for the English language review. The research in the authors’ laboratory has been supported in part by grant support from the Japanese Society for the Promotion of Sciences (22890064 and 23790595), Takara Bio Research Grant from JSGT, and Adaptable and Seamless Technology Transfer Program through target-driven R&D, JST to Kenya Kamimura, and from the National Institute of Health (EB002946 and HL075542) to Dexi Liu.
