Liver gene therapy: Will gene therapy deliver to the liver patient?

Abstract

The gene therapy dream started well over a decade ago. Despite its wide and successful application in research, it still has not reached the clinics in a meaningful way. No doubt that successful gene therapy has a lot to offer to patients with inherited, benign or malignant diseases. Several challenges need to be addressed for gene therapy to succeed. The therapeutic gene has to go from the general circulation to the targeted diseased liver cell. It has to go through the cell membrane, escape the lysosome compartment in the cytoplasm and subsequently penetrate the nuclear membrane and reach the nucleosome. Finally, the gene should be expressed highly enough to produce significant level of protein that would change the behaviour of targeted cells and/or affect the function of neighbouring or distant cells. Various imaginative ways were developed to overcome these difficulties, such as the use of liposome to surround the plasmid containing the therapeutic gene, the introduction of the cassette containing the gene in a viral vector such as adenovirus, adeno-associated virus, retrovirus, vaccinia virus and others. These were successful but not enough to have a clinical impact. They were not effective as in non-viral plasmid approach. They were toxic such as in the use of adenoviruses. They were also associated with malignant transformation such as in the use of retrovirus. The most potent viral vector available today is the lentivirus. However due to its inherent danger this vector has only been used today in HIV patients. It is difficult to speculate on its introduction to the clinic in the near future let alone in patients with benign disease. Our group has been involved in several clinical trials with the use of plasmid DNA, adenovirus, retrovirus and vaccinia virus. It is unfortunate that these clinical trials failed. Despite our lack of success our research is still going strong. Currently we believe clinical breakthrough success in the near future could be achieved via one of these three approaches: (1) Hydrodynamic Gene Delivery (fig. 35.1, 35.2, 35.3): This technique was developed successfully in mice. We applied it successfully in pigs and subsequently to man. The principle is the percutaneous introduction of a Gene-Cath via the internal jugular vein or femoral vein to the hepatic vein. Then a large volume of fluid containing the therapeutic gene is injected rapidly. This creates a large pressure on the hepatocyte and produces holes in their cell membrane through which the plasmid gets entry into the hepatocytes. (2) The cholesterol-DNA mixture that allows genes to be introduced into the hepatocytes via a systemic injection. This is a new technique and if successful will contribute greatly to the field. (3) Combination of gene with cell therapy and in particular with adult bone-marrow derived stem cells progenitors. This approach allows the introduction of genes using viral vectors in an in-vitro ex-vivo approach where the cells are exposed to the vector and then the cells are washed. This would remove any virus that is still remaining outside the cells, therefore reducing the risks of potential toxicity and potential immune reactions. Some of the clinical applications potentially could include the following: -Introducing the genetically defective genes in adult bone marrow stem cell progenitors in diseases such as cystic fibrosis, glycogen storage diseases etc. - Introducing siRNA in the stem cells in order to protect them from HBV and HCV infection. -Introducing in the stem cells anti-cancer genes that would produce soluble tumour suppressor genes, cytokines or suicidal genes with by-stander effect. We believe that the above three approaches will have their clinical debut in the near future. If successful this will lead to the introduction of clinical gene therapy in conditions, where transient gene expression could be beneficial in patients with liver disease. Long term gene expression will remain a problem with existing technologies. Currently the only technology that offers gene expression is with integra ing viruses such as retrovirus, lentivirus or some non-viral approaches such as with the integrate system. These approaches succeed in long term expression as they lead to genetic integration in the host DNA. However, the integration is random and can occur at sensitive locus that may lead to activation of an oncogene or inactivation of a tumour suppressor gene leading to cell malignant transformation. At the time of writing long term gene expression remains a clinical challenge that still awaits further scientific breakthrough. It is a reality that sometimes attractive scientific and biological concepts take long time for their clinical exploitation. Translational research can be a long lead way from the basic science discovery to the clinical application such as in the monoclonal field. Once considered as an area that never delivered monoclonal antibodies have provided recently important pharmaceuticals nearly three decades following their d6but in basic research. No doubt the same will happen in the gene therapy area. Its success will be both in its purest basic form with systemic and local delivery of genetic material as well as piggy-backed on the cell therapy application such as stem cell, adoptive T-cell or dendritic cell. When the gene therapy field will succeed it will have a major positive impact in the management of patients with liver diseases. In this paper we summarised recent developments in gene therapy for malignant and non malignant liver diseases. © 2006 Springer-Verlag/Wien.