Human hepatitis B viral infection is widespread and can lead to chronic infection. This virus causes transient and chronic noncytocidal infection of hepatocytes. The chronic liver disease is thought to be largely due to the host immune response, which induces a high level of hepatocyte destruction, leading to disruption of liver function. There is concern that newly emerging viral variants may threaten the efficiency of currently used HBV vaccines and therefore there is a rising need for novel antiviral strategies The new therapy involves ribozyme technology - a new class of drugs that have shown promise in treating a variety of diseases.
HB infection prevention:
Ø A safe and effective genetically engineered vaccine for hepatitis B is available. It is given in 3 subcutaneous injections (just under the skin) generally over a period of 6 months and conveys immunity in 90 to 95% of people treated. At the end of the course of injections a blood test is taken to see if you have developed the required antibodies. For the 5 - 10% of people who do not respond some new research has shown that a repeat course of injections given intramuscularly can create an immune response in between 62-98% (depending on several factors) of those who did not respond or whose response did not last when given subcutaneously.
Ø Once vaccinated present it is important to be periodically tested to ensure that the body has sufficient levels of antibodies to prevent infection and a single booster dose may be required every 5 to 10 years to ensure immunity from infection.
Ø At present vaccines are ineffective for those already infected with the hepatitis B virus.
Theoretically, gene therapy can be targeted to somatic (body) or germ (egg and sperm) cells. In somatic gene therapy the recipient's genome is changed, but the change is not passed along to the next generation. This form of gene therapy is contrasted with germline gene therapy, in which a goal is to pass the change on to offspring. Germline gene therapy is not being actively investigated, at least in larger animals and humans, although a lot of discussion is being conducted about its value and desirability. Viruses have evolved a way of encapsulating and delivering their genes to human cells in a pathogenic manner. Scientists have tried to take advantage of the virus's biology and manipulate its genome to remove the disease-causing genes and insert therapeutic genes. These gene-delivery vehicles will make this field a reality.
Recent gene therapy approaches promise to avoid these repeated injections, which can be painful, impractical, and extremely expensive. One method uses a new vector called adeno-associated virus, an organism that causes no known disease and doesn't trigger patient immune response. The vector takes up residence in the cells, which then express the corrected gene to manufacture the protein. In hemophilia treatments, for example, a gene-carrying vector could be injected into a muscle, prompting the muscle cells to produce Factor IX and thus prevent bleeding. This method would end the need for injections of Factor IX --a derivative of pooled blood products and a potential source of HIV and hepatitis infection
Advances in human gene therapy may allow doctors to treat a disease or abnormal medical condition by turning off a faulty gene and stopping the growth of a cancerous tumor, for example. Or they may allow the body to begin producing a necessary protein or other substance, such as an enzyme, that the faulty gene cannot order the body to produce. One of the most exciting and (and controversial) highly publicized areas in biomedical research today is what is known as human gene therapy - the replacement of a person's faulty genetic material with normal genetic material to treat or cure a disease or abnormal medical condition.
Developed systems for generating recombinant viral vectors of the following viruses: adenovirus, adeno-associated virus (AAV) and lentiviruses. These viral vector systems are utilized in various gene therapy projects. Developing gene therapy technologies for specific targeting of liver cancer cells. These technologies involve the construction of adeno and AAV vectors carrying various suicide genes whose expression is driven by promoters that are active only in cancerous tissues. The system will be tested in tissue culture and subsequently in mouse models.
New vaccines are being developed and some of these promise increased response rates, only require a single injection and some may be effective for people with chronic hepatitis B. The chronic liver disease is thought to be largely due to the host immune response, which induces a high level of hepatocyte destruction, leading to disruption of liver function. There is concern that newly emerging viral variants may threaten the efficiency of currently used HBV vaccines and therefore there is a rising need for novel antiviral strategies.
But although prevention is important, curing those infected is urgent because current treatment has largely been ineffective. Some novel technologies developed for HBV infection gene therapy are summarized here in.
Novel anti-viral strategies
1) RNA interference (RNAi)
RNA interference (RNAi) is a recently discovered phenomenon, which is based on the observation that double stranded RNA can efficiently inhibit gene expression by promoting selective mRNA degradation. The goal of our research project is to inhibit the production of HBs antigen of HBV by expressing a specific dsRNA transcript in virus infected cells. This technology may provide new anti-viral strategies and lead to the development of novel anti-HBV therapies.
2) Ribozyme technology - (a new class of drugs that have shown promise in treating a variety of diseases)
This technique is developed by Prof. Patrick Arbuthnot, an MRC-supported researcher, was recently awarded a multi-million rand grant to perfect a gene therapy against hepatitis B. He has developed and patented a new gene therapy to cure hepatitis B, and has now been awarded a R9, 1 million Innovation Fund grant from the Department of Arts, Culture, Science and Technology.
The main advantage of this technology is that, it is aimed at trying to develop a new approach to curing people who are chronically infected with hepatitis B so that they are not at risk for liver cancer.
The ribozymes are RNA (ribonucleic acid) molecules that behave like enzymes or 'molecular scissors' and catalyse reactions that result in the specific cutting of a different target RNA molecule. They can be designed to recognise, bind to and cut any disease-causing RNA such as hepatitis RNA. They can also stop the virus from reproducing. Ribozyme gene therapy, which has been shown to block the replication of the virus in cell culture models of hepatitis B infection.
3). Target-dependent ribozymes (Targezymes)
This is a new platform technology for the development of biological targeting vectors with the capability to switch on gene expression within target cells. Target-dependent ribozymes (Targezymes), derived from a modified form of self-splicing group I introns, are introduced into the gene of interest. The splicing of these introns is dependent on the presence of a cell-specific RNA (or other biological molecule), that, in turn, causes the activation of the expression of the desired gene.
Modified group I introns are engineered to combine with the desired target and self-splice. In order to accommodate the interaction with the target, matching changes have to be introduced. As a result the splicing activity is usually impaired, resulting in a highly inefficient targezyme. In order to restore the activity to functionally significant levels, the best fitting targezyme is selected from a pool of mutagenized targezymes by in vitro evolution (IVE). Although, the proposed technology can be applied in a large number of diseases, this project will focused on the use of the technology in the treatment of chronic HBV infection. As a model we will apply our in house HBV replication permissive human hepatocellular cell lines. The novel technologies are emerging for the cure and prevention of Hepatitis -B infection. These technologies if implemented, will start a new era in the Genetic Therapy of Hepatitis-B and other infectitious diseases.
-- The authors are with Pharmacy group, Birla Institute of Technology and Science, Pilani