They are the basis of life and hold the keys to unlock the code to counter genetic diseases. Vanessa Mahapatra charts the successes and failures of gene therapy and explores its potential in checking various gene-linked disorders.
Genes have since long been considered the units of all life within our body. But now, scientists are viewing them as the root cause of innumerable disorders. Thus gene therapy comes into existence heralding an answer to numerous genetic woes. “Gene therapy can be applied to any disease where you can identify that some faulty gene causes the disease. There are several diseases like that,” opines Srikumar Suryanarayan, President, R&D, Biocon. Gene therapy follows a process whereby a functional gene replaces an absent or faulty gene, resulting in the restoration of protein action, consequently eliminating the root cause of the disease. Simple as it sounds, gene therapy has been under the limelight for both its notable successes and concurrent failures.
Memoirs of Genes
It all began with W French Anderson, the father of gene therapy, who evolved the concept in a big way leading to the first clinical trial on gene therapy in 1990. Gene therapy was executed on two girls suffering from adenosine deaminase deficiency (ADA), a form of Severe Combined Immunodeficiency Syndrome (SCID). The disease had plagued the immune system of both the girls making them susceptible to repeated infections. Their bodies had extremely low levels of specialised white blood cells, also known as T cells, which are the immune system’s instruments against invading organisms.
Scientists considered that replacing the defective ADA gene would trigger the production of ADA, creating a permanent cure. Therefore, initiating gene therapy on these patients, researchers induced the T cells from their blood to replicate in culture. ADA-bearing retroviral vectors were then transferred into the cultured T cells, which in turn integrated into the DNA and transferred the gene. The enhanced T cells were then reintroduced into the girls. To the researcher’s delight, reported results were remarkable as their immune functions improved progressively. However, one of the patients had to be subjected to continuous treatment as the genetically treated WBCs work for only a few months. She therefore has to be given repeated transfusion of blood containing the ADA gene. The results of the second girl were welcomed more enthusiastically as after a review in 1995 and till date, it has been observed that the other patient has white blood cells bearing copies of the replaced ADA gene.
This optimistic chapter is only one side of the gene therapy story. For this positive, there have been many negatives. There have been quite a few retreats in the research process that have caused caution in proceeding clinical trials. A decade after the first clinical trial, there was a French clinical trial involving 17 children who were suffering with SCID deficiency caused by a defective gamma C gene. Typically called ‘bubble-babies’, these children unlike the other two girls, didn’t have an immune system at all. To counter this, researchers introduced the required gamma C gene into their system with the help of a viral vector. As with the first trial, the results of this clinical trial too were positive, but unfortunately only for the first couple of years. In 2002 one of the children involved in the trial developed leukaemia, followed by another in 2003 and one in 2005. The reason—correct gene reached the wrong target. A general misconception with gene therapy is that the functional gene that is fed into the system is exchanged for the dysfunctional one. However, contrary to this belief the accurate gene is generally not swapped for the defective gene. Instead it just replaces it within the system by lodging itself somewhere in the chromosome and still being effective. “So it is a functional replacement rather than a physical replacement,” explains G Padmanaban, Distinguished Biologist, Indian Institute of Science. In this case, the introduced gene lodged itself with another gene called LMO2, a proto-oncogene that can cause cancer. This gene then activated LMO2 resulting in the augmentation of leukaemia. According to Padmanaban, “This may not happen in other diseases. But it is a setback to the field.”
In 1999 too, gene therapy suffered a major hurdle with the death of 18-year-old Jesse Gelsinger. Jesse was participating in a gene therapy trial for ornithine transcarboxylase deficiency (OTCD). Instead of being cured he died from multiple organ failures, four days after the onset of the treatment. A severe immune response to the adenovirus carrier is believed to have triggered his death. Till date this factor is a major hurdle in all gene therapy studies and it has led to apprehensions in ongoing trials. Padmanaban remarks, “Gene therapy is an area that goes one step forward and two step backwards.”
How it works
In most gene therapy studies, a normal gene is inserted into the genome to replace an abnormal or defective gene. A carrier molecule called a vector is generally used to deliver the gene into the target cells, the most commonly used vector being a virus or viral vector. The vector is introduced into the target cells following which, it unloads the gene into the cell, hence restoring the generation of the functional protein. The rectified protein action then brings the cell to its normal state and the cause of the disease is eliminated. This is the general procedure for gene therapy. Apart from this, there are a variety of other methods of gene therapy, for instance:
* An abnormal gene could be swapped instead of being replaced functionally, through homologous recombination
* The abnormal gene could be repaired through selective reverse mutation
* The regulation of a gene could be altered
* The expression of a particular gene could be repressed
These procedures are still being studied under clinical trials. Another question in mind is the delivery of the gene to the desired target, for which many options have come to the fore. Viruses have evolved a way of encapsulating and delivering genes to human cells in a pathogenic manner. Taking advantage of this capability, scientists have conveniently been using viruses as the preferred option of gene delivery. Viral vectors like retroviruses, adenoviruses, adeno-associated viruses and hepes simplex viruses form the typical option in clinical trials, depending on the type of sites they target.
Besides virus mediated approaches, another simple method is the direct introduction of therapeutic DNA into the target cells. However, this effort has its limitations as it can only be used with certain tissues and requires large amounts of DNA. Yet another delivery system is through liposome delivery. A liposome carrying the required DNA is capable of passing through the target cell’s membrane. A novel approach called electroporation has recently held researchers interests for gene delivery. According to this technique, a gene can be pushed into a cell through the application of an electric pulse that forms pores in the cell membrane, creating a way for the gene to enter. The seemingly simple methods are largely theoretical and are still under experimentation.
It has been more than two decades since the study of gene therapy commenced. Yet it hasn’t been approved as a clinical practice. There are many factors that have raised eyebrows and kept gene therapy from becoming a conclusive and absolutely effective treatment for countering genetic diseases. Apart from the obvious failures there are many inherent loopholes that hindered any kind of progress. One of the factors is the short-lived nature of gene therapy, which was observed in the first clinical trial. If the therapeutic DNA that is introduced into the cell does not remain functional for a long time, then all efforts are nullified. In addition to this, the cells containing it must be sustainable before gene therapy can become a permanent cure with long-term benefits; else, most patients under-going it will have to be subjected to multiple rounds of treatment.
An additional hindrance, common to such therapies that involve the introduction of a foreign body, is the response generated by the immune system in opposition to the alien substance. Suryanarayan says, “One major side effect for gene therapy is immunogenic reactions, which caused several trials to halt.” The insertion of a virus into the body could stimulate intense immune response, something that caused Jesse Gelsinger’s death in 1999. Furthermore, this would raise a question to the acceptability of the vector in the body, deterring an important method of gene delivery. Viruses also pose other potential problem to the host in terms of toxicity, inflammatory responses, gene regulation and targeting issues. A virus can lodge itself at a wrong site or may alter the regulation of a gene, creating unwanted side-effects. “We still cannot direct a gene to a particular site. That is physically impossible till now,” says Padmanaban. He believes that the answer to targeting issues lies in a natural phenomenon, homologous recombination. He says that this issue can be countered if there could be some strategies by which one could force the system to undergo homologous recombination.
Last but not the least, gene therapy cannot be developed for multigene disorders. So far, disorders arising from mutations or defects of a single gene have been the best candidates for gene therapy. Unfortunately, some of the most commonly occurring diseases such as heart diseases, high blood pressure, Alzheimer’s, arthritis and diabetes are caused by the combined effects of variations in multiple genes. Treating such diseases would add to the existing complications.
A new approach
Gene therapy is being tested in various forms and for various diseases like SCID, Huntington’s, Parkinson’s. Lesch-Nyhan syndrome and phenylketonuria, among many others. However, scientists have found that this therapy holds a lot of promise for cancer as the treatment for this disorder involves the prevention of the expression of a gene. This can be achieved through the revolutionary anti-sense mechanism for gene regulation or RNAi, which is another form of gene therapy. Even as the research initiatives progress, newer forms and techniques are coming to the fore. “Despite all the setbacks, there are tremendous numbers of research and clinical trials going on worldwide, especially for cancers. Sixty percent of the trials are in some form of cancers,” says Padmanaban.
Recently researchers have found a new channel to focus their energies upon. This phenomenon is called the DNA vaccine, an alternative form of gene therapy. Suryanarayan observes, “I have recently noticed that people are countering immunogenic reactions by actually using DNA vaccines.” According to this method, instead of delivering the disease-causing gene into the body, one needs to introduce an artificially copied and multiplied gene from the disease-causing pathogen. The pathogen’s gene expression ultimately leads to the synthesis of proteins and hence the natural production of antibodies in the host’s bodies. Padmanaban explains, “Instead of introducing engineered proteins from a malarial or influenza parasite, I can introduce the gene itself. This is a DNA vaccine.” A vaccine of this sort would usher a life long immune protection. Since, gene therapy hosted many challenges, researchers at IIS turned to this newfound method. Prof Rangarajan has already developed a DNA vaccine for rabies, which has proved positive. This has now been transferred to the Indian Immunological Institute in Hyderabad for the final stages of trial.
What lies beyond
Though clinical trials are on across the globe, not much work is being done in gene therapy in India. Only Tata Cancer has initiated gene therapy studies specifically for oral cancer. “One of the problems that drug companies in India face is that the regulatory framework and the exposure level of our regulators is still not geared up to international standards so as to easily allow researchers in India to do cutting edge science, which involves taking some risk, balanced with potential benefit to patients and society,” remarks Suryanarayan. “Gene therapy still has some risk associated with it and trials will have to be approved after a lot of scientific consideration,” he adds.
Hundreds of clinical trials are going on all over the world for testing potential methods of developing effective gene-transfer strategies, tailoring them to the dynamics of various cells and tissues, maintaining long-term cell survival and establishing reliable gene expression. The road to the development of gene therapy has been rocky and fraught with controversy. “Yet many researchers still continue work because it has got promise for a permanent cure for certain diseases,” comments Suryanarayan. “It will happen very gradually. But as one of the major alternatives to cure genetic diseases, it is very encouraging,” he adds. Only further research can unravel the secrets to out do the complications and develop the therapy into a miraculous remedy. Suryanarayan stresses on the need to continue studies despite the roadblocks and says, “It hasn’t been a complete disaster. People will not understand how to develop it until they move forward.”