Cutting edge hope for patients with chronic neurological disorders.
By Tom Scott
Viruses are ideal vectors for gene therapy.
What is Gene Therapy?
Gene therapy is the experimental treatment process of inserting a “corrected” or therapeutic gene into an individual’s cells and tissues to replace an “abnormal” disease-causing gene in order to treat a disease, or to use a gene to treat a disease, just like a medicine. Although gene therapy is still in its infancy, it has shown great potential in correcting and replacing defective genes behind many diseases, including cancer.
One way gene therapy is accomplished is through the use of a carrier or “biological vector” that delivers the therapeutic gene to the patient’s target cells. Commonly used carriers are viruses that have been genetically altered to carry normal human DNA. Since viruses are capable of encapsulating and delivering their genes by tricking cells into accepting them as part of their own genetic material, doctors and molecular biologists are using this power to their advantage by
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manipulating and re-engineering viruses and replacing their disease-causing genes with therapeutic ones. During this process, target cells are infected with the re-engineered viral carrier, which then unloads its genetic material into the cells’ nuclei. This is a complex procedure that must be performed in a precise manner so that the viral genome (hereditary information or complete DNA of one set of chromosomes) can be delivered and the therapeutic proteins expressed in the host cells. This has been achieved for many viruses that can now be used as vectors, including the virus causing HIV, and adenovirus.
Clinical Roadblocks
First generation adenoviruses (Ad), non-enveloped viruses containing a linear double-stranded DNA genome that is usually the culprit behind upper respiratory tract infections or “common colds,” have been found to be extremely effective vehicles for delivering therapeutic genes, especially to the brain, and tumors. Ad isn’t the only type of virus that has been studied for use in gene therapy; others include retroviruses, adeno-associated viruses, and herpes simplex viruses. Besides viral vectors, alternative therapeutic gene delivery methods are also being studied. Viral vectors, however, are currently the preferred method and show the most promise. But there have been numerous hurdles in the way of their clinical success.
Research has shown that upon systematic immunization of host organisms against Ad, fi rst generation adenoviral vectors (Advs) usually trigger immune system reactions, and the therapeutic virus, is thus seen as a threat, and are attacked and rendered ineffective 2-3 weeks after they are injected. Since a majority of humans are pre-exposed to Ad, having been infected with the virus during childhood, immune system reactions are expected to occur when the therapeutic gene is delivered through this viral packaging. Furthermore, these reactions can vary from person to person. Although problems were seen in early clinical trials, the doses needed have now been optimized so that toxicity and inflammatory conditions that could be life-threatening can be avoided.
So researchers have been faced with a very big dilemma. How can they provide long-term transgene expression to help treat disease if the immune system can detect and eliminate the therapeutic genes within a matter of weeks? And most importantly, how can they make treatment safer for patients? This has been the main challenge of gene therapy since the first clinical trial began in 1990. There have been some setbacks since that time. One of the biggest occurred in 1999 when an 18 year-old boy died from multiple organ failure 4 days after starting treatment during a clinical gene therapy trial for ornithine transcarboxylase deficiency (a genetic liver disease). Doctors believe his death was caused by a severe immune system response to the Ad carrier. Nevertheless, many patients affl icted by deadly diseases have also been cured by gene therapy, the most impressive being the cures achieved in “bubble boys,” boys born without an immune system, and condemned to live their lives within sterile bubbles.
A Safer, Long-Term Approach
Researchers at the Board of Governors Gene Therapeutics Research Institute at Cedars-Sinai Medical Center (GTRI) led by Pedro R. Lowenstein, MD, PhD, and Maria G. Castro, PhD, have recently developed a safer and more effective method of regulating therapeutic transgene expression within the central nervous system of laboratory mice that may eventually be useful in treating patients with chronic neurological disorders.
The study, which recently appeared online in Molecular Therapy, demonstrated in an animal model for the fi rst time that engineered novel gutted high-capacity adenoviral vectors (HCAdvs) injected into the brain of mice can sustain therapeutic transgene expression for up to a year and is undetectable by the immune system, even in mice that have been systematically immunized against Ad. This is achieved by manipulating HC-Advs so that the entire viral genome is absent (or gutted) in order to make it less immunogenic. Researchers also discovered that the success of transgene expression and the prevention of an immune response relies heavily on the precise location within the brain that the vectors are delivered. If the injection of HC-Advs is limited to the brain parenchyma (functional parts) immune responses do not occur, however, if they reach the brain ventricles a systematic immune response will follow. HCAdvs are also able to infect many different cell types, making them useful in treating a wide range of diseases. GTRI researchers hope that their findings will lead to further clinical investigation of HCAdvs and the potential efficacy and safety of these vectors in future gene therapy research.
Lowenstein and his team will be concentrating their clinical efforts on testing these new vectors primarily on brain tumors (of which a bulk of research is already being conducted) and degenerative disorders such as Parkinson’s disease since the pathophysiology of both are clearly understood and the therapeutic targets are easier to identify. If future trials, especially on Parkinson’s disease, are successful, Lowenstein is optimistic that the vectors may eventually be used in patients with spinal cord injuries and disorders. “By studying HC-Advs’ impact on diseases that offer no known cure and very little in the way of treatment we have an opportunity to analyze for the first time the benefits of these vectors as well as to study their impact on survival rates in these patient populations. In regards to MS, Lowenstein cautions that before studies can take place researchers must first fully understand the disease’s pathophysiology. Without more knowledge on what specifically causes MS, there is no way to know what exactly to target therapeutically. Lowenstein admitted that gene therapy has received enormous scrutiny in the past but believes some of the controversy could have been avoided. “Five to ten years ago people within the field were very vocal. At that time researchers rushed into clinical trials and basically tried to run before they could walk. It’s similar to what you currently see with stem cell research. Many scientists were over-enthusiastic and it led to a lot of very bad publicity. Now we are seeing the reverse side of the coin. Everyone in the field is very quiet and taking a much more cautious approach, despite big successes throughout the world and major breakthroughs. Nobody wants to repeat past mistakes,” Lowenstein said.
Although obtaining government grants for gene therapy research is difficult at times, Lowenstein foresees a more concentrated clinical effort within the field that will help prove that the principles of gene therapy, in light of these new findings, can safely work in various models of disease. “We have very good clinical models to work with now, it’s just a matter of us now focusing on a novel wave of safer and more effective clinical trials.”
Tom Scott is staff editor.
