Due to improvements in lab-based technologies and a widespread availability of biomedical data, the past two decades have yielded significant progress in the field of human medicine. Despite the vast advancements, drug treatment success rates for common disorders still remain astonishingly low. According to a 2001 study by Spear et. al, drug effectiveness ranges between 25-70%, with complex disease such as cancer resting at the bottom of the spectrum. The low efficacy may be due to the fact that many drugs only mask the symptoms of a disease without working to fix the root cause. A significant proportion of conditions are created by an underlying genetic mutation. Disorders such as cancer, Alzheimer’s, Parkinson’s, cystic fibrosis, hemophilia, and muscular dystrophy are a few examples of inherited conditions with no known cures. Currently, the only available treatments for many diseases rely heavily on pharmaceuticals.
Since the year 2000, there has been an increase in the amount of genetic data available due to studies such as The Human Genome Project, The International HapMap Project, and an abundant pooling of research data available on bioinformatics databases. Using this newly accessible information, researchers are looking to develop treatments that not only correct the clinical symptoms of a disease, but also work at the genetic level to eradicate it. A technique that aims to treat or prevent a disorder by targeting the problem at a molecular level is known as gene therapy.
The basic strategy of gene therapy is to insert a normal, functioning gene to replace a malfunctioning one. The therapeutic gene is delivered to the cell using a carrier molecule known as a vector. The new DNA can produce a functional protein product or change the regulation of another mutated gene. Thus, returning the cell to a correctly functioning state.
Gene therapy is split into two different categories: in vivo and ex vivo. During in vivo therapy, a patient directly receives the therapeutic genes into their body. Ex vivo trials take a different approach by first culturing a patient’s cell line in the lab, and then injecting DNA into the external cells which are later re-introduced into the patient’s body. The choice of gene introduction method is heavily based upon the disease and target tissue of interest.
Another important consideration of gene therapy trials is the choice of vector. A vector is simply the delivery vehicle for the therapeutic gene. Due to their advantageous structure and natural function, viruses are commonly used for this purpose. Viruses are composed of a proteinous shell called a capsid that contains the viral genome. They have evolved to transfer their own DNA into a host cell in order to proliferate and spread throughout the organism. When a virus is used as a therapeutic vector, the pathogenic DNA is removed and replaced with the therapeutic gene. The virus is no longer able to replicate its own DNA, but utilizes its capsid shell as a gene transportation vessel.
Several different types of virus are available for use as a vector. Retroviruses and Lentiviruses both use the enzyme reverse transcriptase to transcribe therapeutic genes into the host genome. Retroviruses are successful for ex-vivo studies, while Lentiviruses have shown promise for in vivo trials. Adenoviruses and Adeno-associated viruses are other viral vectors that have the capability to carry double stranded DNA into the cell via endocytosis. These viruses are particularly useful because they can enter cells in both dividing and quiescent stages.
While viral vectors have yielded promising results, there are several disadvantages associated with their use. Due to the structure of a virus, only a limited amount of DNA can fit into the vector. Additionally, introduction of a virus can trigger an adverse immune response, resulting in rejection of therapeutic cells.
To combat these challenges, a variety of non-viral gene delivery methods have been investigated. Non-viral methods package the therapeutic DNA in a circular bacterial chromosome called a plasmid. One non-viral method uses direct injection of the plasmid into the target cells. This strategy is useful because it is fairly straightforward in practice; however, it is limited to easily accessible tissues and requires a large quantity of DNA. Another non-viral method involves chemically linking DNA to cell receptors so it is passed into the cytoplasm via a cell’s natural active transport system. While this method has slightly more precision than non-linked direct injection, many problems can occur with the receptor binding resulting in loss of the DNA.
The most promising non-viral method involves the use of an engineered vector called a liposome. A liposome is an artificial lipid sphere that encompasses the DNA. Due to the chemical properties of the liposome, it is able to safely pass through the cell membrane and deliver the DNA to the nucleus. Non-viral methods of DNA introduction are beneficial because there is no limit in the size of DNA that can be introduced, and they are less likely to trigger an immune system response. Unfortunately, non-viral methods have experienced less clinical success than their viral counterparts. More research is being conducted in order to identify safe and effective vectors.
The first gene therapy trial took place in 1990 by Drs. Anderson Blaese and Kenneth Culver. They used an ex vivo method with a retroviral vector to treat two young girls suffering from a severe immunodeficiency disorder known as adenosine deaminase deficiency (ADA). After two years of continuous treatment the girls’ condition improved dramatically, and therapy was considered successful.
Since the first gene therapy trial the field of genetic therapy has experienced a fair number of both successes and failures. In 1999 gene therapy received its first setback with the death of 18 year-old patient, Jesse Gelsinger. Jesse was participating in a trial for omithine transcarboxlase deficiency (OTCD). Unfortunately, his body suffered an adverse immune response to the adenovirus vector, and he died of organ failure four days later. He was the first person to die as a result of gene therapy.
In 2002, another tragedy occurred when two children participating in a trial for another autoimmune disorder commonly referred to as “bubble boy syndrome” both developed a rare form of leukemia from the therapy. In response, the FDA placed a halt on all gene therapy trials using the retroviral vector. By April 2003 the ban had been lifted, but the cautionary tale inspired researchers to investigate non-viral vector methods. Later that year, the University of California published a paper demonstrating the use of liposomes as vectors for transferring genes into the brain.
Gene therapy trials have shown promising results for both simple and complex genetic disorders. In 2008, the UCL Institute of Ophthalmology reported experimental treatment for Leber’s Congenital Amaurosis, a retinal dystrophy causing blindness in young people, improved eyesight dramatically. In 2009, The School of Pharmacy in London effectively used nanoparticles as vectors to treat cancer cells in mice. Other trials have shown advancements in treatment for Huntington’s, Hemophelia, Parkinson’s, leukemia, cystic fibrosis, cancers, and autoimmune disorders.
While gene therapy has made great progress, there are still many challenges to overcome. Developments in gene delivery and activation, as well as safety measures to prevent against immunotoxicity are among the leading problems preventing therapeutic success. Gene therapy is currently only being tested for the treatment of diseases with no other cure, and the FDA has not approved any methods past the clinical trial phase. Excluding the laboratory complications, there are several political and ethical considerations that must be discussed before this technique becomes a widespread clinical practice. Despite the long road ahead, gene therapy is a revolutionary technique that with further research may one day lead to the successful treatment and prevention of genetic disease.
- Published in Genetic Technologies