Alpha-1 Gene Therapy Program
What is Gene Therapy?
Gene Therapy is the transfer or delivery of normal or healthy genes to replace, manipulate or supplement nonfunctional or misfunctioning genes 1. Genes are located along chromosomes in the nucleus of the cells of our body. Genes are composed of deoxyribonucleic acid (DNA) carrying the code or instructions needed to make the proteins in our body. It is estimated that we have 30,000 to 60,000 genes. Nonfunctional or misfunctioning genes contain damaged DNA, lacking the instructions to produce the proteins needed and depending on the seriousness of the damage may cause disease. Some diseases have been identified as being caused by the deficiency of a protein controlled by a single gene; Examples are cystic fibrosis, severe combined immune deficiency (SCID), sickle cell anemia, hemophilia, and alpha-1 antitrypsin deficiency. Most diseases, such as diabetes and asthma, however, are caused by a complex interaction of more than one gene along with environmental factors. Even the diseases identified as being caused by one particular gene may be influenced by other genes and environmental factors varying severity or symptoms from one individual to another. Gene Therapy at its essence is relatively simple. By supplying a good copy of the gene that encodes for the protein to the cell, the problem in theory is solved. This is often easy to do to cells in the Petri plate in culture in the laboratory. It is, however, more difficult to reproduce that gene transfer in an intact living person.
Vehicles of Transfer
In order to do that we need a carrier or “vector” to insert the normal gene into the cell. Usually viruses are used as the vehicle to carry the gene as they are very efficient in obtaining entrance into our body cells. They have evolved mechanisms for efficient gene transfer. The virus is altered to carry the good gene and to hopefully retain fewer of the potentially pathogenic or undesirable properties of the virus. The viruses do though retain important properties of the virus from which they were made. Many of the earliest gene transfer experiments were done with retroviruses; and to work, those retroviruses required cells to be actively dividing, thereby limiting their use. The adenovirus vector, although altered to retain fewer properties of the virus, can cause inflammation. Another virus, the adeno-associated virus (AAV) is a common virus that infects people but does not make them sick. It does insert its genes into cells in a way that lasts for a very long time. These characteristics make it very useful in gene transfer. The AAV vector does not appear to cause inflammation or other immediate side effects, but the use of this vector system in humans has been very limited up to this point. Also, the vector is small, a limiting factor in some situations. Other viral vectors are being developed and studied in the laboratory. Non-viral vectors are also being developed as vehicles for gene transfer. Lipids (fat molecules) and proteins containing the DNA have also been engineered and are being studied. Different vectors have unique characteristics and different diseases have varying target properties, making it difficult to get the right vector to safely and efficiently transfer the normal gene to the desired target. The development of a growing number of and variety of vectors is encouraging and necessary for the advancement of gene therapy.
Place of Transfer
The characteristics of the target-cell and the specific goal of therapy determine the place of delivery. Gene transfer may take place outside or inside the patient’s body. In ex vivo gene therapy, cells are removed from the patient’s body and the normal or healthy gene is introduced into the cells. Once the transfer has taken place, the cells are then returned to the body. This can only occur when the target cell is capable of being reintegrated into the body. An example of this is removal of bone marrow, infecting the stem cells of the bone marrow with the vector, and then injecting into the blood stream as was done in patients with SCID. In contrast, gene transfer for cells of the bronchial epithelium of the airways of patients with cystic fibrosis must occur in vivo, that is within the body. A vector under study for this disease is being administered by inhalation. In diseases where the missing protein is normally secreted into the blood stream, a vector with the necessary gene may be injected into muscle cells stimulating the cells to make the protein and secreting it into the blood stream.
Ultimate Goal of Gene Transfer
The ultimate goal of the transfer is for the replacement gene’s coded (DNA) information to be incorporated into the targeted cell and to function as it is suppose to function to ameliorate or cure disease. The focus of clinical trials in gene transfer to date is on making corrections, replacing or manipulating the somatic (non-reproductive) cells, thereby correcting the disorder within the individual. The genetic information of somatic cells is not passed on to offspring, thus the genetic disorder may still be passed on to offspring.
Application of Gene Therapy
Our knowledge of genetics has increased rapidly with the Human Genome Project and many different disease-causing genes have been found. Researchers have encountered many problems. Scientists are working hard to understand the processes of the diseases and the viral and non-viral vectors in order to develop vectors that will provide safe and efficient gene transfer. Researchers, agencies and federal regulatory agencies are working to provide the optimal clinical design for gene transfer carried out in a responsible manner.
More than 500 clinical gene-therapy trials involving about 3500 patients have been identified worldwide (June 2001); 78% in the United States and 18% in Europe.2 The vast majority of trials are for cancer. French researchers and more recently English researchers report having successfully treated young boys with a disease called severe combined immune deficiency (SCID). Most of the human clinical trials involving gene transfer; however, are still in the early stages of research.
Development of Gene Therapy for AAT Deficiency
Investigators at the Powell Gene Therapy Center at the University of Florida have developed a new alpha-1 antitrypsin (AAT) gene therapy vector, utilizing a gene vector adeno-associated virus (AAV) that appears to be effective when injected directly into the muscle. Previous studies of this vector in animals have been very encouraging, in that a single injection of the vector into the muscle appears to lead to very prolonged production of the normal AAT protein. The regulatory review process governing human trials is long and complex, but all animal studies performed to date have been reassuring with regard to the potential safety of this agent, indicating that it may be suitable for use in future trials.