Gene Therapy: Promises and Problems
Gene Therapy: Promises and Problems
By: Alexander Pfeifer
By: Inder M. Verma
Source: Annual Review of Genomics and Human Genetics 2 (2001): 177-211.
About the Author: Alexander Pfeifer, professor of medicine at the Ludwig-Maximilians University in Munich, Germany, was a researcher at the Salk Institute for Biological Studies in La Jolla, California, at the time this article was published. Inder M. Verma is a professor of molecular biology and researcher at the Salk Institute.
Genetics is the ultimate life science frontier for the understanding of the innermost mechanisms regulating the origin and evolution of all life forms on Earth, from the simplest unicellular organisms to highly complex multicellular mammals. The remarkable advances of the last twenty years in molecular biology, cytogenetics, and biochemistry, as well as the development of new biotechnologies, have led to the deciphering of the genetic code of several species, including humans. These advances shed new light on the physiology of organisms and on the laws of inheritance of genetic traits from one generation to the next.
The Human Genome Project, an international research consortium initiated in 1990 involving research centers on almost every continent, and Celera Genomics of Rockville, Maryland, a private enterprise, succeeded in sequencing the DNA of human chromosome 21 by 2000. (The genome is the total set of DNA sequences encoded by genes that characterize a given organism.) In 2002, the Human Genome Project completed the sequencing of all human chromosomes, thus opening the way for the next step in molecular genetics, known as proteasomics or proteomics, the characterization of all proteins encoded by human genes.
Proteins are the structural and functional products of genetic activity, responsible for the development of organisms, tissue repair, and cell functioning. Errors either in gene sequences or in protein synthesis are at the root of a great number of diseases, whether inherited or acquired after birth. Research in molecular genetics already has led to the development of new classes of medications, such as molecular-targeted anticancer drugs (agents that interfere with cancer cell proliferation in a selective manner). Several drugs of this class can be taken orally. The adverse effects they cause are fewer and milder than those caused by traditional chemotherapy, and these drugs also are more effective in controlling several types of cancer. However, gene therapy offers a much bolder therapeutic approach for cancer and other diseases involving gene deregulation or mutation. Gene therapy aims to identify defects in a specific gene or the absence of a given gene associated with a specific dysfunction and to transfer therapeutic genetic material to that segment of DNA to restore normal function.
The article, "Gene Therapy: Promises and Problems," presents a thorough review of the state of the art in gene therapy research. It relies on research from the Salk Institute laboratories and other important seminal research that contributed to the design and development of several approaches to gene therapy.
… Gene therapy can be broadly defined as the transfer of genetic material to cure a disease or at least to improve the clinical status of a patient. One of the basic concepts of gene therapy is to transform viruses into genetic shuttles, which will deliver the gene of interest into the target cells. Based on the nature of the viral genome, these gene therapy vectors can be divided into RNA and DNA viral vectors.
… The majority of RNA virus-based vectors have been derived from simple retroviruses like murine leukemia virus. A major shortcoming of these vectors is that they are not able to transduce nondividing cells. This problem may be overcome by the use of novel retroviral vectors derived from lentiviruses, such as human immunodeficiency virus (HIV). The most commonly used DNA virus vectors are based on adenoviruses and adenoassociated viruses. Although the available vector systems are able to deliver genes in vivo into cells, the ideal delivery vehicle has not been found. Thus, the present viral vectors should be used only with great caution in human beings and further progress in vector development is necessary.
… Biologists will remember Monday, June 19, 2000, as an historic day. Flanking Bill Clinton, the 42nd President of the United States of America, were Francis Collins of the National Institutes of Health (NIH), leader of the publicly funded Human Genome project, and Craig Venter, CEO of Celera Genomics of Rockville, Maryland, to announce the near-completion of the sequencing of the human genome. Imagine: The entire 3 billion nucleotides of our genome are decoded—an impossible task just a few years ago. The estimate of the number of genes ranges from a low of 35,000 to a high of more than 100,000. What a bonanza for gene therapy. The science of gene therapy relies on the introduction of genes to cure a defect or slow the progression of the disease and thereby improve the quality of life. Therefore, we need genes. Suddenly, we have tens of thousands of them at hand. Though gene therapy holds great promise for the achievement of this task, the transfer of genetic material into higher organisms still remains an enormous technical challenge. Presently available gene delivery vehicles for somatic gene transfer can be divided into two categories: viral and nonviral vectors. Viruses evolved to depend on their host cell to carry their genome. They are intracellular parasites that have developed efficient strategies to invade host cells and, in some cases, transport their genetic information into the nucleus either to become part of the host's genome or to constitute an autonomous genetic unit. The nonviral vectors, also known as synthetic gene delivery systems (45), represent the second category of delivery vehicles and rely on direct delivery of either naked DNA or a mixture of genes with cationic lipids (liposomes). In this review, we focus on viral vectors and highlight some examples of their use in clinical trials. A complete, constantly updated list of human gene therapy trials in the United States is available at the Office of Biotechnology Activities, NIH (http://www4.od.nih.gov/oba/rdna.htm).
Gene therapy aims to correct or slow the progression of certain diseases by supplying the involved tissues with the necessary missing genes or with other regulatory genes that control a given gene expression. In spite of the elegance of the theory behind gene therapy, technical difficulties and safety issues involving the best systems to deliver genetic material to cells remain. Two delivery systems initially were considered: the use of modified viruses as transporters (vectors) and the use of non-viral vectors, known as "synthetic gene delivery systems."
Viruses depend on their host cells' genetic machinery to replicate themselves. They have developed the ability to invade host cells where they sequester certain cellular organelles to replicate their genetic information and, in some cases, to transport and integrate the viral sequences into the host DNA. Approximately 50 percent of the sequences present in human DNA consist of residues of integrated viral sequences. Long tandem repeats (LTRs) are one example. Viral vectors fall into two categories: 1) integrating viral vectors such as retroviruses and adeno-associated virus (AAV), which have the potential to provide life-long expression of the delivered genes (known as transgenes); and 2) non-integrating viruses, such as adenoviruses. Since these non-integrating viruses have medium-size, single or double DNA strands that are bigger than the single or double RNA strands of retroviruses and AAVs, they can transport human transgenes with longer sequences than retroviruses can.
However, a long list of obstacles must be overcome before viral vectors can be safely and efficiently used to deliver transgenes on a regular basis. Gene therapy researchers are studying with such obstacles as: 1) tissue specificity; 2) inserting transgenes in safe, site-specific DNA regions; 3) averting the risk of blood disorders, such as leukemia or other cancers due to uncontrolled gene expression; 4) viral infections; and 5) overcoming a systemic (throughout the body) immune response against viral proteins. In addition, transgenes must be delivered to a great number of targeted cells to effect a cure or to arrest disease progression.
In order to optimize the safety and efficiency of viral vectors, researchers have identified the function of each gene in the genome of vector viruses and have also studied the properties of its proteins and enzymes. Scientists have manipulated the genome of several viruses to knock out those genes that produce disease and, in some cases, have also manipulated specific gene sequences in vector viruses to enhance the ability of the transgenes to integrate into targeted cells.
Gene therapy using vectors other than viruses has presented other initial difficulties, including poor efficiency in delivering transgenes to cells and the fact that an inability to sustain transgene expression within the target. In recent years, this approach is gaining a new impetus thanks to advances understanding of viral genomics and to new bioengineering technologies. The utilization of certain viral enzymes in synthetic delivery systems, instead of viral vectors, has led to the production of new vectors that soon may be used to promote the safe integration of transgenes in specific genomic locations. For example, Rep integrase, an enzyme isolated from AAV was able to promote the integration of a transgene into a safe site on human chromosome 19 without the use of a viral vector. This is just one example of the promising non-viral vectors presently under development, thanks to the extensive research of viral genomes in recent years. In the near future, it is hoped that other advances will make long-term therapeutic gene integration and expression in human cells possible, without the risks posed by viral vectors.
The spectrum of diseases that can be targeted by gene therapy is immense, including several neurode-generative diseases, cancers, diabetes, hyperlipemias, and X-linked severe combined immunodeficiency (X-SCID). In fact, X-SCID is the only disease for which clinical trials with viral vectors have been successful since 1999. At least seventeen children have been cured thus far, but unfortunately three children also have died due to leukemia-like disorders related to the vectors used.
The National Institutes of Health and the U.S. Food and Drug Administration have established strict guidelines for gene therapy research and for clinical trials to ensure the protection of volunteers enrolled in trials and the quality of such experimental therapies.
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