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Gene Therapy

Gene Delivery

Whether given as pills or injections, most conventional drugs simply need to reach a minimal level in the bloodstream in order to be effective. In gene therapy, the drug (DNA) must be delivered to the nucleus of a cell in order to function, and a huge number of individual cells must each receive the DNA in order for the treatment to be effective. The situation is further complicated by the fact that a given gene may normally function in only a small portion of the cells in the body, and ectopic expression may be toxic. Thus, successful gene therapy often requires highly efficient delivery of DNA to a very restricted population of cells within the body.

In 1999 doctors at the Ohio State University Medical Center prepare a gene therapy injection for 36-year-old Donovan Decker. He is the first patient to ever receive gene therapy for muscular dystrophy.

To achieve these goals, many researchers have turned to viruses. Viruses are parasites that normally reproduce by infecting individual cells in the human body, delivering their DNA to the nucleus of those cells. Once there, the viral DNA takes over the cell, converting it to a factory to make more viruses. The cell eventually dies, releasing more viruses to continue the cycle. Scientists can remove or disable some of the genetic material of the virus, making it unable to reproduce outside of the laboratory. This genetic material can then be replaced by the gene needed to treat a patient. The modified (or recombinant) virus can then be administered to the patient, where it will carry the therapeutic gene into the target cells. In this way, scientists can take advantage of the virus's ability, gained over millions of years of evolution, to deliver DNA to cells with tremendous efficiency. One of the most commonly used is a cold virus called adenovirus. Recombinant adenoviruses have been used in experimental gene therapy for muscle diseases, and can deliver genes to almost all of the cells in a small region surrounding the site of injection. Unfortunately, while adenoviruses excel at gene delivery, evolution is a double-edged sword, and the many mechanisms our own bodies have evolved to combat harmful viral infections are also used against therapeutic viruses, as will be discussed in more detail below.

Recombinant adenoviruses cannot be used to transfer DNA to all cell types, because they cannot reproduce themselves outside of the laboratory. When a cell with a recombinant adenovirus in it divides, only one of the two resulting cells contains the virus and the therapeutic gene it bears. The treatment of some diseases requires gene transfer to a stem cell, a cell that actively divides to create many new cells. For example, white blood cells live for only a short time, and must be constantly replenished by the division of precursor cells called hematopoietic stem cells. Gene therapy to treat an immune disease affecting white blood cells would thus require targeting these rapidly dividing cells. Researchers use a different kind of virus to accomplish this: retroviruses, so called because they contain RNA (a different kind of genetic material) rather than DNA.

When a retrovirus infects a cell, it converts its RNA to DNA and inserts it into the chromosome of the target cell. As the cell subsequently copies its own DNA during cell division, it copies the viral DNA as well, so that all of the progeny cells contain the retroviral DNA. At some later time, the viral DNA can liberate itself from the chromosome, direct the manufacture of many new viruses, and go on to repeat its life cycle. Recombinant retro-viruses are engineered so that they can enter the target cell's chromosome, but become trapped there, unable to liberate themselves and continue their life cycle. Because all progeny cells still carry the recombinant retrovirus, they will also carry the therapeutic gene.

This is a great advantage over adenoviruses as a tool for gene delivery to dividing cells, but retroviruses have some drawbacks as well. They can only infect cells that are dividing quickly, and in most cases this infection must be carried out in the laboratory. Cells must be removed from the patient, infected with the recombinant retrovirus, grown for several weeks in the lab, and then reintroduced to the patient's body. This process, called ex vivo gene transfer, is extremely expensive and labor intensive. Nonetheless, this form of gene therapy has been used in one of the most successful clinical applications to date, the treatment of two patients with severe combined immune deficiency (SCID) caused by a defect in the adenosine deaminase gene.

Before treatment, these patients had essentially no immune system at all, and would have been required to live as "bubble children," completely isolated in a sterile environment. While their treatment did not completely cure their genetic disorder, it restored their immune systems enough to allow them to leave their sterile isolation chambers and live essentially normal lives. Many other viruses are being engineered for application to gene delivery, including adeno-associated virus, herpes simplex virus, and even extensively modified forms of the human immunodeficiency virus (HIV), to name just a few.

Many researchers are also exploring nonviral methods for gene delivery. One of the most successful of these methods consists of coating the therapeutic DNA with specialized fat molecules called lipids. The resulting small fatty drops called vesicles can then be injected or inhaled to deliver the DNA to the target tissue. Many different lipid formulations have been tested and different formulations work better in different tissues. These approaches have the great advantage that they do not stimulate the serious immune response that some viral vectors do. However, in general, these nonviral methods are not as efficient as viruses at transferring DNA to the target cells. No clearly superior method for gene delivery has yet emerged, and scientists are still actively developing both viral and nonviral methods. It is likely that many different methods will eventually be used, with each method specifically tailored to work best in a specific tissue or organ of the body.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 2Gene Therapy - Disease Targets, Gene Delivery, Longevity Of Gene Expression, Examples Of Gene Therapy Applications