Genetic manipulations, such as replacing defective or missing genes with healthy ones, can be used to alter germ cells (egg or sperm) and somatic cells. Theoretically germ-line gene therapy appears to have more advantages since it aims at preventing a genetic defect from being transmitted to future generations. However, the prospects of germ-line gene therapy look more remote due to many unresolved ethical and social problems as well as technical obstacles. (2) What is presently understood as gene therapy is, mostly, somatic cell gene therapy. By altering the genetic material of somatic cells onetime cures of devastating, inherited disorders may be potentially achieved. But, "in principle, gene therapy should be applicable to many diseases for which current therapeutic approaches are ineffective or where the prospects of effective treatment appear exceedingly low." (1) However, gene therapy is still extremely new and highly experimental. The number of approved clinical trials is small, and relatively few patients have been treated to date.
The U.S. Government was providing annually about $200 million in research grants for gene therapy projects. To evaluate the effect of these investments, a special panel was set up by the National Institute of Health. Report and recommendations of the panel released in December 1995 summarized the recent progress of research in gene therapy. It also stated that the importance of this research for clinical medicine has been greatly overestimated by the scientists and industry, and further exaggerated by scientific press and mass media. According to the report, scientific evidence of consistent improvement in patients was lacking, although some patients have reported gains. There were too many poorly coordinated research projects. Recommendations included a strict control over selection of gene therapy projects through peer review. Among other things, the report brought attention to the possibilities of applying gene therapy to the treatment of other diseases, including those that are infectious.
Retroviruses are the first and most used vectors in human gene therapy. They can infect important cellular types of diverse animals and humans with a high efficacy of infection. Retroviruses, particularly a disabled mouse virus, are proven to integrate and replicate the genetic material of interest in the genome of the host cell. Their disadvantage is that the integration of the viral genome only occurs during cell division, and they are not effective in tissues in which cells are not dividing. The integration of the viral genome into the cell creates an insertional mutation, and there is a possibility that a gene controlling cellular division may be affected leading to a cancerous growth.
Another well-developed vector is adenovirus that can be used to insert new genes into many kinds of human cells and provide effective gene expression. But the duration of gene expression is limited to about 8 weeks, because by that time the host immune system develops antibodies against the cells harboring the adenovirus. It was recently reported that a patient died while undergoing adenovirus-mediated gene therapy against ornithine transcarbamylase deficiency at the University of Pennsylvania in Philadelphia. If it is proven that adenovirus is the cause of death, the future use of this vector may be problematic. (4, 5)
The body's immune system is a factor of concern in all cases of gene replacement because antibodies are formed against any substance that is new to it. The normal protein produced by the correct gene may nevertheless be treated as foreign by the patient's immune system.
One of possible dangers is that a virus with correct gene might inadvertently get into the patient's eggs or sperm. The virus could insert itself correctly, remedying a genetic disease in the patient's descendants, or the insertion could occur in the middle of a gene, disrupting the gene's function and causing an inherited defect.
In order to limit body's immune defenses, the work is in progress on a new delivery system based on a small virus, so called adeno-associated virus. It has only two genes, both of which can be removed, leaving just its head and tail as a shell to carry therapeutic genes into target cells. It has been used to improve the condition of dogs with hemophilia B. Following a single injection of the gene for Factor IX, the missing blood clotting protein lasted for 20 weeks. Human trials with this gene therapy are due to begin this year. Adeno-associated virus has been also used on monkeys to insert the gene for hormone erythropoetin, which stimulates production of red blood cells in bone marrow. The construct includes a two-part switch activating the gene only in the presence of the drug rapamycin. Vectors based on adeno-associated viruses, appears safe and reasonably effective, but they are limited in the size of the genes they can carry.
Lentiviral vectors gained attention because they can integrate into nondividing host-cell genomes, evade the body's immune defenses and carry large genes. They are widely used as effective gene delivery tools in cells from liver, retina, skeletal muscle and the central nervous system. Recently a number of efficient lentiviral vector systems were introduced that use human immunodeficiency virus type 1 (HIV-1) in combination with a packaging and transducing vectors, and an envelope-encoding plasmid. These may be used in both dividing and nondividing cells at similar efficiencies. New potentially less pathogenic HIV-2 based vectors have been described also. (6, 7)
Viral-based, infectious vectors have significant limitations in their expression characteristics, lack specificity in targeting tumor cells for gene transfer, and pose safety concerns regarding induction of secondary malignancies and recombination to form replication-competent virus.To avoid these problems, researchers turn to nonviral DNA-mediated gene transfer techniques such as liposomes. Although generally not as efficient as viral vectors, such nonviral systems have the potential advantages of being less toxic, nonrestrictive in cargo DNA size, potentially targetable, and easy to produce in relatively large amounts. More important, lipidic vectors generally lack immunogenicity, allowing repeated in vivo transfection using the same vector. Three types of lipidic gene transfer vectors have been described: 1) DNA/cationic liposome complexes, 2) DNA encapsulated in neutral or anionic liposomes, and 3) liposome-entrapped, polycation-condensed DNA (LPDI and LPDII). (8)
A major obstacle to successful gene therapy is the relative inefficiency of the targeting process in mammalian cells. Gene targeting may be accomplished by two different mechanisms: homologous recombination and mismatch correction of DNA heteroduplexes. The second mechanism has been improved by using the recombinogenic activity of oligonucleotides and, especially, specifically designed chimeric RNA/DNA oligonucleotides. The use of proteins like active recombinase to stimulate searching for homology and forming stable DNA heteroduplexes between oligonucleotides and chromosomal DNA may improve the gene targeting events. (9) The chimeric molecules have been demonstrated to be effective in the alteration of single nucleotides in episomal and genomic DNA in cell culture, as well as genomic DNA of cells in situ. This is a potentially powerful strategy for gene repair for the multitude of hepatic genetic diseases caused by point mutations. (10)
A new area is In Utero Gene Therapy (IUGT) using stem cells technology. Stem cells are multi-purpose embryonic cells from which each organ is originally formed. Inserting corrective genes into these cells, once they are identified could prove more effective than using viruses and other vectors for DNA insertion. A number of genetic disorders result in irreversible damage to the fetus before birth. In these cases, as well as when patients may benefit from therapy before symptoms are manifested, in utero gene therapy could be beneficial. Some successes with in utero gene transfer have been reported in animals. But stem cell technology is in its infancy, and a number of technical and ethical questions need to be answered before any clinical tests are allowed. (11)
The focus of gene therapy has shifted from inherited diseases toward more common ailments like cancer, AIDS and heart disease - all areas that could prove more profitable. Many genetic disorders, and there are about four thousand of them, affect anywhere from a handful to a few thousand people worldwide, hardly a commercially promising prospect for pharmaceutical companies. Genes for conditions like obesity and baldness have much more support for further investigation than rare disorders. Of the 244 gene therapy trials registered since 1989 with the Recombinant DNA Advisory Committee at the National Institutes of Health, about 150 are for cancer and another 23 are for H.I.V. Only 33 are for diseases caused by a defect in a single gene, and 16 of those are for cystic fibrosis, the most common inherited disease. Seventeen tests cover 12 other genetic diseases. Among trials registered since the beginning of 1997 the balance is even more lopsided - 53 for cancer and 8 for hereditary diseases. (12)
Funding problems leave many scientists only a hope that their work on cancer and other widespread diseases will eventually produce more effective technologies, and they will revisit their research projects on single gene defects armed with better knowledge. It appears to be a popular option under present U.S. Government's policy of selective support and regulation of gene therapy based on the 1995 Panel Report and recommendations (1).
I agree that cancer is a tragedy that can strike anyone, and there is a great public interest in effective treatment of this disease. There are people with inherited heart conditions and there are innocent victims of HIV infection and Alzheimer's disease who need society's help. Giving priority to gene therapy of these diseases is quite appropriate. But whatever is the aggregate distress of millions of bold and obese people, it is not comparable to sufferings of one child having cystic fibrosis or other potentially fatal genetic disease. And it sounds like the Government tells these people: "You have to wait." It would be better, in my view, to leave totally the development of gene therapy for not life-threatening conditions to private industries. Also, for some of the widespread diseases with genetic component such as diabetes and some heart problems there are reasonably effective conventional therapies and surgical procedures which makes them lower priority areas for gene therapy.
Some people have reservations about the prospect of big private companies monopolizing and profiteering on gene therapy using the results of publicly funded research. But the shift in Government's policy towards gene therapy is, in a way, recognition of the enormity of the problem and government's inability to tackle it on its own. Attracting private investments is of crucial importance to the success of this endeavor, and it is the law of the land that private capital is invested for profit. If public is well informed, and there is political will, the Government could introduce some controls to avoid a hike in costs of gene therapy or apply antitrust laws to stimulate competition among smaller companies and disrupt creation of monopolies providing gene therapy for specific diseases. I also think that when industry's involvement in gene therapy is sufficient, the government funding priorities should be reversed so that the people suffering from rare genetic disorders get more attention.
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