Human Gene Therapy

Melanie Sifford

Copyright 1998

Introduction

Deoxyribonucleic acid (DNA) was discovered in 1944 by Avery and colleagues. Avery identified DNA as the primary genetic material. Watson and Crick later discovered the double helix structure of DNA. Leder and co-workers deciphered the triple nucleotide code that designated the amino acids from which proteins were built. The science of molecular biology was born (Sokol, Gewirtz, 1996). In 1990 a four year old girl who was suffering from severe combined immunodeficiency (SCID) was the first to undergo gene therapy. White blood cells were removed from the girl and the cells were inserted with normal copies of the defective gene and returned into the girls circulation. Her condition improved with four treatments and follow-up treatments (Anderson, 1995). Cystic fibrosis (CF), the most common fatal genetic disease among Caucasians in the United States, afflicts about 30,000 people worldwide. The faulty gene, CFTR, transfers salts across cell membranes, which causes mucus buildup in many tissues, particularly in the lungs. Infections lead to early death, usually by age 30. Cystic fibrosis currently has no effective treatment. Since the cystic fibrosis gene was cloned in 1989 this has led researchers to look for treatments through gene therapy ( Stanford, 1996). CF appears to be suitable for treatment by gene therapy:

  1. It is a monogenic, recessive disorder;
  2. The function of the CFTR gene product is known, facilitating measurement of gene transfer;
  3. The principal target cells, the airway epithelia, are accessible by minimally invasive procedures;
  4. Several transgenic mouse models for developing and testing procedures prior to clinical evaluation are available;
  5. here is a relatively large cohort of patients willing to participate in clinical studies (Gill, Hyde).
Gene therapy is one of the most exciting technologies for the future.

Technical Aspects

There are three techniques that researchers are working on. The first and most common is ex vivo ( or "outside the living body") therapy. The defective cells are removed from the patient and replaced with the normal DNA before returning to the body. This therapy targets the blood cells because many genetic defects alter the functioning of one type of these cells or another. But since blood cells have limited life spans follow-up treatments are required. Future efforts will most likely target stem cells of the bone marrow. Stem cells are ideal for gene therapy because they appear to be immortal. Researchers have obtained stem cells from human bone marrow, but they are having difficulties getting genes into the cell as well as inducing the cells to produce many new blood cells (Anderson, 1995).

The second technique is the in situ (or "in position") therapy. Cells carrying corrective genes are introduced directly into the tissue where the genes are needed. This treatment is good for conditions that are localized, but it cannot correct systemic disorders. In situ treatment is being explored for diseases like cystic fibrosis, muscular dystrophy, and cancer. This treatment is still hindered by a lack of safe and effective ways for implanting correct genes into various organs (Anderson, 1995).

The third technique is in vivo (or "in the living body"). This therapy does not exist yet but is the therapy of the future. Researchers hope to inject gene carriers into the bloodstream. Once in the body the carrier genes will find their target cells, deliver their genetic information efficiently and safely while ignoring other cell types. Progress has been made developing toward vectors that will home to specific cell types. But researchers have had little success creating delivery units that can insert their genetic information efficiently for this technique (Anderson, 1995).

Scientists have developed several methods for transporting genetic material for ex vivo therapy. The most effective therapy uses modified viruses as carriers. Viruses are useful because they are able to penetrate cells and insert the genetic material that they contain into the new host. First the disease causing protein of the virus must be removed. Then the corrective gene is inserted into the virus and the virus transports useful genes into the cells but cannot cause illness (Anderson, 1995). Some of the gene delivery systems are retro viruses, adenoviruses, adeno-associated virus and herpes virus.

Some of the approaches being studied for CF gene therapy are; one using adenoviruses as a vector to slip the corrected gene into the patientís lung cells. The adenovirus is an excellent candidate as a vector because, during the course of its evolution, it has developed sophisticated means of inserting its DNA into human lung cells (Genzyme,). Other advantages of the adenoviruse are they produce high titre viral stock. Adenoviruses also accommodate large cDNA inserts facilitating the insertion of complete gene sequences. Although they are common human pathogens, they produce little morbidity and have not been associated with human malignancies. The main disadvantage is the generally short duration of gene expression. Adenoviruses are also reduced significantly following a second treatment of the virus that may be directly related to the immunogenic response (Sokol, Gewirtz, 1996).

Another approach is to use the adeno-associated virus. Advantages of the adeno-associated virus are their ability to integrate site specifically into chromosomes, their broad host range and their benign activity. Also, the p40 promoter is particularly strong in some cell types, allowing constitutive expression even after integration. Some disadvantages are lack of information on long-term consequences of integration and gene expression from the adeno-associated virus (Sokol, Gewirtz, 1996). Another disadvantage of using viruses is that the bodyís immune system recognizes them as invaders and has potent weapons to defeat them. Research is being done to solve this problem (Genzyme,).

Another therapy for CF is to use liposomes, tiny bubbles of lipids, or fats, to deliver the therapeutic gene. Lipid vectors appear to carry fewer risks than viruses and they are less likely to be viewed as invaders by the immune system. This therapy has shown low rates of success (Genzyme). Most of these therapies target the nostrils or lungs

. A study being done by Stanford for CF gene therapy also uses modified viruses as vectors to introduce healthy genes into the patientsí cells but they target the maxillary sinuses. This is for safety and convenience. The lining of the sinuses is similar to that of the lungs, but the site is smaller and more accessible. Also, the treatment can be contained, and two different doses, one in each sinus, can be studied very precisely in one patient (Spector, Malone, 1996).

Genetic research is advancing steadily and sometimes rapidly on many fronts. Discoveries in genetics have led to novel strategies for treating diseases. The bottom line in any kind of biomedical research lies in the realm of treatment and prevention. The ultimate step in that direction is gene therapy- the deliberate transplantation of genes to treat or even prevent human disease like Cystic fibrosis (Shmeck, 1991).

Ethics Debate

For somatic cell gene therapy there seems to be a lot of support. Somatic cell gene therapy represents a logical extension of existing treatments. For example somatic cells, such as bone marrow cells, involve changes limited to the cells of the person being treated. The difference of somatic cell gene therapy and standard medical treatments would be that the gene therapy would be a permanent treatment and change in the body rather than requiring repeated applications of an outside force or substance (Nichols, 1988).

Germ line gene therapy, where genetic changes would be passed on deliberately to future generation, have caused more of a debate. Leroy Walters, director of the Center for bioethics at Georgetown University, explains that there would be two basic rationales for using germ line gene therapy to treat genetic diseases. One would be if some genetic diseases were resistant to somatic gene therapy. The second would be one of efficiency. For example if somatic gene therapy became a successful treatment for some genetic diseases like, cystic fibrosis, the treated patients would constitute a new group of homozygous "carriers." If two treated patients with the same genetic disease had children all would be affected by the disease. But patients and their doctors might view germ line gene therapy as a more efficient alternative, because some of the children would inherit the inserted gene from their parents (Nichols, 1988).

Arguments against germ line gene therapy are; one the potential risk of gene therapy. Is it ethical to use a technique in which unanticipated problems or mistakes could be passed on to the future generation? The biggest concern that people have about gene therapy is that it will alter genetic traits in "normal" individuals. It would be a technique for parents to use to produce the "perfect"child, for example selecting certain trait over others like higher IQís or better physical fitness, is extremely disturbing (Nichols, 1988).

Ethical issues raised by somatic cell gene therapy are similar to those raised by any other new forms of therapy. Prior to clinical trials, researchers will have to demonstrate that the benefits out weigh the risks; that therapies known to be effective will not be withheld for purposes of conducting the trials; that the process of selecting patients will be fair; and that patients and their families will be informed about all aspects of the treatment including any side effects that may be irreversible (Nichols, 1988).

Religious, scientific and governmental organizations have concluded that it would be unethical to withhold somatic cell gene therapy from severely ill patients, as long as it can be shown to be safe and effective, just because other forms of gene therapy in the future might be misused (Nichols, 1988). Oversight for gene therapy occurs both at local and national levels. At the local level there is the Institutional Review Boards (IRB) to ensure that the research complies with Department of Health and Human Services (DHHS) regulations. Experiments that involve gene insertion must be approved by an Institutional Biosaftey Committee (IBC) (Department of health and Human Service, and Culvert, 1996).

At the national level the National Institutes of Health (NIH) must approve each human gene therapy proposal. The NIH director gets advice form the Recombinant DNA Advisory Committee (RAC). The membership of the RAC includes clinicians, scientists, attorneys, ethicist, theologians, patient advocates, and business persons. The diversity in the membership of RAC is so that the best interest of patients, society and the investigators can be served (Culver, 1996).

Personal View

I believe gene therapy should be continued. We should learn what we can about genetic diseases and how to treat them the best way possible. Gene therapy may hold a cure for many genetic diseases. I donít believe there would be dramatic changes in the gene pool if germ line gene therapy was available now as a treatment. There is a good oversight program for gene therapy and research and experiments are being well regulated.

References

Anderson, W. F.1995. Gene therapy. Scientific American 124-128. Culver, K. W. M.D. Gene Therapy a Primer for Physicians. 1137-140 (Mary Ann Liebert, Inc. 2 Madison Avenue, Larchmont, NY, 1996).

Dr. Gill, D.; Dr. S. Hyde. 1997. Cystic Fibrosis Research Group: Gene Therapy for Cystic Fibrosis. http://www.ndcb.ox.ac.uk/group1.html.

Nichols, E. K. Human Gene Therapy. 162-164. (Harvard University Press, 1988).

Schmeck, H. 1991. The future of genetic research. Howard Hughes Medical Institute. http://www.mit.edu:8001/afs/athena/course/other/esgbio/www/mg/future.html.

Sokol, D. L., A. M. Gewirtz. 1996. Gen therapy: basic concepts and recent advances. Critical Reviews in Eukaryotic Gene Expression, 6(1):29-57.

Walters, L. 1996. The Ethics of Human Gene Therapy. Nature 225-227.

Gene Therapy for Human patients Information for the General Public. 1990 Department of Health and Human Services. Public health Service National Institutes of Health.

Genzyme, http://www.genzyme.com/company/lines/rdgt/welcome.htm.

Spector, R.; M. A. Malone. 1996. Stanford University Medical Center Office of communications. http://www.med.stanford.edu/center/Communications/Pressrel/October96/cfgene.html.


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