Human Gene Therapy

Kiran Oberoi

Copyright 1999


Gene therapy is the use of genes to treat disease. It represents a quantum leap in our approach to the treatment of human disease and will have a significant effect on medicine over the next ten years. William French Anderson, Michael Biase, and Ken Culver performed the first successful gene therapy on a human in 1990. They developed a protocol for treating Adenosine deaminase (ADA) deficiency, severe combined immune deficiency, also known as the" Boy in the Bubble disease". ADA deficiency is a result of inheriting two copies of the defective ADA gene (in other words it is a recessive disease). Possession of a normal gene leads to the continuous, regular production of ADA in cells throughout the body. Without at least one properly functioning gene, children have no way of converting deoxyadenosine (a waste product) into inosine. This leads to the rapid build up deoxyadenosine in the system, which becomes phosphoralysed into a toxic triphosphate which kills T-cell. The result is an almost complete failure of the immune system and early death.

Concept of Gene Therapy

The term gene therapy originally referred to proposed treatments of genetic disorders that would involve replacing a defective gene with its normal counterpart Current usage of the term now extends to include all treatments in which there is an introduction of genetic material into body cells to treat a variety of diseases. Gene therapy utilizes two theoretically possible approaches:

1) Somatic gene therapy entails the transfer of a gene or genes into body cells other than germ (egg or sperm) cells with effect only on the patient. The new genetic material cannot be passed on to offspring. Examples of Somatic gene therapy have already proven to be clinically effective. The first successful treatments of adenosine deaminase deficiency took place in 1990 in 1991 with two patients aged 4 and 11. Both are thriving with continuing treatment. The first successful treatment of familial hypercholesterolemia, a genetic condition, which affects the livers regulation of cholestrols in the blood, took place in 1992 of a 29-year-old woman. Her improvement was stable for the 18 months of the study and liver biopsy demonstrated activity of the inserted gene and no discernible abnormalities. Five patients have been treated as of 1994.Current research involving Somatic gene therapy is focusing on a number of areas. Clinical trials are being performed on a treatment for cystic fibrosis, a chronic genetic disorder.

2) Germline gene therapy would involve the genetic modification of germ cells. Such therapy would change the genetic make up of the egg or sperm of an individual and would be carried on to future generations. This would offer the possibility of removing an inherited disorder from a family line forever. This could be achieved by other methods, such as, at present, diagnosis when there is a known risk before embryo implantation during IVF. Germ line therapy is a remote prospect and general opinion is strongly negative; such therapy is currently illegal in most of Europe. Somatic and Germ line gene therapy raise different issues. Somatic gene therapy offers the prospect of effective treatment and cure for previously fatal disorders. Until now it has only been used experimentally for a small range of genetic disorders; even in these cases treatment is complex, difficult and success uncertain.

Technical Aspects of Gene Therapy

The most fundamental requirement for gene therapy to be successful is that a therapeutic gene can be effectively delivered to a target cell. Once delivered that gene must enter the nucleus of cell wall where it will act as a template for production of protein molecule. The protein then exerts the primary therapeutic effect. This may be, for example, cell killing in the case of tumor therapy or cell preservation in the case of neurodegenerative disease.

There are several ways to get genes into cells. The most efficient of these uses disabled, engineered viruses. These systems are efficient because viruses have evolved over long periods of time to deliver their own genes to cells. Whenever, we get a viral disease, be it a cold or AIDS, the particular virus concerned is placing its genes into our cells in order to reprogrammed our cells to produce more virus. When we use viruses for gene therapy we disable them so that they are unable to cause disease and we engineer them in such a way that they pick up and deliver the genes of our choice rather than their own genes. These derivatives of viruses that are used for gene delivery are known as viral vectors. The most frequently used viral vectors are of two types. The vectors based on adenovirus are generally used for therapeutic strategies that require the therapeutic gene to be active for only a short time. Gene delivery by adenoviruses is very efficient but because the gene does not become integrated into the chromosomes of the target cell the gene is lost overtime.

This not a disadvantage for some therapeutic strategies such as cell destruction in the treatment of some cancers, restinosis or inflammatory disease. However, it is a disadvantage where sustained gene activity required for many months such as in the treatment of some tumors, neurodegenerative disease and HIV infection.

The second major type of vector is generally used and this is based on the retrovirus, murine leukemia virus (MLV). When genes are delivered by derivatives of MLV they become integrated into the chromosomes of target cell and are maintained for as long as the cell remains alive. Gene activity is easy to control and continues over long periods of time. Many clinical trials have been conducted with these MLV based systems and has been shown to be well tolerated with no adverse side effects.

One of the major difference between adenovirus vectors and MLV vectors is that the former can deliver genes to cells that are not multiplying by cell division whereas the latter cannot. Until recently this has meant gene therapy strategies that demand long term gene activity in cells that are not dividing have been feasible. Examples of important target cells that do not divide are neurons, certain cells of the immune system and certain epithelial cells.

Lentiviruses are a subgroup with in the general family of retroviruses but they are distinct from the MLV like viruses in that they are able to infect non-dividing cells. The best studied of the lentiviruses is HIV and when observation was made, about 10 years ago, that HIV could infect terminally differentiated macrophages, which do not divide, there was a move within the research community to develop gene delivery vectors from HIV. There were number of early technical difficulties and first generation vectors could not be used in the clinic as they had potential to generate infectious HIV. Over past two years we have seen new HIV based vectors emerge that are severely disabled containing only the few HIV components that are required for efficient gene delivery to non dividing cells. These so called minimal vectors are now candidates for gene delivery vehicles for clinical use in gene therapy.

The technique, called Chimeraplasty, was developed for mammalian gene therapy. It has an advantage over current genetic engineering methods in that it can seek out any specific gene and cause tiny mutations with high precision. Instead of adding a new gene to trick a plant into doing something it would not normally do, Chimeraplasty simply switches on or off function for which the plant already has a gene.

Until now, an entire gene had to hitch a ride into the nucleus on a defused viruses, which has the ability to insert itself into the genome. However, the virus could settle anywhere on the genome, sometimes choosing a location that is less than optimal for the replication of new gene. Technique also eliminates the danger from inserting large sections of genes with potentially undesirable side- effects, such as poisoning beneficial insects.

For Chimeraplasty, researchers start with small chunks of artificial genetic material, called oligonucleotides or "oligos", with about 25 bases each. They mirror one specific plant gene except for a mismatch of a few bases. The chunks are hooked up to tiny gold particles, which are then shot into nucleus of cell with a particle gun. When the oligos attach their counterparts in the cell, the DNA repair machinery tries to "fix" the mismatch, using the new sequence of the bases as blueprint.

Boosting blood cell production does little good for patients, whose blood cells are malformed, such as those of sickle cell anemic. The ultimate goal of gene therapy is not to compensate for genetic diseases but to erase them completely. Preliminary work published in the September 6 issue of science offers a reason to hope that goal may be possible .A team led by Allyson Cole-strauss and Kyonggeum Yoon of Thomas Jefferson University in Philadelphia experimented on cells containing a mutant gene that causes sickle cell anemia .To make their genetic drug they combined DNA for the normal version of this gene with RNA for the same gene. When they injected the drug into the diseased cells, the RNA/DNA particles homed in on the particular stretch of the genome that matched their codes and formed triple stranded DNA that covered the mutation. The cells normal DNA repair machinery then apparently replaced the mutation with the normal code thus permanently curing 10 to 20 percent of cells. The researchers still have to demonstrate that this technique works in human cells and in human bodies.

In about half of lung cancer cases, a gene called p53 has mutated and thus falls to encode a protein that oversees programmed cell death. In the absence of this protein, Which helps to curb the growth of damaged or abnormal cells, cancer can gain a foothold. Replacing such defective p53 genes with fresh ones has shown promise against a variety of cancers in animal experiments and studies of a few patients.

Scientists now report further progress in such localized gene therapy. By enlisting a virus to deliver p53 to tumor sites in 28 people with lung cancer, they temporarily stabilized or reversed the course of the cancer in more than half patients. The patients, average age 65, had lung cancer that was either inoperable or was no longer responding to radiation treatment or chemotherapy. The researchers injected the tumors with an adenovirus engineered to contain p53 genes. The virus was modified to prevent it from replicating and thus causing the upper respiratory infection that it might otherwise bring about.

During the 6 months treatment period patients received one to six monthly injections of the modified virus. The researchers delivered a range of doses -from 1 million to 100 billion viral units to gauge any toxicity of the treatment. 3 of the 28 patients died of cancer before doctors could make a 1-month follow up examination. Among the 25 others, tumors shrank in 2 patients, stabilized in 16 and continued to grow in the other 7.

The dose of virus mattered; cancer progressed unabated in three of five patients who received injections of 10 million or fewer viral units. In contrast, only 4 of 20 patients getting larger dose experienced cancer growth.

Argument of Human Gene Therapy

Consider what a nation would gain by permitting parents to genetically enhance their children. By assumption, the genetic enhancement technology increases the ability of children to learn and perform cognitive tasks, and thus to acquire and generate knowledge. Permitting or facilitating genetic enhancement would therefore increase the collective human capital embodied in nations workers .The increasing prevalence of high ability genotypes would also multiply the effects of other national investments in education, training, and scientific or engineering research. Because genetic enhancements are heritable, the effect of these investments on the stock of human capital are cumulative unless enhanced offspring or their descendants emigrate. Finally, permitting genetic enhancement would be a cheap way for a state to increase aggregate human capital, because competition between parents would lead some parents to pay for it out of their own pockets. If expanding stock of nations human capital brings increasing returns in productivity and economic growth, it means that in economic competitions among nations, small initial differences in the distribution of able people can multiply, over time, to large international differences in the rate of economic growth. Thus nations have an incentive to defect from an international ban on genetic enhancement to get a jump on others in the accumulation of human capital.

It is said that the day is not far of when parents will be able to browse through gene catalogs to special order a hazel eyed, red headed extrovert with perfect pitch. Every new discovery gives shape and bracing focus to a debate we have barely begun. Even skeptics admit it's only matter of time before these issues become real. If you could make your kids smatter, would you? If everyone else did, would it be fair not to?

It's an ethical quandary and an economic one, about fairness and fate, about vanity and values. Which side effects would we tolerate? What if making smarter kids also made them meaner? What if only the rich could afford the advantage? Does god give us both the power to re-create ourselves and moral muscles to resist? Self-improvement forever been an American religion, but norms about what is normal keep changing.

While gene therapy shows a great deal of potential, opponents, biomedical reductionism will undoubtedly have several responses. The way a person experiences a disease involves many social and psychological elements (such as emotional impact of the disease, the stigmatism attached to it, the cost and employment implications, etc). These important aspects of disease are neglected by therapeutic approaches aimed strictly at the genetic level.

Today in many countries governments sets up committees, which included not only scientists and doctors but also religious leaders, lawyers and ethicist, to consider the matter (Nichols 1988). A distinction should be drawn between making genetic changes in somatic cells and in germinal cells. The purpose of somatic gene therapy is to treat an individual patient, e.g. by inserting a properly functioning gene into patients bone marrow cells in vitro and then introducing the cells into patients body. They differ, however, in that gene therapy an inherent and probably permanent change in the body rather than requiring repeated applications of an outside force or substance. An analogy is organ transplantation, which also involves the incorporation into an individual of cells containing DNA of foreign origin. Germinal gene therapy, in which the changes would be made in germ cells and would be passed on to the offspring, is not allowed.

Personal Opinion

In my personal opinion gene therapy is both beneficial and potentially harmful depending on how it is applied. Somatic cell gene therapy will have great benefits for some of the diseases that human beings suffer. I think before carrying a gene therapy every researcher should keep following questions in mind

If proper guidelines and policies are made gene therapy can be utilized to its fullest potential. Everyone irrespective, of wealth, sex and so on should have same opportunity to make the best use of gene therapy. The government, the public groups and the scientific society should cooperate and walk hand in hand to encourage gene therapy applications and make the best use of it.

The advantage of gene therapy is to cure someone who is born with a genetic disorder or who develops deadly disease like AIDS, cancer and so on. The technique has the ability to cure so many diseases that have affected our society for years I think the advantages are more than disadvantages. Genetherapy has brought revolution in the field of medicine and has also redefined the role of both doctors and patients.

As more and more discoveries are made, the scope of gene therapy is getting wider and wider. Government regulators and scientist must take an initiative to solve the issues related to gene therapy and enlighten people about the benefits of gene therapy.


1. "Oxford Biomedical News" item
2. Science, vol 219 (1983), pg-138-140
3. Scientific American: explorations
4. Time, sept14 1999,pg 59
5. EFB briefing 3 the applications of gene therapy
6. Science news, vol 155 .may 15 (1999), pg 310
7. ethics and genetics paper gene therapy and the concept of genetic disease.
8. genomics.html

Student Essay List

Course Homepage