Gene Therapy: A new generation of molecular medicine

Erick De Wolf

Copyright 1997


In September of 1990, eight year old Ashanthi DeSilva made medical history when she received the first authorized human gene therapy treatment. Ashanthi has been born with a defective gene that normally produces an essential enzyme adenosine deaminase (ADA). If left untreated the inability to produce this enzyme results in the fatal malfunction of the immune system. Four years after receiving the first of many treatment of transgenic cells containing functional ADA genes, Ashanthi was still doing well. Since that first treatment, the National Institutes of Health (NIH) has been spending hundreds of millions of dollars to develop and evaluate new gene therapy protocols for a variety of genetic diseases (Marshall 1995).

Gene therapy has made a profound impact not only in the treatment of genetic disease but also in the way that we look at human disease as a whole. The following text describes the technical and ethical aspects of gene therapy. In the final section a synthesis of the author's personal opinion is presented.

1 Technical Aspects

Recombinant DNA (rDNA) technology has made the transfer of genes from one organism to another possible. The use of rDNA technology in conjunction with humans and other mammals is referred to as gene therapy. Gene therapy can be classified into two categories, germinal and somatic (Sections 1.1 & 1.2, respectively). In general, gene therapy involves augmenting the functions of an absent or dysfunctional gene by the introduction of a functional gene into the cells of the individual. If the transfer is successful, the result is a transgenic individual, who is now free of the effects of the dysfunctional gene as long as the new gene is expressed (Griffiths et al. 1996). Gene therapy has been successful in mammalian models, and potential applications for treatment of human genetic disease such as Cystic Fibrosis, ADA and various types of cancer are now being aggressively persued by researchers. Clinical trials have already begun (Section 1.4) (Kolberg 1994, Gibbs 1996 & Marshall 1995).

1.1 Germinal gene therapy

Germinal gene therapy involves the introduction of genes into both somatic cells and the germline of an individual. The result is not only that the individual is cured of the genetic disease but some of the gametes of the individual may also carry the altered trait. Germinal gene therapy in mice has been successful, but thus far protocols developed for use in mice have not been effective in humans. One protocol that might be feasible in human germinal gene therapy involves the removal of an embryo during the blastocyst stage of development and injection with transgenic cells. As the embryo develops the injected cells then become integrated into tissues of the body, including the germ line and ultimately the gametes of that individual (Griffiths 1996).

1.2 Somatic gene therapy

Somatic gene therapy focuses on the correction of genetic disease by treating non-reproductive or somatic tissues. This method of gene therapy involves the removal of some of the dysfunctional cells and inserting a cloned wild-type gene. The transgenic cells are then reintroduced into the patient's body where they provide the corrected gene function. In most cases not all of the affected cells need to be altered but only enough to relieve the symptoms of the disease. In contrast to germinal gene therapy, somatic gene therapy technology has advanced rapidly and is presently being evaluated in numerous clinical trials (Griffiths 1996). Somatic gene therapy currently focuses on treating those diseases that are limited to the one tissue, including CF and ADA. Soon somatic therapy may be used on multigenic diseases (Msezane 1997, Marshall 1995). However, the weakness of somatic gene therapy lies in the delivery of the wild-type gene and sustaining expression (Verma 1997).

1.3 Gene delivery

At present, there are two types of gene delivery vehicles, non-viral and viral. Non-viral gene delivery includes approaches such as direct injection or mixing the DNA with compounds that allow the gene to cross the cell membrane, but these types of gene delivery systems have not been efficient. Gene delivery with viral vectors has been more successful and most current protocols involve this type of gene delivery. Viral gene delivery involves the use of a virus that has been pathogenicly disabled, but still maintains its natural ability to transfer DNA into living cells (Verma 1997, Msezane 1997, Crystal 1995).

Retroviruses are a group of RNA viruses that convert their genome into DNA in the infected host cell. Researchers have identified regions of the viral genome that are required for integration into the host genome, and control gene expression. By manipulating these regions of the viral genome and replacing portions with transgenes, researchers have been able to use the viruses to effectively deliver wild type genes. The retroviral vectors only have the ability to infect rapidly dividing cells, and cannot infect the cells that make up muscle, brain, lung, and liver tissue. In a procedure called ex vivo gene therapy, cells from the target tissue are removed and grown in vitro where they can be infected with the altered retro viral vector. The transgenic cells are then transplanted back into the patient where they express the desired gene product. However, gene expression ceases after about a week despite continued presence of the cells that posses the transgenic DNA. Longer gene expression has been observed in other systems, and the solution may involve finding the correct enhancer-promoter combination for a given cell type. Lentiviruses are viruses that belong to the retrovirus family, but can infect both dividing and non-dividing cells. The best known lentivirus is the human immunodeficiency virus (HIV), which has been disabled and developed as a gene delivery vector in vivo. Lentiviruses can be manipulated in a similar fashion to the retroviruses, and appear to sustain expression for over six-months. Another advantage of the Lentiviruses is that they do not elicit an effective cellular immune response (Verma 1997, Msezane 1997, Crystal 1995).

Adenoviruses are a family of DNA viruses that can infect both dividing and non-dividing cells. Upon infection the adenovirus genome is not integrated into the host DNA, but is replicated as extra chromosomal elements in the host nucleus (Crystal 1995 Verma 1997). The replication process of these viruses can be disabled and the genome manipulated so that the virus expresses a desirable gene much like the vectors that have been discussed previously. However, in contrast to the retroviruses the Adenoviruses can infect cells in vivo and express the trans gene products at high levels. Host immune response usually results in a short (< 10 days) duration of gene expression and in the production of antibodies that make additional treatments with the recombinant virus almost useless. Moreover, a large percentage of the human population already has antibodies for Adenoviruses because of prior naturally occurring infections. Immunological difficulties may be overcome in time, but at the present time Adenoviruses have limited application (Verma 1997).

One final family of viruses that warrants mention as potential vectors in gene therapy is the Adeno-associated viruses. Adeno-assiciated viuses are single stranded DNA viruses that are non-pathogenic. These viruses contain relatively few genes and require additional genes to replicate (usually an adenovirus). While more information about this family of viruses is needed, preliminary results indicate that therapeutic amounts of gene product can be produced in vivo for up to six months (Verma 1997).

1.4 Clinical trials

As mentioned previously, the evaluation of gene therapy as a treatment for genetic disease such as cancer, ADA, CF, and other genetic diseases has begun in the United States (Kolberg 1994, Gibbs 1996 & Marshall 1995). Many of the trials have resulted in short term improvement in patients receiving treatments, but effects have not been sustained. In some cases clear assessment of the effects of gene therapy have been difficult because the therapies have been used in tandem with other treatments (Marshall 1995, Verma 1997).

2 Public Debate

The science of genetics, and particularly recombinant DNA technology, has seemingly always been in the media since the early 1970's (Berg and Singer 1995). The impact of the involvement of the media, both positive and negative, has shaped how the public views not only genetics, but science as a whole. Gene therapy has not been an exception and the following sections address only some of the issues that have emerged during recent years regarding the ethics and potential of this technology.

2.1 What is a genetic disease?

In recent years the concept of what can be considered a genetic disease has been changing. The term genetic diseases is no longer restricted to diseases like Huntington's, Phenylketonuria (PKU), or Sickle-cell anemia that are the result of a defect in a single locus and have 100% heritably. Rather, genetic disease has been expanded to include any trait that has a genetic component even though the heritably is reduced. As a result, cardiovascular disease, some types of cancer and diabetes are now considered to be genetic diseases. Ultimately, behavioral and psychological disorders such as alcoholism, schizophrenia, obesity and even criminal behavior have come to be considered genetic diseases. The concern over this trend is that too much emphasis will be placed on gene therapy as a potential treatment of disorders that are not entirely genetic in origin, but have important associated environmental factors (Magnus 1997).

2.2 Safety of gene transfer

The safety concerns of the gene transfer that are required for gene therapy to be successful are not trivial. Patients must risk the possibility of vector-induced inflammation and immune responses. The complementation of replication deficient vectors could occur resulting in undesirable viral replication, infection, or insertional mutagenesis. There have been reports of adverse events in human gene therapy including inflamation of the airway and central nervous system induced by the administration of viral vectors. However, when the total number of individuals involved in the gene therapy trials is considered, adverse events have rare and, in most cases, related to dose or method gene delivery (Crystal 1995).

2.3 Ethical issues

It is clear that gene therapy holds both great promise and potential for misuse. The promise of gene therapy is that individuals with previously untreatable genetic diseases may be effectively and safely treated with gene therapy (Crystal 1995). Any delay in the development of this technology would only result in the needless suffering and possibly needless deaths. However, the present state of gene therapy is irreversible, and this technology should not be approached without prudent and judicial caution. In the case of germline gene therapy, not only is

the individual receiving the treatment affected, but also the offspring of that individual (Msezane 1997).

Potential misuse of gene therapy is undeniable and concerns of gene therapy being used for enhancement of desirable qualities such as intelligence, cosmetic appearance, or physical performance need to be addressed (Macer 1995, Brownlee 1994). Perhaps the most difficult question to answer is who defines what or who is normal and what is considered genetic disease (Brownlee 1994).

3 Personal Opinion

3.1 Hype and disappointments

The media has played a pivotal role in the public perception of what the goals of these trials truly are. This can result in hype about treatments that have not been fully evaluated (Wadman 1995). Whether clinical trials can be considered a success or a failure depends on one's expectations. For example, if one expected the trials to result in the complete relief of patient symptoms without side effects or the need for further treatments, the treatments would most certainly be a failure. However, if the expectation was that trials would allow important insights into how human genetic diseases might be treated with gene therapy or other treatments, the trials may appear to be a success. Scientists need to clearly convey their goal to the public and avoid hype that can lead to public disillusion.

Since the Alisomar conference in 1974, scientists have shown that they feel a sense of moral and social responsibly. As a result, committees and government agencies alike have played an important role in the evaluation of the ethics of recombinant DNA technology including gene therapy. The positive and negative potentials of gene therapy are not uncommon for developing technology. No amount of regulation could prevent all potential misuses of this technology and a total ban of gene therapy research would not prevent misuse but rather would delay the development of gene therapy treatment of genetic disorders.

3.2 Need for basic research

Researchers have made great strides in the treatment of human genetic diseases with gene therapy, but a renewed emphasis in basic research is needed to sustain these efforts. Researchers must address short comings in the understanding of critical components of virology, immunology and cell biology if gene therapy is to advance. Perhaps gene therapy will never result in a cure for genetic disease, but what we learn about the human genome, gene expression, virology, immunology, and cell biology may one day bring treatments for not only genetic disease but for many other diseases as well.

Literature Cited

Berg, P. and Singer, M. F. 1995. The recombinant DNA controversy: twenty years later. Proc. Nat. Acad. Sci. U.S.A. 92:9011-9013.

Brownlee, S. 1994. A new eugenics: the narrowing of normality. U.S. News and World Report. Aug. 22, 67.

Crystal, R. G. 1995. Transfer of genes to humans: early lesions and obstacles to success. Science 270:404-410.

Gibbs, W. W. 1996. Gene therapy Scientific American:Explorations. 10/21/97.

Griffiths, A. J. E., Miller, J. H., Suzuki, D. T., Lewontin, R. C., and Gelbart, W. M. 1996. An introduction to genetic analysis, sixth edition. W. H. Freeman Company, New York.

Kolberg, R. 1994. The bystander effect in gene therapy: great, but how does it work? The journal of NIH Research. Feb. 10/21/97.

Macer, D. R. J. 1995. How much enhancement is acceptable? Gene Therapy Newsletter 9:7.

Magnus, D. 1997. Gene therapy and the concept of genetic disease. 10/21/97.

Marshal, E. 1995. Gene therapy's growing pains. Science 269:1050-1055.

Msezane, L. 1997. Gene therapy: the ultimate form of molecular medicine?> 10/21/97.

Verma, I., and Somia, N. 1997. Gene therapy- promises, problems and prospects. Nature 389:239-242.

Wadman, M. 1995. Hyping results could damage gene therapy. Nature 378:655.

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