Human Gene Therapy

Human Gene Therapy

Panos A Ioannou The Murdoch Childrens Research Institute, Parkville, Victoria, Australia

Introductory
doi:10.1038/npg.els.0000994




Human gene therapy is the treatment of human diseases with DNA fragments comprising whole genes or parts of genes.




The Promise of Gene Therapy

Studies in the first half of the twentieth century demonstrated that bacteria were able to exchange genetic material, resulting in permanent and heritable changes in the properties of the recipient strain. The subsequent understanding of the molecular basis of inheritance in the second half of the twentieth century naturally led to the concept of using deoxyribonucleic acid (DNA) fragments comprising whole genes to overcome the effects of genetic defects in various human diseases. The experiences with the transfer of genes between bacteria (transformation) encouraged the development of a fairly simplistic view of gene therapy for human diseases: a single introduction of a normal copy of a defective gene into affected cells should result in long-term, stable production of the missing protein, leading to a complete cure! The promise for 100% cure seemed to be very different from anything else available in the armamentarium of twentieth-century medicine and caught the imagination of many scientists, as well as the attention of the news media and of the investors. Rather than treat disease by repeated administration of pharmacological agents, gene therapy offered the prospect of complete cure after a single application and with no side effects! This concept has formed the premise for much of the gene therapy research that has been conducted over the last 30 years, and underlies much of the ?~hypê?T that has been associated with it. See also: Bacterial genetic exchange; Human genetics: principles


Gene therapy was originally envisaged as a method for delivering normal copies of genes for the treatment of patients with various rare genetic diseases. This concept has gradually changed to include the use of gene fragments, including PCR (polymerase chain reaction) fragments as well as chimaeric and antisense oligonucleotides. Furthermore, the increasing understanding of the molecular mechanisms underlying cancer development and other somatic gene diseases is opening day by day many more applications for gene therapy. This is indicated by the fact that gene therapy trials for cancer now comprise about 70% of all gene therapy trials. These developments have further excited interest in the potential of gene therapy to transform the quality of life of most human beings in the twenty-first century.


On the other hand, it did not take very long for gene therapy research to come up against some serious obstacles. By the early 1980s it was clear that most human genes were much more complex in organization than anything encountered in bacteria. While bacterial genes are composed of continuous coding sequences and adjacent regulatory elements, the coding sequences of human genes (exons) were found interspersed with noncoding intervening sequences (introns), ranging in size from a few base pairs to hundreds of thousands of base pairs. By what wondrous mechanisms could coding sequences placed apart over such distances be brought accurately together, again and again, to make continuous coding sequences in the mature messenger ribonucleic acids (mRNAs), so as to ensure the faithful production of the thousands of proteins needed by each cell? Studies on the regulation of the expression of the Ỵ-globin locus have also revealed not only the presence of regulatory elements in the introns and the untranslated sequences immediately adjacent to each globin gene, but also regulatory elements conferring tissue and developmental specificity as far as 50 kb (50 000 base pairs) away from the target genes. See also: Genome organization: human; Genes: definition and structure; Genetic networks; Globin synthesis


The early concept of gene therapy could thus not be applied directly to the treatment of human diseases, as there are still no methods for packaging and efficiently delivering DNA fragments in the 100?"300 kb range, which is the size of most human genes with their natural regulatory sequences. The discovery of the technique to reverse transcribe mRNA back into complementary DNA (cDNA) seemed to offer a way out of the difficulty. Minigenes produced in this way could be readily cloned and manipulated in bacteria before delivery into human cells. Furthermore, such minigenes were easily accommodated in various viral vectors, thus facilitating efficient delivery into different cell types. However, such synthetic minigenes carried few if any of their natural regulatory elements and, not surprisingly, failed in most cases to approach normal levels of expression under physiologically relevant con***ions. With the exclusion from the viral vectors of most of the essential regulatory elements that underlie the developmental, tissue and locus specificity of gene expression, the goal of gene therapy proved much more elusive than was ever anticipated. See also: Protein production for biotechnology; Transcriptional gene regulation in eukaryotes


The task of identifying one by one individual regulatory elements followed by the inclusion and testing of combinations of these elements in the context of synthetic minigenes in viral vectors has proved very slow work. Thus, although the sequence of the intact Ỵ-globin locus has been known for about 15 years and numerous elegant studies have examined the regulation of the expression of the different globin genes, we still do not have a detailed picture of globin gene expression under various physiologically relevant con***ions, while it has proved very difficult to achieve a therapeutic level of globin expression with any viral constructs. Our understanding of the mechanisms underlying the developmental, tissue and locus specificity of expression of all the other human genes remains of course at an even lower level. Although the completion of the sequencing phase of the Human Genome Project has now revealed the sequences of the regulatory elements for most genes, it will probably take research during most of the twenty-first century before a good understanding of the regulatory mechanisms for most human genes is achieved. The development of effective viral gene therapy vectors is of course con***ional upon such an understanding for each gene of interest. In contrast, the delivery of large DNA fragments carrying intact functional loci and/or the direct correction of mutations in genomic DNA are not con***ional on such detailed knowledge and should enable the faster exploitation of knowledge of the human genome for the effective therapy of genetic and somatic gene diseases. See also: Human genome project; Gene delivery by viruses


The development of several high-quality, large insert genomic bacterial artificial chromosome (PAC/BAC) libraries for the Human Genome Sequencing Project is for the first time opening the real possibility of using intact human genes for the treatment of gene diseases. These resources have already proved extremely popular for gene mapping, isolation and sequencing. Similarly, the use of fully sequenced genomic fragments from PAC/BAC libraries for functional studies is already shifting the attention of many researchers to packaging, delivery and long-term maintenance of intact human genes in human cells as independent minichromosomes or episomes, or by targeted integration into specific sites in the genome using homologous or site-specific recombination mechanisms. See also: Artificial chromosomes; Genome mapping; Site-specific recombination: uses in biotechnology


At the same time, it is also likely that new methods will be developed for the targeted correction of mutations by enhancing the endogenous mismatch repair and/or homologous recombination mechanisms of the cells. The high fidelity of these mechanisms should reduce the risks associated with random integration of viral vectors in the human genome, while enabling the corrected gene to function under the control of the endogenous regulatory mechanisms. See also: Homologous genetic recombination in eukaryotes; DNA mismatch repair: eukaryotic


Finally, it should be noted that no drugs have been developed so far to treat human diseases by modifying the expression of specific genes so as to complement other defective genes, or by modifying the way specific mutations interfere with gene expression. However, the completion of the Human Genome Sequencing Project in combination with DNA chip expression profiling is expected to lead to an unprecedented degree of understanding of genetic networks under different physiological states, thus facilitating the identification of genes playing key regulatory roles in health and disease. The coupling of this knowledge with technologies for high-throughput screening for agents acting on the mechanisms that underlie the developmental, tissue and locus specificity of gene expression should facilitate the development of drugs that modify the expression of specific genes in a tissue-specific manner, or that are able *****ppress or reverse the effects of specific types of mutations. Although such approaches may not involve delivery of any DNA fragments and would not be strictly speaking in the realm of gene therapy, they could provide a basis for the effective and safe management of most genetic and somatic gene diseases. See also: Multispot array technologies; Pharmacogenetics




Ex Vivo Gene Therapy with Retroviruses

Viruses have naturally evolved efficient mechanisms to deliver their nucleic acids into eukaryotic cells. Typically, a few particles per cell are sufficient to lead to productive infection for most viruses. Many different viruses are now known, ranging in the type of nucleic acid they use (RNA or DNA), the size of their genomes and their species and host cell specificity. Many of these viruses, including murine retroviruses (MMLV, Moloney murine leukaemia virus), lentiviruses, adenovirus, adeno-associated virus (AAV), Vaccinia virus, Herpes simplex type 1 virus (HSV1), have been intensively studied as potential gene delivery systems. See also: Virology; Viral replication; Viral genome


Murine retroviruses were the first to be studied as gene delivery vectors and still remain very popular because of their high transduction efficiency, their ability to infect rapidly dividing cells and their efficient integration into genomic DNA. The maximum size of the therapeutic insert with murine retroviruses is about 8 kb, thus precluding the use of most genes with their natural regulatory elements. This is also true for most of the other viral vectors. Lentiviral vectors, despite the obvious safety concerns, are also being used increasingly in gene therapy research, because of their ability to integrate therapeutic genes into nondividing or quiescent cells, a property that may be particularly useful for therapeutic gene delivery into haematopoietic stem cells and neuronal cells. See also: Retroviral replication; Haematopoiesis


In retroviral vectors the therapeutic gene is inserted between the long terminal repeats (LTRs), in place of the viral genes that are needed for replication and packaging. The packaging of the recombinant retroviral vector is carried out in a packaging cell line that provides all the viral proteins that are required for the formation of mature viral particles. A major concern in this approach has been to minimize the chances of generating replication-competent retroviruses by recombination between the therapeutic vector and the helper sequences in the packaging cell line. The latest packaging systems have been designed to make the production of wild-type virus through homologous recombination highly unlikely. However, cells have a remarkable ability to join together DNA molecules through nonhomologous recombination mechanisms, and thus the generation of modified replication-competent retroviruses remains a small but not negligible possibility. See also: Retroviruses in human gene therapy; Retrovirus culture


Retroviral vectors are best suited for ex vivo gene delivery to cultured cells (e.g. cultured haematopoietic stem cells). After transduction, the recombinant retroviral construct is efficiently integrated into the chromosomes of the recipient cells. However, the random nature of the integration events could lead to a number of undesirable complications: See also: Gene transfer and expression in tissue culture cells


The expression of the therapeutic gene may be affected by the overall organization of the chromatin in the region of integration, or by nearby regulatory sequences, leading to variability of expression and/or silencing.
One or more integration events may take place near a gene involved directly or indirectly in the cellular processes leading to cell division. Modification of the expression of such genes by the LTR elements or other regulatory sequences in the recombinant vector may lead to tumorigenesis. Since there have not been systematic long-term studies to assess the magnitude of this risk, this needs to be kept constantly in mind as gene therapy reaches the stage of clinical effectiveness with these vectors.
The ability of retroviral and lentiviral vectors to achieve efficient integration may be linked to equally efficient mechanisms for replicational excision and re-integration at secondary sites in the presence of complementing functions, thus enhancing considerably the risk of complications.
An integration event may take place near one or more of the numerous retrovirus-like elements that are scattered in the human genome, with the potential to activate a dormant element into a new type of transmissible virus.
Another cause for concern is the potential of interaction between an ongoing viral infection and a recombinant viral vector, to produce a virus with a new potential. This is of particular concern in gene therapy trials on HIV-infected patients, where the high rate of virus production and the high rate of spontaneous mutagenesis are already challenging all the tools of modern medicine.

Ex vivo gene therapy allows in principle the possibility of careful evaluation of the effectiveness and safety of the procedure before returning the cells to the patient, although other considerations may necessitate the return of the cells to the patient before a thorough evaluation can be carried out. It is hoped that advances in stem cell research will enable long-term maintenance of human stem cells in culture after gene delivery, to allow a thorough evaluation to be carried out. Similarly, the possibility to direct de-differentiation and re-differentiation of various types of stem cells is opening new avenues for the combination of ex vivo cell and gene therapy approaches.

In Vivo Gene Therapy

The potential for ex vivo gene therapy is currently limited to haematopoietic stem cells. This approach is not applicable to most other cell types, either because the tissue cannot be cultured or because the tissue forms too large a part of the body to be effectively repopulated after gene therapy of stem cells in culture. Besides these practical limitations to the application of ex vivo gene therapy for genetic diseases, in vivo gene therapy is a necessity for cancer and other somatic gene diseases.


Besides the difficulties encountered with ex vivo gene therapy as discussed above, in vivo gene therapy also faces the ad***ional challenge of delivering enough therapeutic vector to the target tissue without toxicity to other tissues. While in some situations the correction of a defect in a proportion of the cells in a tissue may be sufficient to restore health, in the case of cancer most, if not all, cells have to receive the therapeutic gene. Thus a high degree of specificity during in vivo gene delivery is essential. This may be achieved by one of the following methods. See also: Cancer


Altering the glycoprotein coat of the recombinant vector so that it can only get into cells carrying the appropriate receptor. Such modifications can be carried out either by insertion of a peptide on the glycoprotein coat, to alter its cell specificity, or by chemical modification (e.g. chemical attachment of lactose to the surface of MMLV vectors facilitates transduction into hepatocytes via the asialoglycoprotein receptor).
Inclusion of a tissue-specific promoter to drive a therapeutic gene may ensure that the gene is expressed only in certain tissues, although the vector may be delivered to a wider range of tissues.


Nonviral Gene Delivery

Nonviral approaches to gene delivery date back to the middle of the last century, when Avery, MacLeod and McCarthy showed in 1944 that genes were transferred between bacteria by nucleic acids. Calcium phosphate-mediated transfection was developed in the early 1970s as the first nonviral technique for eukaryotic cells and is still the method of choice for the production of recombinant viral vectors. The size limitations and safety concerns of viral-based gene delivery approaches have spurred on efforts to develop safer, nonviral vectors, without limitations on insert size. A variety of nonviral approaches are now available, although the efficiency of the most effective nonviral techniques in stable gene expression is still less than that of transduction with retroviral vectors. The key differences between viral and nonviral approaches seem to lie not so much in getting DNA across the cellular membrane but in the efficiency of transport into the nucleus and the stable integration of the therapeutic genes. Viruses have evolved over a long period to go through these steps very efficiently, while nonviral delivery systems have paid little attention to these aspects of gene delivery. See also: Avery, Oswald Theodore; Macleod, John James Richard; McCarty, Maclyn; Transfection of DNA into mammalian cells in culture


Microinjection

This procedure is labour intensive, as DNA (or RNA) is injected into the nuclei of individual cells under a light microscope. The method is proving particularly useful in the generation of transgenic animal models with large genomic fragments from YAC, PAC and BAC clones. About 10% of the surviving embryos normally carry the microinjected transgene integrated randomly into the genome. Since such clones carry most genes as intact functional units, transgenic animals generated in this manner can be very useful in unravelling the mechanisms that underlie developmental, tissue and locus specificity in gene expression. See also: Transgenic animals


Electroporation

Electroporation usually involves the application of high voltage to a mixture of DNA and cells in suspension, although techniques for the electroporation of DNA into muscle and other tissues have also been described. The high voltage opens small holes in the cell membrane, through which the DNA enters into the cytoplasm. In bacteria this process is very efficient, resulting in over 10 billion clones per microgram of DNA for small plasmids. Although the efficiency decreases considerably for large plasmids, the process is efficient enough to make the generation of high-quality genomic PAC and BAC libraries (clone size range 100â?"300 kb) a rewarding endeavour. See also: Electroporation; Plasmids


Electroporation of mammalian cells is much less efficient because starting numbers of cells are much smaller and the DNA is delivered naked into the cytoplasm, where it can be subject to degradation. It is possible that complexing the DNA with agents that may protect it from degradation and facilitate its uptake into the nucleus will overcome some of these limitations.


Liposomes

Liposomes are formed by a variety of amphiphilic lipids. Chemical synthesis of a large variety of liposomes has allowed gene delivery in vitro and in vivo in various cell types and tissues, under con***ions that overcome most of the safety concerns with viral vectors. Major advantages of the liposome technology over viral vectors include the defined chemical composition of the liposomes and the absence of any obvious size limitation in the DNA fragments that can be packaged. Thus, as the liposome technology improves, it should be possible to package and deliver intact genomic loci, if not whole chromosomes, into specific tissues with good efficiencies. See also: Liposomes


Cationic liposomes interact with DNA electrostatically to form condensed particles, which can then be taken up by cells. Once inside the cells, the DNA is slowly released and some of it makes it to the nucleus where it can be transiently expressed. In a very small proportion of cells the DNA is eventually integrated into the chromosomes, leading to the establishment of stable cell lines. A large variety of cationic liposomes are commercially available and the effectiveness of these seems to vary greatly with different cell types and the con***ions for lipofection. In general, lipofection uses 105â?"106 small plasmid molecules per cell. Thus the efficiency of lipofection is several orders of magnitude lower then transduction with viral vectors. The difference between the two systems may largely be due to differences in the uptake of the DNA from the cytoplasm into the nucleus, rather than the delivery into the cytoplasm. See also: Primary cell cultures and immortal cell lines


Cationic liposomes are generally unsuitable for in vivo delivery, since they tend to interact nonspecifically with many tissues. In contrast to cationic liposomes, neutral liposomes entrap DNA inside the liposome particles. Although such liposome formulations have generally been less efficient than cationic liposomes, they are potentially more useful in vivo, since they can stay in circulation for much longer and can thus be targeted more effectively to various tissues. The compaction of DNA with peptides before encapsulation to facilitate nuclear uptake may eventually enable the in vivo delivery of large genomic fragments to specific tissues.


Naked DNA injection

A variety of tissues show transgene expression after delivery of naked DNA, with expression in striated muscle persisting for long periods after a single injection. The naked plasmid DNA appears to persist primarily as free plasmid, with no significant integration into the host genome after intramuscular injection. The efficiency of DNA uptake and expression in striated muscle is affected by various parameters, including age of animals and species. Plasmids up to about 20 kb in size have been successfully expressed after naked DNA delivery to muscle. A major limitation of this approach is the low percentage of expressing myofibres. However, expression of the erythropoietin gene under such circumstances seems to be sufficient to bring about changes in the haematocrit levels of the treated animals. Similarly, injection of DNA into muscle can be used to induce the production of antibodies to the encoded protein.


Ballistic DNA injection

The â?~gene-gunâ?T was originally developed for the introduction of DNA into plant cells, but it has since been modified to transfer genes into mammalian cells both in vitro and in vivo. The technique is restricted to skin, muscle or other organs that can easily be exposed surgically. The most exciting application of ballistic plasmid DNA injection is in DNA-based immunization. In contrast to immunization with foreign antigens, which generates only an antibody-mediated immunity, DNA-based immunization is more likely to result in a cell-mediated immune response and thus it may be more effective in immunization against a variety of viruses. See also: Microprojectile bombardment; Vaccines: DNA



Homologous Recombination

The ideal form of gene therapy involves the direct correction of the underlying genetic defect, enabling the mutant gene to recover its activity under its normal regulatory mechanisms. This type of repair may be achieved through homologous recombination, a process that involves the exchange of genetic information between two similar DNA sequences, with or without the involvement of mismatch repair. In mammalian cells this process allows the introduction of specific changes into the genome by interaction of the endogenous sequences with homologous sequences in DNA constructs that are delivered into the cells by a variety of vectors. See also: Homologous genetic recombination in eukaryotes; Recombinational DNA repair in eukaryotes


A major obstacle *****ccessful gene therapy through homologous recombination is the low efficiency of the targeting process. A large number of different proteins are involved in homologous recombination. The mechanisms are best understood in bacteria, where inducible homologous recombination systems have recently been developed. Similar studies in eukaryotic cells have yielded only marginal improvements. Further advances in our understanding of the mechanisms of homologous recombination will no doubt open novel possibilities for the therapy of genetic diseases. See also: Homologous genetic recombination during bacterial conjugation


Correction of mutations directly in the genome may also be achieved through the enhancement of the endogenous mismatch repair mechanisms. In order for this process to become operational, it is necessary to induce heteroduplex formation over the site to be corrected. Short PCR fragments, homologous sequences delivered with the AAV vector and specially designed chimaeric RNA/DNA oligonucleotides have all been used to induce mismatch repair correction, with variable results. See also: DNA mismatch repair: eukaryotic


Site-specific recombination mechanisms are well known in bacteria and a small number of such systems have been adapted to eukaryotic cells. Each bacterial integrase has a large recognition site and thus there are only a limited number of potential endogenous recognition sites in the human genome. As more and more integrases become characterized, it may be possible to have a whole spectrum of different integrases for targeting therapeutic constructs into specific regions of the genome. See also: Site-specific recombination; Integrase class site-specific recombinases



The Problems of Gene Therapy

Gene therapy research has gone through many ups and downs in recent years. The hype driven by the concept of gene therapy as an â?~elixirâ?T for genetic diseases, and by the rush of investors to make a quick profit, has given way to a more careful assessment of potential risks and benefits. The death of a patient taking part in a gene therapy trial in September 1999 has renewed safety concerns over viral vectors and has rekindled the debate on the ethical aspects of gene therapy. At the same time, the apparent successful therapy of a number of patients with severe combined immunodeficiency (SCID)-X1 disease, after ex vivo retroviral transduction of bone marrow cells, has given new momentum to the field. These recent developments underscore the potential of gene therapy in treating human diseases but also demonstrate that there are still some major obstacles to overcome before gene therapy becomes an essential tool of modern medicine. See also: Bioethics of gene therapy; Immunodeficiency: severe combined immunodeficiency


The main challenges still facing gene therapy research are the following.


What sequences to deliver?

Use of cDNA sequences in therapeutic vectors can hardly ensure the regulated expression of the cDNA under a variety of physiologically relevant con***ions. While some cDNA constructs may have useful applications, the availability of most genes as intact functional units in genomic DNA fragments from PAC/BAC libraries should encourage efforts at delivering intact functional loci. The delivery of an intact functional locus should overcome expression problems, as it should come under the endogenous regulatory mechanisms, even if it is integrated at a different site. Similarly, the direct correction of defects in genomic DNA by the delivery of short PCR fragments or chimaeric oligonucleotides will also restore the ability of the gene to function appropriately under a variety of physiologically relevant con***ions. See also: Antisense nucleic acids in biotechnology


How to deliver?

Viruses have evolved a number of complex mechanisms that endow them with extraordinary efficiency and specificity in delivering their genomes into the nuclei of different cell types. Similarly, some viruses have very efficient mechanisms for integration into the genome, while others can maintain their genomes extrachromosomally. The design of viral vectors for various gene therapy applications has concentrated primarily on taking advantage of some of these properties while deleting potentially harmful viral DNA sequences from the final constructs. Thus most retroviral, lentiviral and AAV gene therapy vectors retain only the corresponding LTR sequences. However, in a number of viral vectors, some of the viral proteins and mRNAs from the packaging cell lines that may be included in the viral particles may precipitate a variety of short-term acute reactions in the recipient host cells. These short-term acute reactions and the continuing need for a careful assessment of the long-term risks to individual patients and to society further support the development of alternative gene delivery systems. See also: Gene delivery by viruses; Adenoviruses


The development of nonviral gene delivery systems is aiming to take advantage of our increasing understanding of the mechanisms for the compaction of DNA in various physiological states, as well as for the packaging and delivery of therapeutic sequences across the cellular and nuclear membranes. The incorporation of ad***ional mechanisms for the maintenance of the therapeutic DNA as independent minichromosomes or episomes, or for its targeted integration through homologous or site-specific recombination mechanisms, should enable nonviral gene delivery to become a safe and indispenable tool of medicine in this century.


Target cell specificity

Delivery of therapeutic genes to a specific tissue and/or cell type in vivo represents one of the major hurdles in gene therapy for somatic gene diseases. While the design of viral vectors has often taken advantage of the natural tissue-specificity of different viruses, various strategies have also been developed to modify such targeting specificity, so as to modify the types of cells that are susceptible to transduction by each viral vector. Some of these approaches are readily adaptable for use in nonviral delivery systems. Since many cDNA constructs do not display a high degree of cell specificity in the expression of the cDNA, a high degree of cell specificity in the delivery is desirable to reduce unwanted effects on other cell types. In contrast, since expression of intact fucntional loci is highly cell-specific, it is anticipated that there will be less need for cell targeting during delivery of intact functional loci.


Fate of DNA in cells

Retroviruses integrate very efficiently into the genome of dividing cells and this has been one of the main reasons for their development as gene therapy vectors. Lentiviruses appear better in this respect, since they are also capable of gene delivery and integration in nondividing cells. The only elements that appear to be needed for integration are the LTR sequences and the viral integrase. It has generally been assumed that integration is not reversible. However, the recent demonstration that lentiviral vectors can be mobilized to integrate at ad***ional sites in the course of HIV infection is of particular concern, since it highlights the potential for interaction between viral vectors and other ongoing viral infections in patients. See also: Viral replication


It is clear that the random integration of viral vectors into patientsâ?T chromosomes cannot provide a safe approach for the widespread use of gene therapy to alleviate human disease. The enhancement of homologous or site-specific recombination mechanisms to facilitate targeted integration of therapeutic genes into specific sites in the genome, or their maintenance in independent artificial minichromosomes or episomes, may provide a safe alternative approach. See also: Knockout and knock-in animals



Summary

Gene therapy is emerging as one of the most potent tools of medicine for the treatment of genetic and somatic gene diseases. Although the problems associated with the design, delivery and fate of therapeutic constructs into patient cells have been grossly underestimated and oversimplified, leading to unrealistic and overoptimistic expectations, the completion of the sequencing phase of the Human Genome Project is expected not only to catalyse the development of effective therapies for many diseases in the first half of this century but also to provide a solid basis for the biological emancipation of mankind by the end of the twenty-first century.

Originally published: April 2001


Further Reading

Cavazzana-Calvo M, Hacein-Bey S, de Saint Basile G, et al. (2000) Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease. Science 288: 669â?"672.

Evans JT and Garcia JV (2000) Lentivirus vector mobilization and spread by human immunodeficiency virus. Human Gene Therapy 11: 2331â?"2339.

Felgner PL (1997) Nonviral strategies for gene therapy. Scientific American 276: 102â?"106.

Fox JL (2000) Gene-therapy death prompts broad civil lawsuit. Nature Biotechnology 18: 1136.

Friedmann T (1997) Overcoming the obstacles to gene therapy. Scientific American 276: 96â?"101.

Friend SH (1999) How DNA microarrays and expression profiling will affect clinical practice. British Medical Journal 319: 1306.

Kresina TF (ed.) (2001) An Introduction to Molecular Medicine and Gene Therapy. New York: Wiley-Liss.

Michael A (1996) Financing gene therapy beyond phase II. Gene Therapy 3: 1035â?"1038.

Morgan RA and Blaese RM (1999) Gene therapy: lessons learned from the past decade. British Medical Journal 319: 1310.

Orford M, Nefedov M, Vadolas J et al. (2000) Engineering EGFP reporter constructs into a 200 kb human globin BAC clone using GET recombination. Nucleic Acids Research 28(84): 1â?"8.

Penn SG, Rank DR, Hanzel DK and Barker DL (2000) Mining the human genome using microarrays of open reading frames. Nature Genetics 26: 315â?"318.

Thompson L (2000) Human gene therapy. Harsh lessons, high hopes. FDA Consumer 34(5). [http://www.fda.gov/fdac/features/2000/500_gene.html]

Walther W and Stein U (eds) (2000) Gene Therapy of Cancer: Methods and Protocols. Totowa, NJ: Humana Press.

Zhao S (2001) A comprehensive BAC resource. Nucleic Acids Research 29: 141â?"143.

Bioethics of Gene Therapy

David B Resnik East Carolina University, Greenville, North Carolina, USA

Introductory
doi:10.1038/npg.els.0003480




Gene therapy in human beings raises a variety of important ethical and social issues, including risks to patients and future generations, interference with natural human reproduction and the natural human form, the difference between therapy and enhancement, and human subjects protection.




Introduction

Gene therapy emerged as a hot topic in bioethics during the 1990s, but commentators had discussed many of the ethical, social, legal and policy issues surrounding this new technology for over thirty years. At the beginning of the 1970s, the notion of genetically treating or altering human beings was little more than science fiction, but by the 1990s this idea became a scientific and clinical reality. To understand the bioethics of gene therapy, it will be useful first to review the history of this new treatment modality and its biomedical, social and political context. See also: Human gene therapy



History

In the early 1970s molecular biologists developed techniques for transferring genes known as recombinant DNA technology, because the techniques recombine DNA from different cells, organisms or species. While recombinant DNA was still in its infancy, scientists and concerned citizens began to express concerns about its safety. What would happen if a genetically engineered organism escaped from the laboratory? Could they harm plants, animals, people or the environment? The media became aware of these issues and the public soon saw cartoons depicting genetic engineering accidents. Religious leaders warned against tampering with nature and ?~playing God?T. In February 1975, over 140 scientists from around the world who were developing recombinant DNA techniques met in Asilomar, California to discuss these questions. They considered the benefits and risks of recombinant DNA as well as methods for preventing accidental contamination. Shortly thereafter, the US government formed the Recombinant DNA Advisory Committee (RAC) to oversee and regulate recombinant DNA. See also: History of molecular biology; Impact of genetically modified organisms (GMOs)


After this initial caution, scientists continued developing recombinant DNA techniques during the 1980s, and they were able to transfer genes into microorganisms, plants and animals. Biomedical researchers began to realize that genetics, molecular biology and biotechnology held the key to understanding human health and medical therapy. Agricultural researchers began to see the potential benefits of genetically modified crops, poultry and livestock. The public also began to view disease and psychosocial problems in genetic terms. People became comfortable with genetic explanations of diabetes, hypertension, cancer, alcoholism, obesity, homo***uality and crime. See also: Transgenic animals; Transgenic plants; Microorganisms: applications in molecular biology


Private and public money soon began flowing into biotechnological research and development (R & D), culminating with the initiation of the Human Genome Project (HGP) in 1989. In the legal realm, the courts helped ensure that private firms would have economic incentives to invest in biotechnological R & D. In 1980, the US Supreme Court ruled that a genetically engineered bacterium developed by Ananda Chakrabarty could be patented. In 1987, the US Patent and Trade Office (PTO) awarded Harvard University and Dupont a patent on a genetically modified mouse. Congress and the Reagan Administration formulated laws and policies, such as the Bayê?"Dole Act, that encouraged university?"industry partnerships. See also: Human genome project; History of biotechnology; Biotechnology intellectual property ?" bioethical issues


Meanwhile, significant developments also took place in assisted reproduction technology (ART). On 25 July 1978, Lesley Brown gave birth to Louise Brown, the first child conceived via in vitro fertilization (IVF). The physician who treated Mrs Brown, Patrick Steptoe, performed the procedure under the guise of medical treatment, and thus avoided the layers of regulation that accompany human experimentation. At the time, IVF had never been tried in humans, and its risks were not well documented. Fortunately, the child was born healthy and is alive today. During the next twenty years, over 100 000 ?~test-tubê?T babies were born, and ART became a flourishing, multibillion dollar industry. Today, couples have a cornucopia of ART methods available to them, including sperm and oocyte donation, surrogate pregnancy, amniocentesis, choronic villi sampling, selective abortion and preimplantation genetic diagnosis (PGD). Despite its tremendous growth, the ART industry conducts many of its interventions with little regulation. See also: Infertility: causes, investigation and management


ART raised many ethical issues concerning human reproduction. Some commentators pointed out that children created in this fashion face potential medical risks, while others worried that ART children could feel intense pressure to meet parental expectations. Some suggested that ART could have a negative effect on how society views handicapped people. In the era of designer babies, how would we view people who are less than perfect? Others worried about the effects of ART on family integrity, parenting and marriage. The familiar criticisms that doctors were tampering with nature and ?~playing God?T also were heard. Last, but not least, ART had important implications for the emotionally charged abortion issue, since some techniques used selective abortion to terminate defective fetuses and other techniques often involved creating and discarding embryos and pre-embryos. In response to these concerns, in 1984 the Reagan Administration enacted a ban on the use of US government funds for research on human embryos. The Clinton Administration revised the ban in the 1990s. Today, US scientists receiving money from the federal government can conduct research on human embryos that have been discarded (e.g. through an abortion) but they cannot use federal money to create embryos for research purposes. See also: Bioethics of new assisted reproduction


One last element of this preliminary history needs to be mentioned, the debate about protecting human subjects in research. By the 1970s, the USA, as well as many other countries, had established systems for reviewing and regulating human subjects research. The US system involves primarily two agencies, the National Institutes of Health (NIH), which oversees all federally funded research, and the Food and Drug Administration (FDA), which oversees all research on drugs and medical devices that may be marketed to the public. Even though countries and research organizations have developed many different policies and codes pertaining to human subjects research, the social, legal, political and ethical debates about human research continue to this day. Ongoing controversies include risks and benefits, informed consent, research on vulnerable populations, and the use of placebos. See also: Ethics of research: protection of human subjects; National Institutes of Health (NIH)


Thus, by the time human gene therapy clinical trials began in the early 1990s, many of the issues surrounding this idea, such as the dangers of recombinant DNA, tampering with nature, ?~playing God?T, unnatural reproduction, discrimination, the ethics of human subjects research, and so on, had already been discussed. Nevertheless, researchers, clinicians, patients and the public revisited these issues during the gene therapy era.


The world?Ts first human gene therapy experiment began on 14 September 1990, when a 4-year-old girl with adenosine deaminase (ADA) deficiency was injected with some of her own, genetically modified, bone marrow stem cells. People with this disease have a malfunctioning ADA gene, which leads to the destruction of T cells due to the build-up of toxic levels of deoxyadenosine. The cells had been removed from her body, cultured in the laboratory, and treated with a vector (a retrovirus) in an attempt to transfer an ADA gene into her malfunctioning cells. French Anderson, who led the team of clinician/researchers who conducted this pioneering experiment, had defended gene therapy research in public for several years before the US government gave the team permission to begin clinical trials. He argued that ADA deficiency is an ideal candidate disease for gene therapy because it results from a known genetic cause, is progressive and fatal, and has no known cure. Previous animal studies on transferring genes into T cells had also achieved promising results. The experimental protocol also required that the patients with ADA deficiency would receive polyethylene glycol (PEG)-ADA in conjunction with gene therapy. PEG-ADA is a version of the ADA enzyme that is somewhat effective in reducing toxic levels of deoxyadenosine in T cells. See also: Retroviruses in human gene therapy; Immunodeficiency, primary: affecting the adaptive immune system


The results of gene therapy trials for ADA deficiency have thus far been favourable. Anderson?Ts first two subjects showed improvement according to laboratory measures of immune system function as well as clinical measures of overall health and morbi***y. A research group led by Claudio Bordignon has produced similar results. While the patients have improved, it is difficult to determine how much of their progress is due to gene therapy and how much is due to receiving PEG-ADA, since both treatments affect deoxyadenosine levels in T cells. Although gene therapy has not produced a ?~curê?T for ADA deficiency, it does offer patients some hope for a useful intervention.


Since these initial experiments, somatic gene therapy (SGT) has grown steadily. Thus far, it is estimated that several thousand patients have been enrolled in SGT experiments worldwide. The US government alone has approved over 250 SGT protocols. The diseases that researchers have attempted to treat with SGT now include severe combined immune deficiency (SCID), cystic fibrosis (CF), familial hypercholesterolaemia, Canavan disease, coronary artery disease, arterial restenosis, rheumatoid arthritis, Gaucher disease, alpha-1-antitrypsin deficiency, Fanconi anaemia, various forms of cancer, and HIV/AIDS. Of the protocols approved in the USA 70% are for cancer, 12% are for HIV/AIDS, 8% are for CF. Proposed future SGT targets include Lesch?"Nyhan syndrome, phenylketonuria (PKU), haemophilia A or B, Duchenne muscular dystrophy, and Huntington disease, among many others. Ad***ionally, public and private funding for gene therapy experiments has risen from several million dollars per year to several hundred million dollars per year since 1990. If one includes genetic research that has applications for SGT, such as the HGP, then one can multiply financial support for this research enterprise tenfold.


By the late 1990s, some of the initial optimism for gene therapy began to fade as scientists and clinicians came to terms with some of the technical difficulties of SGT. The two main technical problems with SGT, which have been present since its inception, are: (1) developing safe and effective vectors to deliver genes to targeted cells or tissues in vivo or ex vivo, and (2) ensuring that transgenes are expressed appropriately in cells and function properly. All of the vectors that have been used so far, such as retroviruses, adenoviruses, lentiviruses, liposomes and naked DNA (no vector), have their strengths and weaknesses. Researchers have not developed a sure-fire method for inserting the proper gene into the proper place in the genome, in the proper cell, in such a way that it is expressed at the proper time and functions properly. Although SGT can now offer some patients some benefits, it has not yet lived up to its billing as the new way to treat diseases. However, it must be remembered that gene therapy is still in its infancy and it may take many more years before the technology begins to realize its promise. See also: Retroviruses in human gene therapy; Gene delivery by viruses; Liposomes


In the late 1990s, several developments took place that amplified some of the bioethical debate surrounding gene therapy. On 22 February 1997, Ian Wilmut announced the birth of Dolly, the world?Ts first cloned sheep. Richard Seed, an iconoclastic physicist, offered to clone a human being and said he would set up a private cloning clinic. Politicians and the public reacted with shock, amazement, and at times revulsion, to this story. Some countries passed laws banning human cloning. The USA almost passed such as law, but settled for a ban on the use of federal funds to clone humans and a call for a voluntary moratorium in the private sector. Currently, scientists have succeeded in cloning several mammalian species, and human cloning is now feasible. In 1998, scientists were able to grow embryonic stem cells in the laboratory. Since embryonic stem cells can differentiate into any tissue or even develop into a fetus, they could theoretically be used in replacing dead or damaged tissues, in engineering human organs, or in human cloning. Mammalian cloning and stem cell research both have important impacts on gene therapy because they are key technologies that could be used in genetically engineering human beings.


On 17 September 1999 the gene therapy research community suffered a key setback when Jesse Gelsinger died in an SGT trial at the University of Pennsylvania. Gelsinger, who was receiving treatment to correct a deadly liver problem, died when his immune system reacted to the high levels of adenovirus vectors that were injected into his liver in an attempt to transfer a gene to produce ornithine carbamoyltransferase (OCT) enzyme. The FDA issued the university a warning letter and temporarily shut down its SGT trials. Soon Congressional hearings took place and the NIH began investigating the incident. Although deaths are not uncommon in clinical trials, Gelsinger?Ts death caused a stir, in part, because gene therapy is in the mediâ?Ts spotlight. Thus far, the US government has not created any new regulatory agencies to monitor gene therapy, although some people have argued that it should. In the wake of this incident, many researchers are urging more carefulness and caution in SGT clinical trials. Some researchers are concerned that gene therapists are going too fast and that the high financial and political stakes can lead to bad science and bad medicine. See also: Adenovirus culture; Gene delivery by viruses

Somatic Versus Germline

Ever since the first SGT trials began, researchers and policy makers have distinguished between somatic gene therapy (SGT) and germline gene therapy (GLGT). SGT interventions target somatic cells or tissues in the body; GLGT interventions target germ cells or tissues, such gametes, zygotes, early stage embryos, testes or ovaries. To illustrate the difference between these modalities consider gene therapy for ADA deficiency. The team led by Anderson transferred the ADA gene into the patient?Ts bone marrow stem cells. A GLGT intervention, on the other hand, would attempt to transfer an ADA gene into germ cells in order to prevent a person from being born with this disease. The most feasible targets for GLGT at this time would be early stage embryos.


From a purely technical perspective, it may be easier, though not necessarily safer, to perform GLGT than SGT because GLGT can target a single cell (or clump of cells). If a gene can be successfully transferred to the cell, then it is likely to be transferred to every cell in the future patient?Ts body. If the gene is not transferred, then one can discard the cell and try again. For comparison, GLGT has already been successfully performed in mammals. Scientists have transferred human genes into sheep to make them produce human hormones in their milk. To conduct SGT, on the hand, one must penetrate the body?Ts defences and target specific cells in the body. As we have seen, it is often very difficult to accomplish this task. Sometimes genes will not get to the targeted cells at all, sometimes only a few will hit their targets, sometimes those that hit their targets will be destroyed, and often those that are not destroyed may not function properly.


The advantage of GLGT, from a clinical and ethical perspective, is that it offers parents a way of preventing their children from being born with devastating genetic diseases, such as ADA deficiency. Parents have a right ?" some would say an obligation ?" to ensure that their children are born healthy. Of course, many genetic diseases can be prevented by other means. Autosomal recessive disorders, such as sickle cell anaemia, can be prevented through genetic counselling or PIGD. Other disorders, such as Down syndrome, can be prevented through genetic testing and abortion. Some genetic diseases, such as PKU, may respond well to conventional therapy or SGT. One might even argue that parents can always adopt children if they are worried about passing on a genetic disease. However, if one assumes that parents have a right to try to give birth to genetically related children who are free from devastating genetic diseases, then it make sense to allow parents to attempt GLGT when this procedure would be the safest, cheapest, and most effective way of achieving this goal. GLGT offers the added advantage that it would allow patients and families to avoid the pain and suffering caused by a genetic disease. See also: Chromosomal syndromes and genetic disease; Genetic counselling; Bioethics of genetic testing; Genetic screening and testing


Despite the clinical and technical advantages of GLGT, many researchers and ethicists have argued that SGT is morally acceptable while GLGT is not. There are several reasons why people hold this view. First, GLGT poses greater risks to patients than does SGT. SGT targets specific tissues or cells in a patient?Ts body; the effects of SGT are not usually systemic. If genes are inappropriately expressed in these tissues, then these effects can, in theory, be limited to the targeted cells or tissues. Moreover, it may often be possible to reverse the effects of SGT. For example, an inappropriately expressed SGT transgene could cause cancer in the targeted area. If this occurs, then the cancer could be surgically removed or treated with radiation or chemotherapy. Since GLGT targets an entire genome before or after conception, its effects can be systemic. GLGT mistakes therefore have the potential to affect development, cell differentiation, growth, and many aspects of the patient?Ts physiology or behaviour.


There are many ways that GLGT transgenes could have adverse effects on a patient. For example, GLGT transgenes could be expressed at the wrong time in development; they could be over-expressed or under-expressed; and they could affect the expression of neighbouring genes on a chromosome or other genes in the genome. Moreover, the effects of GLGT may not be reversible, especially if they occur during crucial stages of development or if they affect almost all the cells in the patient?Ts body. Although science has made great strides in identifying, sequencing, and analysing human genes, it will take centuries before scientists understand the complex set of genomic, epigenetic, metabolic, physiological and environmental factors that produce phenotypes. One might argue that it is foolish and irresponsible to try to alter or manipulate a complex system we barely understand.


Second, GLGT poses far greater risks to future generations than does SGT. Although there is a small chance that some SGT transgenes will accidentally incorporate into germ cells or tissues, this risk can be reduced by using methods to target specific somatic cells. (SGT experiments on fetuses, however, have an increased chance of impacting germ cells.) Since SGT does not target germ cells, it is likely to have only a minimal impact on future generations. The risks of SGT, therefore, should be limited to SGT patients. GLGT, on the other hand, is designed to have an impact on future generations. Therefore, GLGT is much riskier than SGT for the simple reason that it can affect more people. Generations of individuals could be born with artificially induced genetic defects. GLGT could even result in a kind of anti-eugenics where the entire human gene pool is corrupted by human attempts to control reproduction. Furthermore, it is difficult to estimate the current risks of GLGT because errors may not manifest themselves for several generations. See also: Eugenics


Proponents of GLGT have responded to these risk and safety issues. Concerning the risks *****bjects, proponents have argued that many of the severe defects created by GLGT will show up in utero. Many of these fetuses will fail to develop and will terminate naturally; others could be aborted if the defects are detectable. Proponents have also argued that it may be possible to develop methods that could minimize the risks to patients posed by transgenes inserted in the germline. Allowing a gene to insert itself randomly into the genome poses tremendous risks for patients because randomly inserted genes can disrupt the genome and can be improperly expressed. However, a method known as targeted gene replacement (TGR) may greatly reduce the risks of gene insertion. In TGR, one replaces the targeted gene with a new gene. The transgene would therefore be in the right place in the genome, since it would be in the same place as the old gene. It would also have a good chance of being expressed at the proper time in the proper amount because it would be surrounded by the coding region that was responsible for expressing the replaced gene. Another promising method uses a bacterial artificial chromosome (BAC) to carry genes. In this method, a patient would have an extra chromosome carrying specific genes designed to perform specific functions. Since the transgenes would reside in the BAC, they should not interfere with the rest of the genome. Concerning the problem with reversibility, proponents have argued that it may be possible to develop SGT (or other interventions) designed to counteract GLGT mistakes. Proponents also point out that many conventional therapies, such as radiation, chemotherapy and hormone therapy, have been developed despite reversibility issues. When these treatments have resulted in adverse effects, clinicians and researchers have developed ways of mitigating, reversing or counteracting those effects. See also: Artificial chromosomes


Concerning risks to future generations, GLGT proponents have argued that there may be ways of minimizing these risks. First, it may be possible to develop transgenes that affect the patient but are not heritable. To keep the transgene in the population, it would be necessary to reinsert it in each generation. Second, it may also be possible to design transgenes that can be triggered when needed. These genes would be designed to have no effects on future generations unless a patient receives a specific drug. Concerning dangers to the human gene pool, GLGT proponents point out that the effects of transgenes on future generations are not likely to be widespread because the population of GLGT subjects is likely to be relatively small. Since there are over six billion humans at present, there is little chance of corrupting the human gene pool by introducing a few transgenes into a small population. It might be possible to affect a small, genetically isolated, human subpopulation, however. See also: Genetic variation: human


Thus, the moral differences between SGT and GLGT appear to boil down to technical issues concerning safety. If these technical problems can be surmounted, then would GLGT still be morally problematic? GLGT opponents argue that there would still be some important moral differences between SGT and GLGT. For some, moral concerns about ?~playing God?T would be paramount. In response to the ?~playing God?T argument, GLGT proponents counter that all of medicine (and science) is an attempt to ?~play God?T. The issue is not whether human beings should use their power and knowledge to change the world; the issue is how they should use it. If GLGT offers human beings important benefits, such as the prevention of genetic diseases, then it is appropriate to use the technology for those purposes.


For others, the question of interfering with natural reproduction would still be a major concern. In response, GLGT proponents counter that this is an objection to all forms of ART, not just an objection to GLGT. If one accepts the idea of assisted reproduction, then one should accept (at least in principle) the idea of GLGT. Finally, opponents also counter that GLGT involves morally problematic procedures, such as cloning, the destruction of embryos, and abortion. Proponents would admit this much. For the foreseeable future, the most feasible methods of GLGT would involve the insertion of transgenes into cloned cells removed from early stage (eight-cell) embryos. These cells would be tested for the presence of the gene and some could be discarded. If a GLGT defect manifests itself in pregnancy, the fetus could be aborted. GLGT proponents admit that GLGT is appropriate only for people who also accept these procedures.


But there is still one more powerful argument against GLGT, namely, that it would take us down a ?~slippery slopê?T toward genetic enhancement. We might begin on this slope by attempting to prevent genetic diseases, but we would start using GLGT for nonmedical or enhancement purposes. Parents would attempt to create ?~perfect?T children with enhanced intelligence, height, athletic ability, beauty or strength. Those not lucky enough to be enhanced would be regarded as inferior or defective. To understand and address this argument, we need to consider the question of genetic enhancement.



Therapy Versus Enhancement

Since the initial gene therapy discussions, scientists and policy makers have drawn a distinction between genetic therapies, i.e. interventions that are designed to treat or prevent diseases, and genetic enhancements, i.e. interventions that are designed to enhance or alter the human form. Genetic therapy is morally acceptable because it serves the well-accepted medical goals of disease prevention and disease treatment. Genetic enhancement is morally unacceptable because it promotes goals that are not as well accepted and could lead to dire consequences. Although this argument has been used to attack GLGT, it is also an objection to SGT, since interventions that target somatic cells could also be used for enhancement purposes.


To clarify these issues, one needs to say a bit more about this distinction. The difference between ?~therapy?T and ?~enhancement?T is based on the difference between ?~medical?T and ?~non-medical?T interventions: therapy is a type of medical intervention designed to achieve medical goals, e.g. disease prevention or treatment or health promotion; enhancement is a nonmedical intervention. The distinction has some intuitive plausibility: there seems to be a difference between taking a steroid to recover from orthopaedic surgery and taking a steroid to increase onê?Ts athletic performance. Likewise, there seems to be a difference between inserting a transgene into a patient in order to produce a vital liver enzyme and inserting a transgene in order to help the patient grow hair on his head. There also seems to be a difference between inserting a gene in an embryo in order to prevent a child from being born with CF and inserting a gene in an embryo in order to ensure that the child will grow to be exceptionally tall.


But this distinction becomes less clear when one considers borderline cases, such as breast implants for women who have mastectomies, growth hormone for short children, antidepressants for people who want to feel better, ritalin for children whose parents want them to perform better in school, vitamin and mineral supplements beyond what is required to avoid metabolic deficiencies, Viagra for men with ***ual dysfunction, immunizations, glasses for people whose vision is not perfect but falls within the statistically normal range, and even cochlear implants for congenital deafness.


What these borderline cases all have in common is that they raise important questions about social values. Many clinicians and scholars define health and disease in biomedical terms: diseases result from environmental, developmental or genetic factors that cause the body to function abnormally. In a healthy individual, all of the body?Ts organs and tissues perform the functions that natural selection designed them to do. For example, a person with congestive heart failure (CHF) has a heart that is not functioning normally; they have a disease, CHF, or a diseased heart. A person with a broken ankle has an ankle that does not function properly.


But how do we define ?~normal?T human functioning? In some local populations, many of the con***ions considered to be ?~diseases?T, such as HIV, alcoholism, tooth decay, hypertension, obesity, prostate cancer and malnourishment, are so common that they could be considered statistically normal. Yet we would say that people with these con***ions, though normal, are not healthy; they should not have these con***ions. This implies that normal human functions are not defined in a statistical sense; normal functions are normative ideals for health. Who determines these normative standards? Clearly, clinicians have a great deal of input, but one might argue that society has a strong influence. In the USA, dyslexia is considered to be a fairly serious learning disorder, and educators, paediatricians and psychologists do a fairly good job of diagnosing and treating this con***ion. In a country that emphasizes oral communication dyslexia may not be diagnosed, treated, or even regarded as a problem. Thus, one might argue that ?~health?T and ?~diseasê?T are not entirely objective concepts. For many con***ions, the difference between a healthy human being and a sick or diseased human being depends on social and cultural norms. These considerations imply that the distinction between genetic therapy and genetic enhancement is also not entirely objective: an intervention may be therapeutic in one society but not in another.

Suppose that we can at least make some crude distinction between therapy and enhancement, why should we believe that enhancement is morally problematic? After all, many of the ways that one could genetically alter the human form could offer individuals and society important benefits. Although many of these potential benefits are beyond the scope of our current technology because they involve the manipulation of complex, polygenic traits, they may be feasible sooner than we think. For example, some researchers and ethicists have proposed that GLGT or SGT could be used to make people that are â?~healthierâ?T than normal. Some enhancements could boost immunity against viruses, such as HIV; others could help the body ward off cancers or neurodegenerative disorders. Other enhancements might increase intelligence, athletic or musical ability, or longevity. Society already accepts and encourages many nongenetic forms of human enhancement, such as education, travel, athletic training and information technology. Why is it wrong to use genetic instead of nongenetic means to enhance human functioning?


Opponents of enhancement can offer a variety of replies to this argument. First, there is the familiar argument that enhancement is â?~playing Godâ?T, but, as we saw earlier, this argument is out-of-step with societyâ?Ts reliance on science and technology; for better or worse, there is no way to avoid â?~playing Godâ?T.


Second, some would counter that enhancement is wrong because it is unnatural. Therapeutic interventions are designed to promote or restore the natural human form, but enhancement aims to change it. But is it inherently wrong to alter the human form? Several hundred years of science and technology have completely changed the natural human order that existed for several millions years by increasing lifespan, height, weight and literacy, and by decreasing mortality due to various diseases. Even low-tech societies routinely alter the human form through body piercing, circumcision, tattoos and paints. One might argue that there is nothing inherently wrong with altering the human form, although some kinds of alterations would be unethical because they cause harm.


Third, opponents could counter that enhancement is wrong because it can be easily abused by parents or governments. Parents might attempt to make super-children and totalitarian regimes might attempt to create super soldiers or a passive servant class. Proponents would admit this point but counter this is not a good reason to oppose enhancement, since any new technology can be abused. See also: Eugenics: historical


Finally, opponents have argued that enhancement is wrong because it would exacerbate existing social (economic and political) inequalities because enhancements would only be available to those who can afford them. Those who can afford enhancements will have enhanced children, who will purchase more enhancements, and the gap between the rich and poor would widen. Some have theorized that the widespread use of enhancement could even produce distinct castes or subspecies.


Proponents have several replies to this argument. First, the effects of enhancement may not be significant enough to have a significant effect on social inequalities. Second, if enhancements do have a significant effect on social inequalities, then this effect may only be temporary, since this technology, like other technologies, will eventually â?~trickle downâ?T to poorer members of society as it becomes cheaper and more accessible. Third, society may decide to adopt regulations or policies to ensure that enhancement technologies are distributed fairly. In assessing these issues, it is important to note that both opponents and proponents of enhancement can only speculate about the effects of genetic technology on society; we do not know, at this point, how genetic enhancements will affect society. However, we can be fairly confident, based on our understanding of free market dynamics and the history of technology, that it will be very difficult to prevent people from using genetic enhancement technologies once they are developed. The recent history of genetics tells us that day may be fast approaching.

Human Subjects Protection

The protection of human subjects in SGT or GLGT experiments is an important thread that runs through all discussions of gene therapy. As noted earlier, the USA has a fairly extensive system for regulating gene therapy. Regulatory agencies for gene therapy include local Institutional Review Boards (IRBs) as well as the RAC (research appraisal checklist) and NIH or FDA. In theory, almost all gene therapy research protocols should be covered through this network, although it is possible that some privately funded gene therapy research could slip through the system. Some commentators have argued that the USA needs a separate agency to regulate gene therapy, while others have argued that this agency would be redundant and would add extra layers of red tape. One point has emerged from the Gelsinger incident: gene therapy research has been hampered by lack of adequate communication. IRBs, government agencies and researchers should take ad***ional steps to report adverse events and share and monitor data. They should also do a better job of communicating results (both good and bad) to the public. See also: Ethics of research: protection of human subjects


It is not possible to give a full assessment of the ethics of SGE and GLGT clinical trials here, but there are several points worth considering. The first point concerns risks and benefits. Gene therapy, as we have seen, poses considerable risks *****bjects, including death. These risks do not invalidate gene therapy, since all human experiments involve risk and many involve significant risks. However, researchers need to ensure that their protocols have an acceptable risk/benefit ratio. Since gene therapy is a risky procedure, it should be performed only when (1) it has a reasonable chance of benefiting the subject and (2) there are no other effective treatments. The first SGT protocol met these two criteria. Although most SGT experiments performed thus far fit these criteria, gene therapy researchers need to consider risks and benefits carefully in light of the problems that have occurred in delivering transgenes. Concerning GLGT protocols, one might argue that a GLGT experiment would have a justifiable risk/benefit ratio only if (1) it has a reasonable chance of benefiting the future child and (2) there are no other effective treatments for the child. A GLGT protocol that would meet these criteria might be an intervention designed to help a person with Huntington disease produce a normal child.


The next point to consider would be informed consent. Adults who consent to gene therapy must understand the risks and benefits of gene therapy as well as alternative treatments. Given the complex nature of genetic interventions, it may be difficult to explain to patients how gene therapy works, so researchers may need to spend a great deal of time educating patients. Researchers also need to avoid taking any coercive measures; they need to speak honestly and openly with their patients and they should not attempt to ?~oversell?T or ?~hypê?T gene therapy. Although incompetent patients, such as minors, cannot give consent, consent can be obtained from appropriate surrogates, such as parents or guardians, who should make decisions according to the ?~best interests standard?T. SGT should be performed on a child only if it is in the child?Ts best interests to take part in the study. Since ?~best interests?T can be understood in terms of benefits and risks, understanding the benefits and risks of a study will provide the key to making decisions for incompetent patients.


Finally, since private corporations have sponsored many gene therapy trials, there have been concerns about conflicts of interest in gene therapy research. James Wilson, who led the experiment that resulted in Gelsinger?Ts death, was accused of having a conflict of interest because (1) he held equity in Genovo, a private firm that he founded, which also sponsored the trial, and (2) he has patented several gene therapy techniques. ?~Conflict of interest?T is a complex ethical problem that we cannot discuss in depth here. There are several steps gene therapy researchers should take to deal with conflicts of interest. First, they should disclose all potential conflicts of interest, such as personal and financial interests, to the appropriate parties, including patients, journal e***ors, funding agencies and IRBs. Second, if researchers have a conflict of interest, they should avoid situations that could compromise their professional judgement, such as recruiting patients for clinical trials. Third, researchers should avoid forms of remuneration that are especially problematic, such as stock options, or direct payment for enrolling patients in clinical trials. Finally, to minimize the impact of conflicts of interest, all research should be reviewed by an independent agency, such as an IRB. Moreover, IRBs should also disclose potential conflicts of interest and should take steps to avoid or manage them.


Even if the use of gene therapy for experimental purposes is sufficiently regulated, the use of gene therapy for non-experimental purposes may not be sufficiently regulated. US regulations distinguish between research and clinical practice. In theory, a physician could perform gene therapy in a clinical setting without the degree of regulation associated with research. As noted earlier, much of the ART industry is under-regulated in the USA. It is conceivable that an ART specialist could perform GLGT to help a couple reproduce and that this intervention, like the first test-tube baby, would take place outside the reach of US laws. A scientist could follow Richard Seed?Ts lead and offer to genetically engineer a human. There might be no way to stop him.



Originally published: February 2001


Further Reading

Anderson WF (1989) Human gene therapy: why draw a line? The Journal of Medicine and Philosophy 14: 81?"93.

Friedmann T (2000) Principles for human gene therapy studies. Science 287: 2163?"2165.

Marshall E (2000) Gene therapy on trial. Science 288: 951?"957.

McGee G (1997) The Perfect Baby. New York: Rowman and Littlefield.

Peters T (1997) Playing God. New York: Routledge.

Resnik D, Steinkraus H and Langer P (1998) Human Germline Gene Therapy. Austin, TX: RG Landes Bioscience.

Rifkin J (1983) Algeny. New York: Viking Press.

Stock G and Campbell (eds) (2000) Engineering the Human Germline. New York: Oxford University Press.

Suzuki D and Knudtson D (1989) Genethics. Cambridge, MA: Harvard University Press.

Walters L and Palmer J (1997) The Ethics of Human Gene Therapy. New York: Oxford University Press.