Gene transfer research builds on technical advances in many fields, notably genetics, molecular biology, and clinical medicine. Hundreds of clinical trials have generated better understanding of the technology’s promise and risks and the increasing evidence of clinical benefits in various areas. This chapter explores the scientific foundations of gene transfer as well as how the field has advanced over time, outlining recent notable clinical developments in the basic techniques of gene transfer and illustrating the potential of gene transfer as a therapeutic approach. The current chapter also outlines the scientific community’s initial understandings of risks involved in clinical gene transfer research and describes how those theoretical risks and uncertainties look today, summarizing how and why many risks and uncertainties have been minimized and noting those that persist today.
GENE TRANSFER RESEARCH
Gene transfer can be broadly understood as the introduction of genetic material, through a vector, into cells with the intent of altering gene expression (Kay, 2011). Approaches to gene transfer fall into three categories, sometimes used in combination: adding a functional gene, correcting a dysfunctional gene, or altering the expression of a naturally occurring gene. Most clinical studies rely on the introduction of a functional gene in individuals possessing a nonfunctional or mutated gene (Kresina and Branch, 2001). The hypothesis is that the new functional gene (or corrected gene) will be translated into a protein that will allow for the restoration of a biochemical pathway interrupted in the disease
and therefore will eliminate or lessen the clinical manifestation (Kay, 2011).
A variety of gene transfer vectors have been developed over the years and studied extensively, and researchers are currently developing alternative non-viral strategies for gene delivery to overcome some of the limitations associated with viral vectors (Al-Dosari and Gao, 2009; Pathak et al., 2009). Much of the effort, however, to develop clinical gene therapy has focused on viral vector systems (Nienhuis, 2013). Naturally occurring viruses have evolved sophisticated strategies to infect specific target cells and co-opt cellular machinery to express viral genes stably and heritably (Vannucci et al., 2013). The use of viral vectors for gene transfer seeks to exploit these abilities with the intent to genetically modify the target natural cell (Vannucci et al., 2013). The basic strategy is to eliminate viral genes that are essential for replication and pathogenicity while making space for the therapeutic genes (Vannucci et al., 2013).
Gene transfer and gene therapy are not synonyms. Gene transfer is a broad category encompassing technique of introducing a genetic sequence with any function, for example, the transfer of a fluorescent protein marker into a cell for diagnostic purposes. The process of gene transfer may or may not have a therapeutic purpose or demonstrated therapeutic effect. Gene therapy is the clinical application of gene transfer, with the intent to produce beneficial health consequences. In research ethics, the term therapy is generally reserved for a product or intervention with demonstrated safety and efficacy—it is not applied to interventions that are still being investigated, a distinction that is important, given that all gene transfer products are investigational; none has received a Biologic Licensing Application1 (BLA) approval by the U.S. Food and Drug Administration (FDA) thus far. Conceptually, gene transfer is disarmingly simple. Introduce a gene into a cell, tissue, or organ, and a functional protein product will express a protein useful for clinical diagnostics or nonfunctional pathway to ameliorate treatment of a disease. The translation of the concept into effective therapies has not, however, been simple (see, generally, Bersenev and Levine  and Kay ).
1A Biologic License Application is a request for permission from FDA to introduce, or deliver for introduction, a biologic product into interstate commerce. The BLA is regulated under the Code of Federal Regulations 21 § 601.
Although the idea of gene transfer has been explored by the scientific community for decades, it was launched into the national spotlight with the advent of recombinant DNA (rDNA) technology in the early 1970s. An rDNA molecule is made up of DNA sequences that have been artificially modified or joined together so that the new genetic sequence differs from naturally occurring genetic material. The recombination of genetic material can happen as a result of normal biochemical processes in nature, but the term rDNA is typically reserved for genetic sequences that have been engineered in a scientific laboratory. Recombinant DNA technology is the product of advances in enzymology, biochemistry, and molecular genetics, and it has provided the foundation for other important technologies and applications, including genetic sequencing, sophisticated medical diagnostics, the elucidation of the molecular basis of many diseases, new avenues of disease prevention, and, in some cases, new and precisely targeted treatments of serious medical conditions (Berg and Mertz, 2010).
EARLY EXPERIENCE IN HUMAN GENE TRANSFER RESEARCH
In the 1960s, about one decade after the discovery that viruses could transfer genetic material between bacteria, it became apparent that viruses might be used to deliver genes into cells of interest (Wirth and Ylä-Herttuala, 2013). Before this technology could be utilized, scientists first had to learn how to remove the virus’s natural ability to cause illness. Unfortunately, a technology to engineer such a virus did not exist until the 1970s, and early experiments utilized wild-type viruses.
The first series of direct human gene transfer experiments were performed between 1970 and 1973 when a wild-type Shope papilloma virus was introduced into two young research participants with genetic deficiency in arginase, an enzyme that degrades arginine and prevents it from accumulating in the bloodstream (Rogers et al., 1973; Terheggen et al., 1975). The procedure was carried out with the goal of transferring a functional arginase gene to the research participants. Unfortunately, the subjects showed neither improvement in arginase levels nor any other clinical benefits. Years later, sequencing of the Shope papilloma virus genome revealed that it does not actually code for arginase (Wirth and
Ylä-Herttuala, 2013). This first attempt at human gene transfer with therapeutic intent involved only a wild-type virus and no rDNA techniques, but as will be discussed further in Chapter 3 and Appendix B, these experiments prompted public concerns about the risks and uncertainties of gene transfer (Friedmann and Roblin, 1972).
In 1980, sentiment within the U.S. scientific community leaned toward favoring tighter research regulations in the name of patient protection when a U.S. researcher conducted gene transfer experiments in Italy and Israel without the Recombinant DNA Advisory Committee (RAC) or institutional review board (IRB) approval. Martin Cline attempted gene therapy in two research subjects who had beta thalassemia, a hereditary blood disorder (Jacobs, 1980). Cline allegedly failed to disclose to the Israeli IRB that the proposed gene transfers involved rDNA, and Italy did not have an IRB system at the time (see discussion in Rainsbury ). The Los Angeles Times published an article reporting on the details of his activities (Jacobs, 1980; Rainsbury, 2000). Although the expression of therapeutic genes was not achieved, there was no evidence of further harm to the already gravely ill research participants. Ultimately, in 1981 Cline resigned as department chair at the University of California, Los Angeles (Fredrickson, 2001, p. 272). Cline’s behavior led to a decline in confidence in the scientific community, which increased the willingness of many scientists to tolerate tighter regulations in the name of research participant protection (Rainsbury, 2000).
The first clinical gene transfer protocol approved by the RAC in December 1988 was proposed by Rosenberg and colleagues, who were developing specialized white blood cells known as tumor-infiltrating lymphocytes (Merrill and Javitt, 2000; Rosenberg et al., 1990). The protocol was not designed to induce a therapeutic outcome. Instead, Rosenberg mapped the in vivo distribution and survival of the marked tumor-infiltrating cells in cancer patients. Rosenberg concluded that the procedure was safe and feasible (Rosenberg et al., 1990).
In 1995, Michael Blaese and colleagues proposed a gene transfer protocol designed to test an experimental intervention for adenosine deaminase severe combined immunodeficiency disorder, a single-gene disorder that severely compromises immune system function (Blaese et al., 1995). In research approved by the RAC, two children with the condition were infused with their own white blood cells, which had been modified ex vivo to express the normal adenosine deaminase gene. One of the research participants, who has since been identified as Ashanti DaSilva, exhibited a temporary increase in functional enzyme production, but the
effects of the gene transfer experiment were difficult to differentiate from the effects of enzyme replacement therapy she was receiving simultaneously (Wirth and Ylä-Herttuala, 2013).
In 1995, as gene transfer research progressed, the National Institutes of Health director convened a multidisciplinary panel, co-chaired by Stuart Orkin and Arno Motulsky, to review the field of gene therapy. By then, 5 years had passed since the first research participants received genetically modified cells, and the RAC had reviewed and approved more than 100 clinical protocols (NIH, 2013). The panel concluded that most clinical gene transfer protocols were not sufficiently well designed to answer fundamental biological questions about gene transfer and that, despite anecdotal claims of success, the protocols lacked statistical power to demonstrate clinical efficacy (Orkin and Motulsky, 1995). The panel also concluded that actual progress in gene transfer research had been oversold, and it recommended that the field focus on understanding the basic biology of vector systems, target cells, and tissues; accumulating preclinical evidence of effective protocol design; and developing strategies to improve targeted and sustained gene expression (Orkin and Motulsky, 1995).
In sum, the first few years of clinical research experience showed that developing effective gene transfer strategies was more technically demanding than originally anticipated (Mountain, 2000). Inadequate vector performance demonstrated significant gaps in knowledge, and the duration of clinical experience at the time was as yet too short to rule out long-term adverse effects from gene transfer protocols. The combined absence of foundational knowledge and experience presented a serious ethical challenge in that researchers were unable to identify genuine hazards or make judgments about acceptable levels of risk.
UNDERSTANDING RISKS IN GENE TRANSFER
Risk is ubiquitous in clinical research, and discussing risk requires an understanding of what it is and how it is perceived. Risk can be defined as the probability of an adverse outcome within a defined period of time (Deakin et al., 2009). In gene transfer and other biomedical research, basic biological and preclinical studies provide objective knowledge about probable outcomes and their severity. Ultimately, determining an acceptable level of risk is a value judgment that takes into account other factors beyond absolute risk, most notably the nature of potential benefits.
Furthermore, research involving human subjects with life-threatening terminal or severe diseases who have few if any other treatment options are likely to deem a much higher degree of risk as acceptable compared to diseases that are less severe or have existing successful therapies or that address lifestyle conditions or even genetic enhancement (Kimmelman, 2007). The acceptability of the risks posed by gene transfer research should be understood as a function of acceptable risk rather than any absolute measure. Early gene transfer studies were being undertaken at a time when the absence of scientific knowledge regarding the various agents and processes involved in gene transfer exposed the human subjects, third parties, and society at large to an unacceptable amount of theoretical and uncertain risk. Some early risks of human gene transfer were real and continue to drive research, and others were found to have no scientific basis.
Understandings of Risk and Uncertainty in Initial Gene Transfer Research
In the early years of gene transfer research, the members of the scientific community generally agreed that gene transfer research had new risks and contained many uncertainties. Many concerns were also brought to attention by the public involving the potential social and ethical implications of the idea of gene transfer, while scientists articulated scientific uncertainties and technical and safety concerns.
Social and Ethical Concerns
Many of the public’s ethical concerns were captured in the 1982 report by the President’s Commission for the Study of Ethical Problems in Medicine and Biomedical Research and Behavioral Research. The report, titled Splicing Life, noted that the interchangeability of genetic material raised questions in the scientific community about the unpredictable consequences of genetic engineering (President’s Commission, 1982). The report also described public unease about the “Frankenstein factor”—“the notion that gene splicing might change the nature of human beings, [which was] compounded by the heightened anxiety people often feel about interventions involving high technology that rests in the hands of only a few” (President’s Commission, 1982, p. 16). Furthermore, germline research (modification of gametes [sex cells] that would then persist
across generations) was a particular concern and sometimes led to a “slippery slope” argument. An extreme version of the argument contended that if society began to permit germ-line genetic engineering, this would lead before long to coercive social policies mandating genetic enhancement of all embryos (Kresina and Branch, 2001). Another version of this argument expressed the concern that, over time, the collective social attitudes—peer pressure—would subtly induce individuals to choose germ-line therapy and enhancement. Scientists were uncertain about the possibility that new genes could be integrated into the germ line inadvertently, potentially affecting future generations (Marshall, 2001). Finally, there was concern about risks to people involved in the research enterprise and to the environment, including the potential for research participants to shed viral vectors that might go on to infect research staff or escape from the laboratory and into the environment with unknown but conceivably serious consequences (Spink and Geddes, 2004).
Over the years, commentators disagreed about whether the existing knowledge about gene transfer was sufficiently robust to justify moving into clinical trials. Theodore Friedmann and Richard Roblin wrote an opinion piece in 1972 in which they opposed attempts at gene therapy in human patients until technical advances were made and regulatory structures were put into place. Referencing Rogers’s failed Shope papilloma experiment, Friedmann and Roblin argued that “our understanding of such basic processes as gene regulation and genetic recombination in human cells is inadequate” (Friedmann and Roblin, 1972). They proposed ethical and scientific criteria that gene transfer should satisfy before moving forward to human trials, acknowledging that physicians would consider the risks and potential benefits for each individual patient. There was also reasonable belief among some that starting clinical trials was appropriate (see, generally, President’s Commission ). The fact that this was the subject of disagreement highlights how difficult it is to set clear boundaries between what is wise and unwise, ready and unready.
Scientific and Technical Concerns
In addition to these ethical and social issues, early gene transfer research was characterized by a high degree of technical uncertainty and its associated risks. The use of this new investigational agent, rDNA enclosed in a vector, was supported by relatively little pre-clinical experience or testing of similar agents. Therefore, gene transfer carried
more uncertainties than chemical drugs, for example, which were able to draw predictability from more than a century of pharmacology (Kimmelman, 2008).
Scientists’ ability to quantify risks to subjects was further complicated by the permanent nature of changes to the human genome intended by gene transfer, and these changes had yet to be explored. The prospect of an individual undergoing continuous, life-long exposure to transgenes and vectors meant that long-term side effects might not be detected for years, even decades, after the end of a trial—and few to no investigations had taken place to evaluate this. Gene transfer research was perceived to be fundamentally different from research with chemical drugs, which have a finite kinetic lifetime in a research subject (although these chemicals can also produce adverse consequences that are not evident for years). At the time, few biologics (e.g., bone marrow transplant) had been studied for life-long effects.
Another area where scientific understanding was not well developed was the interaction of new genes and vectors with host immune systems. Very little was known about what might trigger a host’s immune response to vectors or to the transgene products or how researchers might mitigate any potential adverse events.
CURRENT UNDERSTANDING OF THE SAFETY OF GENE TRANSFER
Since the first RAC-approved gene transfer trial in 1988, hundreds of clinical gene transfer trials have been initiated and completed. Most trials have been early-stage phase I or phase I/II trials (Ginn et al., 2013). Phase I trials are primarily designed to generate safety data that will allow investigators and research review bodies to assess whether it is safe to pursue further clinical investigations. Phase I/II trials begin to develop evidence that the agent has the hypothesized physiological effects. These early-stage trials are critical to assessing safety for the research subject. These assessments have been the major focus of gene transfer experiments and constitute a fundamental requirement for the government approval of any medication. Central to progress in the field have been advances in vector design and the accumulation of long-term follow-up data. As gene transfer science has matured, so too has the understanding of potential harms.
Some Potential Hazards Found to Not Be a Problem
Many original uncertainties have been replaced by scientific clarity, and fears have been alleviated by decades of experience. Some of the early concerns about gene therapy, such as the perceived danger of creating transmissible pathogens, accidental germ-line modification, and unspecified xenogeneic dangers, have not been verified by clinical experience (Deakin et al., 2010). In general, risks that gene transfer might originally have been thought to pose to third parties and society at large have been determined to be minimal (Deakin et al., 2010). This is due in large part to the effectiveness of techniques developed to render viral vectors incapable of replicating. Since the early 1980s, researchers have devoted a great deal of effort into determining how to rearrange the viral genome in order to impede the replication or generation of infectious viral particles while still maintaining the virus’s ability to deliver nucleic acids (Vannucci et al., 2013). These technological advances, along with the ever-growing knowledge of molecular virology and virushost cell interactions, have constantly improved the safety profile of viral vectors that are now used in gene therapy.
The safety profile of most viral vectors has been considerably enhanced by advances in vector design strategies as well as better understanding of molecular virology and virus–host cell relationships (Vannucci et al., 2013). For example, the risk of adenoviral vectors regaining the ability to replicate was reduced considerably with the deletion of select genome components found to play an important role in adenovirus-specific immunity (Campos and Barry, 2007). Another improvement was a strategy known as pseudotypization, in which the spectrum of infectable (or transducible) cells by retroviral and lentiviral vectors is controlled by separating select viral genes into separate constructs (Vannucci et al., 2013).
In addition to advances in safety, long-term follow-up data demonstrating effectiveness have also been generated from many gene transfer clinical trials for a variety of diseases. For example, researchers have shown that intravenous injection of recombinant adeno-associated virus particles resulted in long-term production of human factor IX in patients with hemophilia with only minimal, effectively managed complications from treatment up to 3.3 years after treatment (Nathwani et al., 2011).
Clinical Research Successes
A number of clinical research successes have emerged in gene transfer since 2008. For example, patients with Leber’s congenital amaurosis, a genetic blindness for which there is no alternative therapy, have shown modest but sustained improvements in subjective and objective measurements of vision following a gene transfer experiment, with the greatest improvements noted in children enrolled in the study, all of whom gained ambulatory vision (Maguire et al., 2008).
Gene transfer trials involving hematopoietic stem cells show particular promise in the treatment of blood disorders, especially Wiskott-Aldrich syndrome and beta thalassemia, as well as metabolic disorders, such as X-linked adrenoleukodystrophy and metachromatic leukodystrophy (Booth et al., 2011). In 2010, clinical trials were initiated using lentiviral vectors to transfer functional genes to young patients with metachromatic leukodystrophy, and 3 of these patients experienced no further disease progression for up to 24 months. These children were predicted to otherwise experience disease onset in 7 to 21 months without treatment (Biffi et al., 2013). Long-term follow-up data on 90 research participants who received the gene transfer treatment to treat inherited primary immunodeficiencies in the past decade show a survival rate of more than 90 percent and indicate that most experience significant clinical benefit (Seymour and Thrasher, 2012). Promising results are also being recorded in the treatment of several degenerative conditions. For example, patients with Parkinson’s disease who received dopamine-biosynthetic enzymes using lentiviral vectors have shown signs of improvement, as measured on the Unified Parkinson’s Disease Rating Scale. Long-term benefits have been observed, with some patients experiencing sustained improvements up to 3 years after treatment (Eberling et al., 2008; Marks et al., 2010).
CHARACTERISTICS OF GENE TRANSFER TRIALS
The Journal of Gene Medicine has compiled summary data on gene transfer research, beginning with trials approved or initiated from January 1989 through January 2013. The summary provides information on approved, ongoing, or completed gene transfer clinical trials worldwide and outlines indications addressed, vectors used, gene types transferred, and clinical indications (Ginn et al., 2013). Most trials (63 percent) were
undertaken in the United States.2 Sixty percent of gene transfer clinical trials included in the database are phase I trials, designed to gather safety data (n = 1,171). Another 20 percent of trials are phase I/II studies (n = 376), with the smallest proportion at phases II and III (Ginn et al., 2013).
To date, most gene transfer trials have targeted cancer (64 percent). The next most common target conditions are a diverse array of monogenetic diseases (9 percent), cardiovascular disease (8 percent), and infectious diseases (8 percent) (Ginn et al., 2013). Gene delivery can be accomplished with viral or non-viral vectors. Vectors used most commonly in clinical gene transfer trials from 1989 to 2013 are adenoviral (23 percent), retroviral (19 percent), and plasmid DNA (18 percent). The most clinically relevant viral vectors for gene transfer today include retroviral, lentiviral, adenoviral, and adeno-associated viral vectors. Viral vectors offer the best efficiency in terms of gene delivery, but they carry risk of extreme immune response and insertional mutagenesis, which may lead to the development of cancer (Molina, 2013). Non-viral vectors may be safer, but are limited by very low transfection efficiency (Molina, 2013). Viral vectors currently dominate clinical gene transfer trials (Ginn et al., 2013).
REGULATORY STATUS OF GENE TRANSFER PRODUCTS
Gene transfer is currently coming to fruition as a therapeutic strategy with the potential for broad application. As of November 2013, FDA has not yet approved a gene transfer product for marketing, but several products have advanced to late-stage trials that could serve as the basis for such approval in the near future. Therefore, gene transfer products may be available as a therapeutic strategy with the potential for broad application beyond clinical trials in the near term. Three gene therapy products have been approved outside the United States. Two were approved by China’s State Food and Drug Administration: Gendicine, which involves a non-replicative virus for squamous cell carcinoma treatment, was approved in 2003; and Oncorine, which delivers genetic material through a conditionally replicative adenovirus to treat nasopharyngeal carcinoma in combination with chemotherapy, was approved in 2005 (Wirth and Ylä-Herttuala, 2013). To date, U.S. approval has not been granted.
2The Journal of Gene Medicine database does not present information on trials’ sponsors (e.g., governments, industry).
The third gene therapy product approved is Glybera, which received marketing authorization from the European Commission in November 2010 on the basis of the evaluation and recommendation of the European Medicines Agency (Bryant et al., 2013). Glybera is designed to treat severe lipoprotein lipase deficiency, a rare inherited condition associated with increased levels of fat in the blood; the product uses an adeno-associated viral vector (Hildegard, 2013; Watts, 2012). The company launching this product commercially is expected to pursue FDA approval (UniQure, 2012).
REMAINING CONCERNS IN GENE TRANSFER RESEARCH
Although dramatic advances have taken place regarding the techniques of human gene transfer and mechanisms of action, some gaps in scientific knowledge remain. First, although active viral vectors are designed to be replication incompetent and no longer pathogenic, predicting severe immune response remains difficult. Second, each component of a transfer carries its own risk, thus complicating risk assessments. For example, the cases of leukemia that arose in a trial for X-linked severe combined immunodeficiency (discussed above) may have been attributable to the combined toxicity of the vector and the transgene (Baum et al., 2003). Third, permanent genetic modification may involve life-long exposure risks. There are few long-term studies of any low-level toxic effects of a transgene product or assessments of cumulative effects on the health of a research participant over time. Many features of human gene transfer, although not unique, raise concerns and present complex risks for research participants.
Even with these pivotal advances and dramatic examples of clinical success, risk assessment remains difficult in gene transfer research (Deakin et al., 2010). Some of the theoretical risks have been invalidated, and some genuine risks have exceeded expectations or were never uncovered by preclinical studies. For example, as discussed above, the risk of extreme immune reaction and death was not fully appreciated by preclinical studies in the case of Jesse Gelsinger.
The risks of gene transfer products (and cellular therapy products) can be different from those typically associated with other types of pharmaceuticals, as seen in current draft guidance from FDA (2013). Nevertheless, the risks and uncertainties associated with gene therapy are not altogether unique, although some risks may be amplified or arise more often (Kimmelman, 2005). For example, genotoxicity is a high-profile concern in gene therapy, but it also occurs with chemotherapy and radiation (Deakin et al., 2009). Still, unlike research on many small molecule pharmaceuticals, the complicated logistics and feasibility of manufacturing a cell and gene therapy product sometimes influence the design of the clinical trials and further complicate assessment of risk. In addition, the preclinical data generated for cell or gene therapy products may not always be as informative as for small molecule pharmaceuticals, particularly because it usually is not feasible to conduct traditional preclinical pharmacokinetic studies with cell and gene therapy products. Gene therapy products, along with cell therapy products, often involve consideration of clinical safety issues; preclinical issues; and chemistry, manufacturing, and controls issues that are encountered less commonly or not at all in the development of other pharmaceuticals.
Several characteristics of gene therapy products (as well as cell therapy products) present increased risk for patients, including researchers’ relatively limited clinical experience with these therapies, the persistence of the transgene in humans for an extended period after a single administration, the potential to elicit an immune response (immunogenicity), the potential for the integration into host DNA and its interference with normal function of existing genes (genotoxicity), and the possibility that viral or bacterial matter could be transmitted to other individuals (FDA, 2013). To add to these considerations, the committee recognizes that in vivo gene therapy can inadvertently target transgene expression to an unintended and clinically unaffected cell or tissue type, with a potential for toxicity. Further, some gene transfer vectors, such as adeno-associated virus (AAV), introduced into non-dividing cells, such as neurons or striated muscle, present the potential for life-long persistence of vector and transgene expression (Lee et al., 2013). This may also produce a potential for toxicity, particularly if the sustained function of gene-modified cells alters relationships with unmodified cells.
Additional considerations regarding gene therapy include the following:
a) In vivo gene therapy can inadvertently target transgene expression to an unintended and clinically unaffected cell or tissue type, with a potential for toxicity.
b) Some gene transfer vectors, such as adeno-associated virus, introduced into non-dividing cells, such as neurons or striated muscle, present the potential for lifelong persistence of vector and transgene expression (Lee et al., 2013). This may also produce a potential for toxicity, particularly if the sustained function of gene-modified cells alters relationships with unmodified cells.
The potential for off-target toxic effects of modified cells must be considered. For example, one study involving the infusion of T cells that were genetically engineered to target tumors resulted in unexpected off-target cardiac toxicity. The engineered T cells bound to cardiac muscle tissue, and the resulting toxic effects on the heart were lethal to the research participant. Available preclinical models did not demonstrate this risk (Ertl et al., 2011; Morgan et al., 2010).
Insertional mutagenesis (genotoxicity) was a predicted risk, and although it is not unique to gene transfer, in some cases it presented more problems than expected in clinical studies. In a 2008 trial involving 12 patients with X-linked severe combined immunodeficiency disorder, 4 patients developed vector-induced T-cell leukemia (Aiuti and Roncarolo, 2009). Immediate treatment with chemotherapy to all four patients with leukemia sent three into remission, but one died. Although 11 participants in the trial survived and regained normal immune function, the trial results were a major setback for the field (Aiuti and Roncarolo, 2009; Hacein-Bey-Abina et al., 2008).
Some scientific hurdles—such as the absence of efficient delivery systems, difficulty with sustained expression, insertional mutagenesis and host immune reactions—remain formidable challenges to the field (Kay, 2011). Some practical limitations associated with even the most successful gene transfer techniques remain to be resolved before any gene transfer procedure can be demonstrated to be a safe and effective
therapy (Grigsby and Leong, 2010). Many of the major hurdles have to do with providing efficient gene delivery.
First, the vector uptake and distribution must be tightly controlled so that expression of the vector-encoded gene remains within the therapeutic range—if expression is too low, the functional protein product may not be produced at a high enough concentration to effectively restore the intended biochemical pathway, and if expression is too high, the research subject may experience toxic effects. Transcription of the new genetic material must also remain stable so that the transgene is expressed as long as necessary to treat the disease. For a given patient, this could range from a limited period to life-long expression (Kay, 2011). The degree to which the vector containing the corrective gene is taken up in a sufficient number of target cells is influenced by vector size and stability, the extent of target tissue vasculature, and the efficiency of interactions between vector and host cell receptors. The ideal vector would be cell-type specific, but the design of either non-viral or viral vectors that successfully target a specific cellular receptor has been elusive despite a great deal of effort. To date, re-engineered viral vectors are often too large, too unstable, or otherwise unable to reach the nucleus of some cell types (Kay, 2011). Non-viral vectors are attractive because of their suitability for pharmaceutical considerations such as scale-up, storage stability, and quality control; however, non-viral gene delivery remains prohibitively inefficient for most therapeutic applications (Grigsby and Leong, 2010).
Second, a substantial proportion of the population has been exposed to viruses from which vectors have been derived (or engineered), especially adenoviral and adeno-associated viral vectors. Exposed individuals thus have circulating antibodies that can interfere with transduction of closely related recombinant vectors. If researchers or clinicians must control an unanticipated immune response that arises in a research participant, this could then be complicated by the challenge of “turning off” expression of transgenes driven by constitutive, non-conditional promoter sequences specifically designed to always be “on” (Bessis et al., 2004; Jooss and Chirmule, 2003).
Third, gene transfer involves the interaction of many agents. The combined risk factors associated with the individual components, risks that may be amplified by their interaction, complicate risk assessments (Kimmelman, 2005). For example, the cases of leukemia that arose in the X-linked severe combined immunodeficiency disorder trial may have
been the result of the combined toxic effects of the vector and the transgene (Baum et al., 2003).
Finally, permanent genetic modification may expose patients to lifelong risks. Few long-term studies have been conducted to detect potential low-level toxic side effects of gene transfer products or assess cumulative effects on patient health over time (Hedman et al., 2011).
The committee concluded that, although not without challenges, the field of gene transfer research has experienced dramatic advances in scientific knowledge and that somatic gene transfer clinical investigations may today be considered part of an established scientific research enterprise. Whereas the field of gene transfer research was characterized in its early years by considerable uncertainty and concern about theoretical risks—on the part of the public as well as the scientific community—this field has matured to a state in which some of the early concerns about risk, and uncertainty overall, have been minimized. With the experience of more than 40 years of gene transfer trials and nearly 1,700 currently approved clinical trials; much has been learned about potential adverse events and how to ensure the safety of research participants. The committee concluded that many gene transfer clinical trials pose acceptable risks and are fast becoming an established modality of modern medicine.
Although the state of gene transfer research is constantly evolving, not all of gene transfer research can still be considered a completely new scientific enterprise or novel technology. This conclusion has significant repercussions for the oversight required for research projects to proceed; the committee’s assessment of the current regulatory structure is the subject of the next chapter.
Considerations of an appropriate regulatory structure, one that protects human research subjects while not adding to researchers’ administrative burden unnecessarily or impeding scientific advancements, focus to a large extent on how to assess risk. Given that such questions about risk assessment are not unique to gene transfer research—they are shared by other cutting-edge scientific and clinical research—the committee considered the question of oversight with a broader lens. In the following two chapters, the committee explores how the regulatory considerations for the evolving field of gene transfer research may shine light on potential needs in all emerging areas of clinical sciences.
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