Gene therapy has attracted intensive coverage from the media and likewise interest from the pharmaceutical and biotech industry. Indeed, genetic intervention is a fascinating topic as it may provide us with cures for a large number of ailments. Virtually all inherited diseases such as Huntington‘s, cystic fibrosis and severe inherited immunodeficiency could theoretically be treated, along with diseases acquired later in life—AIDS and cancer being the most prominent. This dream is not unrealistic if all the players in this field become seriously committed to it: the scientific and medical communities, biotech companies, stockholders, patients’ associations and even the media. In principle, gene therapy is no different from any other medical research, but its complexity and multifaceted interfaces with differing fields in biology and medicine can be counterproductive in several ways. Too much hope and expectancy based on short‐term results have led to disappointment and scepticism. There is also the danger that economic pressure on clinical researchers might lead to failure and cast a bad light on this emerging field.
Too much hope and too much expectancy based on short‐term results have led to disappointment and scepticism
Gene therapy is a complicated matter. It requires inserting the right gene into the right cells and starting gene expression at the right time. Gene expression should be as persistent as possible, ideally lifelong in the treatment of inherited diseases. Expression of the inserted gene also has to be tightly controlled to keep the level of the gene product within a therapeutic window. The toxicity related to transgenic expression, the vectors used and immune responses must be kept below a level of ‘acceptable risk’. Altogether, this is a formidable task, and all of these problems have to be solved specifically for each disease. This might explain why only a limited amount of clinical success has been achieved so far, in spite of the high promises from the scientific community. For more than 20 years now, researchers have elaborated on various possible strategies to deliver therapeutic genes. This has generated a set of distinct vectors, different promotors and a number of transduction protocols. However, while all these techniques are available, the complexities mentioned above make it difficult for the clinical scientists to assemble the right therapy for a specific disease from this set of tools.
Gene therapy requires competence in genetics, molecular biology, virology, chemistry, immunology, physiology and medicine. Unfortunately, a number of investigators have largely underestimated this degree of complexity. I was amazed to observe how naïve several attempts of therapeutic gene transfer have been. The acute immune responses to viral proteins that take place in pre‐immunized patients, for instance, leave very little hope that ‘classical’ viral vectors such as adenovirus can be used to establish a sustained gene expression. The assessment of gene therapy by the NIH advisory committee in 1995, therefore, rightly stressed the need for more pre‐clinical research devoted to vectors, target cells and host responses, in order to avoid a fatal immune response by the patient. These problems have to be solved before designing therapies and having them tested in clinical trials (Orkin and Motulsky, 1995).
In the meantime, advances based on empirical results have been quite remarkable. For instance, gene transfer into haematopoietic stem cells has become an important strategy to tackle a number of inherited disorders of blood cell and immune system development. It can also be used in the treatment of leukaemia and other cancers if drug resistance genes are inserted into non‐malignant stem cells before the tumour cells are attacked with chemotherapeuticals (Abonour et al., 2000). As the range of diseases involving the haematopoietic system is large, and blood stem cells are easily accessible targets for gene therapy, a number of important results have been achieved within the last 10 years. These include efficient gene transfer into murine haematopoietic stem cells, design of new vectors such as lentiviruses and improved onco‐retroviral vectors, and the improvement of ex vivo transduction protocols using cytokines and stroma surrogates (Halene and Kohn, 2000).
Unfortunately, a number of investigators have largely underestimated the complexity of gene therapy
High‐rate virus production in specific cell lines and pseudotyping these viruses with foreign proteins, such as Gibbon ape leukaemia virus and RD114 feline retrovirus envelope, significantly increase the ability of these vectors to specifically transfect their target cells. Researchers have modified retroviral vectors to enhance expression and avoid silencing of the gene after its insertion in the target cell's genome. Another important finding is that fibronectin and CH‐296, a recombinant fragment of this protein, can significantly improve the efficiency of retroviral gene transfer. In addition, a better understanding of how growth factors regulate the development of blood and immune cells enables researchers to direct their vectors to the right stem cells. For instance, it has recently been shown that a combined use of various cytokines can induce the proliferation of human CD34(+) CD38(−) cells. The pre‐treatment makes these primitive haematopoietic cells permissive for integration of onco‐retroviral provirus into their genome without inducing further differentiation. Stem cells with the therapeutic gene do not differentiate and thus get lost, but produce transduced daughter cells for a long time. Finally, Chad May and his co‐workers from Cornell University, the University of California and Memorial Sloan‐Kettering Cancer Center in New York have used HIV‐derived vectors with large segments of the locus control region integrated into their genome to enhance erythroid‐specific expression of human β‐chain haemoglobin in thalassemia mice (May et al., 2000). This is an important step toward gene therapy that requires tissue‐specific expression of the transferred gene. However, clinical applications using this technology will have to wait until the safety problems associated with HIV‐derived vectors can be solved.
Although they come with limitations, these findings have provided sufficient results in gene therapy research to make clinical applications possible in selected settings. We demonstrated the medical potential of these advances when we used gene transfer to treat children who had a severe inherited immune deficiency. These patients are at risk from all kinds of infections. The disease is caused by mutations of the gene encoding the γ‐c cytokine receptor, which is required for the delivery of survival and proliferation signals to T‐ and NK‐lymphocyte progenitor cells (Cavanazzo‐Calvo, 2000). By transferring a normal copy of the gene, we were able to correct the mutation fully and thus enable expression of the correct γ‐c cytokine receptor in four of our patients. This result has remained stable for more than a year in two of the patients whom we first treated. We think that two important factors have contributed to this encouraging result. The improvements made in the transduction of human haematopoietic cells mentioned above allowed us to design a vector and transduction protocol that is able to deliver the corrective gene to the right place. Furthermore, the disease itself offers a favourable model for gene therapy because expression of the correct γ‐c cytokine receptor gene confers a strong selective advantage for transduced cells. Daughter T cells derived from transduced cells live longer, so the effect of the gene transfer persists.
Indeed, the growth advantage that results from the expression of transduced genes is a very useful effect in treating disorders with gene therapy. Rafat Abonour from the Howard Hughes Medical Institute and his co‐workers recently demonstrated that ex vivo transfer of the MDR‐1 gene into haematopoietic progenitor cells led to positive selection of the transduced cells after chemotherapy in vivo (Abonour et al., 2000). In this context, the development of lentiviral vectors holds great promise as they can transduce most haematopoietic stem cells that are not proliferating. Sustained transduction of quiescent cells has also been demonstrated by using the xenogenic NOD/SCID transplantation model (Miyoshi et al., 1999).
This short survey shows that rapid advances have taken place in gene therapy, particularly in the therapy of blood stem cells. But it must be emphasized that this progress is a result of extensive basic and applied research in a wide range of fields in biology as well as in medicine. As gene therapy requires biological and medical expertise, it could perhaps become a model for how to close the gap between physicians and biologists (Editorial, 2000).
Progress in gene therapy is a result of basic and applied research in a wide range of fields in biology and medicine
The first applications of gene therapy are now within reach. As there is a huge medical and economic potential for these cures, gene therapy researchers find sufficient funding for their projects from industry and government. But the further development of this field requires the solving of a number of scientific and logistic problems. As discussed above, gene therapy research involves experts in multiple fields, so projects have to be designed carefully and carried out as collaborations between biologists and physicians. This raises logistic problems that cannot always be solved easily, at least in the setting of the traditional university hospital structures found in most European countries. Obviously, much effort is still necessary to turn gene therapy into a commonly used medical tool.
The ethical problems that are associated with gene therapy are not particularly specific to it, but they must be discussed openly. Given the attention that this field has received from the media and the public, a strict adherence by the scientific and medical communities to ethical guidelines is necessary as the economic pressure on gene therapy research is high, particularly in the USA. Several scientists developing gene therapies for a number of diseases are employed by, or have other forms of financial interest in biotech companies (Marshall, 2000). Nevertheless, the expectations of companies and their shareholders to make profits from gene therapy should not impair access to scientific and medical information, or have any impact on the design and realization of gene therapy trials.
Interestingly enough, the American Society of Gene Therapy published a manifesto in April this year to address this question. Their policy statement notes that ‘clinical investigators must be able to design and carry out clinical research studies in an objective and unbiased manner, free from conflicts caused by significant financial involvement with the commercial sponsors of the study.’ Adopting such principles would demonstrate a better awareness of the ethical and economic issues by the scientific community and, if applied, should have positive consequences for medical research. Savio Woo, professor at Mount Sinai School of Medicine in New York and president of the American Society of Gene Therapy, said in a statement in December last year: ‘Adherence to oversight principles is important in maintaining public credibility about a newer form of biotechnology that still seeks hard evidence for success [...].’ Everyone concerned should follow the outcome of such efforts with careful interest. In the end, as in all medical research, the development and testing of gene therapies must be done with the best interest of patients and sufferers in mind.
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