Gene Therapy

David Cooper, Prof Nick Lemoine, David Cooper, Prof Nick Lemoine

• Éditeur: Garland Science
• Année: 2020
• ISBN:
  9781000144680
• Édition: 1
• Sujet: Physical Sciences
• Sous-sujet: Chemistry

Gene Therapy describes the delivery systems now available to target a given tissue with specific gene or oligonucleotide sequences, and explores the utility of animal modules as test systems. In the context of selected disease states, it summarises in vitro and in vivo studies and clinical trials performed to date.

1

Scope and limitations of gene therapy

Karol Sikora

1.1 Introduction

Gene therapy can be defined as the deliberate transfer of DNA for therapeutic purposes. There is a further implication in that it involves only specific sequences containing relevant genetic information; that is, transplantation procedures involving bone marrow, kidney and liver are not considered a form of gene therapy. The concept of transfer of genetic information as a practical clinical tool arose from the gene cloning technology developed during the 1970s. Without the ability to isolate and replicate defined genetic sequences it would be impossible to produce purified material for clinical use. The drive for the practical application of this technology came from the biotechnology industry, with its quest for complex human biomolecules produced by recombinant techniques in bacteria. Within a decade, pharmaceutical-grade insulin, interferon, interleukin-2 (IL-2) and tumour necrosis factor (TNF) were all undergoing clinical trials. The next step was to obtain gene expression in vivo.

1.2 Suitable target diseases

Genetic disorders were the obvious first target for such therapies. Abortive attempts were made in the early 1980s to treat two patients with thalassaemia. These experiments were surrounded by controversy as the preclinical evidence of effectiveness was not adequate and full ethical approval had not been given. We now know that thalassaemia — a disorder in which there is transcriptional dysregulation of the globin chains of haemoglobin — may not be an ideal target for gene therapy as we still do not have good methods of regulating the expression level of inserted constructs.

Table 1.1 lists the features of a suitable target disease for gene therapy approaches. Clearly, the disease must be life-threatening so that the potential risk of serious side-effects is ethically acceptable. The gene must be available and its delivery to the relevant tissue feasible. This may involve the ex vivo transfection or transduction of cells removed from a patient, which are returned after manipulation. This approach is only possible with a limited range of tissues and most trials so far have used bone marrow. Ideally, a short-term surrogate end-point to demonstrate the physiological benefit of the newly inserted gene should be available. The electrical conductance change in the nasal epithelium after insertion of the cystic fibrosis transmembrane regulator (CFTR) gene is a good example. Other diseases may have more complex, and therefore more difficult to measure, end-points. Finally, there must be some possibility that the disability caused by a disease is reversible. Some of the tragic mental and physical handicaps caused by some genetic metabolic disorders may never be improved by somatic gene therapy, however successful a gene transfer protocol.

Table 1.1. Features of a disease suitable for gene therapy

Life-threatening

Gene cloned

Efficient gene transfer to relevant tissue possible

Precise regulation of gene not required

Proper processing of protein product

Correct subcellular localization of protein

Persistence of gene expression to avoid repetitive dosing

Measurable surrogate end-points

Effects of disease must be potentially reversible

The aim of gene manipulation varies in different diseases. In haemophilia, all that may be required is a suitable ‘protein factory’ to produce enough circulating coagulation factor to be effective physiologically. In cystic fibrosis, enough CFTR product must be selectively expressed by those cells where its lack causes pathological damage, such as in the lung and gastrointestinal epithelia. A variety of metabolic disorders result from genetic abnormalities in liver proteins. Some form of tissue targeting may be necessary before effective therapeutic strategies can be developed. With cancer, the problem is the requirement to target every single malignant cell. Although a variety of ingenious methods are currently being examined, it would seem more realistic to use systems that do not require the correction of the somatic defect resulting in malignancy.

Over the past seven years, a growing number of protocols have been approved by regulatory authorities throughout the world, the majority in the USA. Protocols for cancer gene therapy now far outstrip all others. This reflects the difficulty in treating patients with advanced cancer by conventional chemotherapy, the low risk-benefit potential of gene therapy and the relatively high level of research funding in this area of biomedical research. Figure 1.1 outlines gene therapy protocols currently in use from the NIH Recombinant DNA Advisory Committee (RAC) and the European Working Group on Gene Therapy (EWGGT) databases. Out of 163 currently active trials, 119 (65%) are for cancer.

Figure 1.1 Gene therapy: current 163 approved protocols from US and European databases.

1.3 Germ-line gene therapy

So far, no government has seriously considered germ-line gene therapy. The technology is relatively straightforward as the problem of targeting can be solved by direct manipulation. The very sophisticated developments in in vitro fertilization together with the growing experience in the generation of transgenic and knockout animals and plants mean that sooner or later we will have to discuss the ethics of such approaches. Techniques for homologous recombination — the targeted replacement of old genes with new — are now possible, at least in mice. Furthermore, the likely explosion in genome information coming both from wholesale sequencing of the human genome and the genetic dissection of functionally related genes in simpler organisms as diverse as nematodes, zebrafish and yeast will almost certainly open up new possibilities for genetic intervention that could transcend generations. It is this permanency that is most frightening to the ethicists, especially if unforeseen problems arise. At some time in the future, a policy of ‘genetic cleansing’ may become a serious option for governments trying to contain spiralling healthcare costs. Ethics, like beliefs, values and cultures, change with time — so perhaps today’s heresy will be tomorrow’s routine.

1.4 Ethical and safety considerations

Perhaps the biggest risk from gene manipulation in vivo is the possibility of insertional mutagenesis and the activation of oncogenes leading to neoplasia. Such risks are clearly important factors in the consideration of the ethical basis for gene therapy for disorders such as cystic fibrosis, haemophilia and the haemoglobinopathies. For patients with metastatic cancer, the risks are low. Such patients are often desperate for some form of treatment and are already searching for the gene therapy programmes described in the media. Therapies with even minimal potential benefit will be avidly considered. In this situation, the biggest problem is offering false hope. It is unrealistic to expect such new strategies to be effective immediately. The first patients entering trials will provide much information in return for little personal benefit. This must be recognized by both the investigator and patient in order to reduce the ‘breakthrough’ mentality that surrounds novel treatments (Table 1.2).

Table 1.2. Taking gene therapy to the clinic

Safety:	Must demonstrate lack of toxicity, lack of immunological response, lack of viral replication
Efficacy:	Must demonstrate effective delivery and expression leading to phenotype correction
Practicalities:	Issues of scale-up must be addressed Regulatory authorities must be satisfied Therapy must be cost-effective

Various countries have now established regulatory bodies for gene therapy. Most are modelled on the US model of the RAC, which works closely with the country’s existing drug regulatory body — the Food and Drug Administration (FDA). The success of the FDA in taking over much of the paperwork has recently led to the disbanding of the RAC. A parallel system has been established in the UK where the Gene Therapy Advisory Committee (GTAC) together with the Medicines Control Agency (MCA) evaluate proposals from a scientific, ethical and safety standpoint. The creation of the European Medicines Evaluation Agency (EMEA), a single agency for drug and biotechnology product evaluation in Europe based in...