Gene therapy

Introduction
The human genome project has led to the isolation of genes which determine the risk of developing common diseases such as cancer, Alzheimer's disease, diabetes and psoriasis, as well as genes which are mutated to cause the less common Mendelian diseases like cystic fibrosis or muscular dystrophy. The structure of each gene can be studied easily and cheaply for any individual using gene amplification techniques, giving predictive information. Scientists hope that genes can be introduced into relevant tissues in the body (somatic gene therapy) to alter function so as to prevent or treat disease.

The introduced gene may replace a defective gene, or code for a protein which acts on other cells.

Genetic testing
Until each human gene could be isolated by cloning it into a bacterial virus, geneticists only understood the aetiology of a few genetic disorders. These were mainly single gene disorders, such as phenylketonuria or sickle cell disease where the affected protein could be studied easily. Once genes could be studied directly and for each individual and family, the mutations causing many other diseases were found. In most cases, including cystic fibrosis and Huntington's chorea, these genes coded for proteins which were not known to exist before the genes were cloned. Interest rapidly turned to the use of the new genetic information for

– screening

– prevention by antenatal diagnosis

– treatment.

Once each gene was isolated by cloning, genetic testing both of affected persons and carriers became an important part of the investigation of families with an inherited disorder (see 'DNA i. approach and techniques' Aust Prescr 1995;18:45-8 and 'DNA ii. clinical applications' Aust Prescr 1995;18:76-9).

Somatic gene therapy
The first attempts at somatic gene therapy took place only a few years ago. Ethically, this experimental form of treatment had to be tested first on diseases that were well understood and severe, and had no effective treatment. The affected organ system also had to be accessible so that the response to the gene delivery could be assessed easily. There were problems because the only vectors that were available to deliver the genes into the cells were based on retroviruses or adenovirus. These vectors retained some of their pathogenic potential; a situation which still exists to some extent, and limits the usefulness of gene therapy. The retroviral vectors were thought particularly to pose a risk, albeit small, of causing lymphoma if injected into a patient. This made ex vivo treatment essential. Cells were taken from the patient and put into culture to avoid possible treatment-related illness.

Early trials
The first trials were for an inherited form of severe combined immunodeficiency caused by the absence of the enzyme adenosine deaminase (ADA) in lymphocytes. The researchers stimulated bone marrow to produce lymphocytes by giving natural ADA to the patients. The lymphocytes were then taken ex vivo and a copy of the coding portion of the human ADA gene cloned into a mouse tumour virus was introduced. The retrovirus integrated into the human lymphocyte genome in some cases. The DNA sequence coding for ADA was transcribed and translated to give an active enzyme which allowed the cells to function normally when they were put back into the patients. Since this was autologous transplantation, rejection was not a problem. However, most of the cells die in a few weeks, and the treatment has to be repeated. More recent approaches have tested bone marrow as well as peripheral lymphocytes, to try to achieve a lasting effect by transfecting stem cells, and these have achieved some success.^1,2

There have been many similar trials for other single gene disorders such as cystic fibrosis, Gaucher's disease and Canavan's disease. However, ADA deficiency is still the only disease for which somatic gene therapy is an efficacious clinical treatment.

In cystic fibrosis, several methods have been tried to replace the mutated gene. These include introducing the normal gene with adenovirus (for which there were safety problems due to inflammation of the lungs) and liposomes (non-toxic lipid envelopes encapsulating the gene preparation). Research shows that liposomes can transfer enough gene to allow transient and partial correction of the ion transport defect in the epithelial cells lining the upper airways of patients with cystic fibrosis. Since liposomes are far less toxic than virus vectors, this is a very encouraging result, but still many years away from clinical application.^3,4

Problems
Many viruses and liposomes are being studied for gene transfer. In addition to retroviruses and adenovirus, the trials include vaccinia, herpes and adeno-associated virus. Targeting is also being attempted using the interaction between antibodies or other proteins or ligands, with cell surface antigens or receptors, to achieve uptake using normal cellular entry mechanisms. Although each new approach shows promise in some systems, two overwhelming features recur time and again.

The first is that no one system will meet all somatic gene therapy requirements. The required response to gene therapy and the number of cells to be transfected will vary. Some corrections (as for cystic fibrosis) only require a low level of synthesis in a proportion of accessible cells, while others (such as thalassaemia) will need high expression, probably from stem cells. In patients with cancer, it will be important to reach a high proportion of the affected cells, and to use gene therapy in conjunction with other forms of therapy. For prevention of cardiovascular disease, the transfection of a few liver cells with the gene for the low density lipoprotein receptor might perhaps be enough to lower cholesterol. In spite of the hopes of the biotechnology companies for a 'magic bullet' based on one agent, there will need to be a multiplicity of vectors tailored for each cell type and each disease.

Although single gene disorders such as muscular dystrophy and cystic fibrosis are `high profile' in the genetics world, the second problem is whether it will be possible to use gene transfer for treatment (or prevention) of more common diseases, which may be multi factorial or even entirely environmental. In one sense, delivering a normal copy of a human gene to a human tissue is an attractive way to offer a natural product as a pharmaceutical. For example, the gene for insulin could be transfected into cells from the patient which are then implanted under the skin in the form of an organelle, a packet of encapsulated cells connected to the circulation. Provided such genes can respond physiologically, this could have great advantages over injecting insulin, as the response could be accurately regulated by the body's needs.

Such systems require a gene to be under the control of normal signals, most of which are not in the coding sequence. Consequently, there is now a great deal of interest in developing human artificial chromosomes, which might be delivered to the cell and replicate each time the cell divides.⁵ Another promising approach would encourage homologous recombination of a normal sequence to replace the mutation, reconstituting a non-mutated gene. These systems are for the future, but with increased understanding of ways of achieving site-specific recombination and of handling very large DNA sequences, they have promise.

Ethical issues
Ever since gene therapy was first proposed, there has been interest in and serious concern about its ethical implications. There are particular concerns about the possibility of germ line therapy or the use of gene therapy for enhancement ('the perfect baby'). Such uses are impossible at this time and are also illegal in most countries. Somatic gene therapy, as researched and applied in all countries today, is confined to serious diseases, and is never proposed for use so that the genetic changes might be passed on to children. Much of the discussion around gene therapy in this context has no basis in reality.

Conclusion
How soon will gene therapy come? There are already a few patients with ADA deficiency who benefit. Some cancer treatment protocols, such as those using a retroviral vector to deliver a thymidine kinase gene to enable tumours to be treated with acyclovir as a prodrug, are in Phase III trials. The real advances will be seen when new vector systems are developed which combine the best features of the current viral, liposome and receptor approaches. It is to be hoped that (in spite of our low level of investment in commercial biotechnology and pharmaceutical research) at least some of the fundamental advances will be made by the very strong biomedical sciences research community in Australia.