Feature

Juggling Genes

What is actually happening in the world of gene technology and should we be concerned?

John Marbrook and Jenny Rankine

Regulation of genetic research, particularly where human subjects are involved, has been a major concern to both researchers and the general public. Manipulation of genes, transferal of genetic material for therapeutic purposes and other forms of genetic "tinkering" have been been the subject of on-going debate.

Scientists are currently able to select, cut, splice and amplify sections of DNA in the test tube. The extension of this DNA manipulation to clinical situations has been called translational research. However, the progression from laboratory experiments to altering complex human genetic systems is an enormous conceptual jump, and there have yet to be fruitful results from research translated in this way.

Gene Therapy

Several hundred patients have been involved in attempts to affect various genetically-based disorders, but the DNA transfers involved have not led to definite clinical improvements. Attempts to reverse the effects of genetic disorders in models of human diseases in animals have also been unsuccessful.

The rationale for attempting gene therapy, however, seems compelling. Single gene disorders, especially where the mutation is at a single known site, would lend themselves to therapeutic correction if we could replace the deficient genetic material. In addition, a group of acquired diseases, including HIV infection, cardiovascular disease and various forms of cancer are also being considered for genetic manipulation.

If a genetic defect is the result of a dominant mutation, merely adding a functional normal gene would not be enough. Such a dominant gene would also need to be silenced, which is not yet feasible.

All scientists involved in genetic manipulations with therapeutic potential realise that there must be a greater detailed knowledge about basic biological systems before genetic technology can be clinically useful.

Genetic Transfers

Several ways have been used to transfer genetic material into cells, each having advantages and disadvantages. It is relatively easy to splice the DNA of the gene involved into viral DNA, for example. The natural entry of a virus into a cell introduces the spliced gene. However, choosing the target cell and integrating the transported gene with the cell DNA provide additional problems.

Retroviruses have been used in cell marker studies, for treating cells outside the recipient (ex vivo) and to enhance immunity by, for example, introducing genes coding for crucial immune system regulatory molecules. The introduction of retroviruses is well understood, although they only infect dividing target cells. However, it is difficult and expensive to produce the high concentrations of retrovirus preparations needed for these manipulations.

Adenovirus, herpes and pox virus are other likely candidates. Some of these would be likely to stimulate the immune system and thus be rejected. Herpes viruses are naturally neurotropic -- they have a special affinity for nerve cells -- and would be considered for introducing genetic material into deficient nerve cells. These viruses provide different modes and degrees of integration of viral nucleic acid and the additional normal DNA into the genome of recipient cells.

Although large amounts of specific DNA sequences can be prepared easily, its uptake into target cells is very inefficient compared with viral infection. Introduced DNA is poorly integrated into recipient DNA, but its instability and non-persistence give it a high level of safety. Being associated with liposomes is one way of enhancing the uptake of specific DNA into cells. In theory, liposomes can be made to be attracted to specific cell types, although there is no mechanism for increasing DNA persistence.

Targeting Cells

Major problems in gene therapy include ensuring that the cell targeted for genetic modification is the cell that actually uses the product of the introduced gene, and getting the correct DNA into this cell.

To do this, scientists need to understand the molecular topography and kinetics of the disease process in detail. For example, it is the precursors and not the mature red blood cells that would be the critical targets in attempts to reverse disorders associated with haemoglobin malfunction. Similarly, to alleviate cystic fibrosis scientists need to know which and how many lung cells must be transformed with the critical normal DNA. In contrast, clotting factors are continually circulating, so it may not be as critical to target a particular factor production site.

From existing knowledge of the blood cell system, we could imagine a situation where the product of a crucial gene is only functional in a cell with a very short half-life. Successful gene therapy would then need to target the population of cells from which these short-lived cells are continually recruited. We still don't have enough knowledge of stem cell biology or the factors influencing the growth and differentiation of many cell lineages to confidently identify target cells.

Ethical Issues

According to generally agreed ethical principles, gene therapy in humans is only permissible for the benefit of the recipient. Thus, transfer into reproductive cells (the germ line) is unacceptable. This may only be achieved if gene therapy is ever attempted on fertilised eggs or on the foetus. Presently, scientists are required to establish that cells being targeted do not include reproductive cells.

The use of animal models in determining the molecular pathology of disorders has been of great value, and helps bridge the gap between laboratory science and clinical therapeutic use of genetic material. It is not always easy to establish that an animal model is appropriate for the use to which they are being put. Biological experience in the last two decades, including the group of autoimmune mouse strains and animal models of diabetes, shows the need for multiple models to cover the range of manifestations of each disorder.

Research which leads to the introduction of new treatments is assessed by regional or organisational ethics committees before it begins. The Health Research Council of New Zealand (HRC) Ethics Committee is responsible under the HRC Act for accrediting these committees, some of which have to assess both treatments and research. HRC Ethics Committee members have been concerned, however, that some regional ethics committees did not have the expertise to assess complicated genetic research and treatment proposals.

In late 1995, the HRC set up the Gene Technology Advisory Committee to examine proposals involving gene technologies and advise regional ethics committees. This two-tiered system is similar to that operating in Australia, the US and several European countries. This ensures transparent monitoring by scientists from a range of disciplines, taking responsibility for the results of scientific enquiry.

Professor John Marbrook works in the Department of Molecular Medicine at Auckland University.
Jenny Rankine is the Public Relations Officer with the Health Research Council of New Zealand