Cutting Edge: Dave Barlow

August 17, 2001

The development of gene therapies may bring relief to those with cystic fibrosis or Aids and can also give rise to a little wine tasting.

Gene therapy has the potential to treat many different diseases, even those previously thought impossible to cure.

For inherited diseases such as cystic fibrosis and muscular dystrophy, the idea is to transfer a gene, which makes the crucial protein that is lacking, into the patient's body so that the protein can be produced and the patient returned to normal health.

With infectious diseases such as Aids, the proposed strategy is to deliver a gene that will make a protein that kills the invading organism directly or else will trigger the body's natural defence system to destroy it.

Although the potential range of clinical applications of gene therapy is enormous, we do not yet have an effective method of gene delivery. Early efforts to transfer genes to a patient's cells by packaging them inside viruses have proved disappointing.

More recently, researchers have focused on inserting the genes into particles known as liposomes. These are bubble-like structures made from detergent-like compounds that, like viruses, can penetrate cell membranes.

Exploiting these liposome-gene constructs, however, depends on knowing precisely how the liposomes interact with the added genes, and how they then behave when they encounter the body's cells. If the liposome-gene complex sticks to the cell surface and then collapses, the DNA making up the gene will be rapidly digested and so will be ineffective; whereas, if the complex sticks to the cell surface and remains intact, it is more likely to be taken inside the cell as required.

Unfortunately, understanding liposome-gene interactions is not straightforward, and the situation is made worse by the fact that different research groups have used different kinds of liposome (made from different types and combinations of detergents) and have used different experimental techniques to study their effectiveness in gene delivery.

Some of the systems studied appear promising when used to deliver genes to isolated cells in laboratory culture, but prove useless when it comes to delivering to live animals. There is then the added complication that if the liposomes are made using more than one type of detergent, the ratio of the different components used is found to affect their efficacy in gene delivery, as does the quantity of DNA added.

What is crucial, therefore, is to be able to visualise these systems at a molecular level so that we can better understand how all the ingredients should be fitted together to provide a package that can deliver genes to their target sites inside a patient's body.

Surprisingly, there have been few detailed studies looking at how the structures of different kinds of liposomes influence the way they interact with genes, and how this in turn affects their interactions with cells and thus their effectiveness as gene deliverers.

Our research group at King's College London, in collaboration with colleagues at the Institut Laue-Langevin (ILL), Grenoble, is doing precisely this. We are using the neutron-scattering facilities available at the ILL (and at the Rutherford Appleton Laboratory in the United Kingdom) to study the structures of various liposome-gene systems when they are simply dissolved in water, and then use the same facilities to look at what happens to these systems when they approach the surfaces of artificial cell membranes.

The beauty of the neutron-scattering experiments, quite apart from the fact that they provide a wonderful excuse to spend a few days each year sampling fine wines and French cuisine, is that we are able to build up a picture of what these complicated gene delivery systems look like step-by-step.

At each stage we focus on one particular component, with all the other components in the system made in effect "invisible". We do this by exploiting the technique known as contrast variation. At its simplest, this involves putting the liposome-gene constructs into different mixtures of normal water and heavy water. For example, if we carefully arrange to disperse the gene delivery constructs in a mix of 32 parts normal water and 68 parts heavy water, we can then use the neutrons to "see" just the detergent in the system, and with a mixture very nearly 100 per cent normal water, we can arrange to see just the DNA.

This method of contrast variation is also used when we want to look at how the gene-delivery systems interact with our model cell membranes. When our studies are complete, we will be better placed to know what kind of liposome would be best for use in gene delivery. This will take us another step forward in the fight against disease.

Dave Barlow is a senior lecturer in the pharmacy department at King's College London.

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