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Wednesday, 23 May, 2001, 16:17 GMT 17:17 UK
Faulty human genes repaired
Chromo BBC
By BBC News Online science editor Dr David Whitehouse

A new technique to insert healthy genes into human cells shows promise as a method to repair minute but devastating genetic flaws.

Currently, gene therapy experiments focus on delivering a complete healthy gene into a cell rather than repairing the one that has gone wrong - but it is an approach that has several problems.

The new method uses a virus, genetically engineered to include a correct copy of a human gene, to fix the defect at the site of the problem.

Although at an early stage, scientists say the novel approach has great promise and could make gene therapy easier to carry out.

Control DNA

Using current techniques, when a replacement gene is inserted into a malfunctioning cell the new gene rarely works properly. This is because it is controlled by stretches of DNA that can lie far away from the gene on a particular chromosome - and a gene delivered without its control DNA will not work.

Genome BBC
There is also the possibility that when a gene is randomly inserted into a chromosome, it may trigger or disrupt the action of other genes.

Success with a method that bypasses these problems is reported in the Journal of Gene Therapy and Molecular Biology, by a team led by Dr David Russell of Cornell University in the United States.

The researchers used viruses to deliver deliberately altered copies of the hypoxanthine phosphoribosyl transferase (HPRT) gene into human cells. The experiment was carried out six times, with each virus delivering a slightly different version of the HPRT gene.

Base swap

Following their natural tendency to cut into a cell, travel to its nucleus and interact with the genes there, the viruses were able to replace the normal HPRT gene in human cells with their modified versions.

When a virus reached the nucleus of the human cell that contained a normal HPRT gene, it changed just one of the gene's base pairs - swapping adenine for guanine - at a specific location on a particular chromosome.

To further demonstrate the power of the technique, the team showed that it could be used to fix a genetic defect by repairing a faulty alkaline phosphatase gene in human cells.

The scientists say that their results suggest that it is possible to correct a single DNA defect among the three billion or so base pairs that make up the human genome.

A single DNA defect can result in a debilitating disease such as sickle-cell anaemia.

At present, the researchers do not know exactly how the virus repairs the faulty gene. It could insert its healthy gene directly into the genome, or it could act as a template for the cell to repair itself.

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See also:

27 Apr 01 | Health
Gene therapy restores dogs' sight
06 Mar 01 | Health
Heart failure damage reversed
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