New Gene Therapy Attacks AIDS




Biologists at the University of Texas have invented a novel and apparently far-reaching method of singling out precise regions of DNA. The method can be used both to disrupt particular genes and to add new genetic material at a specific site.


As a first use, the biologists have shown that they can disrupt both the AIDS virus and the human gene for the receptor used by the virus to gain access to human cells.


The chief inventor, Dr. Alan Lambowitz, said if further laboratory tests succeeded, he wanted to take T-cells from AIDS patients, disrupt the cells' receptor gene and return the cells to the patients, in the hope the cells resisted further attack just like those of people naturally unaffected by the virus.


T-cells are the infection-fighting cells of the immune system and a principal target of the AIDS virus.


Lambowitz's method could also boost gene therapy, the concept of inserting corrective genes into patients to treat a wide variety of diseases. This field still has many obstacles, and one major drawback is the viruses now used to deliver new genes to human cells insert their cargo at random sites in the chromosome. In delivering new genes to a precisely designated site, the new method, if it works, would be safer and more effective.


The idea of being able to insert new genes into a "safe harbor'' in the genome "would be very important for future gene therapy,'' said Dr. Ronald Crystal, a gene therapist at New York Hospital.


Dr. Savio Woo, a gene therapist at Mount Sinai Medical School, said the new method was "in principle very exciting, because it provides a specific means of inactivating a particular gene in a human genome.''


The technique was developed by Lambowitz and Dr. Huatao Guo at Texas and by colleagues at other institutions, and is described in Friday's issue of Science.


Their work is based on a discovery about introns, pieces of DNA that are found inserted between the working parts of genes in many species. The cell strips out the intronic DNA and splices the remaining segments together before using the information to make a protein.


Lambowitz had been working with a kind of intron found in bacteria. The intron did not always sit passively in the bacterium's chromosome but would instead sometimes spring free and reinsert itself at another site in the bacterial DNA.


In 1995, Lambowitz found that in choosing its new site, the intron looked for a string of 14 DNA letters that matched 14 letters in its own structure. He realized that by changing some of these 14 letters in the intron's own structure, he might be able to make it insert itself at any desired point in a DNA molecule.


"We were carrying out basic research on Group II introns and how they related to gene structure,'' he said. "Any kind of practical applications was the furthest thing from our minds.''


In Friday's article, he and colleagues described how they designed introns that disrupted the human CCR5 receptor, a protein that studs the surface of human cells and is exploited as a trapdoor by the AIDS virus to gain entry to the cell. People with a certain variant of the receptor are immune to AIDS.


The artificial introns all inserted themselves in the intended position in the receptor gene, the scientists reported. Other introns attacked specified points in the form of the AIDS virus that lay latent in cells.


These experiments were conducted in human cells grown in the laboratory. The receptor gene and the AIDS virus were not in their usual position in the chromosomes but rather, for ease of experiment, were carried in small rings of DNA known as plasmids. Lambowitz said he now needed to test whether the attacking introns worked as well against targets in the chromosomes.


Ohio State University, where work on the new method started, has applied for a patent on it, as has the University of Texas. A company, InGex, has been formed to exploit the method. Lambowitz, who is a scientific adviser to the company, said practical applications included disrupting deleterious genes in the microbes used to make wine and cheese and making new vaccines by knocking out the pathogenic genes in harmful bacteria.


Besides disrupting target genes, the introns can be used to insert new genetic material into DNA. The longest piece of inserted material so far is 2,000 DNA letters in length. Most human genes are far longer, but Lambowitz said the full carrying capacity of introns had not yet been assessed.


He said the method should be useful in any kind of genetic engineering where genes must be inserted at a specific site, and in any genome project where the function of an unknown gene could be ascertained by disrupting it.


"We couldn't have designed a better way of inserting DNA,'' Lambowitz said. "If you set out to do it, you never would have thought of it. But nature did it in the course of evolution.''