Doctors at Baylor College of Medicine have succeeding in copying a gene that causes two human diseases when it is defective, an important step in the study of how genes can cause disease when accidentally altered, according to Dr. C. Thomas Caskey.

Caskey, head of the five-member team at Baylor, reported "cloning" or making multiple and identical copies of a gene that manufactures a crucial enzyme in the body, hypoxanthine phosphoribosyltransferase, or HPRT for short.

HPRT helps govern production of an important chemical in the body, the lack of which causes Lesch-Nyhan syndrome, in which babies are born severely mentally retarded. Such babies often bite and scratch themselves and beat their heads with hard objects.

The other, milder, disease caused by HPRT deficiency is gouty arthritis and/or serious kidney problems, the result of excess uric acid produced by the faulty gene. Doctors say about 10 percent of all gouty arthritis is the result of the HPRT deficiency.

The cloning of the HPRT gene by Caskey along with John Brennard, A. Craig Chinault, David Konicki and David Melton, was reported in the March issue issue of the Proceedings of the National Academy of Sciences.

"We feel we've done something pretty significant," Caskey said. "We are working toward being able to correct the genetic defect."

The work of Caskey's team should facilitate the study of genes near the HPRT gene on the chromosome, such as the one that causes hemophilia when defective. Hemophilia prevents blood from clotting normally so that even small scratches or bruises can result in fatal hemorrhaging.

The most important aspect of the advance is that it allows researchers to study these genes, which apparently are highly susceptible to sudden accidental mutation, or structural change. Because genes direct the making of enzymes, when a gene's structure is damaged the enzyme it manufactures is often misshapen and can fail to perform its biological function.

Unlike most genetic diseases, which are passed from generation to generation over many years, the ones caused by HPRT deficiency occur in families that have never had a history of the disease.

By studying the genes and chromosomes, Caskey hopes doctors will be able to identify women who are carriers of the diseases from chromosomal studies performed before they become pregnant.

Then, before the women have babies, they could be told of their chances of having infants with some physical defects. Replacing defective genes is farther in the future, spokesmen said.

It is not possible yet to insert normal gene copies for defective ones in the body, but Caskey's research brings the possibility closer.