New "decoy" may improve the success of organ transplants.

SAN DIEGO--(BUSINESS WIRE)--Nov. 9, 1994--The success of organ transplantation to treat life-threatening disease may one day be further improved through a new kind of drug created with the tools of biotechnology at UC San Francisco.

The potential drug, called a "gene regulating decoy," is designed to stop donated organs from being rejected by the host's immune system, a nagging long-term threat to organ recipients.

The decoy, invented by Anthony Hunt, Ph.D., professor of pharmacy and pharmaceutical chemistry at UCSF, is based on a novel drug design strategy to prevent cells from making undesirable proteins. The decoy is designed to stop the cells -- at the earliest stage in the process -- from activating specific genes and producing the proteins encoded by these genes.

Carol S. Lim, a graduate student on Hunt's laboratory team, presented a poster on the new technology Wednesday at the annual meeting of the American Association of Pharmaceutical Scientists in San Diego.

The technique, still in its infancy, now is being tested on cells growing in the laboratory. Hunt hopes to use the decoy as a sort of stealth technology that would keep a transplanted organ from displaying telltale proteins on its cell surfaces that reveal the organ to the recipient's immune system as a foreign invader and subject it to attack.

Hunt believes that physicians may be able to stop transplanted organs from being rejected by bathing the donor organs with the new drug before transplantation, until few of the proteins that elicit attack by the host immune system remain.

There are three main steps every living cell must complete to produce proteins from the genes which encode them -- promotion, transcription and translation. The first step, promotion, is the most sensitive, Hunt said, and it is this step that should be targeted to most efficiently stop unwanted proteins from being made.

The cell's construction of proteins resembles the efforts of a work crew, Hunt said. During promotion, a crew of molecules assembles at a specific destination on a gene, housed on one of the many rod-shaped chromosomes within the cell's nucleus.

At this target site the work crew attaches to the DNA and initiates the DNA unwinding process, which allows DNA to be read. When the gene is read, it is transcribed into another long-stranded molecule called RNA, which travels outside the cell's nucleus into the surrounding region, called the cytoplasm, where it is finally translated by other cellular machinery into protein.

"There is usually a crew boss -- an essential promoter protein -- that must be present for the cellular crew to properly congregate and work at the target DNA site on the genes," Hunt explained.

"The assembly site on the gene is a relatively short bit of DNA, and there are typically only one or two copies of it within a cell," he said. The researchers mass produce copies of this small stretch of DNA in the laboratory. When the researchers introduce a large number of these decoy copies into the cells of the organ to be transplanted, the decoys should overwhelm the natural target.

The decoy is intended to attach to all the promoter proteins, preventing any from being available to attach to their natural target site on the gene. As a result, production of protein encoded by the gene would not be initiated. It is hoped that none of this protein would then appear on the cell surfaces of the donor organ, and that the host's immune system would not attack.

Hunt developed a way of chemically capping the ends of the decoy so that it is not chewed up by enzymes within the cell before it can do its job. In addition, to enhance their ability to stop protein production, Hunt has created modified decoys that are many times more successful in attracting promoter protein away from its natural target.

For organ recipients, a variety of drugs are now used to stop the immune system from attacking the donated organ. The drugs work by suppressing immune system function, but immune suppression increases the risk of infection. In addition, the drugs are not always successful in preventing organ rejection.

The strength of the organ rejection response depends on the "foreignness" of the donated organ, which is why close relatives are considered the best prospective donors of kidneys, for example, according to Hunt. Immediate relatives share many genes, and therefore a high proportion of identical proteins. Fewer "foreign" proteins are present to incite an immune response in an organ donated by a close relative.

In a donated organ, the most likely proteins to be attacked by the host immune system are called major histocompatibility complex (MHC) proteins. When the organ was still with the donor, these MHC proteins were regarded as part of the "self" by immune system cells patrolling the donor's bloodstream. In the recipient's body, these MHC proteins are more likely to incite attack by the immune system.

The decoy strategy works well at preventing MHC proteins being made in cells grown in culture dishes. Hunt and his main collaborator, Marvin Garovoy, M.D., professor of surgery and director of the Immunogenetics and Transplantation Laboratory at UCSF, will next test decoys in islet cells of the pancreas, which are attacked by the immune system in diabetes.

Hunt is optimistic about the technique's prospects, based on what is known about a similar shutdown in MHC protein production that occurs in a rare genetic disease. Children born with the disease, called Bare Lymphocyte Syndrome, have a defective promoter protein that does not attach to its target on a key MHC gene. As a result, the children do not make MHC protein.

This suggests that a small molecule like the decoy may indeed serve as a useful drug to prevent promoter protein from attaching to MHC genes, thereby stopping the encoded proteins from being produced from the genes and making the donor organ a less likely candidate for rejection.

CONTACT: University of California, San Francisco 

Jeff Norris, 415/476-2557