Science: Conquering Inherited Enemies

When Chad Wilkinson, 10, is not strapped down in his padded bed, he must be bound to his wheelchair. Otherwise he would try to chew off his fingers or bite his arms. All his teeth have been removed so that he cannot gnaw through his lips and tongue. His body is racked with palsy, and he is mentally retarded. His parents can no longer take him to restaurants in their hometown of Allen, Texas, because he might yell out abusively or spit food at other diners. “You always pray for a miracle,” says his mother Valda. “But you don’t hold out much hope.”

Chad is a victim of Lesch-Nyhan syn-drome, a rare inherited disorder triggered by a defect in a single gene out of the 100,000 in his chromosomes. His cells do not manufacture hypoxanthine-guanine phosphoribosyl transferase (HPRT), an enzyme needed to metabolize basic compounds. As a result, crystals of uric acid have collected in his kidneys and joints, while his brain has decayed from a lack of HPRT, triggering his bizarre array of symptoms.

Until now, Chad and others like him have had few options for treatment beyond inadequate medication or risky therapies; most Lesch-Nyhan patients die before their 20th birthday. But that bleak picture may soon change. Genetic engineers at a handful of U.S. laboratories are getting ready to embark on the first trials of human gene therapy, a revolutionary approach to conquering inherited ailments. Employing the subtlest available techniques of recombinant DNA, the scientists will attempt to inject healthy copies of the affected gene into the bone-marrow cells of a victim of a genetic disorder. If all goes well, the good genes will begin producing enough of the missing enzyme to cure the disease. That will be cheering news for the hundreds of thousands of patients who suffer from the 3,000 known genetic disorders. Says Ihor Lemischka, a researcher at the Whitehead Institute for Biomedical Research in Cambridge, Mass., one of the several labs involved: “It will be a scientific tour de force of unparalleled proportions.”

Anticipating a blizzard of activity in the next several months, a subcommittee of the National Institutes of Health (NIH) has approved guidelines for physicians during the initial gene-therapy trials. The directives are posed as questions to doctors considering the experimental procedure. Among them: Is the illness selected for treatment severe enough, and is the new approach likely to be better than conventional efforts? Are the risks to the patient tolerable? Do the patients and their families understand what the ordeal will entail? Explains guidelines Panel Chairman LeRoy Walters of Georgetown University’s Kennedy Institute of Ethics in Washington, referring to the infant who received a transplanted baboon heart: “We wanted to avoid future Baby Faes.”

Scientists are also eager to avoid blunders, and they have worked arduously to smooth the way for tests on humans. Among the most difficult tasks has been designing an efficient delivery system, or vector, to get the interloping DNA into cells. An ideal cell type to target for combatting disease is the so- called stem cell, an immature marrow cell that further divides into white and red blood cells, including the many “housekeeping” cells responsible for producing critical enzymes. Stem cells, however, are the rarest and most elusive cells in the marrow, so scientists must settle for getting their foreign genes into all the cells in a patient’s marrow sample and thereby hope to hit a few stems. To accomplish such widescale insertion, biologists have enlisted the aid of nature’s most successful, if fearsome, invader: the virus. “The virus has solved the problem of infecting cells,” says William Nyhan of the University of California, San Diego, one of the researchers who identified the Lesch-Nyhan syndrome. “That’s how it does business.”

Working with viruses, however, is tricky: the germs must be made complete-ly benign before being used as DNA carriers. One master manipulator of viruses is Richard Mulligan. In a technique employed at the Whitehead Institute, Mulligan commandeers a mouse leukemia virus, chemically carves out its dangerous ability to replicate and replaces it with a human gene and the appropriate signals to regulate it. That construction is then mixed with a kind of helper cell, which assists in turning the transformed leukemic virus into a functioning vector.

Next, several million marrow cells are removed from mouse bones and incubated with the viral vector; within 48 hours the viruses have deposited their genes in the nucleus of every cell. The mouse is then irradiated to destroy its resident marrow cells, leaving plenty of room for new marrow growth. Only two weeks after the altered cells have been reinjected into the mouse, the stem cells among them have divided and begun to repopulate the bone. One question that remains: Are the new genes expressed, or turned on, in the living mice? “We’re not there yet,” admits David Williams of Harvard Medical School and a collaborator with the Whitehead group. “But we’re getting close.” Other laboratories are at more or less the same stage. Theodore Friedmann of the University of California, San Diego and Inder Verma of the Salk Institute for Biological Studies in La Jolla, Calif., have been using mice to study HPRT genes. Perhaps most dramatic of all, W. French Anderson of the NIH has begun similar gene-transfer experiments on four rhesus monkeys. He has removed a portion of their bone marrow, added human genes to the cells, irradiated the monkeys and reinserted the engineered cells. If his tests are successful, the procedures will be almost directly applicable to human beings.

For the first targets of gene therapy, many researchers have singled out three rare diseases that affect on the order of one birth per 100,000. In all cases the genes have already been isolated and extensively studied. What is more, biologists hope that for at least two of the disorders, only a tiny amount of the enzyme need be produced to alleviate the worst symptoms. The & likeliest candidates: Lesch-Nyhan syndrome; adenosine deaminase deficiency, an immune disorder of the type that killed the famous “Bubble Boy”; and purine nucleoside phosphorylase deficiency, another illness of the immune system. As investigators learn ever more about gene regulation, however, they may tackle ever more complicated hereditary diseases, including sickle-cell anemia, diabetes and even cancer. Meanwhile, the increasing genetic adeptness of researchers worries some observers, who fear the application of recombinant DNA to human beings. Some extreme critics even evoke visions of Hitlerian attempts to engineer a new master race. Most scientists dismiss these fears, pointing out that the new therapy will be used only on patients’ body cells; it will not alter their sex cells, and hence cannot affect future generations. “It’s not a way of altering the genetic pool,” says A. Dusty Miller of the Fred Hutchinson Cancer Research Center in Seattle. “It’s just a novel and clever way of administering a drug.” Of course, it is just of such novel and clever tricks that miracles are often made.

All advances in gene therapy depend on having the gene in hand, ready for insertion into a cell. But isolating that gene can be extremely difficult. As a result, only a few hundred of the 100,000 human genes have been separated and cloned.

Scientists, however, have been making remarkable progress in a technique that can sharply focus their search for a desired gene. Known as restriction fragment length polymorphism (RFLP), the method relies on enzymes that slice DNA in distinctive patterns; families with a history of a genetic disease will tend to have similar configurations, permitting scientists to zero in on the likeliest site of the offending gene. In recent weeks biologists have announced the discovery of RFLP distinctive patterns, or “markers,” for cystic fibrosis, which afflicts about 30,000 Americans; cardiovascular disease susceptibility; polycystic kidney disease; and muscular dystrophy. Says Manuel Buchwald of the Hospital for Sick Children in Toronto, one of the co-discovers of the cystic fibrosis marker, “It’s the first handle we have on the disease.”

The latest batch of DNA markers will serve as important stepping-stones to the isolation of genes. But RFLPs can only approximate the gene’s position; biologists must progressively snip away the intervening fragments before they can fish out the gene and begin manipulating it. At least one marker, however, may be immediately useful. John Baxter of California Biotechnology, the firm where the heart-disease markers were found, believes the RFLP could help in alerting people to their tendency in time to change their behavior. Warns Baxter: “If you have this marker, it’s equivalent to having blood pressure of 190.”