Gene Discovery: The Giant First Step Toward Therapy

After a grueling search, the gene causing a disorder is identified. Now what? While hope for rapid development of a cure fixes itself in the public's mind, in the scientific sector, discovery of a health-related gene catalyzes a frenzy of research activity, geneticists say. The first steps following gene discovery have become somewhat routine: completing sequencing of the gene, matching the sequence to a known protein pattern (if possible), deciphering gene expression through tissue and time, and finding new mutations. Then, the direction of research branches. Many geneticists working with newly discovered genes are looking to developments in cystic fibrosis (CF) research for guidance. Consider a sampling of current CF research efforts: To Screen Or Not To Screen At Imperial College in London, geneticist Robert Williamson is evaluating the advisability of screening the general population for CF carriers (R. Williamson, Nature Genetics, 3:195, 1993). In the United States, the American Society of Human Genetics (ASHG) cautioned against general screening just months after the gene's discovery, advising the test only for those with affected relatives. In March 1990, a National Institutes of Health panel echoed that advice, which was reaffirmed by ASHG in December 1992, after nearly 200 variants of the CF gene had been identified. For now, CF carrier testing a "cheekbrush test for the 16 most common mutations, costing under $100 is suggested for affected families only. Information The Office of Technology Assessment published a 300-plus-page report, "Cystic Fibrosis and DNA Tests: Implications of Carrier Screening" in August 1992. And genetic counselors, who deal directly with the interface between research and people (Ricki Lewis, The Scientist, Aug. 31, 1992, page 1), are preparing patient information pam- phlets, such as "For Adults with Cystic Fibrosis: Facts on Reproduction," by Amy A. Lemke, a genetic counselor at the University of Chicago. Animal Models A milestone in CF research was the creation of a "knockout" mouse, in which the animal's version of the CF gene is inactivated by gene targeting, or swapping a nonfunctional gene for a functional one (J.N. Snouwaert, Science, 257:1083, 1992). Researchers like University of North Carolina, Chapel Hill's Richard Boucher, Oliver Smithies, Beverly Koller, and colleagues can now observe the effects of complete absence of CF gene activity. The mice are small and die prematurely of intestinal blockage reminiscent of that seen in human newborns who have CF. Surprisingly, the knockout mice have normal pancreases, livers, and male reproductive tracts, all structures that are impaired in the human disorder. The mice also lack the lung infections that make the disorder life-threatening in people. Scientists have also created transgenic mice bearing the human CFTR gene, and evaluated the gene's expression in various tissues. "This mouse model is useful in considering the options for trying to develop ways to introduce the gene or protein into patients," says Richard Gregory, principal scientist at Genzyme Corp. of Framingham, Mass. Preimplantation Diagnosis In March 1993, Chloe O'Brien celebrated her first birthday. Chloe was tested for the CF gene that each of her parents carries when she was an eight-cell pre-embryo. When a DNA probe revealed that she was free of the abnormal gene, she was carried to term, as was another pre-embryo found to be a carrier (A.H. Handyside, et al., New England Journal of Medicine, 327:905, 1992). New Treatments Once the CFTR protein was known, new treatments began being tested. Amiloride, a drug approved in 1967 as an adjunct to diuretic drugs, is being tested in CF because it thins lung secretions by blocking sodium uptake. An enzyme familiar to geneticists, deoxyribonuclease, clears lungs by degrading DNA from white blood cells that clog infected lungs. The nucleotides ATP and UTP are also being evaluated as CF treatments because they stimulate chloride secretion from lung cells. Gene Therapy In December 1992, the NIH Recombinant DNA Advisory Committee approved three gene therapy protocols to introduce a functional CFTR gene into the respiratory tracts of patients via viral vectors. The trials are currently being conducted by Ronald G. Crystal at Cornell University Medical College in New York, James Wilson at the University of Pennsylvania in Philadelphia, and Michael Welsh of the University of Iowa in Iowa City (J.F. Engelhardt, et al., Nature Genetics, 4:27, 1993; L.G. Johnson, et al., Nature Genetics, 2:21, 1992). --R.L. Thanks to the recent rush of discoveries of disease-inducing genes, opportunities for researchers in human genetics and the many scientific cottage industries it spawns are more numerous and diverse than ever. As DNA double-helix co-discoverer James Watson put it at a recent news conference, the '90s are "a golden age of human genetics." Added to the genome map since January 1 are familial Alzheimer's disease (G. Schellenberg, Science, 258:668, 1992); Huntington's disease (Huntington's Disease Collaborative Research Group, Cell, 72:971, 1993); amyotrophic lateral sclerosis, known as ALS or Lou Gehrig's disease (D.R. Rosen, et al., Nature, 362:59, 1993); adrenoleukodystrophy (J. Mosser, et al., Nature, 361:726, 1993); and colon cancer (P. Peltomak, et al., Science, 260:810-11; L.A. Aaltonen, et al., Science, 260:812-15; S.N. Thibodeau, et al., Science, 260:816-18, 1993). But the headlines often overshadow the fact that identifying a disease-causing gene and its protein product is only a first, if monumental, step (see accompanying story). "It is important to recognize that the road from gene to therapy is as hard, if not harder, than the road to discovering the gene," says Michael Hayden, a professor of medical genetics at the University of British Columbia in Vancouver, whose report on several candidate genes for Huntington's disease (HD) appeared (Y.P. Goldberg, et al., Nature, 362:370, 1993) just a day before the paper in Cell pinpointed the gene. In some instances, the path to discovery itself has been easier for some genes than for others. "In the case of cystic fibrosis (CF), there had been a ton of work done on the physiology and biochemistry of that disease before the gene was discovered, and the gene research benefited from that," Hayden says. Other recent gene discoveries have been previously suspected, as well. Menkes' disease, a neurological deterioration that is fatal in childhood, was known to be a derangement in copper utilization in enzymes that need copper to function. It wasn't a great surprise when the gene product was found to be a metal-binding protein (C. Vulpe, et al., Nature Genetics, 3:7, 1993; J. Chelly, et al., Nature Genetics, 3:14, 1993; J.F.B. Mercer, et al., Nature Genetics, 3:20, 1993). For some disorders, the gene's protein product is a surprise, but one that makes sense. The revelation that ALS is caused by a mutation in a gene-encoding superoxide dismutase (SOD1) seems logical. This enzyme normally detoxifies free radicals, unstable chemicals that are implicated in other neurological disorders. "Finding that it was this enzyme led immediately to a new approach in treatments, using antioxidants," says Hayden. But, cautions geneticist Kevin Davies, editor of Nature Genetics, "We know that SOD1 causes ALS, but why is the damage selective to motor neurons, when SOD is vital everywhere? Is it overexpressed in the disease state or underexpressed? There is still a lot to be done." Davies did his doctoral work with Robert Williamson at Imperial College in London, where he pursued SOD1 as a contender for the CF gene. The protein that causes adrenoleukodystrophy (ALD), the disorder featured in the recent movie Lorenzo's Oil, was also a surprising discovery, but with a logical basis. The gene product was hypothesized to be VLCFA synthetase, an enzyme that degrades certain fatty acids in subcellular structures called peroxisomes. Instead, the cause is a protein that transports the enzyme. CF, Menkes' disease, ALS, and ALD have familiar gene products. And then there is Huntington's disease. The DNA sequence of the HD gene in its entirety resembles no known gene sequence. The mutant gene has extra copies of the DNA base triplet CAG. Such a genetic stutter has been found in only three other disorders and is not well understood. The unusual nature of the HD gene may be why 10 years elapsed between the discovery of a genetic marker (a DNA sequence near the disease-causing gene that is inherited along with it) and the actual gene. "There are an awful lot of proteins that we don't know about yet. So it isn't unusual to clone a gene and not know what it does," says Margaret Wallace, an assistant professor in the division of genetics at the University of Florida, Gainesville. As a graduate student, Wallace was part of the team that found the HD marker; as a postdoc, she discovered the gene behind neuro- fibromatosis type I (M.R. Wallace, et al., Science, 249:181, 1990). Adds Hayden, "There is nothing known about the [HD] protein, and this is a completely new area. A huge amount of basic research now needs to be done on the gene." Although no one knows precisely how many scientists are searching for human genes--Sharon Dunham of the National Center for Human Genome Research at the National Institutes of Health says such a figure is too difficult to track--signs of success and employment growth in the field are abundant. "The most visible spin-offs are the new private biotech companies," says Davies, speaking of genome startups created to bridge gene discovery and clinical applications. Some companies focus on a particular part of the genome. Myriad Genetics Inc. of Salt Lake City, Utah, for example, concentrates on cancer genes. Other companies, such as Palo Alto, Calif.-based Incyte Pharmaceuticals Inc., are amassing complementary DNAs (DNA sequences that are expressed in a particular cell type) for future gene discoveries. Providers of enabling technologies that support gene research are already abundant, including suppliers of polymerase chain reaction (PCR) reagents and equipment (Ricki Lewis, The Scientist, Dec. 9, 1991, page 23); manufacturers of sequencers; and research contract firms that specialize in gene manipulation methods, such as transgenic technology, gene targeting, and antisense technology. Many companies offer software to analyze gene and protein sequences (Ricki Lewis, The Scientist, May 31, 1993, page 17). A demand seems to be developing for those who can streamline a facet of the gene discovery process. This was the case for St. Louis-based Genome Systems Inc., created by geneticists David Smoller and Paul Goldwhat. They parlayed their expertise in constructing a DNA library--a collection of pieces of a genome, usually maintained in microbial cultures--into a much-needed service. "Genome Systems was founded to make genome technology accessible," says Smoller. "People send us a probe, and we send back a big piece of DNA. Our clients range from small academic labs to medium-to-huge companies to government labs." Many labs can afford to let someone screen their probes against a library because funding for genetic disease is often provided by government agencies as well as medical organizations. For example, the search for the ALS gene was supported by NIH, the Howard Hughes Medical Institute, the Muscular Dystrophy Association, the ALS Association, and the Les Turner ALS Foundation. Scientists are also stretching their research dollars by pooling resources. "Some problems are so big, and the stakes so high, that no one group can go it alone. With HD, it clearly helped to pool resources and talents," says Davies, referring to the Huntington's Disease Collaborative Research Group, an international team of 58 scientists working in six groups. "There aren't too many labs that know how to do everything--molecular genetics, biochemistry, in vivo assays. You have to collaborate," says Wallace. In tracking the ALS gene, data sharing was paramount, because inherited ALS affects only a few dozen families worldwide, with different teams of researchers working on different families. New academic gene therapy facilities may also help collaboration efforts. These include the University of Pennsylvania's Institute for Human Gene Therapy and the Gene Therapy Center at the University of California, San Diego. In some cases, the unpredictability of gene discovery has led to new alliances. "When we found that neurofibromatosis involved a GTPase, all groups whose main research was these proteins instantly could pick up and start working on it," says Wallace, who is characterizing different mutations in the gene. Several new publications fill the gap that has formed between established general journals that emphasize basic genetic research, such as Science and Nature, and the more clinically oriented outlets, such as the American Journal of Human Genetics. These include Human Mutation (New York, Wiley-Liss), Human Gene Therapy (New York, Mary Ann Liebert Inc.), Human Molecular Genetics (New York, Oxford University Press), Nature Genetics (Washington, D.C., Nature Publishing Co.), and, debuting next fall, Gene Therapy (Nature Publishing). Nature Genetics was created because editors at Nature noticed that papers that they had rejected for space reasons were ending up in other prestigious journals, such as Cell, according to Nature Genetics editor Kevin Davies. "Sometime in the fall of 1991, [Nature editor] John Maddox, [Nature deputy editor] Barbara J. Culliton, and I did some research and determined that the genetics community would relish the chance to publish in another good genetics journal," Davies says. "With all the excitement of genome programs, and more and more genes being isolated, specialty journals in genetics were becoming too few and too slow." Last month, when Johns Hopkins University geneticist Bert Vogel- stein announced that his group had found a key colon cancer gene, the news was reported on National Public Radio and the "Today" show before it appeared in Science. Such communication with the public is becoming more important to continued research support because citizens influence government funding; they also exert input as consumers of medical products that result from basic research. In addition to media scrutiny, today's geneticists must respond to anxious disease victims and their families. This can be difficult, especially if one's work isn't directly applicable to improving the quality of life for sufferers. Richard J. Gregory, principal scientist at Genzyme Corp. of Framingham, Mass., is part of a group--working with mice that have the human cystic fibrosis gene--who are attempting to delineate where, when, and to what extent different tissues require the gene's product. This knowledge is essential for implementing gene therapy (J.A. Whitsett, et al., Nature Genetics, 2:13, 1992). "Patients often call and ask, `When can I get this [gene therapy]?' " Gregory says. "It's a bit of a problem. It's hard when working at the bench to answer that kind of question." But at least for CF, with three gene therapy trials in progress, carrier tests available, and several new treatments spawned directly from the gene discovery in 1989 (J.M. Rommens, et al., Science, 245:1059; J.R. Riordan, et al., Science, 245:1066; B-S Kerem, et al., Science, 245:1073, 1989), answers are possible. HD lies at the other end of the spectrum in terms of diagnostic and therapy development, and researchers should be sensitive to that, says Hayden. "For HD, it is very important to dampen expectations for families," he says. "Otherwise, there is expectation, disappointment, and depression." Ricki Lewis, a freelance writer based in Scotia, N.Y., is the author of a biology text and a human genetics text to be published in November.

Gene Discovery: The Giant First Step Toward Therapy

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