Scientists Uncovering Mechanisms Of Rheumatoid Arthritis

Affecting more than 2 million Americans, rheumatoid arthritis (RA) remains an enigma. The chronic swelling and pain brought about by RA can be debilitating in severe cases, and, as is true of most autoimmune diseases, the mechanisms and risk factors that influence onset are poorly understood. But long-suffering patients and eager biotech companies need not give up hope; recent insights into the autoimmune response and a new project that aims to ferret out genetic risks for the disease promise to open up new research and therapeutic approaches. PROGRESS: Immunex's Mike Widmer says that "we have now come to understand at least one molecule involved in the pathogenesis" of rheumatoid arthritis. CLINICAL RESEARCH ABOUNDS: Alabama's Larry Moreland notes that at least 60 drugs for rheumatoid arthritis are in industry pipelines. Although genetic causes have long been suspected, researchers trying to pin down the risk genes have been frustrated so far. Work in the mid-1980s by Gerald Nepom, currently the director of the Virginia Mason Research Center in Seattle, made it clear that various alleles of major histocompatibility complex (MHC) genes confer susceptibility to RA (G.T. Nepom et al., Proceedings of the National Academy of Sciences, 80 :6962-6, 1983; Lancet, 2 [8514]:1002, 1986; Arthritis and Rheumatism, 32 :15-21, 1989; this work builds on P. Stastny, New England Journal of Medicine, 298 :869-71, 1978). MHC proteins are presented on the surface of cells along with foreign antigens in order to alert the immune system to the presence of an invader. But studies of twins indicate that more risk genes must be involved: The concordance rate among identical twins-that is, the likelihood that a second twin will develop RA if the first one is afflicted-is higher than the concordance rate among siblings who have identical copies of the MHC risk genes. Researchers are therefore convinced that somewhere lie additional risk factors, as yet unrevealed. In early September, the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) and the Atlanta-based National Arthritis Foundation announced the launch of the North American Rheumatoid Arthritis Consortium (NARAC). The 12-center screening program will collect samples from siblings with RA and use genetic markers to search for new candidate genes. "Any gene that contributes to the progression of RA is automatically a therapeutic target," says Nepom, whose center is not participating in the consortium. "We're going to come up with drug targets and candidate therapeutics that we aren't even thinking about now." The centers will collect medical information and blood samples from siblings with RA and their parents. They will send them to the North Shore University Hospital in Manhasset, N.Y., where lead investigator Peter Gregersen and his team will scan for regions of DNA shared by RA-affected siblings at a suspiciously high frequency. Mendelian genetics predicts that siblings will share the same sequence in a given region 50 percent of the time-but if they consistently share a region at some higher frequency, it's a good bet that the region contains an RA trouble spot. To pinpoint the regions, the team will scan for shared repeating base units-microsatellite markers. The number of repeats in these sequences is inherited but also varies widely within a population, giving researchers a method of tracking inheritance within a family. If the number of repeats is identical in each sibling more than 50 percent of the time, chances are that both are inheriting some trait in the vicinity of the marker that is influencing their susceptibility to RA, explains Gregersen. If an affected parent also has the same number of traits, the case is even stronger, since that parent likely passed the trait on to both siblings. Homing in on the genes themselves will take more refinement, according to Gregersen, since the microsatellite markers are spread about 15 million to 20 million base pairs apart, a mere drop in the 3-billion-base-pair bucket that is the human genome. But he expects that the detailed maps and increased numbers of markers generated by the Human Genome Project will make that job easier in the near future. The task-to consider 350 markers within 1,000 sibling pairs and a parent, or 3,000 people total-seems monumental. The shotgun approach taken by the project has its critics, but Gregersen sees it as the first step in getting at the complex genetics of RA. "People are critical of the obvious thing to do. They want the elegant, insightful experiment-that's going to come later." For a thorough review on this topic, see the entire issue of Rheumatic Disease Clinics of North America, 18(4), 1992, published by W.B. Saunders Co. of Philadelphia (www.wbsaunders.com), which contains a number of essays and review articles summarizing the field. American Association of Immunologists Executive Director: M. Michele Hogan 5,500 members 9650 Rockville Pike Bethesda, Md. 20814-3994 (301) 530-7178 Fax: (301) 571-1816 http://www.scienceXchange.com/aai/ Journal: Journal of Immunology American College of Rheumatology Executive Vice President: Mark Andrejeski 6,000 members 60 Executive Park South, Suite 150 Atlanta, Ga. 30329 (404) 633-3777 Fax: (404) 633-1870 http://206.65.85.201 http://www.rheumatology.org/educ/nm97/nm97.html Journals: Arthritis and Rheumatism, Arthritis Care and Research American Society of Human Genetics Executive Director: Elaine Strasf 6,000 members 9650 Rockville Pike Bethesda, Md. 20814-3998 (301) 571-1825 Fax: (301) 530-7079 http://www.faseb.org/genetics/ashg/asgmenu.htm Journal: American Journal of Human Genetics Arthritis Foundation 1330 W. Peachtree St. Atlanta, Ga. 30309 (404) 872-7100 Fax: (404) 872-8694 http://www.arthritis.org Siblings diagnosed with rheumatoid arthritis who wish to participate in the NARAC screening program are requested to call the coordinating center at North Shore University Hospital, (800) 382-4827. Centocor Inc. http://www.centocor.com Immunex Corp. http://www.immunex.com Cambridge, Mass.-based Millennium Pharmaceuticals Inc. is combining a similar human genetics study-including 200 affected families-with genetic libraries derived from two types of "helper" T lymphocytes, Th1 and Th2. The two cell types influence different arms of the immune response-with Th2 cells more directly involved in RA-and they differ in which genes they express. Some 1,600 genes distinguish them, and Millennium has created arrays for both cell types using cDNA from every known variety of cell in the two classes. To the arrays, researchers then hybridized mRNA from Th1 or TH2 cells derived from a number of RA patients to determine, first of all, which genes are up-regulated or down-regulated as compared with normal cells. With 1,000 RNA samples collected from patients, animals, and cell cultures, the company can determine which of these differentially expressed genes are common to all or most of the RA patients or animal models. That cuts the list of candidate genes down to about 100, notes Carlos Gutierrez-Ramos, director of the inflammatory diseases division at Millennium. Researchers there are in the process of "knocking out" each gene individually using antisense or antibodies and studying the impact on the cell's function. They hope to home in on four or five candidate genes that control a crucial immune response function and that play a pivotal role in disease progression. "The goal is to find the important genes as soon as possible in the discovery process," says Gutierrez-Ramos. Although he applauds the mission behind NARAC and agrees that the shared linkage studies are valuable, he is concerned about putting exclusive emphasis on the approach. "Human genetics can be deceiving," he says, noting that a costly, exhaustive, five-year search for a susceptibility gene could turn up a candidate whose biopathways are too poorly understood to be of much use to drug developers-or, worse, the process might yield a known gene that has been well studied already. "At Millennium, we believe that it's best to combine" genetics and expression studies, he contends. CLEAR AIM: Virginia Mason Research Center's Gerald Nepom says that "any gene that contributes to the progression" of rheumatoid arthritis is a therapeutic target. Millennium's approach is more limited than that of NARAC, focusing as it does on genes that have an obvious-and well understood-impact on the immune response. The NARAC study is designed to find targets heretofore unknown. "Cytokines, kinases, phosphatases, transcription factors-[there are] lots of possibilities," notes Nepom. "That's why we need a whole-genome approach." He adds that looking for specific candidate genes "would just be guesses, whereas [NARAC] will take a very unbiased, whole-genome look at it." Nepom likes Millennium's project, too, noting that it "is not strictly a random, guesswork, 'candidate gene' approach," since the company pares down the potential genes using the differential expression arrays. "Theirs is a more sophisticated candidate gene strategy than the usual one," he says. And in case anyone believes the genetics behind RA isn't complicated enough, Nepom and others maintain that environmental factors-in the form of viral infections-likely play a role. Since the MHC molecules escort viral antigens to the surface of the cell-thus provoking an immune response from passing T cells-Nepom believes that autoreactive T cells might be amplified in response to a risk allele's popping to a cell's surface with an antigen in tow. "It's like turning up the gain on an amplifier," Nepom says. He adds that infections may "change the likelihood that the susceptibility gene will lead to a disease outcome" (E.D. Harris, Jr., "Etiology and Pathogenesis of Rheumatoid Arthritis," in W.N. Kelley et al., eds., Textbook of Rheumatology, 4th ed., Philadelphia, W.B. Saunders Co., 1993). Fortunately for RA patients, researchers are not as stymied as the clouded genetic picture might lead one to believe. Most of the drugs available today were developed empirically, having started out with different indications: Methotrexate (Rheumatrex, from Wyeth-Ayerst Laboratories of St. Davids, Pa.) reduces swelling and pain in affected joints but began clinical life as a cancer drug; the nonsteroidal anti-inflammatories relieve joint swelling and pain but are more commonly used for osteoarthritis and soft-tissue pain. Methotrexate's nausea and the gastrointestinal problems caused by the nonsteroidal anti-inflammatories have left rheumatologists still searching for an ideal treatment. But a new class of drugs may be just around the corner. These developments stem from the recent discovery that tumor necrosis factor (TNF)-named for its ability to kill tumor cells in mice-plays a key role in the events that lead to destruction of the joints. Once autoreactive T cells have been "primed" by contact with a foreign antigen, they release several cytokines-including TNF, interleukin-1 (IL-1), and others-that stimulate other immune cells to attack joints. TNF occupies the top of this pyramid, directing amplification and activation of T cells and attracting other immune cells to the area. It even triggers the release of collagenase enzymes that attack connective tissues directly (J. Saklatvala et al., Journal of Experimental Medicine, 162 :1208-15, 1985; J.M. Dayer et al., J. Exp. Med., 162 :2163-8, 1985). IL-1 plays a similar role in the immune cascade, but studies have shown inhibition of TNF shuts down production of IL-1 (F.M. Brennan et al., Lancet, 2 [8657]:244-7, 1989). Not surprisingly, experiments by Malvern, Pa.-based Centocor Inc. showed that TNF administered to mice could eliminate RA symptoms. Its lofty perch has made TNF a popular target among drug developers, and two approaches are nearing submission to the Food and Drug Administration (FDA). Seattle-based Immunex Corp. has developed a soluble TNF receptor (called ENBREL) that ties up free TNF, apparently down-regulating the immune response in RA patients. In September, the company announced results of a Phase III clinical trial that showed statistical improvements in pain and swelling. The data back up the results of a Phase II trial (L.W. Moreland et al., New England Journal of Medicine, 337:141-7, July 17, 1997). Centocor's humanized anti-TNF antibody (cA2) also shows good efficacy in clinical trials, having recently passed a similar Phase III test. Both companies plan to file applications with FDA next year. Though some researchers express concern that cA2 could draw its own immune response-and have its effectiveness diminished-the company claims it has seen no sign of diminished effectiveness in clinical trials. Both companies concede that blocking TNF-though apparently effective-is not the whole answer to therapy. "We have now come to understand at least one molecule involved in the pathogenesis of RA," states Mike Widmer, vice president of biological sciences at Immunex. "With that understanding comes the desire to examine what happens when you block its effect." Resulting changes in the activity of IL-1 and other cytokines could interfere with future immune responses-placing a high priority on research to unveil these relationships. At least 60 anti-RA drugs are in pharmaceutical and biotech pipelines, according to Larry W. Moreland, an associate professor of medicine at the University of Alabama at Birmingham, who has conducted a number of clinical trials with drugs that target TNF. "Right now there is more clinical research going on with arthritis patients than has gone on for years," says Moreland. Future Prospects Richard McCloskey, Centocor's vice president for medical affairs and disease management, predicts that in the near future, "there will be several classes of TNF [antagonists] . . . and possibly other antibodies aimed at other parts of the cytokine cascade." He also expects to see small molecules aimed at the signaling mechanisms of cytokines. Widmer has a similar vision of the future: "It is quite clear that in the next five or so years the medical profession will have ways to treat RA that were not available before, and some will be of startling efficacy," he says. Moreland sees many of these therapies curbing the actions of the specific cytokines or immune mechanisms, but he suspects many previous RA therapies have failed because they are circumvented by the action of other cytokines. "I'd like to be able to use IL-1, TNF, and [protease] inhibitors together . . . but we won't be able to find out until all these drugs are approved whether they will work better in combination," he says. Perhaps the biggest hurdle, Moreland believes, will be in finding quality clinical investigators to run the trials. There are so many new therapies competing for subjects that it might be difficult to attract enough patients to do a well-designed trial. "[Companies] can find good investigators to help out with that," he says. As for genetic studies, Gregersen sees a bright future. NIAMS has already committed to similar linkage studies for lupus and juvenile rheumatoid arthritis, and other autoimmune diseases are in the works. "I think there is an increasing appreciation that genetics offers a powerful way to get at these things." The NARAC project could also help sort out some of the inevitable confusion arising from new therapies. The presence of specific risk genes-and even regions associated with more severe forms of the disease-could help rheumatologists select appropriate therapies. Centocor's McCloskey, a practicing rheumatologist, looks forward to that day: "I would be very anxious to have some method of deciding which of the new therapies to use." As genetic factors are uncovered, and more details of the autoimmune cascade emerge, the result is likely to be heightened research efforts and more jobs in arthritis R&D. FDA approval of new therapies in the next few years won't hurt either, says McCloskey, applauding the emergence of RA research from the dark days of recycled cancer drugs and anti-inflammatories: "People get tired of doing research on things they can't do anything about." James Kling, a science writer based in Bellingham, Wash., can be reached online at jkling@nasw.org.

Scientists Uncovering Mechanisms Of Rheumatoid Arthritis

Become a Member of

Meet the Author

James Kling

Published In

October 1997

Related Research Resources

Bunsen Burners and Bad Hair Days

Research Resources

Podcasts

Webinars

Videos

Infographics

eBooks

Taking the Guesswork Out of Quality Control Standards

PFAS: The Forever Chemicals

Unlocking the Unattainable in Gene Construction

Comparing Analytical Solutions for High-Throughput Drug Discovery

Products

Product News

Thermo Scientific™ Centrifuges with GreenCool Technology

Singleron Biotechnologies and Hamilton Bonaduz AG Announce the Launch of Tensor to Advance Single Cell Sequencing Automation

Zymo Research Launches Research Grant to Empower Mapping the RNome

Cytosurge Appoints Magid Haddouchi as Chief Commercial Officer