But joints can take only so much stress. Damage resulting from injury, obesity, overuse, repetitive movements or improper joint alignment such as bowlegs or knock-knees can contribute to cartilage degeneration and the development of osteoarthritis—the most common type of arthritis. A leading cause of disability among adults, osteoarthritis accounts for billions of dollars in health-care costs and lost wages, not to mention considerable pain and suffering each year.

Researchers used to think that osteoarthritis, whose incidence increases with age, was a simple case of age-related wear and tear. Recent studies, though, suggest that the disease process is more complicated.

THE JOINT-GENE CONNECTION
In his laboratory and in the clinic at Case Western Reserve University School of Medicine in Cleveland, HHMI investigator Matthew L. Warman studies rare hereditary disorders that affect the joints. Through these studies, Warman hopes "to understand what the essential biological processes are that get a joint to last for an entire lifetime of use and not fail." Following that, the challenge is "to figure out how to intervene in these biological pathways to minimize the risk of joint failure in common joint diseases."

Once you find a [rare-disease-causing] gene," Warman says, "it gives you an entrée to a biological pathway." That pathway is also likely to be involved in maintaining healthy joints for all people.

There is strong precedent for using unusual genetic diseases as a starting point for insights into more common diseases, notes HHMI investigator David M. Kingsley, a developmental biologist at Stanford University School of Medicine. One of the strengths of the genetic approach, Kingsley says, is that "genetics is great for taking a complex problem and breaking it into manageable bits." Genetics also allows researchers to create model organisms for human diseases, which can be used to test potential treatments or explore the effects of various gene mutations on disease.

The study of rare genetic syndromes that cause severe heart disease in young children helped researchers identify the mechanisms and pathways that control cholesterol and lipid metabolism and the role that problems in these pathways play in adult heart disease. For example, Michael S. Brown and Joseph L. Goldstein's Nobel Prize–winning work on severe familial hypercholesterolemia, which results in abnormally high levels of cholesterol in the blood, led to new approaches for treating and preventing atherosclerosis. "We're at a very early stage in the arthritis field," says Kingsley. "But having seen the impact of genetic approaches in heart disease, I'm optimistic that a concerted attack on what may look like rare or more severe forms of arthritis … will help provide molecular targets that can be manipulated to make joints healthier or repair [damaged] joints."

THE PROMISE OF LUBRICIN
Warman's studies of the "one-in-a-million" inherited joint disorder known as camptodactyly–arthropathy–coxa vara–pericarditis syndrome, or CACP, exemplify how genetic approaches are providing new perspectives on what keeps joints healthy and how things can go wrong. CACP can cause joint deformities such as permanently bent fingers and symptoms similar to those of osteoarthritis, including stiff, painful and swollen joints and limited mobility. In youngsters with CACP, symptoms of joint swelling and stiffness begin in mid-childhood, and joints are destroyed at an early age. Cells in the synovium—the thin layer of tissue that lines the joint and produces the synovial fluid that nourishes cartilage—grow abnormally in these children. When joint symptoms become so severe that they limit the activities of everyday life, the only effective treatment is joint replacement surgery.

In 1999, an international consortium of researchers led by Warman found that mutations in a particular gene on human chromosome 1 cause CACP. Around the same time, other scientists, including Gregory D. Jay of Brown University, discovered that this same gene is switched on in human synovial cells and codes for lubricin, long thought to be a key joint lubricant. This convergence of research not only showed that inherited defects in lubricin could lead to damaged joints, but also underscored that lubricin is important for the health of joints generally.

"This protein is a major contributor to reducing friction in joints," Warman says. "When lubricin is genetically deficient, joints wear out from the surface down." In the future, he hopes, doctors may be able to treat CACP by replacing lubricin directly or through gene therapy.

To study lubricin in action, Jay—an emergency physician and bioengineering researcher—uses a friction apparatus that simulates the mechanics of abutting cartilage surfaces in a joint. His findings suggest that lubricin made and secreted by synovial cells and chondrocytes in the joint normally reduces friction and wear by coating the cartilage layer and "keeping the two surfaces apart at the nanoscale," he says. Warman likens the apparent effects of lubricin at the joint surface to that of Teflon on a nonstick frying pan. "Teflon firmly adheres to the metal underneath," he says. "It's not floating on the surface of the frying pan, like oil might be."

Results of recent studies in mice by Warman, John D. Carpten of the National Institutes of Health and Jay lend support to the importance of lubricin in joints. Mice in which the lubricin gene has been "knocked out" via genetic engineering have signs and symptoms like those of humans with CACP; studying them can lead to new insights into the protein's role in maintaining joints. For example, Jay found that there was increased friction in the limb joints of these mice. Warman's studies revealed that lubricin may also normally keep the brakes on synovial cell growth. In the absence of lubricin, Warman says, "we think the synovial cells … become much more aggressive and can potentially invade the cartilage surface," a phenomenon also seen in rheumatoid arthritis. This inflammatory autoimmune disease affects more than 2 million Americans, causing pain, swelling, stiffness and progressive loss of function in the joints.

Extrapolating from what goes wrong in CACP and from knowledge of how lubricin works, Warman says, it's not hard to imagine how acquired, nongenetic defects in lubricin might play a role in common joint diseases. He and Jay have joined forces to test the hypothesis that breakdown of lubricin by enzymes released in inflamed or injured joints, or diminished production of lubricin in aging joints, might contribute to joint damage in osteoarthritis and rheumatoid arthritis. If enzymes are chewing up lubricin, impairing its ability to keep joints moving smoothly or to curb uncontrolled synovial cell growth, drugs that inhibit these enzymes might help prevent disease. In addition, if lubricin levels are low, Warman says, doctors might someday be able to prevent or treat disease by injecting new lubricin into people's joints or giving them drugs that cause cells in the joint to pump out more lubricin.

Recent work in Jay's laboratory suggests that lubricin breakdown may play a role in the early stages of osteoarthritis that develops as a consequence of joint injury. As an emergency physician, Jay sees people whose joints are swollen with fluid as a result of sports injuries or other blows to the joint that could lead to osteoarthritis somewhere down the line. It turns out that lubricin in the fluid removed from these patients' joints is degraded and has reduced lubricating capacity. In addition, he says, their articular cartilage (cartilage in the joint) shows signs of early erosion. The injection of new lubricin into a joint after injury could help prevent arthritis from developing in patients like those Jay sees.

Interest in lubricin, a protein that "had not been on the radar screen a few years ago," is growing, Warman says. At this year's annual meeting of the Orthopaedic Research Society, an entire session was devoted to the biology of lubricin. Scientists described studies on the distribution of lubricin in osteoarthritic cartilage and the susceptibility of lubricin to be broken down by enzymes that may be found in arthritic joints, and they reported on efforts to tailor lubricin production and secretion in tissue-engineered cartilage being developed to repair damaged joints. Researchers also revealed that simulating natural joint motion in engineered articular cartilage increases lubricin expression by chondrocytes. This finding may not only enhance the quality of engineered cartilage but could also help explain why physical therapy involving passive motion is beneficial for injured and arthritic joints, Warman says.

TARTAR CONTROL
At his lab in Palo Alto, Stanford's Kingsley is also dreaming up new ways to use genetic approaches to understand joint maintenance and improve arthritis treatment. About three years ago he and his colleagues identified the gene responsible for a severe progressive form of arthritis in mice that shares many features with human arthritis, and they showed that a single mutation in this gene, known as progressive ankylosis (ank), causes the disease.

In mice with the ank mutation, crystals of calcium phosphate form in most joints, triggering inflammation and erosion of articular cartilage. "In the latter stages of the disease, the articular cartilage is heavily damaged," Kingsley says. "But even worse than that, [the joints] form osteophytes—bony struts that go from one bone to another all the way across the joint," making the joint rigid. By the time these mice reach six months of age, most of their joints are completely frozen, and they die.

Kingsley and his colleagues found that the ank gene codes for a previously unknown protein, ANK, which spans the cell membrane and is produced in articular cartilage and other tissues. Cell-culture and biochemical studies showed that the ank mutation leads to a drop in extracellular levels of a small molecule called pyrophosphate—the active ingredient in tartar-control toothpaste. Pyrophosphate is known to inhibit the formation of calcified mineral deposits typically found in tartar, and in the crystals that cause arthritis in ank mice. Kingsley's findings suggest that ANK normally provides the equivalent of tartar-control for the joints by stimulating transport of pyrophosphate out of cells and into joint fluid, where it acts to prevent crystal formation.

More recent studies indicate that defects in this tartar-control system also play a role in human joint disease. In the October 2002 issue of the American Journal of Human Genetics, an international collaboration led by Kingsley reported that mutations in the human version of the ank gene cause a rare hereditary form of a common joint disease known as chondrocalcinosis, or "pseudogout." Among people with this disease, which is less severe than the mouse disorder, calcium-containing crystals build up in the articular cartilage of some joints before age 40, causing pain and inflammation. "In the disease state, the ANK protein is either overly active, which stimulates one type of crystal formation, or it's not active enough, which triggers the formation of a different type of crystal. But in both cases, you end up with joint disease," Kingsley says. "ANK activity has to be within a window—too much may be bad, too little may be bad."

He notes that 60 percent of people with osteoarthritis also have an excess of one or both of these types of crystals in their synovial fluid. However, scientists have long debated whether this is a cause or an effect: Do the crystals play a role in triggering osteoarthritis or are they a by-product of other damage in the joint? In the case of the ank mouse, it is clear that the defect in crystal formation is the primary cause of joint disease in the animals, not a by-product of joint damage. Moreover, "regardless of whether crystal formation is a primary or secondary event," Kingsley says, "once the crystals are formed they can act in an amplification loop that probably increases the severity of disease."

For those with the genetic form of chondrocalcinosis, which appears to result from an overly active ANK protein, "a compound that inhibited ANK activity might be something you could give to prevent the formation of crystals and try to prevent that sort of joint pain," Kingsley says. What researchers don't yet know is whether changes in ANK activity play a role in the common, nonhereditary forms of chondrocalcinosis, or in osteoarthritis or other joint diseases. If this turns out to be the case, drugs that affect the protein's activity might be useful in these diseases as well.

As baby boomers age, the human and financial costs of osteoarthritis will only escalate. Still, as Roland W. Moskowitz, a rheumatologist at Case Western Reserve University and president of the Osteoarthritis Research Society International, says, "it may be a while" before safe and affordable treatments for this joint disease are available. Researchers are just starting to understand the complex interplay of factors involved in osteoarthritis, not to mention figuring out reliable ways to diagnose the disease in its earliest stages and measure its progression. Moskowitz may be disinclined to forecast immediate cures, but he has hope for meaningful treatments in the not-too-distant future. Of that promise, he says, "I think we can see light at the end of the tunnel."

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Photos: Daniel Levin, Kay Chernush

Images: Patrick Smits and Veronique Lefebvre; From Ho, A.M., Johnson, M.D., and Kingsley, D.M. 2000. Science 289:265-270. © 2000 by The American Association for the Advancement of Science

Reprinted from the HHMI Bulletin,
June 2003, pages 8-13.
©2003 Howard Hughes Medical Institute