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Bone and joint disease

Treatments for osteoporosis and inflammatory diseases such as rheumatoid arthritis, ankylosing spondylitis, Sjogren's syndrome and lupus are all being developed using animals. These diseases and their complications affect millions of people.

Osteoporosis and related conditions
Rickets
Osteoarthritis
Inflammatory joint disease
Modifying the immune response
Protection and prevention
Rheumatoid arthritis
Ankylosing spondylitis
Lupus
Sjogrens syndrome
More Information
References

Osteoporosis and related conditions

Osteoporosis is a common condition of elderly men and women. It is caused by lack of load-bearing exercise, ageing, and oestrogen deficiency. As oestrogen is manufactured in fat, it affects thin people more. Bones become thin, as there is a shortage of the organic scaffolding that holds calcium. Women patients can be treated with oestrogens, and both sexes are advised to take load-bearing exercise such as walking and running, but this is often not feasible and sufferers have to cope with spinal deformity (the dowager’s hump), increased risk of fractures, and pain.

Research on sheep showed that 20 minutes a day of mild vibration to their back legs for a year was harmless and significantly increased their bone density, and this finding could be used to improve the bone structure of osteoporotic patientsANCHOR. Researchers have identified a gene, Alox15, which produces an enzyme  affecting skeletal development in mice. It seems to stimulate a pathway that favours more fat deposition but less bone formation, and an experimental drug that blocks the pathway reduces bone lossANCHOR.

Amylin, a hormone made by the same pancreatic B-cells that produce insulin, prevents bone loss, and explains why people with early-onset diabetes are susceptible to osteoporosis, a finding that has therapeutic implications. Amylin-deficient mice have lowered bone weightANCHOR.

In 2003 it was suggested that there is an immunological component to osteoporosis, at least in mice. Oestrogen-related bone loss is associated with too many T-cells in the blood, and this is associated with oestrogen deficiency. T-cells produce tumour necrosis factor, which increase the number of osteoclasts, cells that break down bone (bone undergoes a constant cycle of formation and breakdown, but until recently no-one knew how this was regulated). In turn, T-cell regulation is kicked off by an immune system protein called interferon gammaANCHOR.

Rickets

Hypophosphataemia is an inherited form of rickets found in boys. It is caused by excessive phosphate excretion by the kidney, and a similar disease occurs in mice. Researchers have found that indomethacin, a widely-used anti-inflammatory painkiller, prevents this phosphate loss, and the drug is already being tested in children with the diseaseANCHOR.

Osteoarthritis

Osteoarthritis is a common condition in older people and is the most common reason for hip and knee replacement. It also causes painful swelling of the fingers. It has a genetic component and is more common in women. The earliest indication of the disease is a gradual loss of large molecules called proteoglycans from the surface of the joint cartilage resulting in a decrease in the mechanical strength of cartilage. At the same time, other cells proliferate and form clusters. Then cracks in the cartilage gradually develop and the cracks are filled with fibrous tissue, probably as a result of unsuccessful attempts by the cartilage cells to repair the cracks. Finally, bony structures, called osteophytes, are formed at the periphery of the joint. The end result is loss of joint function.

Using naturally occurring mutant mice with a defective collagen gene, scientists have identified a signalling molecule involved in osteoarthritis.

Although the causes of osteoarthritis are diverse, mutations in two types of collagens, type IX and XI, are linked to early-onset osteoarthritis. Mice with a mutation in type XI collagen have age-related osteoarthritis and scientists found that these mice had increased amounts of the protein called DDR2 in their knee joint cartilage. Inhibitors of DDR2 signalling may be useful as arthritis drugsANCHOR.

In March 2005, two research groups showed that shutting down a single gene can prevent cartilage destruction in a genetically modified mouse with osteoarthritis. During the human disease, the cartilage in the joints gradually breaks down and a key component of cartilage called aggrecan, which helps the tissue bear load and resist compression, is chewed up by enzymes of the ADAMTS or aggrecanase family. Mice that lack a part of one such enzyme, ADAMTS5 (aggrecanase-2), are largely protected from cartilage destruction. The studies are the first to show that mutations in a single gene can halt cartilage degradation, and suggest that drugs designed to inhibit the human form of ADAMTS5 might help fight osteoarthritisANCHOR ANCHOR.

Inflammatory joint disease

The inflammatory joint diseases include rheumatoid arthritis, ankylosing spondylitis, Sjogren’s syndrome, and lupus. There are some features and treatments that are common to all of these conditions. With the exception of ankylosing spondylitis, women are more often affected than men. Skin and other organs are often affected, and the conditions are now understood as being caused by autoimmunity – that is, the body attacking itself. All of these conditions have a genetic component. More specific details of research in these conditions are detailed below.

Modifying the immune response

Both types of immune cells, called B-cells and T-cells, take part in the attack on oneself that forms the autoimmune response. As these are also essential for fighting off infection, it is hard to damp them down without doing harm. In 2001 scientists found a molecule that does that in mice, and which works on both types of immune cellANCHOR. Another promising line of research comes from understanding and modifying T-cells, the white blood immune cells that attack joints inappropriately. This can be prevented in mice by blocking the cells’ response to low oxygen levels by inactivating a protein called HIF-1, a finding that could lead to the development of new treatmentANCHOR. Another approach consists of making a ‘magic bullet’ that carries a toxic drug, doxorubicin, and targets only those T-cells that attack the jointsANCHOR.

Research on mice explains why the severity of inflammation varies between individuals. Different types of a protein called RAGE (receptor for advanced glycation end-products) exist in joints, and RAGE binds to other proteins called S100/calgranulins. Inflammation can be greatly decreased by preventing this binding. RAGE may also be associated with other autoimmune genesANCHOR.

Protection and prevention

A natural constituent of cannabis, cannabidiol, which has no effect on mood or consciousness, may be useful for treating inflammatory joint disease; scientists have found that it protects mice from severe joint damage by suppressing key parts of their immune responseANCHOR.

Rheumatoid arthritis

One line of research comes from studying the joints of patients with rheumatoid arthritis (RA), which have excessive amounts of an enzyme called synoviolin. When mice were genetically engineered to overproduce this enzyme they got the disease, and when they were engineered to underproduce it, they were protected from RA. Overproduction also impaired a process called apoptosis, whereby damaged or unwanted cells self-destructANCHOR.

A study in rats found that a drug, M40403, mimicking superoxide dismutase (SOD), a naturally occurring compound in the body, halved the damaged to joints and reduced the amount of inflammation by over 70%ANCHOR.

Injection of an antibody called anti-CD40 controls progression of the disease in mice with RA, and reduced pain and swelling. The antibody normally boosts the immune system, so the discovery is puzzlingANCHOR. Another gene, that makes interleukin-13, when injected into mouse joints, reduced inflammation in RA mice and prevented it in normal miceANCHOR.

About 1 in 40 RA patients has a mutation in a gene called ZAP-70. Mice with this mutation have a type of T-cell that is normally lost during development, and suffer from RA, so studying them might help some patientsANCHOR.

One of the more important discoveries in recent years is the finding, in 2001, that injections of a hormone called vasoactive intestinal peptide or VIP prevented joint swelling and destruction in mice, and the benefit stayed for some time after stopping injectionsANCHOR. This treatment is now being studied in human RA patients.

The most effective therapy in mice, to date, is the discovery in 2004 of an experimental agent that blocks the CD139 protein on the surface of T-cells and thereby decreases the production of destructive antibodies that damage the joint. Clinical trials are awaited with interestANCHOR.

Ankylosing spondylitis

Ankylosing spondylitis is an inflammatory disease of the tendons that link the vertebrae together. These gradually become ossified and the spine collapses, causing deformity. Patients often suffer additional arthritis in their arms and legs, and have disease in other organ systems. The disease is strongly genetic, but often triggered by infections. Mice with a defective copy of a gene called ank have a similar disease and it has now been discovered that this defective gene causes a shortage of a chemical, pyrophosphate in the joints. Humans have a similar gene, and the finding may lead to new approaches to treatmentANCHOR.

Lupus

Lupus affects skin, joint, blood vessels and other organs and tissues. It has a genetic component, is often triggered by viruses, and can be life threatening. A similar disease occurs in a strain of mice, and recently – from 2003 onwards – this has generated results that have greatly increased the understanding of human disease.

There is a strain of mice with lupus, and these have been shown to lack an enzyme called Dnase1, which removes rubbish from cellsANCHOR. Removal of an immune-system signalling protein, SLAM-associated protein, or SAP, also causes lupusANCHOR. In 2001 a lupus-causing gene was identified in mice. It causes a fault in an enzyme called alpha-mannosidase IIANCHOR. Scientists now need to know more about how these substances interact.

Scientists have used gene therapy to prevent the development of lupus in mice by boosting levels of an immune system component. A receptor gene known as  Fc acts like a gatekeeper, helping to maintain a healthy immune system instead of one that turns on itself, as is the case in autoimmune diseases.

In mice and in humans with lupus, production of the Fc receptor is reduced. Gene therapy reversed this and prevented the disease in a lupus-susceptible strain of mice, but not in untreated mice. The scientists expect their finding to apply to humans, since there seemed to be no serious adverse effects from the therapy in mice25.

A protein molecule that interferes with the site where antibodies act reduces the mortality in mice with genetic lupus from 80% down to 10%, and virtually eliminates kidney damageANCHOR, and a tranquilliser-like drug, Bz-423, reduces the incidence of renal complications by 85%ANCHOR, as does an anti-cancer drug, SAHAANCHOR.

Lupus patients often have neuropsychiatric problems, and their memory is sometimes affected. Mice with lupus have shown why this might be: the immune system sends defensive antibodies into the brain and selectively kills memory cells. This effect can be blocked by the drug memantine, which is used for Alzheimer’sANCHOR.

Sjogrens syndrome

Patients with Sjogren’s syndrome not only have joint symptoms but also have dry eyes and insufficient saliva because the body’s immune system attacks moisture-producing glands. By studying a strain of mice with the disease, scientists have identified the protein, ICA69, which causes it. By removing the gene that produces the protein, scientists removed the tear gland disease and reduced the salivary disease. They have now produces a vaccine that stops disease progressionANCHOR.

More Information

National Osteoporosis Society

Arthritis Care UK
Arthritis Research Campaign
National Ankylosing Spondylitis Society

May 2005


References

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