Non-Oncology Applications: 11 of 14

Musculoskeletal Disease

This category covers diseases of bone, joints, cartilage, and muscle. It includes osteoarthritis, rheumatoid arthritis when the focus is on joint damage rather than the immune process, osteoporosis, and muscle diseases.

Osteoarthritis shows why a blood test could be useful. It is usually diagnosed partly with an X-ray, which shows the space between bones narrowing as cartilage wears away. By the time that change is visible, however, the cartilage damage is already permanent and may have been developing for years. Any treatment meant to protect cartilage would need to start earlier, when there is currently no reliable way to identify who is already on that path.

X-rays also do not line up neatly with how a person feels. A joint can look badly damaged and cause little pain, while another can look only mildly affected and hurt a great deal. That makes the image an imperfect guide to the patient's actual experience.

Many of the markers being studied are fragments of the tissues themselves. Bone and cartilage contain large structural proteins. As those structures are broken down and rebuilt, fragments enter the bloodstream, where their levels can reflect the rate of tissue turnover. Researchers also study short RNA pieces and packages of material released by cells. Joint fluid, which can be drawn directly from an affected joint with a needle, gives a sample much closer to the source.

Blood has a basic limitation here: one joint contributes very little to the total material circulating through the body. A signal from one damaged knee can be drowned out by material from every other bone and joint. A marker may show that joint tissue is being damaged somewhere without telling you where. Exercise also changes bone and cartilage turnover, so the same person's result can shift depending on what they have been doing recently.