Nervous System Health

The genetic links between different diseases

Diseases that strike different parts of the body—and that don't seem to resemble each other at all—may actually have a lot in common.

Scientists have identified the genetic basis for many separate diseases. Now, some researchers are looking at how the genes interact with each other. They are finding that a genetic abnormality behind one illness may also cause other, seemingly unrelated disorders. Sometimes diseases are tangentially linked, having just one gene in common. But the greater the number of shared genetic underpinnings a group of diseases has, the greater the likelihood a patient with one of the illnesses will contract another.

Researchers have found evidence, for example, that there is a close genetic relationship between Crohn's disease, a gastrointestinal condition, and Type 2 diabetes, despite the fact the two conditions affect the body in very distinct ways. Other illnesses with apparently close genetic links are rheumatoid arthritis and Type 1 diabetes, the form of the disease that usually starts in childhood, said Dr. Joseph Loscalzo, chairman of the department of medicine at Brigham and Women's Hospital in Boston.

This network approach, known among scientists as systems biology, could change the way medical specialists view and treat disease, according to some researchers. Rather than only looking to repair the parts of the body that are directly affected by illness, "we should be looking at what the wiring diagram [inside of cells] looks like," said Dr. Albert-László Barabási, a physicist at Northeastern University's Center for Complex Network Research in Boston.

The information could help better predict a person's risk of developing diseases, researchers say. It also could aid drug development. By figuring out which proteins are most critical to the normal functioning of the body, pharmaceutical companies could target those key proteins to treat disease. In some cases, drug companies may want to avoid interfering with key proteins to avoid too many unintended side effects, says Marc Vidal, director of the Center for Cancer Systems Biology at Dana-Farber Cancer Institute in Boston.

Since all the DNA in the human body was first sequenced in 2000, some 4,000 diseases with a known genetic basis have been identified, according to the National Institutes of Health. But only about 250 of those diseases have treatments, leaving many genetic puzzles left to untangle.

Scientists have long known that proteins and other molecules in the body don't act alone. In order for the body to operate efficiently, biological substances must bind to or pass chemical messages to each other to start and stop working. The system is complex: Each gene is thought to produce, on average, five separate substances, mostly proteins, and these products interact with each other. When a protein, or group of proteins, malfunctions, it appears to give rise to a variety of distinct illnesses.

Barabási and his colleagues set out to see which diseases shared genetic underpinnings. They used information from a vast database at Johns Hopkins University in Baltimore that pulled together research from around the world on diseases and genes they were linked to. The scientists then mapped out a network indicating which diseases were seemingly connected to each other through common genes.

Of the 1,284 diseases mapped, nearly 900 had genetic links to at least one other disease. And 516 of these formed a so-called disease cluster, in which illnesses, mainly cancers, were linked to each other through multiple genetic connections.

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