Walking and dancing. Typing on a keyboard or climbing Mount Everest. You use your "sixth sense" — your body's uncanny ability to sense where it is in space — to perform everything from normal activities to great feats of athleticism.
Scientists have known about this ability, called proprioception, for more than a century, but they weren't sure how it worked. You might think dancers or athletes would hold the answer, but it was researchers studying a rare genetic disorder who have shined a light on it.
The researchers studied a young girl and a woman who completely lacked this sixth sense. Their unusual set of symptoms included an extreme lack of coordination, difficulty walking, and a lack of sensation when objects were pressed against their skin. They both also have an unusual curvature of the spine, as well as feet, hips and fingers that bend at unusual angles. The results of their study were published in the New England Journal of Medicine.
The researchers learned that both patients didn't start walking until they were between 6-7 years old and they both had problems learning to feed and dress themselves. Neither patient was able to walk with their eyes closed; they could only take steps if they could see their limbs as they moved.
Genetic analysis revealed a genetic mutation in a gene called PIEZO2, which has been associated with the body's sense of touch.
One of the researchers, study co-author Alexander Chesler from the National Center for Complementary and Integrative Health, had been studying PIEZO2 in mice for years. But he'd never found a good way to study the gene in people — until now.
Trying to understand proprioception just by experimenting with mice was like trying to understand Beethoven by reading sheet music, Chesler told NPR. "But when I talked to the patients, it was like going to the symphony," he said.
When researchers began studying these two patients, they were able to demonstrate that the PIEZO2 gene was responsible for proprioception, as well as sensations of touch. They learned much of what it was able to do by studying what the patients were not able to do.
In experiments, the patients were unable to walk blindfolded. They also weren't able to move a finger from their nose to a targeted point if they were blindfolded. When researchers moved a particular limb for them, if they couldn't see, they were unable to tell which way the limb was being moved. Once the blindfolds were removed, the patients were able to walk, touch the target and see the direction of their moving limbs.
There's one other potentially interesting thing researchers may be able to learn from this new discovery: understanding if variations of the PIEZO2 gene contribute to whether a person is klutzy or coordinated.
“Could a finely tuned PIEZO2 gene contribute to superior athletic performance, or a poorly tuned one to clumsiness?” co-author pediatric neurologist Carsten Bönnemann of the National institute of Neurological Disorders and Stroke told Science. “I think it’s not impossible.”