Two people with a rare and little-understood disorder have helped scientists identify a gene that may be responsible for our “sixth sense” of body awareness.
In a study published Wednesday in The New England Journal of Medicine, scientists at the National Institutes of Health describe the cases of a 9-year-old girl and a 19-year-old woman with rare neuromuscular symptoms. Both patients seem to lack proprioception ― that is, an awareness of the position of one’s body in space. When blindfolded, they couldn’t walk or keep their balance. They also couldn’t track the position of their arms and legs without seeing them.
To understand how naturally this comes to most people, just close your eyes and bring your finger to your nose. Easy, right? Being drunk can temporarily take away your ability to do this. But if the two people in the study closed their eyes and tried to touch their noses, they wouldn’t be able to do it at all.
The researchers discovered that the patients had mutations on a gene called PIEZO2, suggesting that this gene contributes to the sense of touch and proprioception.
“Our study highlights the critical importance of PIEZO2 and the senses it controls in our daily lives,” Dr. Carsten Bönnemann, an NIH researcher, said in a statement. “Understanding its role in these senses may provide clues to a variety of neurological disorders.”
The patients were examined by Bönnemann, who specializes in studying unknown genetic illnesses in children. The woman and the girl both had similar difficulties with walking, coordination and body growth and development. Their symptoms included bent fingers and feet and curved spines.
The woman and the girl could each sense pain and temperature, but certain other kinds of touch went almost ignored by their nervous systems. One patient who underwent brain imaging showed no neural activity in response to someone brushing on her palm. She also reported that a gentle brush on her forearm, which feels comforting to most people, felt “prickly” to her.
It turned out Bönnemann and his colleague had zeroed in on a specific gene that was already being investigated in mice at another lab at the NIH. So the two teams of researchers joined up.
The mutation in the gene appears to block the production of a protein by the same name. This protein is believed to be mechanosensitive ― which is to say it generates nerve signals in response to the mechanical force of touch on the skin, allowing us to sense the touch.
“As someone who studies Piezo2 in mice, working with these patients was humbling,” Alexander Chesler, researcher at the National Center for Complementary and Integrative Health and a co-author of the study, said in the press statement.
“Our results suggest [these patients] are touch-blind,” Chesler said. “The patient’s version of Piezo2 may not work, so their neurons cannot detect touch or limb movements.”
The woman and the girl appear to compensate for a lack of a sense of touch and proprioception by relying on vision and other senses, the researchers said.
The researchers suspect the PIEZO2 gene may also be related to skeletal growth and development. Now that the mutation has been identified in two people, they hope they can more easily find other people who might have it.
It’s also possible that there are other variations of the gene, each one producing different sets of symptoms or simply mild variations in how our bodies develop and how we move about in the world.
“Could a finely tuned PIEZO2 gene contribute to superior athletic performance, or a poorly tuned one to clumsiness?” Bönnemann told the journal Science. “I think it’s not impossible.”