Scientists have identified a gene which controls specific aspects of human touch and a 'sixth sense' that describes awareness of one's body in space.
With the help of two young patients, one nine and the other 19 years old, having a unique neurological disorder, scientists found that the gene called PIEZO2 controls specific aspects of human touch and the 'sixth sense' called proprioception.
Mutations in the gene caused the two to have movement and balance problems and the loss of some forms of touch. Despite their difficulties, they both appeared to cope with these challenges by relying heavily on vision and other senses.
"Our study highlights the critical importance of PIEZO2 and the senses it controls in our daily lives," said Carsten G Bonnemann, from the US National Institutes of Health's National Institute of Neurological Disorders and Stroke (NINDS).
"The results establish that PIEZO2 is a touch and proprioception gene in humans. Understanding its role in these senses may provide clues to a variety of neurological disorders," he said.
Bonnemann's team uses cutting edge genetic techniques to help diagnose children around the world who have disorders that are difficult to characterise. The two unrelated patients in this study have difficulties walking; hip, finger and foot deformities; and abnormally curved spines diagnosed as progressive scoliosis.
Working with Alexander T Chesler from NIH's National Centre for Complementary and Integrative Health (NCCIH), the researchers discovered that the patients have mutations in the PIEZO2 gene that appear to block the normal production or activity of Piezo2 proteins in their cells.
Piezo2 is what scientists call a mechanosensitive protein because it generates electrical nerve signals in response to changes in cell shape, such as when skin cells and neurons of the hand are pressed against a table.
Studies in mice suggest that Piezo2 is found in the neurons that control touch and proprioception. "Our results suggest they are touch-blind. The patient's version of Piezo2 may not work, so their neurons cannot detect touch or limb movements," said Chesler.
Further examinations suggested the young patients lack body awareness. Blindfolding them made walking extremely difficult, causing them to stagger and stumble from side to side while assistants prevented them from falling. They were also less sensitive to certain forms of touch. They could not feel vibrations from a buzzing tuning fork as well as the control subjects could.
Nor could they tell the difference between one or two small ends of a caliper pressed firmly against their palms. Brain scans of one patient showed no response when the palm of her hand was brushed. The study appears in the New England Journal of Medicine.
