May 2, 2012
Where Touch Meets Hearing
Touch sensitivity is hereditary and linked to genetic mechanisms that support hearing
Vision and hearing are so crucial to our daily lives that any impairments usually become obvious to an affected person. Although a number of known genetic mutations can lead to hereditary defects in these senses, little is known about our sense of touch, where defects might be so subtle that they go unnoticed.
In the 1 May issue of the online, open-access journal PLoS Biology, Gary Lewin's laboratory at the Max DelbrÃ¼ck Center for Molecular Medicine (MDC) in Berlin demonstrates that differences in touch sensitivity arise from genetic factors that can also be inherited. Some of these factors influence hearing as well, meaning that a single mutation may impair both senses.
There are good reasons to suspect that hearing and touch might have a common genetic basis. Sound-sensing cells in the ear detect vibrations and transform them into electrical impulses. Likewise, nerves that lie just below the surface of the skin detect movement and changes in pressure, and generate impulses. The similarity suggests that the two systems might have a common evolutionary origin–they may depend on an overlapping set of molecules that transform motion into signals that can be transmitted along nerves to the brain.
In the new study, Henning Frenzel from Lewin's lab and collaborators at medical schools in Berlin (CharitÃ©), Hannover, and Valencia, Spain (Hospital Universitario La Fe) carried out a classical "twin study" to try to discover a hereditary basis for touch sensitivity. The project compared the touch and hearing abilities of identical twins–who have identical sets of genes, including any mutations that might cause defects–with those of fraternal twins, other family members, and a wider set of subjects. They discovered a significant hereditary trend in touch sensitivity, and this correlated strongly with hearing ability. Thus the better the touch sensitivity the better their hearing, while poor hearing correlated with poor touch.
During the tests, subjects were exposed to a high frequency vibrating stimulus; and in another experiment the investigators had them press their finger onto fine gratings with ridges spaced at intervals ranging from a less than millimeter to almost a centimeter. The better the touch acuity, the finer the grating that could be perceived.
"We found a strong correlation between touch and hearing acuity in healthy human populations," Lewin said. "Additionally, about one in five young adults who suffered from congenital deafness had poor touch sensitivity." Blind subjects used as controls, on the other hand, often had enhanced touch perception. This made sense because the genetic basis of vision depends on proteins called photoreceptors that detect light rather than motion.
One group of subjects suffering from Usher's syndrome, a hereditary condition that leads to both deafness and blindness, had a significantly impaired sense of touch. This suggests that the gene USH2A, which is mutated in the syndrome, contributes to sensations of both touch and sound. There may be many more genes that play a role in both types of perception.
The scientific literature reports about 60 mutations in known genes that have been linked to hearing impairment, and about 60 more alterations in DNA with a similar effect that haven't yet been clearly linked to a gene.
"Our next task will be to investigate some of these other cases to see if they are also correlated with problems in touch," Lewin said. "This will give us a better understanding of the genetic mechanisms that underlie both types of perception."
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