July 25, 2012— -- It's an exciting day for deaf, blind mice. Scientists are reporting success with treatments that seem to restore sight and hearing in mice born without those senses.
The treatments have a long way to go before the researchers will know if they can help humans, but the early results are encouraging.
Scientists from the University of California, Berkeley studied mice with blindness similar to the inherited and age-related blindness that affects humans. When a person or mouse goes blind, the cells in the retina of the eye that respond to light, known as rods and cones, die off, leaving the eye without the ability to detect light. The team used a chemical, called AAQ, to target the remaining living cells, which normally aren't activated by light.
When AAQ is struck with light, it turns those non-sensitive cells into light-sensing signal senders for the brain's vision circuitry.
"The first drug candidate or prototype that directly restores photosensitivity," said Richard Kramer, one of the study's authors. "With a drug, you can adjust its dose, discontinue it, or use in combination with other therapies. And this being a simple chemical, you can use chemistry to make it even better."
But the mice weren't permanently cured. The chemical lasts for a few weeks before the mice need another injection. Dr. Marco Zarbin, chair of ophthalmology and visual science at the New Jersey School of Medicine, who was not affiliated with the study, said if the treatment proved to work in humans, the need for repeated injections isn't a major flaw.
"Injecting something into the eye is something that surgeons do all the time," Zarbin said. "The idea that one would have to periodically repeat the injection is not a deal breaker."
Scientists have been testing many avenues to correct blindness in both mice and men – everything from gene therapy to retina transplants to electronic chips implanted in the retina that stimulate defunct cells. But those methods are invasive and permanent, and none have been proved to restore perfectly normal vision. Kramer said the AAQ chemical he used on his blind mice is a valuable alternative simply because the solution can be stopped at any time if, for example, better treatments came along.
Zarbin agreed, and said the approach also appears to target a majority of the retina's million cells, rather than a few thousand that could be stimulated by an electrical chip.
"That's got to improve a patient's light sensitivity," he said. "I'm as encouraged as I could reasonably hope to be by these findings. But you never really know until you've tried it in a person."
Another team of scientists from the University of California, San Francisco, set out to help hearing-impaired mice.
The scientists used gene therapy to correct defects in tiny hair cells in the inner ear in mice that were born deaf. By injecting a gene, called VGLUT3, into the inner ears of the mice, the scientists were able to prompt the hair cells to send signals to the brain, restoring the mice's hearing. The effects lasted about nine weeks in newborn mice and at least seven weeks in adult mice. Two mice still had their hearing after one and a half years.
"This is the first time that an inherited, genetic hearing loss has been successfully treated in laboratory mice, and as such represents an important milestone for treating genetic deafness in humans," said study author Lawrence Lustig in a press release.
Zhen-Yi Chen, a hearing researcher at the Massachusetts Eye and Ear Infirmary, said he considers the findings a breakthrough for deaf humans as well.
"Before we really didn't know if this was doable, even in the animal model. Now we know that it is," he said.
But the study's authors were cautious in applying the findings to humans, noting that although the same genetic mutation is tied to hearing loss in mice and humans, the condition may be entirely different in the animals than it is in people.
Chen said if the approach proved effective in humans, it could represent an improvement over current approaches to treating hearing loss – wearable hearing aids and cochlear implants, neither of which restore hearing to normal levels.
Both of the studies were published today in the journal Neuron. Although they may be a giant leap for mousekind, researchers say it will be a while before the therapies can be tested, proven safe and used in people.