A major breakthrough that could revolutionise future treatment for genetic hearing loss has been discovered. For the very first time, deafness has been prevented in mice using gene editing.
Progressive deafness in baby animals that normally lose their hearing was prevented once they received a single injection of a gene editing formula, one study found.
Prof David Liu, who led the study at Harvard University and MIT said, “We want to see our research become the basis for the further development of a cure for certain forms of genetic deafness in people.”
Current treatment options for genetic deafness are sparse; which is shocking when almost half of all cases of deafness have some form of genetic root. This might not be the case for much longer. Emerging high-precision gene editing tools - like Crispr - are making the probability of a new class of therapies that target the underlying problem seem much more reachable.
The mutation in a gene called Tmc1, is what the study published in the journal Nature, is focused on. It’s a single wrong letter in the genetic code which causes the loss of the inner ear hair cells over time.
These hairs, found in the spiral-shaped organ called cochlea, will vibrate in reaction to sound waves. Nerve cells pick up these waves and then transmit them to the brain, where they are recognised as sound.
Children will suffer with progressive hearing loss is they inherit just one copy of the mutated Tmc1 gene. Hearing loss usually begins within the first 10 years of the child’s life and will eventually suffer profound deafness within 10-15 years.
Scientists wanted to see if eliminating the faulty version of the mutation would be an effective treatment; as in most cases people that have the mutation would have also inherited a healthy version of the gene from their other parent.
Liu and colleagues used Crispr-Cas9 - a gene editing technology - to cut the genome in order to disable the target gene. The gene editing solution is then injected into the inner ears of baby mice which suffer with hearing loss mutation.
Amazingly, hair cells in treated ears looked similar to those in healthy animals after just eight weeks. The hair cells of untreated mice remained looking sparse and damaged, instead of densely packed and tufted with hairlike bundles.
Once injected, a hearing test was conducted on the mice. The researchers placed electrodes on their heads to monitor the parts of the brain that are active when hearing. The team discovered that untreated mice needed to hear higher levels of sounds in order to spark brain activity.
After four weeks, treated ears on average could hear sounds that were 15 decibels lower than those that were untreated. To put that number into perspective, Liu said, “that’s the difference between a quiet conversation and a rubbish disposal.”
Before moving closer to a patient trial, Liu and his team are looking to develop this therapy by testing it on larger animals. This is to ensure that the method is both effective, and most importantly, safe.
Ethical debates have been sparked, when the option to carry out screening for genetic causes of deafness during IVF treatments was available in the UK; as some deaf couples used this to select embryos carrying the deafness gene. In 2008, this screening was banned under legislation in the UK. In light of this ethical conundrum Liu concluded, “Throughout our research and it our work progresses, we understand it is essential that we are aware of cultural considerations within the deaf community.”
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