Prospect of a new class of therapies that could transform future treatment of genetic hearing loss, at the root of nearly half of all cases of deafness

Deafness has been prevented in mice using gene editing for the first time, in an advance that could transform future treatment of genetic hearing loss.

The study found that a single injection of a gene editing cocktail prevented progressive deafness in baby animals that were destined to lose their hearing.

“We hope that the work will one day inform the development of a cure for certain forms of genetic deafness in people,” said Prof David Liu, who led the work at Harvard University and MIT.

Nearly half of all cases of deafness have a genetic root, but current treatment options are limited. However, the advent of new high-precision gene editing tools such as Crispr has raised the prospect of a new class of therapies that target the underlying problem.

Q&A What is Crispr? Show Hide Crispr, or to give it its full name, Crispr-Cas9, allows scientists to precisely target and edit pieces of the genome. Crispr is a guide molecule made of RNA, that allows a specific site of interest on the DNA double helix to be targeted. The RNA molecule is attached to a bacterial enzyme called Cas9 that works like a pair of 'molecular scissors' to cut the DNA at the exact point required. This allows scientists to cut, paste and delete individual letters of genetic code.

The study, published in the journal Nature, focused on a mutation in a gene called Tmc1, a single wrong letter in the genetic code, that causes the loss of the inner ear’s hair cells over time.

The delicate hairs, which sit in a spiral-shaped organ called the cochlea, vibrate in response to sound waves. Nerve cells pick up the physical motion and transmit it to the brain, where it is perceived as sound.

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If a child inherits one copy of the mutated Tmc1 gene they will suffer progressive hearing loss, normally starting in the first decade of life and resulting in profound deafness within 10 to 15 years. However, since most people affected by the mutation will also have a healthy version of the gene, inherited from their other parent, the scientists wanted to explore whether deleting the faulty version worked as a treatment.

Liu and colleagues used gene editing technology known as Crispr-Cas9, which acts as a molecular scissors, snipping the genome to disable a target gene. The team injected the gene editing solution into the inner ears of baby mice with the hearing loss mutation. After eight weeks, hair cells in treated ears resembled those in healthy animals – densely packed and tufted with hairlike bundles. The hair cells of untreated mice, in contrast, looked damaged and sparse.

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Then the researchers conducted a hearing test on the mice by placing electrodes on their heads and monitoring the activity of brain regions involved in hearing. Researchers needed more sound to spark brain activity in untreated mice compared with treated mice, the team found. On average, after four weeks, treated ears could hear sounds about 15 decibels lower than untreated ears. “That’s roughly the difference between a quiet conversation and a garbage disposal,” Liu said.

Simon Waddington, a reader in gene transfer technology at University College London, described the study as an elegant application of new gene editing tools. “Hitherto incurable and often even untreatable diseases are now within the scope of gene therapy,” he said.

The team plans to develop the therapy in larger animals to ensure the method is safe and effective, before moving closer to a patient trial.

Previously, the option to carry out screening for genetic causes of deafness during IVF treatments has prompted an ethical debate, with some deaf couples seeking to use screening to select embryos carrying the deafness gene. In the UK, this was banned under legislation introduced in 2008. Liu added: “We also recognise the importance and remain mindful of cultural considerations within the deaf community as this work moves forward.”