Scientists cured lab mice of deafness using gene-editing - paving the way to a similarly radical technique for humans.
The so-called Beethoven Mice - engineered to have the same hearing loss condition as the composer - were able to hear six months after the research team at Harvard Medical School 'snipped' a mutation out of their DNA.
To do so, they used a new version of CRISPR Cas-9 gene-editing which acts like a pair of 'molecular scissors' to find faulty proteins and remove them, before inserted a healthy version.
The team also tested the technique on human ear cells grown in the lab, with success.
Beethoven Mice - engineered to have the same hearing condition as the composer - were able to hear after the research team at Harvard Medical School 'snipped' a mutation out of their DNA
'Our results demonstrate this more refined, better targeted version of the now-classic CRISPR/Cas9 editing tool achieves an unprecedented level of identification and accuracy,' lead author Dr David Corey said.
The gene at fault is called Tmc1.
It causes the loss of the inner ear's hair cells over time. The delicate hairs sit in a tiny organ called the cochlea and 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.
The Beethoven Mice are completely deaf by six months of age, while mice without the defect retain normal hearing throughout life and can detect sounds at around 30 decibels - a level similar to a whisper.
The deaf mice had one incorrect letter in the DNA sequence of the Tmc1 gene. Instead of a T they had an A. This single error spells the difference between normal hearing and deafness. Disabling, or silencing, the mutant copy would be sufficient to preserve the animals' hearing.
Classic CRISPR-Cas9 gene-editing systems were created from bacteria, which are designed to hunt and destroy viral invaders. Scientists used Streptococcus bacteria, and trained it to hunt down specific proteins or segments of DNA, instead of deciding its own route. That is done using a guiding molecule - gRNA - to identify the mutant DNA sequence. Once the target is pinpointed the cutting enzyme - Cas9 - snips it.
But in some instances, it hasn't been perfectly precise, cutting the wrong DNA.
To minimize the risk of hiccups, the researchers tried using a different type of bacteria - Staphylococcus - to build a modified version of Cas9 that would ensure selective cutting of only the harmful Tmc1 gene.
It worked: remarkably, their system managed to spot a single incorrect DNA letter among three billion in the mouse genome.
First author Dr Bence Gyorgy, who is now at the Institute of Molecular and Clinical Ophthalmology in Basel, Switzerland, explained: 'We