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Scientists cure lab mice of deafness with gene-editing technique – paving for human treatment

AdvertisementGene-editing: Scientists cure lab mice of deafness, paving for human treatment Scientists have cured lab mice of deafness using a radical new gene-editing technique thereby paving way for such technique in humans.

The breakthrough, reported in Nature Medicine, has the potential to change the lives of the millions of people who live with hearing loss.

The research team at Harvard Medical School ‘snipped’ a mutation out of the DNA, of Beethoven Mice – engineered to have the same hearing loss condition as the composer. They were able to hear six months after.

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 inserting a healthy version, Mail Online reports. The team also tested the technique on human ear cells grown in the lab, with success. Lead author , Dr David Corey said: “Our results demonstrate that more refined, better targeted version of the now-classic CRISPR/Cas9 editing tool achieves an unprecedented level of identification and accuracy.”

The gene at fault is called Tmc1. According to Mail Online, 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. “We took advantage of the fact this system recognizes mutant DNA but not normal DNA and uses a dual recognition system for enhanced precision.” First author Dr Bence Gyorgy, who is now at the Institute of Molecular and Clinical Ophthalmology in Basel, Switzerland, explained. “This approach resulted in an unprecedented level of specificity in targeting the mutant gene.”

The therapy was administered shortly after birth the mice were then repeatedly. After a month untreated Beethoven mice could hear low-frequency sounds but had notable hearing loss at high frequencies. By month six they had lost all their hearing. In contrast, treated mice retained near-normal hearing at low frequencies – with some showing near-normal hearing even at high frequencies.

Even more encouragingly, a small subset of treated mice that were followed for nearly a year retained stable, near-normal hearing. An analysis of human diseases also showed the tool correctly identified 3,759 defective variants responsible for a fifth of mutations. 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. The mouse with the greatest hearing preservation was capable of picking up sounds at 25 to 30 decibels – virtually indistinguishable from its healthy peers. The effect of the treatment was then tested on human ‘Beethoven’ cells grown in the lab.

A DNA analysis also revealed it caused editing exclusively in the mutant copy of the Tmc1 gene – and spared the normal one. The approach holds promise for 15 other forms of inherited deafness caused by a single mutation in hearing genes, said the researchers. Nearly half of all cases of deafness have a genetic root but current treatment options are limited.

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. Much more work remains to be done before even such a highly accurate therapy could be used in humans.

But it represents a milestone since it greatly improves the efficacy and safety of standard gene-editing techniques. What’s more the results set the stage for using the same approach for other inherited diseases that arise from a single defective copy of a gene.

Co-senior investigator Dr Jeffrey Holt, a neurologist at Boston Children’s Hospital, said: “We believe our work opens the door toward a hyper-targeted way to treat an array of genetic disorders that arise from one defective copy of a gene. This truly is precision medicine.”

Source: Daily Trust 

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