Although people can lose their hearing for a variety of reasons — old age, as well as exposure to loud noises — genetics are behind a little less than half of all deafness cases, says study co-author David Liu, a professor of chemistry and chemical biology at Harvard, who also has affiliations with the Broad Institute and the Howard Hughes Medical Institute. The hearing-loss disease tackled in this study is caused by mutations in a gene called TMC1. These mutations cause the death of so-called hair cells in the inner ear, which convert mechanical vibrations like sound waves into nerve signals that the brain interprets as hearing. As a result, people start losing their hearing in their childhood or in the 20s, and can go completely deaf by their 50s and 60s.
To snip those mutant copies of the gene, Liu and his colleagues mixed CRISPR-Cas9 with a lipid droplet that allows the gene-editing tool to enter the hair cells and get to work. When the concoction was injected into one ear of newborn mice with the disease, the molecular scissors were able to precisely cut the deafness-causing copy of the gene while leaving the healthy copy alone, even if the two copies differ by just one base pair. The treatment allowed the hair cells to stay healthier and prevented the mice from going deaf.
After four weeks, the untreated ears could only pick up noises that were 80 decibels or louder, roughly as loud as a garbage disposal, Liu says. Instead, the injected ears could typically hear sounds in the 60 to 65 decibel range, which is the same as a quiet conversation. “If one can translate that 15 decibel improvement in hearing sensitivity in humans, it would actually make a potential difference in the quality of their hearing capability,” Liu tells The Verge.