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Gene therapy reverses genetic deafness in mice

From the left, microscopic images of cells from within the ear of a mouse with a mutation that causes deafness, a mouse that received the gene editing therapy, and a healthy mouse

Gao, X. et al. Nature. 2017

A DNA editing experiment has enabled “Beethoven” mice, which have a genetic alteration that causes progressive deafness, to maintain their hearing (Nature, December 20). The group, led by chemist David Liu, from Harvard University, USA, injected fat particles, or lipids, containing CRISPR-Cas9 directly into the inner ear of the mice. With an RNA segment serving as a guide to help find the mutation that caused the hearing loss (a single letter of the DNA), the Cas9 enzyme deactivated only the defective copy of the gene involved. At four weeks, the treated animals were already more sensitive to sound than the control group. In terms of human hearing, the improvement would be like the difference between being able to hear a quiet conversation, or not. Eight weeks after the treatment, the mice that underwent genetic editing showed a startle response to a sudden 120-decibel noise, equal to that of a chainsaw or a rock concert. The untreated mice, meanwhile, remained impassive. The therapy’s success was not only due to the shutdown of the defective gene. The fact that the lipids are only absorbed locally by the cells that detect sound, rather than traveling through the body, is an important factor. The injected solution has a low durability, reducing the risk of the Cas9 enzyme eliminating the normal versions of the gene. The results offer a promising step forward in the search for similar treatments for humans. Liu is one of the founders of a company that aims to develop treatments based on gene editing.