Researchers have used CRISPR-Cas9 genome editing to partially prevent hearing loss in mice with a genetic form of deafness. Traditional gene therapy, adding a functional gene to cells containing a missing or non-functional one, has been used before to treat genetic hearing loss in animals. But the new approach disrupts a bad gene instead of adding a good one.
The study offers a possible treatment for a rare form of hearing loss in people caused by a single-base mutation in a gene called Tmc1. In inner-ear hair cells, the hairs of which sense sound waves, the mutation causes the cells to produce a toxic protein that then kills them. The mutation is dominant, meaning that a single mutation in one of the two gene copies causes people to lose hearing progressively.
Traditional gene therapy cannot correct dominant mutations, so David R. Liu of Harvard University, Zheng-Yi Chen of Massachusetts Eye & Ear Infirmary and Harvard Medical School, and coworkers used CRISPR-Cas9 genome editing to cleave the mutated gene in mice. Eliminating the gene protects hair cells by preventing production of the toxic protein (Nature 2017, DOI: 10.1038/nature25164).
To treat newborn mice with the Tmc1 mutation, the team combined the enzyme Cas9 with a piece of RNA and a lipid. Cas9 is a nuclease that cuts double-stranded DNA, and the guide RNA has a sequence that directs Cas9 to the mutated gene but not the normal gene. The researchers injected the lipid complex directly into cochlea in the mice’s inner ears, where the particles could enter hair cells. In principle, such a delivery method could work in people.
After several weeks, hair cells of mice receiving the treatment looked full and normal, whereas hair cells of untreated mice were damaged. Compared with untreated mice, the treated ones maintained a startle response to loud noises and scored more highly on hearing tests. For example, the animals receiving the treatment could hear sounds of about 15-decibel lower intensity than untreated mice, a substantial hearing improvement.
Genetic therapies are realistic options for people “when hearing loss is progressive and intervention occurs prior to sensory hair cell damage,” says Karen Avraham of Tel Aviv University, whose group in 2002 discovered the Tmc1 mutation in the hearing-impaired mice used in the new study. “The key is to know what causes the deafness. Therefore, proper and early genetic diagnosis is essential for the approximately 50% of hearing loss cases that have a genetic cause.”
The study shows that the genome therapy approach “is a plausible strategy to further develop,” says John V. Brigande of the Oregon Hearing Research Center at Oregon Health & Science University. Its strength “is that the disease gene is functionally knocked out, which is a permanent solution for successfully targeted cells.” However, in this study, probably fewer than 25% of cells had their mutated gene successfully knocked out. Therefore, he says, “it will be essential to maximize the efficiency of target cell editing to optimize therapeutic benefit.”