A novel CRISPR-Cas9 gene editing strategy has shown promise in restoring auditory function in adult mice with progressive hearing loss caused by mutations in microRNA 96. The research, published in Science Translational Medicine, offers insights into treating late-onset genetic hearing loss and highlights the potential of gene therapies for addressing hearing impairments later in life.
Targeting microRNA 96 for Hearing Restoration
Mutations in microRNA 96, specifically expressed in inner ear cells, lead to dominantly inherited hearing loss. Researchers at the Massachusetts Eye and Ear Institute developed an adeno-associated virus (AAV)-based gene therapy to selectively knock out the mutated allele. "We designed a CRISPR-Cas9 system that targets this mutated allele, but not the wild type allele," explained Wenliang Zhu, a postdoctoral researcher involved in the study.
The team packaged the CRISPR-Cas9 system in an AAV vector that specifically targeted inner and outer ear hair cells, the cells in the cochlea responsible for transmitting sound information to the brain.
Improved Hearing Function in Adult Mice
The AAV was injected into the ears of six-week-old mice with a copy of the microRNA 96 mutant allele. Ten weeks post-injection, auditory brainstem response (ABR) tests were conducted to measure hearing improvements. The AAV-injected ears showed a significantly reduced ABR threshold at 10, 14, and 20 weeks after injection compared to non-injected ears, indicating improved hearing sensitivity.
Furthermore, the gene editing therapy promoted the survival of healthy ear hair cells. Microscopic analysis revealed that hair cells from injected ears appeared more organized and healthy compared to the disorganized and damaged hair cells from non-injected ears.
Age-Related Limitations
To investigate the therapy's effectiveness at later stages of hearing loss, the AAV was injected into 16-week-old mice. However, no significant difference in ABR threshold was observed 10 weeks later. "This is the fundamental question that everyone is dealing with: if you can treat [using gene therapy] a patient coming in with severe, profound hearing loss," said Zheng-Yi Chen, a biologist at the Massachusetts Eye and Ear Institute. "At the moment, our experiments suggest that it's probably very hard."
Potential for Human Translation
The fact that the same microRNA 96 mutation causes hearing loss in both mice and humans suggests that the CRISPR-Cas9 system could be directly translated to human therapy with minimal modifications.
The team also engineered a dual-AAV system containing CRISPR-Cas9 systems capable of editing multiple microRNA 96 mutations. "There's only three variants that are known for this mutation, and they were able to use this approach to target all three of them," noted Seiji Shibata, an otolaryngologist at the University of Southern California.
Future Directions
The researchers are now focused on preclinical studies to advance the therapy towards human clinical trials. "Hearing loss is a good area where [gene therapy] has shown great efficacy," Chen stated. "It really shows that there is a fundamental importance in translating the research from animal models into humans."
