Auditory impairments are often severe forms of functional challenge that may be genetic in humans and animals. Despite this, there are few direct therapies for ‘genetic’ deafness available. On the other hand, the new wave of gene-editing may soon change that for a huge proportion of those affected by hearing loss. A research article published this week has documented the potential of CRISPR gene-editing in a murine model of autosomal-dominant hearing loss. This pre-clinical study may be a first step in the development of genetic therapies for humans with corresponding disorders.
Hearing loss is a profound public health problem. There are many factors that affect the risk of deafness in both adults and children; genetic disposition is a main example of these. However, genetic therapies for relevant cases are yet to grow in popularity. This situation may be subject to rapid change as CRISPR gene-editing techniques are applied to the problem. A study published in the December edition of Nature has reported that their Cas9-based intervention resulted in reduced hearing loss in a mouse model of genetically-acquired deafness.
This experiment was completed across numerous otolaryngology, neuroscience, chemical and biological engineering centres in Boston and China. The scientists were led by Zheng-Yi Chen of the Department of Otolaryngology and Program in Neuroscience, Harvard Medical School and Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary and David R. Liu, who holds positions at the Harvard Department of Chemistry and Chemical Biology, the Howard Hughes Medical Institute and the Broad Institute. They developed the Beethoven mouse model, which had been engineered to mimic a form of genetic deafness based on damage to the inner ear. This involves the alteration of one base-pair on an allele of the Tmc1 (transmembrane channel-like gene family 1) gene, which gives rise to a dominant mutation that leads to hearing loss.
The scientists designed genome-editing therapies that targeted and corrected this mutation. These complexes were Cas9/guide RNA complexes, which were also conjugated to a lipid vehicle for transport to the right location. These complexes were tested in cultured cells, and found to achieve their intended purpose. The team then treated the cochleae of newborn Beethoven mutation-positive mice with either the Cas9 complex or a control that targeted an irrelevant gene. They found that mice treated with the Tmc1-targeting complexes exhibited reduced death in inner-ear hair cells, and, thus, an improved conservation of auditory functions compared to mice injected with the control treatment. The mice in the ‘treatment’ group also exhibited improved thresholds in their brainstem responses to auditory stimuli compared to the ‘control’ group. In addition, the team compared ‘treatment’ group mice to completely un-treated Beethoven-positive mice in a startle-based model of auditory response. The control mice displayed considerably impaired reactions in this test compared to their treated counterparts.
This study is one of many that demonstrate the ability of guide-RNA/protein complexes to access and interact with their target genes when introduced to the tissues affected by the genetic defect in question. The team also added the lipid component to this treatment, as it was found to enhance the delivery to the hair cells of the inner ear. In addition, the lipid-carried RNA/Cas9 complexes were also unlikely to migrate away from this specific site, thus reducing the risk of unwanted side-effects in the mice. Therefore, such a treatment just may be able to help human patients with similar disorders. Dr.Liu now hopes that this Tmc1-editing technique may be developed to the level of a therapeutic solution for humans with inner ear-related, genetic forms of deafness.
This experiment also demonstrates that viral vectors were not necessary to deliver the changes in the murine model of deafness mediated by gene-editing. This method is a classic component of genetic therapies, although it may result in the target gene being edited in other tissues besides the actual site of interest. This may result in adverse effects. On the other hand, gene editing facilitated by viral vectors may have an enhanced lifespan compared to the lipid-delivered example in the Beethoven-model study. On the other hand, some researchers argue that the biochemical vehicles for gene-editing therapies should be tailored to each individual condition, as it was in this experiment. This adaptation may make this form of genetic engineering more acceptable for use in human patients.
This study may have demonstrated improvements in the accuracy and efficacy of gene-editing, albeit in pre-clinical studies. The team behind it now plan to propose up-scaled versions of this trial, as well as other similar studies that investigate the effect of Cas9/RNA gene therapies in animal models of genetic blindness. Hopefully, their work will lead to more effective, long-term solutions for people who are predisposed to lose their hearing from birth.
Top image: American Sign Language is the predominant sign language used by the deaf and interpreters in the United States. JBER offers ASL classes to military families as part of the Instructional Youth Program here. (U.S. Air Force Photo Illustration/Airman 1st Class Kyle Johnson) (Public Domain)
Gao X, Tao Y, Lamas V, Huang M, Yeh W-H, Pan B, et al. Treatment of autosomal dominant hearing loss by in vivo delivery of genome editing agents. Nature. 2017.
Ledford H. Gene editing staves off deafness in mice. Nature News. 2017. Available at: https://www.nature.com/articles/d41586-017-08722-3