A team of researchers has corrected a recessive gene mutation that causes deafness and successfully restored hair cell sensory transduction in mice, giving them partial hearing. The treatment targeted the cause of hearing loss using DNA base editors. The researchers corrected a point mutation in Baringo mice, a model of deafness caused by a mutation on the transmembrane channel-like 1 (Tmc1) gene.
Sensory hair cells in the inner ear detect sound waves and are necessary for normal auditory function. In Baringo mice, a point mutation on the Tmc1 gene stops an essential protein from being synthesized, which is necessary for the formation of mechanosensitive ion channels in the hair cells. The lack of this protein means Baringo mice have no auditory sensory transduction. A cytosine base editor used to correct the mutation and was introduced into the inner ear using a dual adeno-associated virus (AAV).
Previous research conducted in 2015 showed that hearing could be restored in deaf mice by replacing the entire Tmc1 gene with a functioning copy. In that case, CRISPR-Cas9 was used to correct a dominant Tmc1 mutation in Beethoven mice. The use of base editors allows function to be restored without cleaving the DNA. One problem with using base editors is they cannot be delivered by a single AAV as they are too large.
“We tested several optimized cytosine base editors (CBEmax variants) and guide RNAs in Baringo mouse embryonic fibroblasts. We packaged the most promising CBE, derived from an activation-induced cytidine deaminase (AID), into dual adeno-associated viruses (AAVs) using a split-intein delivery system,” explained the researchers.
The base editor therefore needs to be split into two AAVs, which are both introduced. The base editor is then assembled inside cells. Both parts of the base editor must be in the cells for the point mutation to be corrected. When both parts were present in the cells, the sensory hair cells recovered 100% of their function. The problem experienced by the researchers was both parts of the base editor were only present in around 25% of the sensory hair cells.
The mice were given a single injection of the base-editor AAVs which resulted in a 51% reversion of the Tmc1 point mutation to the wild type sequence in TMC1 transcripts. 4 weeks after injection, the researchers demonstrated rescued low-frequency hearing. The researchers are now working on improving the efficiency of the process and delivering both AAVs to more cells, which will allow more hearing function to be recovered.
The base editing approach could be used to correct recessive gene mutations in humans and restore hearing and balance. “This research is very important for the pediatric community here at Boston Children’s Hospital and elsewhere because about 4,000 babies are born each year with genetic hearing loss,” said Jeffrey Holt, PhD, director of otolaryngology research at Boston Children’s Hospital and co-senior author on the paper.
You can read more about the study in the paper – In vivo base editing restores sensory transduction and transiently improves auditory function in a mouse model of recessive deafness – which was recently published in the journal Science Translational Medicine. DOI: 10.1126/scitranslmed.aay9101