By showing they could repair a single recessive mutation behind a form of hearing loss, researchers open the door to using base editing to address a broader range of conditions
Base editing restores partial hearing in mice
Using a genome editing technique known as base editing, researchers from Boston Children’s Hospital and the Broad Institute of MIT and Harvard, have restored hearing in mice with a known recessive genetic mutation.
With this technique, researchers repaired one single error in the Tmc1 gene known to cause a hereditary form of deafness. The one-time repair involved switching one incorrect DNA base in the gene with the correct version. While a similar approach has been used previously for other forms of hearing loss, this is the first time base editing has been used for a genetic sensory disorder.
Details about the approach are published in a new paper in Science Translational Medicine.
“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,” says Jeffrey Holt, director of otolaryngology research at the F.M. Kirby Neurobiology Center at Boston Children’s, and co-senior author on the study with David Liu, a core institute member at Broad and director of Broad's Merkin Institute for Transformative Technologies in Healthcare. “And, we feel this is a big step beyond the field of hearing restoration and for the broader field focused on treatment of genetic disorders.
Base editor acts as a spell-check
“In that case, we used a single engineering adeno-associated virus (AAV) to deliver a functioning copy of the Tmc1 gene into the ear,” he says.
A microscope image of the mouse cochlea; cells with repaired Tmc1 are green. (Credit: Olga Shubina-Oleinik/BCH)
This research goes a step further. Instead of replacing a gene, the team repaired a single mutation in the Tmc1 gene converting it back to the correct sequence. “It’s like your spell-checker,” he says. “If you type the wrong letter, spell checker fixes it for you.” When the team fixed the defect in the sensory cells in the ear, the edited cells recovered 100 percent of their function.
But the base editor was too large for a single AAV. The newly designed base editor that engineers the genetic repair required more space. It did not fit into a single AAV. Instead, they split up the base editor sequence into two AAVs.
“Once the cell was infected with these two parts, it was able to reassemble into a single full length sequence and then perform the base editing task we needed,” says Olga Shubina-Oleini of the Holt lab, who was co-first author on the study with Wei-Hsi Yeh from the Liu lab.
It is important to note the approach worked when both AAVs made their way into the cell. But that was the case in about one-quarter of the cells which was enough to provide some hearing to the mice.
“We got it to work but we need to boost the efficiency to make it broadly useful,” says Holt. If only one AAV got into the cell, it did not work. “But the message is that when we got both into the cells, we went from zero function to 100 percent. That tells me all we need to do is get it into more cells and we will recover more hearing function.”
Building on previous success
At least 100 different genes are involved in hearing in the inner ear. Mutations in any one of those can lead to hearing loss.
“We have been developing different strategies targeting several of these different forms of hearing loss,” says Holt. “It really takes a precision medicine approach where we are trying to tailor our strategy specific, not just each gene that is involved, but in some cases the individual genetic mutation in the gene as is the case with this study.”
The Holt lab has a long history of success unraveling these genetic causes of hearing loss and developing gene therapy treatments for genetic forms of hearing loss. In 2011, the team first discovered that the Tmc1 protein is required for hearing and balance. In 2017, the Liu lab joined with researchers from Massachusetts Eye and Ear to develop a CRISPR-Cas9 gene editing-based approach for repairing Tmc1 mutations in Beethoven mice, a model of a dominant Tmc1 mutation. And building on their 2015 success, the Holt team also used CRISPR-Cas9 in 2019 to prevent hearing loss in this system.
Just one of many mutations related to hearing and balance
Over 70 different mutations have been identified in the Tmc1 gene in humans. “We hope this new technique will allow us to pick them off one at a time to restore hearing and balance related to the inner ear,” says Holt.
Along with hearing loss, balance disorders represent a large unmet medical need, though it is present mainly in aging adults. The inner ear houses the cochlea (the auditory organ) and five organs of balance – the vestibular organs. Disruptions in function in any of those five could lead to balance problems.
Other contributors to this research include Bifeng Pan of Boston Children’s; Jonathan Levy, Gregory Newby, Michael Wornow, and Jonathan Chen of Broad; and Rachel Burt of the Murdoch Children’s Research Institute, Australia.
Support for this study was provided by National Institutes of Health, the Harvard Hughes Medical Institute, the Jeffrey and Kimberly Barber Fund, and the Foundation Pour L’Audition.
Yeh W-H, Shubina-Oleinik O, et al. In vivo base editing restores sensory transduction and transiently improves auditory function in a mouse model of recessive deafness. Science Translational Medicine. Online June 3, 2020. DOI: 10.1126/scitranslmed.aay9101.