The ear is a part of the body that’s readily accessible to gene therapy: You can inject a gene delivery vector (typically a harmless virus) and it has a good chance of staying put. But will it ferry the corrected gene into the cells of the hearing and/or vestibular organs where it’s most needed?
Back in 2015, a Boston Children’s Hospital/Harvard Medical School team reported using gene therapy to restore rudimentary hearing in mice with genetic deafness. Previously unresponsive mice began jumping when exposed to abrupt loud sounds. But the vector used could get the corrected genes only into the cochlea’s inner hair cells. To really restore significant hearing, the outer hair cells need to be treated too.
That’s where an improved synthetic vector called Anc80 comes in. Developed by Luk H. Vandenberghe, PhD, and colleagues at Massachusetts Eye and Ear, the vector safely penetrates both kinds of hair cells, as described in the first of two papers yesterday in Nature Biotechnology and as shown below in green.
“Outer hair cells amplify sound, allowing inner hair cells to send a stronger signal to the brain. We now have a system that works well and rescues auditory and vestibular function to a level that’s never been achieved before,” says Gwenaëlle Géléoc, PhD, in the otolaryngology department and F.M. Kirby Neurobiology Center at Boston Children’s.
Géléoc led a second Nature Biotechnology study that tested Anc80 in a mouse model of Usher syndrome, the most common genetic form of deaf-blindness that also impairs vestibular function. The team injected the vector, carrying a corrected gene, into the cochleas of newborn mice.
Of 25 mice tested by measuring responses in auditory brain regions, 19 heard sounds quieter than 80 decibels, and a few could hear sounds as soft as 25-30 decibels.
“Now, you can whisper, and they can hear you,” says Géléoc.
The effects persisted for at least six months, with only a slight decline between six weeks and three months.
Restoring balance, and perhaps someday vision
Mutations in Ush1c (causing one of several forms of Usher syndrome) also damage hair cells in the vestibular organs, so mice with the Ush1c mutation have balance problems. When placed on a rotating rod, they quickly fall off — but after gene therapy, they stayed on longer.
In another test, the mice were let loose to explore a circular field while the track beneath them recorded their path. Below, you can see the mutant mice moving erratically, in repeated circles, while normal controls stay mainly at the periphery. Treated with gene therapy, the mutant mice resumed a near-normal pattern.
“This is a landmark study,” says Jeffrey R. Holt PhD, director of otolaryngology research at Boston Children’s and an author on both papers. “We show, for the first time, that by delivering the correct gene sequence to a large number of sensory cells in the ear, we can restore both hearing and balance to near-normal levels.”
Margaret Kenna, MD, MPH, a specialist in genetic hearing loss at Boston Children’s who does research on Usher syndrome, is also excited by the findings.
“Cochlear implants are great, but anything that could stabilize or improve native hearing at an early age would give a huge boost to a child’s ability to learn and use spoken language,” says Kenna. “In addition, the improvement in balance could translate to better and safer mobility for Usher Syndrome patients.”
While the study didn’t treat the animals’ eyes or test their vision, Anc80 has also been shown to work in the retina, so could possibly help with the blindness that is part of Usher 1c. “Progress in gene therapy for blindness is much further along than for hearing,” notes Vandenberghe.
Down in the hair cells
In the meantime, Géléoc’s team found good things happening at the cellular and molecular level. In both mice and humans, the Ush1c mutation compromises a protein called harmonin, causing sensory hair cell bundles in the inner ear to deteriorate and become disorganized. Normal mice (top row below) have normal hair cells (stained in red) with plenty of harmonin (stained green), while in the mutant mice, harmonin all but disappears.
After gene therapy, the inner and outer hair cells in the cochlea began to produce normal full-length harmonin, so the green reappears. Zooming in further below, you can see the hair cells resuming their proper “V” formation (at right) with three rows of cilia.
More to come
Before this gene therapy approach can be tried in patients, much more work is needed. Of note, the mice were treated right after birth; when treatment was delayed 10 to 12 days, it did not restore hearing and balance. The researchers will do further studies to determine the reasons for this.
“It may be that the vector cannot get into older cells,” says Géléoc.
Another next step: to test gene therapy in larger animals, which may take up the vector differently, and to develop approaches for other forms of genetic hearing loss. More than 100 genes are known to cause deafness, many of which could potentially be transferred with Anc80. For those genes that are too large to fit into Anc80, Géléoc and Holt will seek other gene-transfer strategies.