Almost half of all hearing loss has an underlying genetic cause. Late-onset hearing loss, which occurs after the acquisition of speech, may appear in generation after generation, often progressing to a severe or even profound loss. Or it may skip generations, passing the faulty gene along to unsuspecting offspring.
Those affected, even if they were aware that they might eventually lose their hearing, are usually part of the hearing world and do not know sign language. The loss may be severe enough to be disabling, even with sophisticated hearing technology. Preventing this loss, even when anticipated, has not been possible.
On December 20th, researchers at the Broad Institute of MIT and Harvard and the Howard Hughes Medical Institute published a study in the prestigious journal “Nature” that holds promise for prevention of hereditary loss.
Why do we care about a study on mice? Mice, like all mammals including humans, cannot regrow damaged hair cells. If prevention works on mice, it may work on other mammals. Mice are also the test subject for studies on regeneration of hair cells, which would allow a reversal of hearing loss.
The mutant gene, whimsically called Beethoven by researchers, is found in the hair cells of the inner ear. Because the gene is dominant, it takes only one to cause damage. That also means it may exist alongside a healthy copy of the gene. One of the challenges for researchers was to find a way to target just the mutated gene without disrupting the normal copy.
The technique the researchers used, called CRISPR, is a gene editing technique that the journal “Science” cited as the 2015 Breakthrough of the Year. I won’t try to explain the technique (or even the acronym) but here’s a link to a reader-friendly article in the L.A. Times that laid out the technique – and its potential dangers.
What distinguished the new study, according to the scientists, is that this is the first time a genome-editing protein has been ferried directly into the relevant cells to halt progression of genetic hearing loss.
Direct delivery of the protein allows “exquisite DNA specificity,” according to the press release. The specificity is needed to selectively disrupt the pathogenic copy of the gene without disrupting the normal copy.
Since this type of genetic hearing loss generally manifests as late onset, it would allow researchers to test suspected carriers of the defective gene (the Tmc1 gene) and to treat carriers. As co-senior author Zheng-Yi Chen, associate professor at Massachusetts Eye and Ear said, the later onset allows “a precious time window for intervention.”
The subjects of the study were 100 Beethoven-model mice carrying one copy of the defective gene and one normal copy. Untreated, the mice began to show hearing loss at four weeks, and by eight weeks were profoundly deaf (as measured by auditory brain-stem response). The treated mice, in comparison, responded to sound at about 65 decibels, the level of normal human speech.
Peter Barr-Gillespie, a sensory biologist at Oregon Health and Science University who was not involved in the study, praised it (in the “New Scientist”) as a “pretty significant piece of work.” He noted, however, that the decibel level at which the treated mice could hear was relatively loud compared to the hearing threshold in wild mice, which is 30 to 40 decibels. “It’s nowhere near the threshold of the wild-type mice, [but] the 10-15 decibels could make a huge difference in humans,” he said. “That sort of loss of hearing is very noticeable in people and could make for substantial improvements in quality of life.”
As always with gene editing, one concern is possible undesirable changes in the DNA. Senior co-author David Liu said the researchers had not observed “any off-target editing in the animal.” In the specific cells treated, they found only one modification, in an area not known to play a role in hearing. One major concern, however, would be the potential to develop cancer. Stephen Tsang, a clinical geneticist at Columbia, praised the study as “good for basic research“ (in “Axios”) but, like others, noted that there are many more steps to prove it safe and effective for humans.
This study showed that gene editing prevented, rather than reversed, hearing loss in mice. But for those affected by genetic hearing loss, it’s a promising step. “A lot of additional work is needed before this strategy might inform the development of a therapy for humans,” co-senior author David Liu said, “but at this stage, we’re delighted and excited that the treatment preserved some hearing in the animal model.”
For more information on living with hearing loss, see my books on Amazon.com.