Many factors come into play to cause hearing loss. In childhood, almost 50% of cases can be traced to the patient’s genes. Genetic testing promises to help identify such hearing loss before it has progressed, or potentially even before birth, while also helping to steer treatment—most notably, by identifying candidates for a new groundbreaking gene-editing strategy that selectively removes defective genes and eliminates the risk of inheriting the trait for future generations.
To explore the current status of gene editing in the field of otolaryngology, ENTtoday spoke with a number of leading researchers in the field.
Genes Involved in Hearing Loss
Researchers have identified 46 known genes involved in non-syndromic hearing loss, and expect to discover more. The most frequent genes implicated in recessive non-syndromic hearing loss are GJB2 (connexin 26 gene), which is responsible for more than half of cases, followed by SLC26A4, MYO15A, OTOF, CDH23 and TMC1 (Mutat Res. 2009;681[2-3]:189-196).
Although the majority of hereditary hearing loss is non-syndromic, many genetic syndromes also cause deafness, including neurofibromatosis type 2 and Usher syndrome. In addition, age-related hearing loss is thought to have a genetic component (SNPs rs4932196 and rs58389158), although the exact genetic cause(s) of the disease is still being elucidated (PLoS Genet. 2016;12:e1006371).
The discovery of genetic mutations has come with a whole host of potential genetic therapies; however, “the dominant deafness genes cannot be corrected with gene replacement strategies, so researchers are turning to gene-editing approaches to correct those types of hearing losses: either silencing RNA (siRNA) or CRISPR strategies,” said Hinrich Staecker, MD, PhD, the David and Mary Zamierowsky Professor of otolaryngology at the University of Kansas School of Medicine in Kansas City.
CRISPR, which stands for clustered regularly interspaced short palindromic repeats, uses the Cas9 enzyme to cut a specific target sequence on a mutant gene—cutting both strands of the DNA. Repair enzymes can then be inserted to repair and seal the cut with new genetic information, permanently changing the underlying genetic code. A landmark study in Nature reported the results of using CRISPR-Cas9 to correct a mutation in a gene that causes hypertrophic cardiomyopathy in human embryos (Nature. 2017;548:13–14). Researchers have questioned the findings of that study, however (see “Landmark Gene-Editing Study Called into Question,”).
The Promise of Gene Editing
“Gene editing is now much simpler and better than it has ever been. Even though we have tried to edit genes for a long time, the relatively recent discovery of the CRISPR-Cas technology, in which we can discreetly edit parts of the gene, is very exciting,” said D. Bradley Welling, MD, PhD, the Walter Augustus LeCompte Professor and chair of the department of otolaryngology at Harvard Medical School in Boston, and editor of ENTtoday’s sister publication Laryngoscope Investigative Otolaryngology.
“The CRISPR technique can now target messenger RNA [mRNA], which theoretically could solve a lot of our problems, but there are a lot of practical details that would have to be worked out, at least in the human ear,” said Lawrence R. Lustig, MD, Howard W. Smith Professor and chair of the department of otolaryngology–head and neck surgery at Columbia University College of Physicians and Surgeons in New York City. He noted, for example, that any treatment for hearing loss would have to be performed in the earliest stages of fetal development; once degenerative hearing loss begins, there is no way to reverse it.
To test the viability of early treatment, Dr. Lustig and his colleagues are using adeno-associate virus type 1 (AAV1) vectors very early in the mouse life cycle, day 1 after birth, “which actually translates into a human baby in utero,” he said. In one study, his team used gene replacement therapy to restore hearing in a mouse model by using the AAV1 to deliver vesicular glutamate transporter-3 (VGLUT3) cells into the cochlea of impaired mice. Within two weeks of AAV1-VGLUT3 delivery, the newborn mice had restoration of their auditory brainstem response and partial rescue of the startle response (Neuron. 2012;75:283–293).
Dr. Staecker also underscored the importance of early intervention. “CRISPR will probably have to [be applied to] dominant genes before you have complete degeneration of the hair cell in question,” he said, adding that better screening of patients also would be needed. “It is not enough just to say ,‘You have hearing loss’; we need to define what a patient has on a structural, physiologic, and molecular level,” he explained. “You want to know exactly what is wrong with the ear—whether the problem is in the stria vascularis, the hair cell, or the spiral ganglion—what are the genes involved. We will not be there tomorrow, but it will be much sooner than expected.”
Dr. Staecker was instrumental in developing the first clinical trial of human inner ear gene therapy, including evaluating the efficacy of inserting the Atoh1 gene to help regenerate vestibular hair cells (Laryngoscope. 2014;124:S1–S12). “The trial [yielded] information on how to access the ear, how to deliver treatments, what doses the human ear tolerates, and sets the stage for the more complicated therapies. If we think about gene editing or gene replacement, we may have to treat the patient for the duration of their life rather than a one-time short intervention. So we have to approach all of these things slowly.”
If we think about gene editing or gene replacement, we may have to treat the patient for the duration of their life rather than a one-time short intervention. So, we have to approach all of these things slowly. —Hinrich Staecker, MD, PhD
The CRISPR technique holds the promise of targeting specific mutations in the DNA strand without affecting normal cells. Studies in the mouse model have run into problems, however. “There always is a risk that the technique is not selective enough to prevent the snipping of ‘normal’ portions of the strand or, potentially worse, inserting cells in the wrong place. Other concerns occur when only some of the embryo cells are repaired, called mosaicism,” Dr. Welling noted. Even in the landmark Nature study, the reported success rate was 72.4%. Most scientists agree that before this technique could be used in viable human embryos, the success rate would have to be 100% in the mouse models and larger animal studies.
Patients with more than one mutation pose another hurdle, Dr. Staecker said. “You would have to design a specific editing therapeutic for each one of those spots, so that makes it very expensive from a manufacturing standpoint, and only a fraction of those dominant type alleles would be treatable that way,” he said.
One of the foundations of medical ethics is benevolence, … but we have to balance that with the potential risks, especially when you are talking about changing the DNA of future generations. —G. Richard Holt, MD
Many bioethicists worry that the rapid pace of gene-editing innovation has occurred without enough discussion about the ethics of the procedure. Although the embryos created by the OHSU group were never intended for implantation, the ability to permanently manipulate the genome points to the possibility of using the technique to create designer babies, raising questions of fairness, social norms, and personal autonomy, said G. Richard Holt, MD, MSE, MPH, professor emeritus and clinical professor in the department of otolaryngology-Head and Neck Surgery at The University of Texas Health Science Center at San Antonio, and member of ENTtoday’s Editorial Advisory Board.
The National Academies of Science, Engineering and Medicine recently published a report concluding that “heritable genome-editing … trials might be permitted, but only following much more research aimed at meeting existing risk/benefit standards for authorizing clinical trials and even then, only for compelling reasons and under strict oversight. It would be essential for this research to be approached with caution, and for it to proceed with broad public input” (Human Genome Editing: Science, Ethics, and Governance. Washington, DC: The National Academies Press: 2017).
Dr. Holt agreed that gene editing has to be approached with a careful eye toward bioethics. “One of the foundations of medical ethics is benevolence—you want to help people struggling with medical conditions, such as hearing loss, sickle cell anemia, and muscular dystrophy, but we have to balance that with the potential risks, especially when you are talking about changing the DNA of future generations,” he said.
“Should this technology eventually turn out to be successful and safe, it will likely be very expensive,” Dr. Holt added. “This may mean that only those able to afford it will be able to use it, which raises issues of social justice—providing healthcare resources without regard to social status and other factors. While it is true that healthcare is not always ‘fair,’ bioethical responsibility requires us to address this issue as a substantive concern with emerging medical technology.”
Dr. Welling echoed concerns over the potential downstream risks of the procedure. “If we go off target, it could take decades for the long-term side effects [of gene editing] to unfold,” he warned.
And then he pointed to an even more chilling scenario: “I do believe there will be lot of upsides to this technology, but it is also very accessible [to people with ill intent],” Dr. Welling said. “I know that is one of the concerns the intelligence community [has]—that CRISPR could produce a biologic weapon of mass destruction.”
Financial disclosures: Dr. Staecker disclosed that he is on the surgical advisory board of MedEl. None of the other participants have any financial conflicts to disclose.
Nikki Kean is a freelance medical writer based in New Jersey.