Audiologist Joan McKechnie, BSc Hons Audiology & Speech Pathology, works for UK based hearing aids vendor HearingDirect.com. Joan is registered with the United Kingdom’s Health Care Professions Council and has years of experience in the hearing aid industry. Today, Joan is discussing Hearing Loss and Gene Therapy.
Hearing loss is extremely common in the United States with an estimated 20% of adults having some form of hearing loss (Hearing Loss Association of America I, 2015). Harvard University’s stem cell institute estimates that 17% of Americans have a form of hearing loss that is irreversible given current medical technologies (Hsci.harvard.edu, 2015).
In adults, the most common causes of permanent hearing loss are age-related (presbycusis) and noise related (WebMD, 2015).
The Hearing Loss Association of America (2015) estimates that by the age of 65, one out of three people are suffering from hearing loss. There are a number of possible causes for age-related hearing loss including changes within the cochlea, pathways from the ear’s nerves to the brain no longer functioning correctly, certain medical conditions and the use of certain medications (Nidcd.nih.gov II, 2015).
Noise-related hearing loss usually occurs as a result of continuous or intermittent high level noise exposure (Kirchner et al., 2012). The National Institute on Deafness and Other Communication Disorders (NIDCD) estimates that 15 percent of Americans between the ages of 20 and 69 (26 million Americans) have some degree of hearing loss that may have been caused by exposure to noise at work or in leisure activities (Nidcd.nih.gov, 2015).
Both noise-related and age-related hearing loss are forms of Sensorineural Hearing Loss (SNHL) (Hearing Loss Association of America II, 2015). The cochlea contains two types of nerve-endings, the inner hair cells (IHC) and the outer hair cells (OHC), which help transduce incoming sound vibrations into electrical signals. The electrical signals are then transmitted to the auditory pathway of the brain through spiral ganglion neurons (SGCs). SNHL occurs when the functioning of the cochlea and/or the auditory pathway is impaired.
This form of hearing loss is typically described as irreversible and permanent (Betterhearing.org, 2015). Childhood infections including meningitis, mumps, measles, scarlet fever and Meniere’s disease can also cause sensorineural hearing loss.
Another categorization of hearing loss is Conductive hearing loss. That occurs when problems within the ear canal, ear drum or middle ear cause hearing loss (Hearing Loss Association of America II, 2015). Conductive hearing loss hearing loss can be caused by a genetic malformation, tumour, infection, foreign body or trauma. Some forms of conductive hearing loss are treatable through surgery or medication.
Treating Hearing Loss with Gene Therapy
Genes are commonly referred to as the “building blocks of inheritance” (Nlm.nih.gov, 2015). They are passed down from parent to child and contain instructions which inform the body how to make proteins. Many diseases are caused by malformed genes.
Gene therapy usually attempts to treat a disease by inserting a gene into a patient’s cells, often to replace an abnormal gene (Genetics Home Reference, 2015). The activity of a gene can also be altered. The delivery mechanism for gene therapy treatments can vary, depending upon the condition being treated.
According to the Stanford Initiative to Cure Hearing Loss, 70% of the most common forms of hearing loss are caused by mutations in only three genes (Hearinglosscure.stanford.edu, 2015). If scientists are able to modify or replace those defective genes, many common forms of hearing loss could be treated. Stanford researchers are currently working on understanding the genetic causes of hearing loss so gene therapies can be developed.
Researchers have already begun animal trials to test if modifying or removing mutated genes can restore hearing. Scientists from the Harvard Medical School and Boston Children’s Hospital were able to restore hearing in mice by removing mutated transmembrane channel–like 1 (TMC1) genes. The mice had a type of deafness caused by the genetic mutation, which causes between 4-8% of all cases of deafness in humans (Hms.harvard.edu, 2015). The researchers noted that more than 70 genes are known to causes deafness when mutated.
Gene Therapy May Repair Nerve Endings
The reason why sensorineural hearing loss is considered permanent is that after birth, humans lose the ability to regenerate the hair-like nerve endings in the ear (Actiononhearingloss.org.uk, 2015). Once the nerve endings are damaged or lost, hearing is permanently lost. If gene therapy can be used to tell the body to regrow those nerves, hearing could be restored for millions of people.
One major breakthrough that has occurred in the field is the discovery of a gene that is active in developing nerve endings. Researchers have discovered that enabling the “Atoh1” gene, causes nerve endings to regrow (Actiononhearingloss.org.uk, 2015). Despite the gene being identified more than 10 years ago, researchers have only just developed a mechanism that may be able to modify the gene.
A bio-technology company named Genvec has partnered with Novartis to test a drug which may be able to enable the Atoh1 gene (Genvec.com, 2015). The clinical trial for the drug CGF-166 started in October 2014 and researchers are expected to release the results in 2017. The treatment will involve a one-off dose of the drug, directly injected into the inner ear.
If all goes well, there may be new treatments for many forms of sensorineural and conductive hearing loss in the near future. Gene therapy has some difficult hurdles to cross, but the recent findings are very encouraging.
About the Author
Audiologist Joan McKechnie, BSc Hons Audiology & Speech Pathology, works for UK based hearing aids vendor HearingDirect.com. Joan is registered with the United Kingdom’s Health Care Professions Council and has years of experience in the hearing aid industry.
Image Credit: soflasun. Quoted sources are available upon request!