There are many causes of hearing loss that we know about. Long-term exposure to loud noise, various diseases like rubella and mumps, sudden loud noises that cause permanent damage to the hearing mechanism, ototoxic medications, certain diseases and of course, aging. Hearing loss, caused by nerve deafness, affects 33 percent of people over the age of 65.
Late in 2009, researches published a report showing that a newly-discovered type of gene is the cause of a certain kind of early, progressive hearing loss most prevalent in children beginning in infancy.
The Genetic Connection
Trying not to get too complicated, genes are the building blocks of who we are. Genes make up DNA, found in every cell, and each set of genes creates the unique individuals we are. The color of our hair, skin color, eye color, big muscles, tall, short, bald or prematurely gray – all laid out in an incredibly complex genetic map. Your genetic map is totally you, unlike any other human. That's what makes DNA evidence in legal cases so compelling. The odds of two people having identical DNA are astronomical.
Researchers in Germany and England have now identified a newly-discovered type of DNA, called microRNA, as the cause of the loss of hearing with a laboratory-proven, genetic basis.
Lab results were founded, first, on tests of a new "mouse model" created by German scientists researching genetics in mice. Next, scientists in England studied families with histories of early hearing loss, collected DNA samples and found a direct, genetic corollary between hearing loss in lab rats and worsening hearing loss in children.
Now, there are a number of known hearing loss conditions associated with genetic mutations, but researchers have only scratched the surface in analyzing the complex structure of DNA. Now, with this latest research, a new kind of DNA, labeled microRNA, is reported, and, well, you start to get an understanding of just how complex the problem is and why research into genetic causes of hearing loss continues.
microRNA and Hearing Loss
Researchers at Germany's Helmholtz Zentrum Munchen traced the early loss of hearing to a specific region on the double-helix that forms the structure of DNA. A specific mutation in something called a "seed region" of the double DNA helix causes gradual, early hearing loss in children.
Labeled miR-96, this gene is responsible for the development of the hair-like structures that line a tiny, snail-shaped organ, called the cochlea, located deep within the inner ear.
The cochlea is responsible for converting sound waves into electrical impulses that are then sent to the hearing centers of the brain for interpretation.
Using lab rats, bred to experience genetically-based hearing loss, experiments showed a similar mutation in mouse DNA, offering scientists quantifiable evidence to conclude that, in many instances, early, progressive hearing loss is the direct result of the mutation of a new class of gene – one scientists are just beginning to examine more closely.
Perhaps the most remarkable aspect of this study is the discovery of a new class of gene, one that, through mutation in both mice and humans, is the culprit in cases of early loss of hearing. The connection between microRNA – the new classification of DNA – opens extensive avenues for new, more advanced, disease-specific research. Prior to this study, mutated microRNA had not been related to hearing loss.
What other relationships between hearing loss and microRNA will be uncovered?
Obviously, research into microRNA mutations has just begun but, again, it holds great promise in determining the cause of many genetically-based medical conditions, including hearing loss, and more specifically, hearing loss that affects children born with the mutant miR-96 microRNA. These youngsters:
- Develop fewer of the hair-like projections lining the cochlea and
- These projections "die" at a faster rate, thus leading to gradual hearing loss and, in the most severe cases, deafness.
In order to correct a genetic defect, researchers first have to identify the cause of a health-related condition like early hearing loss. We know this takes years of tireless, dedicated research to find that one genetic sequence within human DNA responsible for the condition.
Bottom line? More questions than answers – a good thing in the case of on-going research into genetically-caused hearing loss. Questions provide the road map to discovery, and if a gene mutation is identified as the cause of hearing loss, it may someday be possible to identify and correct the mutation, preventing the onset of a devastating disease.
What Does The Future Hold For Progressive Hearing Loss?
Promise. And hope.
The research conducted by this international group of distinguished geneticists has provided us with a glimpse of what may someday be possible in the treatment of pediatric, genetically-based hearing loss. But there remains a great deal of research to be done. Unfortunately, we're years away from a genetic "cure" for the mutated miR-96 microRNA.
Still, this critical research, including the development of a new "mouse model", points scientists in new directions.
Rodents and humans working together to find answers to a common, but often poorly understood condition, progressive hearing loss. Progressive hearing loss affects one in 500 children and this newly discovered gene that affects sensory hair cells of the inner ear may offer future medical interventions.
Hope and promise are the results of this research on "mice and men."