Prior to discovering a link between hearing loss and autism spectrum disorder, ASD patients were found to have hearing loss. However, hearing loss in ASD patients leads to an increased, over-stimulation of the senses. Sometimes this intense input of sound can be uncomfortable and overwhelming. How could this hearing loss end up causing an over amplification of sounds?
An interesting, proposed phenomenon, called central gain, is an adaptive response due to decreased auditory input. When the auditory stimulation is lowered, this theory suggests that the body increases neuronal gain. This neuronal gain usually results in excessive sound stimulation that can be both painful and unpleasant.
Hainan Lang, M.D., Ph.D, professor of Department of Pathology and Laboratory Medicine, wanted to find out if this central gain could be causing the brain to respond incorrectly in ASD individuals. He approached this by stating, “We didn’t have a clinically relevant model to directly test this important fundamental question.”
This study is powerful as it was a collaboration between the Department of Neuroscience, the Department of Pathology and Laboratory Medicine, and the Hearing Research Program at MUSC. These departments came together with their knowledge in genetics and cell biology. Dr. Lang was able to partner with Christopher Cowan, Ph.D., chair of Neuroscience, to test her preclinical model. Dr. Cowan’s lab had been testing mice with a very specific gene, MEF2C. In typical neurodevelopment, there are two functional copies of this gene. MEF2C is a transcription factor that aids in development of multiple body systems, such as the heart, brain, immune, and vascular systems.
Dr. Cowan’s lab was looking at this gene to see its implementation in brain development. Their lab had noticed symptoms that were like ASD behaviors, such as repetitive actions and hyperactivity. They appeared when there was only one functional copy of MEF2C. This became even more interesting when mice with one functional copy of MEF2C also had mild hearing loss.
Noting that there was a potential link between hearing loss and autism spectrum disorder, the team began to investigate further. The researchers found that the activity of the auditory nerve was decreased in these mice who had one functional copy of MEF2C. They even recognized that this hearing loss was very similar to the degeneration of hearing that occurs with increased age.
But why was there a reduction in functionality of the auditory nerve? The study concluded that there was increased inflammation around the nerves, vascular degeneration, and immune cell activation. These specific factors caused destruction of the auditory nerve, leading to hearing loss.
“Now we understand that auditory nerve activity can also involve the immune system, and that’s the beautiful new direction we want to continue to study,” Dr. Lang said.
“Now we understand that auditory nerve activity can also involve the immune system, and that’s the beautiful new direction we want to continue to study.”
With these new findings, researchers can continue to investigate the link between hearing loss and autism spectrum disorder. This study helps researchers better understand the MEF2C gene’s role in development and gives a more accurate picture of what is causing the hearing symptoms.