Deafness in A Silent Voice
A Silent Voice premiered in the fall of 2017, and made a big splash in the anime community, and will be released on Netflix soon. The reason I am talking about A Silent Voice is not for its portrayal of bullying and suicide which are very important issues especially for classroom teachers. What I will be talking about today is deafness and a potential cure for it. A doctor who reads my blog pointed me to a potential new treatment for certain types of deafness involving CRISPR, which I have previously covered here.
Hearing loss is a condition where an individual has a lowered ability to hear sound, and the degree of impairment can vary. The degree of impairment can vary widely from a slight impairment like having trouble hearing the people across from you in a crowded restaurant, something I have trouble with, to having a complete inability to understand speech even with a hearing aid. The latter is referred to as deafness and it has a variety of causes. In the case of A Silent Voice, the female lead Shoko Nishimiya, it is most likely that she was born with hearing loss that then progressed to full deafness by the time we see her in elementary school. While the anime never does tell us what caused her deafness, it is possible to narrow down the list a bit based on her symptoms and that the anime is set in modern Japan.
Causes of Deafness
In Shoko’s case, given the young age at which she goes deaf, it is not a stretch to think that she was born deaf or at the very least hearing impaired. While it is true that modern society and the loud noises associated with it have been shown to cause hearing damage in children, it would be unlikely to progress that fast to the point of causing complete hearing loss. Additionally once the damage, is first detected the parents would have taken steps to prevent further damage. There are also other conditions like fetal alcohol syndrome that can cause deafness, which only had 56 reported cases in Japan as of 1995. Not to mention that fetal alcohol syndrome causes other neurological issues that Shoko does not have, such as mental retardation. Diseases like measles and mumps can also cause deafness, but these would be prevented by vaccinations. There is a vaccine gap in Japan caused by a variety of reasons, mainly bureaucratic, and not due to an antivaccine movement, but measles causes hearing loss when the mother contracts the disease while she is pregnant, and Shoko’s mother would have been vaccinated well before the vaccine gap occurred.
This just leaves a laundry list of genetic conditions and the first that come to mind are Down’s syndrome and Charcot–Marie–Tooth disease, of which Shoko shows neither the signs or symptoms. This means that Shoko’s hearing loss is nonsydromic, which means that the only issue is the hearing loss and there are no other symptoms. Now some of you might be thinking about Shoko’s speech impediment, and that is a result of her impaired hearing. There are a number of different genes implicated in nonsyndromic hearing loss:
ACTG1, CABP2, CDH23, CLDN14, COCH, COL11A2, DFNA5, ESPN, EYA4, GJB2, GJB6, KCNQ4, MYO15A, MYO6, MYO7A, OTOF, PCDH15, POU3F4, SLC26A4, STRC, TECTA, TMC1, TMIE, TMPRSS3, USH1C, WFS1, CCDC50, DIAPH1, DSPP, ESRRB, GJB3, GRHL2, GRXCR1, HGF, LHFPL5, LOXHD1, LRTOMT, MARVELD2, MIR96, MYH14, MYH9, MYO1A, MYO3A, OTOA, PJVK, POU4F3, PRPS1, PTPRQ, RDX, SERPINB6, SIX1, SLC17A8, TPRN, TRIOBP, SLC26A5, and WHRN.
If that seems like a bit of alphabet soup, don’t worry, I can simplify things a bit. Despite the number and variety of genes involved, 80% of nonsydromic hearing loss is inherited in an autosomal recessive fashion. This means that the afflicted individual will have to inherit a defective gene from each parent to have hearing loss. The remaining 20% are inherited in an autosomal dominant fashion, which means that the individual only needs to inherit one defective copy to have hearing loss. In the case of Shoko, I think that her hearing loss is autosomal recessive due to the fact that neither her sister, mother, nor grandmother have hearing problems.
Microbiology of Hearing Loss
Our sense of hearing is very similar to how our sense of balance works, meaning that it runs on tiny hairs located in our cochlea. Essentially what happens is that sound is funneled down the outer ear to the middle ear, specifically the ear drum.
The sound causes the eardrum (tympanic membrane) to vibrate and those vibrations are transferred via three connected bones to the inner ear.
The vibrations then travel through the inner ear, which is filled with fluid and small hairs. Different hairs move in response to vibrations of different frequencies and those movements are what we perceive as sound.
On a cellular level each time a hair moves it causes a change in the cell membrane of the hair cell sending off an action potential to the brain. A key protein in this process is made by TMC1 and this protein is a potassium and calcium channel. When the protein is missing, deafness is the result.
Cure for Deafness
Sadly at the present time there is no known cure for this particular type of deafness. There are ways to improve and protect the remaining hearing an individual has but there is no known method to reverse the damage, or at least that was the case until recently. Through the use of CRISPR scientists were able to successfully edit the defective TMC1 gene in neonatal mice. These mice showed significant improvement in hearing over non-edited mice. While these improvements did eventually fade over time, this is still an important first step in treating hearing loss. The authors of the study also said that it was possible to edit the DNA of adult mice, but no results were reported in the study published on March 29, 2018. This is a great step forward, but do keep in mind that animal studies do not always perfectly correlate to humans. In this case the inner ear (cochlea) of the mouse is not fully developed at birth whereas a human inner ear is. Either way I am excited for this development.
I hope you found this discussion on hearing and hearing loss in A Silent Voice interesting. Also, A Silent Voice should be required viewing for any discussion on bullying and the effects it can have on people. Lastly, you will cry while watching this movie, in sadness and in happiness.