by Larik Sonfar

Created on: May 23, 2008

Schultz, J.M., 2005. Modification of Human Hearing Loss by Plasma-Membrane Calcium Pump PMCA2.
New England Journal of Medicine, 352, 1557-1564.

Summary

About 0.1%, or 1 in 1000 children are born with a significant amount of sensorineural hearing loss, a type of hearing loss associated with dysfunction of the auditory nerve. An additional 0.1% of children will develop this amount of sensorineural hearing loss by the age of nine. 50 or more percent of these cases of hearing loss are caused by the children's genetics. It has long been known that humans have hundreds of genes where slight mutations may cause sensorineural hearing loss. However, it is unclear which of these genes caused most of the cases of hearing loss in these children. Recessive mutations in the gene CDH23 is sometimes causes nonsyndromic sensorineural hearing loss.

An evaluation of a family of five siblings all with hearing loss caused by recessive genes revealed that while they all had similar causes to their hearing disorders, the severity of each of their hearing losses was radically different. This established the key causal question for this experiment: Why do some people with genetically recessive sensorineural hearing loss have profound high and low-frequency hearing loss while others only have high-frequency hearing loss?
Multiple hypotheses were presented. One explanation for the difference in hearing loss was mutations in CDH23 or other genes. Another explanation for the different levels of hearing loss was different levels of exposure to environmental factors such as noise or antibiotics. The last hypothesis presented was that genetic mutations and environmental factors together accounted for the difference in frequencies of hearing loss.
In order to test these hypotheses, the original family, along with a control group of unrelated persons, was tested. Medical-history interviews, physical examinations, and several other medical tests were conducted to rule out possible minor causes of the difference in hearing loss. Further interviews and tests with the participants in the experiment revealed no exposure to harmful antibiotics, extreme noise levels, head trauma, or other infections that could possibly affect hearing. Hypotheses two and three, both being at least partially based on environmental factors, were ruled out as possibilities for the difference in hearing loss in the participants.
Blood samples were taken from each of the participants in order to test their DNA for genomic mutations.

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