Researchers at Children's Hospital Boston and Harvard Medical School have just published a new study on children whose hearing loss is caused by connexin 26 mutations. They wanted to know if hearing loss caused by connexin 26 remains unchanged or worsens. They monitored the hearing of about one hundred children for about 10 years to see what happened. Read on to learn what they discovered.
Background: Connexin 26 is a protein made in the cochlea. (It is coded for in our DNA by a gene called GJB2.) The exact purpose of the protein is not known for sure, but it is thought to help regulate the balance of potassium in the supporting cells. If the potassium balance is wrong, the supporting cells die, and there is evidence to suggest that if they die, the sensory hair cells -- those that pick up sound vibrations and stimulate the auditory nerves -- die also.
A cell-level view of the sensory part of the cochlea (from http://www.nature.com/nrn/journal/v7/n1/fig_tab/nrn1828_F2.html) (You don't need to try and understand this picture if you don't want to -- it's just for decoration.)
Connexin 26-caused hearing loss is typically inherited in a recessive fashion. In this case, each parent, usually unknowingly, has one healthy and one mutated copy of the gene. If a child inherits a mutation-carrying copy from both parents, the child will have hearing loss. (More details about how hearing loss is inherited can be found in the booklet you can download here.)
Connexin 26-caused hearing loss is common and could account for 10% or more of all cases congenital, or "born with", hearing losses. In other words, if your child was born with hearing loss (and you haven't yet had any testing to find out the cause) the chances are 1 in 10 that it is caused by connexin 26. If your child does not have any kind of syndrome that is responsible for their hearing loss, such as Waardenburg, the odds that it was caused by connexin 26 are even higher, like 1 in 5:
Causes of hearing loss from Boystown's http://www.babyhearing.org/hearingamplification/causes/genetics.asp website.
The Research Study The new research described here was carried out by a group of geneticists, otolaryngologists, and audiologists. A total of 126 children were included in the study. Of these children, 33% already had profound hearing loss when their parents agreed to take part in the research. Their hearing loss could not get any worse, so its progression couldn't be studied. The rest of the children had hearing losses somewhere in the mild to severe range when they first enrolled in the study.
Does the hearing loss progress? This was the first question the researchers asked. They found that in just over half of the cases (56%) the answer was yes, the hearing loss did get worse. The rest of the children seemed to have stable hearing levels over time.
Does the type of mutation determine whether it will progress? Many different hearing loss-causing mutations have been discovered in the gene for connexin 26 since it was first discovered in 1997. Earlier research showed that these different mutations can have different effects. Those that cause the protein to not be made at all are the most damaging and usually cause more severe hearing loss. Those that cause it to still be made but in an altered form usually cause less severe hearing loss. What was not known is whether certain kinds of mutations are more likely to cause hearing loss to worsen. This new study revealed that there is no correlation between the particular mutations a person has and whether or not the hearing loss will worsen. In other words, there is no crystal ball. However, for the children whose hearing loss levels did worsen, changes were detected on average 13 months after their hearing loss was first discovered. (A graph in the article shows a pattern of fairly rapid progression over the first year or two followed by a gradual leveling off.)
Why might some individuals' hearing losses progress while those of others with the same mutations stay the same? The researchers wrote that they don't know, but that there could be additional, as yet unidentified, modifier genes or environmental factors involved.
What does this mean for parents? While it would have been nice for parents to have a more definitive prediction of whether or not their child's hearing loss would progress based on which connexin 26 mutations they have, at least knowing that the odds of it being stable are 50:50 is something. Also knowing that progression, if it is going to happen, would likely be seen in the first year or two after the initial diagnosis gives parents at least some time-frame over which fret. Inevitably, our (genetically programmed??) individual human natures will determine just how much sleep we lose over this knowledge.
The full article is published in the Archives of Otolaryngology - Head & Neck Surgery:
Audiologic Phenotype and Progression in GJB2 (Connexin 26) Hearing Loss,
If you would like a copy of the full article, email me.
Lydia is the parent of a child with bilateral cochlear implants and hearing loss caused by connexin 26. She is also an audiology graduate student.
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