Tuesday, February 8, 2011

Genetics of Deafness

Taylor has Connexin 26.  This means that Jon and I are recessive carriers of Connexin 26.  What is Connexin 26?

Connexin 26 (Cx26) is a protein found on the (GJB2) gene and is the most common cause of congenital sensorineural hearing loss. Connexin 26 mutations are responsible for at least 20% of all genetic hearing loss and 10% of all childhood hearing loss. In some ethnic populations, more than 80 percent of cases of nonsyndromic recessive deafness result from a mutated Connexin 26 gene. Children born with bilateral (both ears) severe to profound sensorineural hearing losses are typically referred for genetic testing for Connexin 26 mutations.

Connexin 26 mutations are genetically transmitted from parent to child in a recessive manner, which means that an affected individual must inherit one copy of the non-functioning gene from each parent. Carriers who only have one copy of the gene (i.e. one normal gene and one mutated gene) do not manifest any of the signs of the condition, but have a 1 in 2 chance of passing on the defective gene to their children (who would then also be carriers only), and a 1 in 4 chance of having an affected child if the other parent is also a carrier. This explains how Derek can hear perfectly, as Jon and I do.

Connexin 26 mutations are non-syndromic, meaning that the mutation produces only isolated hearing loss. There is NO increased risk for other medical problems that are commonly associated with hearing loss such as blindness, thyroid problems, kidney problems, or balance disorders. However, there is in a minority of cases a higher incidence of skin disorders in patients with a Connexin 26 mutation.

This is the Best Part of knowing it's Connexin 26!  No associated issues with the deafness!

Connexin 26 mutations produce a pre-lingual hearing loss, because it affects the child before they are old enough to develop speech.

Why do we need Connexin 26 to hear?

The GJB2 gene contains the instructions for manufacturing a number of proteins, including Connexin 26. Connexin proteins in general are called “gap-junction proteins” which are necessary for cells to communicate with each other. Without sufficient levels of Connexin 26, the potassium flow from hair cells in the cochlea is disrupted, resulting in extremely high levels of potassium in the Corti s organ, leading to the profound sensorineural hearing loss. Connexin 26 mutations occur primarily in Caucasians, Ashkenazi Jews, and some Asian populations.

This seems like old news, since we found out about this with Taylor within a month or so of finding out she was deaf.  Now, we have another baby on the way and the worry starts anew in that this time we know there's a 25% chance.  Thankfully, we know what to do and how to find out this time...

I pulled this information from the California Ear Institute website.  Harvard's Genetics of Deafness center has a great pamphlet of information that we found to be helpful when we confirmed the Connexin 26.