Tuesday, September 15, 2009

About Connexin 26

  • The most common cause of hearing loss in American and European populations is a mutation in the connexin 26 (Cx26) gene.

  • Cx26 has a carrier rate of 3%, similar to that for cystic fibrosis, and it causes about 20% of childhood deafness.

  • Everyone has two copies of this gene, but if each birth parent has a flawed copy of the GJB2/Connexin 26 gene, the baby may be born with a hearing loss. The hearing loss occurs because the mutation is suspected of disrupting potassium flow in the inner ear.
  • Connexin 26 mutations occur primarily in Caucasians, Ashkenazi Jews, and some Asian people.
  • 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.
  • Recent medical evidence has shown that there is an additional complexity to Connexin 26 mutations. Currently, only about 75% of the mutations predicted to be Cx26 actually show up in genetic testing. In some cases this may be due to mutations in another gene immediately adjacent to Cx26 called Connexin 30 (Cx30) at gene (GJB6). Connexin 26 and Connexin 30 are expressed in the same hair cells of the cochlea. Cx26 and Cx30 are similar, sharing over three-quarters of the same amino acid sequences. These considerations have led researchers to believe that mutations in Cx30 are probably also responsible for hearing loss.
  • 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.

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