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Cochlear Implant Program

If your child has been tested

So far, we have already tested over 300 children with cochlear implants and over 1000 of their family members.  We would like to thank you for taking the time to be tested and to advance our knowledge of genetic causes of hearing loss.

Background

Approximately 1/500 children are born with hearing loss, and half of these are due to genetic changes.   Genetic hearing loss can be non-syndromic (individuals only have hearing loss) or syndromic (individuals have hearing loss plus other affected parts of the body).  If your child has hearing loss but no other parts of the body are affected, then there are several tests that we will perform.  The first of these tests is for a gene called connexin 26.  We test for this gene first because up to 60% of non-syndromic hearing loss is due to changes in connexin 26.

What is Connexin 26?

Connexin 26 is normally found in the cochlea or inner ear and helps to remove potassium from the fluid inside the cochlea.  This fluid normally helps to regulate the environment in the cochlea so that the hair cells and nerves of hearing can send signals to the brain.  When someone inherits a genetic change in connexin 26, potassium can not be removed from the fluid in the cochlea.  This buildup of potassium in the cochlear fluid prevents the hair cells from sending signals to the brain, resulting in hearing loss.

How are these genes inherited?

Every child inherits half of their genes from their mother and half from their father.  For recessive disorders such as hearing loss due to a connexin 26 mutation, usually each parent has normal hearing but carries a connexin 26 mutation on half of their genes.  Approximately 3% of the population are carriers of this genetic change.

 

MOTHER(Aa)

A

a

 Father

  (Aa)

A

AA

Aa

a

Aa

aa

In the diagram above, the big “A” is the “normal” hearing gene and the little “a” is the hearing gene with the mutation in connexin 26.  When two carrier have a child, there are 4 possible combinations of genes that can be inherited.  These are as follows:

AA = ¼ or 25% = child has normal hearing and can not pass down hearing loss genes to their children

Aa = 2/4 or 50% = child has normal hearing but carries the hearing loss gene, and can pass it to their children

aa = ¼ or 25% = child has hearing loss, and his/her children will either carry the gene or have hearing loss

*Remember: Every new brother and sister will have the same chances of inheriting these combinations of genes.  This means that even though the odds of having a child with hearing loss are 1 in 4, it is still possible for all of your children to have hearing loss.  Also, it is possible for none of your children to have hearing loss.

How do I interpret the exact Connexin 26 change?

Every person inherits one half of their connexin 26 gene from each parent.  Each half of the gene is represented on either side of a forward slash.  There are 3 possible combinations that a person can inherit:

  1. None/None  :  meaning that no connexin mutations were found on either half
  2. Connexin mutation/None :  meaning that one half of their genes have a connexin 26 mutation and there are no mutations on the other half
  3. Connexin mutation/Connexin mutation :  meaning that both halves of their genes have a connexin 26 mutation.

What do all the numbers and letters in the connexin mutation mean?

The connexin 26 test can even tell us exactly where the mutation occurred in the gene.  For example, the mutation 35delG means that at position 35, there was a deletion in the DNA sequence of a Guanine residue which is represented by the letter “G”.  There are many different types of changes that we are finding.  Interestingly, the type of change that someone has is related to the country that their ancestors originally came from. 

If my child has a Connexin 26 mutation, what are the chances that my child will have children with (or that my grandchildren will have) hearing loss?

The answer to this depends on who your child has children with!  There are three possible types of people that your child can have children with.  For these answers, please recall that a big “A” is the “normal” hearing gene, and the little “a” is the hearing loss gene due to a mutation in connexin 26. 

Scenario 1

Your child who has hearing loss “aa” has children with another person with hearing loss “aa”.

 

DEAF PARTNER

a

a

YOUR CHILD

a

aa

aa

a

aa

aa

Result: all of their children will have hearing loss “aa”.

Scenario 2

Your child who has hearing loss “aa” has children with someone who has “normal” hearing, but is a carrier “Aa” of the connexin 26 mutation, just like you probably are.

 

CARRIER PARTNER

A

a

YOUR CHILD

a

Aa

aa

a

Aa

aa

Result: Half of their children will have hearing loss “aa”, and half of their children will have “normal” hearing, but will also be carriers “Aa”.  Carriers “Aa” of the genetic mutation, can pass it down to their children.

Scenario 3

Your child who has hearing loss “aa” has children with someone who has “normal” hearing and is NOT a carrier of the connexin 26 mutation.

 

NORMALLY HEARING PARTNER

A

A

YOUR CHILD

a

Aa

Aa

a

Aa

Aa

Result: All of their children will have “normal” hearing.  However, they will all be carriers “Aa” of the connexin 26 mutation, and could pass it down to their children.

If my child has a Connexin 26 mutation causing their hearing loss, who else in the family should be tested?

Parents of children with hearing loss should be tested for this same genetic change so that we can make sure that the gene was inherited. 

Brothers and sisters of children who have a connexin 26 genetic change should also be tested, because even if they have “normal” hearing, there is still a 67% chance that they carry the genetic change and could pass it along to their children. 

We can also test any relatives of children with hearing loss. 

To be tested, call Patt Fuller in the Cochlear Implant Program (416-813-7259) and make a time to meet with her to fill out the forms.  This will only take a few minutes.  She will ask for your Date of Birth and Health Card. 

What does testing involve?

A small sample of blood is taken by trained blood technicians at The Hospital for Sick Children.  The room is located on the main floor, and is the first door to the right of SHOPPER’S DRUG MART (#8 on the map below).  The whole event will take only a few minutes, and is generally well tolerated.  The blood lab is open from Monday to Friday from 7:30am to 4:30pm.   We recommend arriving before 4:00pm.

How will we get these new results?

The Cochlear Implant Program will either telephone you to discuss the results or will mail the results to you.  You may also meet with Dr. Evan Propst or Dr. Blake Papsin to discuss the results by making an appointment through Patt Fuller in the Cochlear Implant Program.  If you would like further information after this, you can also meet with a genetic counselor to discuss your results. 

If our results are negative for a Connexin 26 mutation, what happens next?

If your child is a carrier “Aa” or does not have any mutations in the connexin 26 gene, then we will continue to test for other genes that are known to cause hearing loss.  WE DO NOT NEED ANY MORE BLOOD, SINCE WE CAN KEEP TAKING SMALL DROPS OF BLOOD FROM THE SAMPLE THAT WE ALREADY HAVE.  Every time that we test the sample for another gene, we will contact you with the result, either by telephone or by email.

Thank you for your participation and for helping us advance our knowledge of the genetic causes of hearing loss!

Dr. Evan Propst and Dr. Blake Papsin