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Re: CMT 4

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This is from our Genetics presentation at the SGL Conference March 2010.

All the SGLs were asked to send in Genetics questions from their groups so

a and Carly could address them. This is what they said about CMT 4. (I have

kept the questions here first, then the answers)

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My question(s) -How common is CMT Type 4A? In other words, what percentage of

CMT diagnoses are type 4A?

-How common is it for un-related parents to have a child with CMT Type 4A when

no family history of CMT has been documented on either side?

-How far back can recessive CMT genes go in a family tree without being

recognized?

The actual percentage of people with CMT4A is not known. What we do know is that

the recessive types of CMT (CMT4) make up between 5 and 10% of all CMT. Of

these, it is seeming more and more likely that CMT4C is the most common type,

followed by CMT4A. Out of our clinic population (~800 people with CMT), 7 have a

CMT4, with 3 having CMT4C and 2 having CMT4A. For most families who have a child

with a recessive disorder, there is no family history (unless the family is

related by blood, in which the recessive forms of CMT may be seen more often).

Of the three patients with CMT4C and two with CMT4A in our clinic, three of

these patients had parents who were not related and in some cases, came from

different parts of the world.

Because there are no symptoms associated with being a carrier for a recessive

condition, a family may have a mutation that goes back for generations and

generations without ever knowing it, until a child inherits another copy of the

gene with a mutation, and has CMT4.

It is important to note that everyone has 8 – 10 genes that have mutations in

them, but we do not know it because they do not cause any symptoms – we are

carriers for recessive conditions. It is not until another person has a mutation

in the same gene, which is then passed down to a child along with the mutation

in their partner's gene that a person is affected with a recessive condition.

It is not your fault for not knowing, and it is not your fault for passing on a

gene with a mutation.

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