About CMT types

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THere are many types of CMT. Some can be detected with the genetic test, but

many can not. These comments and websites may help you learn more.

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http://www.geneclinics.org/profiles/cmt1/index.html

Click on Diagnosis

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http://www.neuro.wustl.edu/neuromuscular/time/hmsn.html

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The type is determined by either the genetic defect or a combination of

things including the nerve conduction velocity, heredity pattern, onset, and

symptomology. The most complete listing I have found is at

http://www.neuro.wustl.edu/neuromuscular/time/hmsn.html Many of the symptoms

are common for all

types. Every individual even those with the same type have a difference in the

symptoms and rate of progression. This has even been found with identical twins

having the same genetic makeup.

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Basically they are now sub-typing CMT by the hereditary pattern, what

specific gene carries the trait, and by the pattern of symptoms that are

involved.

Not all physicians agree on the different types. There remains much confusion on

some of the types of CMT. However, a few types of CMT are diagnosed by

genetic testing. These few types that have been identified by the genes can be

easily diagnosed with blood tests. However, a majority of the types of CMT are

diagnosed by family history, symptoms, and frequently by nerve conduction and

electric myography (EMG) studies. The terms neuronal and axonal are used to

describe the differences in of changes seen in the EMG and are utilized to

differentiate between the types of CMT.

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http://www.mdausa.org/publications/Quest/q81cmtds.cfm

Pathological Puzzle: CMT Genes and CMT Types.

Though it's been highly instructive, the emerging genetic complexity of CMT

is wrecking the orderly distinctions among different types of the disease. With

only five major types of CMT, and roughly 20 CMT genes, researchers are

finding that mutations in different genes can cause the same type of CMT.

Not only that: " We're seeing that the same gene, depending on where the

mutation is in the gene, can cause a different [type of CMT], " says geneticist

Jeffery Vance.

Even at the beginning of the CMT gene hunt, it became clear that CMT genes

weren't going to fit neatly into the disease categories that physicians and

scientists had constructed. The hunt first focused on CMT1 — the major

demyelinating type of CMT — and has uncovered at least three different

" myelin " genes.

About 80 percent of CMT1 cases are caused by mutations that affect the PMP22

gene (on chromosome 17), about 5 percent are caused by mutations in the P0 gene

(on chromosome 1) and very rare cases are caused by mutations in the EGR2 gene

(chromosome 10).

" That was a surprise for sure, " says Vance.

Another surprise is that certain mutations in each of these genes can cause

Dejerine-Sottas disease (a.k.a. CMT3). Because it's more severe than other

types of CMT, Dejerine-Sottas has traditionally been considered a distinct

disease. " Now we know that, genetically, Dejerine-Sottas is actually a severe

version

of CMT1, " says Vance.Another " myelin " gene — called Cx32 — has been

implicated in CMTX, and two others have been implicated in the demyelinating

CMTs, CMT

4B and HMSN-Lom. So, at first glance, there's a straightforward relationship

between defective " myelin " genes and demyelinating forms of CMT.

But that relationship doesn't always hold up. " For instance, " says Vance,

" Cx32 mutations can cause a neuropathy that looks very much like the axonal form

of CMT — CMT2. A lot of clinicians have had families that looked like CMT2

[based on NCV tests], but turned out to be CMTX [based on genetic testing]. " In

addition, some families thought to have CMT2 have turned out to have mutations

in the P0 gene.

Although the complex relationship between CMT genes and CMT types gets

confusing, patients shouldn't worry too much about it, says neurologist

Shy.

Genetic testing is now available for mutations in the PMP22, P0 and Cx32

genes, so a patient can sometimes determine the genetic basis of his or her

disorder. Ultimately, patients will find that information more useful than being

told they have CMT1, CMT2 or any other classic type of CMT, says Shy.

" What physicians really have to do is go over the implications of what [the

specific mutation] means in terms of who's at risk in the patient's family,

what's known about the natural history of the disease, and to work with them

about how best to remain independent, " he says.

Still, says Vance, the sometimes unpredictable connection between CMT

symptoms and their underlying cause can make it difficult for clinicians to help

CMT

patients. " If you're a clinician, you want to know: What type of test should I

run on somebody who has this clinical picture? " says Vance. Besides that,

patients will naturally want specific information about how particular mutations

are likely to affect their health.

To improve the understanding of " what particular mutations do to CMT

patients, " Shy is helping to develop a national inherited neuropathy database as

a

collaborative effort between Wayne State University and Indiana University. The

database will collect statistics on CMT patients, and use those statistics to

correlate mutations with CMT symptoms.

To find out more about the CMT database, go to the clinical trials section of

MDA's Web site, find the section titled " Diseases of Peripheral Nerve " and

connect to the link " National CMT Registry. " For physicians, registry

information will also be available with genetic test results from Athena

Diagnostics.

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Kat

Seattle WA USA