From Clear-Cut Endings to Complex Beginnings:
Researchers Probe the Origins of Charcot-Marie-Tooth Disease
by Dan Stimson
"Clinical medicine is first and foremost the study of the
difficult aspects and complexities of diseases. When a patient calls on you, he
is under no obligation to have a simple disease just to please you."
— Jean-Martin Charcot in a lecture to medical students, 1887 |
The renowned 19th-century French neurologist Jean-Martin Charcot advised young
scientists that they should be prepared to deal with complex origins and
manifestations of disease. But even he couldn't have predicted the hidden
complexity of a disease he helped to discover — Charcot-Marie-Tooth disease
(CMT).
CMT — first described by Charcot and his contemporaries Pierre Marie and H.H.
Tooth in 1886 — causes weakness by damaging nerves that are required for muscle
control, and is the most common inherited disorder of the nervous system.
Despite its remarkable history and prevalence, CMT has no treatment, and its
genetic origins were completely unknown until 1992.
The search for a genetic cause of CMT began in the mid-1980s. Around that time,
researchers had discovered that Duchenne/ Becker muscular dystrophy (DMD/ BMD)
was caused by defects in a single gene. The same is true in both Huntington's
disease and cystic fibrosis. In each case, the discovery of a single disease
gene provided a key for understanding disease pathology and for designing
possible treatments that are currently being tested.
Those successes provided CMT researchers with the technical advances and the
optimism they needed to hunt for a CMT gene, but they didn't anticipate a
simple resolution. By the 1970s, several types of CMT were recognized based on
characteristic features of the disease within different families. So,
researchers weren't expecting to find just one CMT gene.
But they've been stunned by the number of CMT genes their search has uncovered.
As is the case for Huntington's, DMD/BMD and cystic fibrosis, a single
defective gene can cause CMT in one person. But there appear to be at least 18
CMT genes among the world population. So far, seven of these CMT genes have
been identified, and 11 more are suspected.
Although CMT is more complex than Charcot could have imagined, modern
researchers are now beginning to unravel that complexity. Studies of CMT genes
and their functions are leading to a better understanding of how nerves work,
how they become damaged in CMT and how to protect them from damage.
A Breakdown of the Peripheral Nerves
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Peripheral nerves control movement by relaying impulses from the
spinal cord (not shown) to our muscles (shown in the forearm). A single
peripheral nerve is composed of many long nerve cell branches — or axons — that
extend from the spinal cord and connect to muscle fibers. Each axon is
surrounded by myelin made from the wrappings of Schwann cells. |
Despite its diverse genetic origins, CMT is universally recognized as an
inherited disorder that affects the peripheral nerves — nervous tissue that
connects the spinal cord to the muscles and sensory organs. Its defining
symptoms are muscle weakness and wasting, and usually some loss of sensation,
in the body's extremities (the forearms, lower limbs, hands and feet). Often,
these symptoms appear in adolescence.
Because of these features, CMT is sometimes called hereditary motor and sensory
neuropathy or HMSN (a neuropathy is any disease of the peripheral nerves). CMT
also has a more old-fashioned name — peroneal muscular atrophy — which refers
to wasting of the peroneal muscle in the lower leg.
Usually, CMT isn't life-threatening. But it can cause significant disability,
including difficulty with routine activities like walking and grasping objects.
Sometimes, a person with CMT requires braces or a wheelchair for mobility. The
muscle weakness of CMT can cause deformities of the hands and feet, and the
loss of sensation can cause minor foot injuries to go unnoticed, sometimes
leading to infected sores called ulcerations. In severe cases, the ulcerations
might require amputation.
 Jeffery Vance |
Understanding how CMT affects the peripheral nerve is key to identifying
treatments for the disease, says geneticist Jeffery Vance, who as an MDA
grantee helped to find one of the first CMT genes, and who continues to study
CMT at Duke University in Durham, N.C. "The peripheral nerve is not a really
well understood organ," says Vance, "and until you understand its normal
biology, it's difficult to understand its disease biology."
The peripheral nerves are bundles of nerve cell tendrils — or axons — which send
electrical signals to muscles, and receive signals from sensory organs. Within
each peripheral nerve, each axon is surrounded by a coating called myelin,
which is produced by supportive cells called Schwann cells (see the illustration). The myelin insulates the electrical signals traveling
through axons, allowing rapid transmission of the signals to and from the
spinal cord.
A 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, CMT4B 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
Michael 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. |
Long before the CMT gene hunt began, clinical examinations of patients suggested
that CMT could be caused by inheritable defects in the axons or the myelin of
peripheral nerves. Those defects ultimately damage the nerves, and are believed
to be especially harmful to longer nerves, which explains why CMT mostly causes
motor and sensory problems in the body's extremities.
Based partly on whether it's caused by defects in axons or in myelin, and partly
on its pattern of inheritance, CMT has been divided into five major types.
CMT1 and CMT2 are the most common types, and both are autosomal dominant,
meaning that a child can inherit the disorder from one parent. CMT1 is caused
by a loss of myelin (demyelination), which physicians can detect by using a
nerve conduction velocity test (NCV) to show a decline in the speed of nerve
signaling (see "Simply Stated," Quest, vol. 7, no. 5).
In contrast, CMT2 is caused by defects in axons (axonopathy), and shows little
or no slowing of the NCV.
The three other types of CMT are all caused by demyelination. CMT3 (usually
called Dejerine-Sottas disease) and CMT4 are both very rare. They're autosomal
recessive, meaning that a child can inherit the disease only if he gets the
gene from both parents. Finally, CMTX is X-linked dominant, meaning that a girl
can inherit the disease from either parent, while a boy can inherit it only
from his mother.
Pretty simple, huh? Not quite. For a more in-depth look at CMT types, see "A
Pathological Puzzle."
Schwann Cells: More than Myelin
Much to the satisfaction of researchers, the identification of CMT genes has
confirmed the idea that CMT can be caused by defects in either the myelin or
the axons of peripheral nerves. Six of the known CMT genes appear to be
required in Schwann cells (which make myelin), and one is known to be critical
for axon structure. (For descriptions of the seven identified CMT genes and
their functions in peripheral nerve, see "The
Players.")
Identification of these genes also helped establish the surprising discovery
that defects in myelin or in axons ultimately produce the same type of damage
in peripheral nerves — destruction of the axons.
 Michael Shy |
"As more and more causes of CMT became apparent, this gave more clues to how
peripheral nerve damage could occur," says neurologist Michael Shy, who's
studying possible CMT treatments at Wayne State University in Detroit with MDA
support. "Investigators have now begun to realize that even in the
demyelinating forms of CMT, disability correlates more with degeneration of the
axons inside the myelin than the demyelination itself." This means that a child
with a family history of CMT might show signs of demyelination during an NCV
test, but won't begin to show symptoms of CMT until the demyelination causes
axonal damage, explains Shy.
How does the loss of myelin eventually destroy axons? Researchers suspect that
some essential communication must normally take place between axons and Schwann
cells. "One thing that researchers are learning is that the Schwann cell isn't
just hanging around making myelin," says Vance. "It's clearly involved in
regulating the health of the axon."
 For axons and Schwann cells, communication is the key to a healthy relationship. Axons send chemical messages (red) that attract Schwann cells and encourage myelin formation, and Schwann cells appear to send messages (green) that nourish and protect axons. |
Nonetheless, the process by which Schwann cells make myelin might reveal clues
to how they nurture axons. In the developing nervous system, the Schwann cells
essentially become mye-lin by lining up along an axon and wrapping themselves
around the axon to form successive layers (see illustration).
The axon sends the Schwann cells chemical messages that tell them to form
myelin, and the Schwann cells appear to respond with their own messages. In the
mature nervous system, continued communication between the axon and the Schwann
cells appears necessary to keep the myelin intact and the axon healthy,
explains Shy.
Mutations in the different "myelin" genes probably cause demyelination in
different ways, Shy points out. "Figuring all these out is likely to be
tricky," he says, "so the current research is beginning to focus on how Schwann
cells communicate with axons and how axonal degeneration occurs." Finding a
means to promote normal communication between Schwann cells and axons might
provide a way to treat CMT, he says.
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