Congenital muscular dystrophy
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Clinical types
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There are various types of congenital muscular dystrophy, most of which are transmitted
by
autosomal recessive inheritance. The central nervous system is also affected
in some cases. The different diseases may be distinguished based on the clinical
and laboratory findings after examination by a geneticist.
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Clinical signs
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Signs of muscle weakness with or without fixation of the joints (arthrogryposis)
appear at a very early age, usually at birth. In contrast to Duchenne muscular dystrophy, this disease
is not progressive, at least not to any significant extent.
In the laboratory, high levels of creatine phosphokinase (CPK) are found in the
blood. This enzyme is found in muscle and it -leaks- into the blood in certain muscular
diseases. Electromyography and muscle biopsy can assist in the diagnosis.
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In about two thirds of cases, there is a mutation in the gene for merosin, a protein
that binds to dystrophin, which is responsible for Duchenne muscular dystrophy.
In these cases, the disease can be diagnosed by a muscle biopsy with immunofluorescent
staining for merosin.
In healthy individuals, whose muscle contains merosin, antibodies bind with merosin,
and a fluorescent glow can be seen that is evidence of the presence of merosin in
the muscle.
In patients with congenital muscular dystrophy, who do not have normal merosin,
there is no binding of the antibody and therefore no fluorescent glow.
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Inheritance pattern
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Autosomal
recessive in most cases.
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Penetrance
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Full
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Associated features that can be demonstrated by imagery
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Usually there are no defects that can be identified by ultrasound examination during pregnancy.
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What is the risk of recurrence in a subsequent pregnancy?
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The family should be referred for
genetic counseling in order to establish exactly which condition is the
one affecting them. In the classic cases, such as in merosin deficiency, the risk
of recurrence is 25% in every pregnancy.
As far as more distant relatives are concerned, the risk of having a similarly affected
child is relatively low. Healthy siblings of patients have a risk of less than 1
in 500 of having an affected child.
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Molecular genetic information
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The gene for the disease
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CMD
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Location
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chromosome 6q22-23.
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Genetic testing
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Diagnostic testing
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Non-genetic diagnostic methods are listed above under clinical signs.
The defect in the gene can be identified using a direct test. This is a more expensive
and time-consuming process that does not offer the couple any great advantage for
prenatal diagnosis. See: Finding
the defect (mutation) by establishing the gene sequence - autosomal recessive diseases.
However, because the risk is low and the feasibility of testing their partners is
limited, these tests are not usually performed.
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Carrier testing
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In order to identify carriers in a patient-s family it is possible to use indirect
tests as described below in the section on fetal testing. See: Indirect testing for genetic
markers in a family that has one or more patients - when there is only one gene
that can cause the disease - autosomal recessive diseases
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Carrier tests for partners of carriers or in the general population
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There are no common mutations that can be specifically looked for, so that in order
to identify carriers in the general population, the entire sequence of the CMD gene
must be examined. This is a time-consuming, expensive test that is not currently
performed as a matter of routine. See: Finding the defect
(mutation) by establishing the gene sequence - autosomal recessive diseases.
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Fetal testing
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When a couple has had a child with a disease caused by a mutation in the merosin
gene, it is possible to test the fetus in a subsequent pregnancy relatively easily
in order to determine whether or not it is affected. This can only be done, however,
if DNA from the affected child is available. Usually performing linkage analysis
is enough. See: Indirect testing for genetic
markers in a family that has one or more patients - when there is only one gene
that can cause the disease - autosomal recessive diseases.
Alternatively,
chorionic villus sampling with fluorescent staining of the villi for merosin
can be performed - an absence of staining indicates that the fetus is affected,
whereas the presence of staining indicates that it is normal, provided that a lack
of merosin was demonstrated in the muscle of the affected child in the family.
If the defect in the patient does not involve merosin, a diagnosis must first be
established within counseling and testing at a genetic institute.
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