Download duchenne muscular dystrophy (dmd) introduction

Georgiana Laura Cioanca
Ákos Bogó
Esther Hazane Leroyer
Molecular Genetics Class Presentations
JPEMS 2015, Szeged
DUCHENNE MUSCULAR DYSTROPHY (DMD)
INTRODUCTION
Duchenne Muscular Dystrophy (DMD) is the most common muscular dystrophy and the most
severe. It takes its name from the French neurologist Guillaume Duchenne, who was the one to
describe the condition. DMD is a single-gene type of disorder, caused by loss-of-function mutations
(usually deletions) affecting the dystrophin gene. This gene is located on the X chromosome, thus the
disorder shows X-linked recessive inheritance pattern, with an incidence of 1 in 3500 males. 1 Female
mutation carriers can also show mild signs in 20°% of the cases.2 Mutations in the DMD gene can also
result in Becker Muscular Dystrophy (BMD), similar condition associated with milder symptoms and
better prognosis.
DMD is characterized by a childhood onset, as the first clinical signs are seen in males
between the ages of 3 and 5 years. As dystrophin is found in the skeletal, heart and smooth muscle,
as well as in the brain, these are the affected organs in DMD. Loss of function of dystrophin leads to
muscle cell degeneration and as a consequence to an increased serum level of creatine kinase (CK).
The progressive proximal muscle weakness, accompanied by calf muscle pseudohypertrophy is the
most distinctive sign. Young affected boys show difficulties in rising from the floor (positive Gower’s
sign, figure 1). Subsequently, patients develop increased lumbar lordosis and scoliosis, leading to a
waddling gait (figure 2). Independent ambulation soon becomes impossible to perform, thus in most
of the cases patients are wheelchair bound by the age of 11 years.1 Patients show breathing
shortness, due to the respiratory muscles wasting. The myocardium is also affected and DMD
patients develop dilative cardiomyopathy. Regarding the brain damage, a mild to moderate
intellectual impairment can usually be observed. In spite of this fact, about 20% of patients present
severe mental retardation. 2
Figure 1. Gower’s sign: patients have
difficulties in rising from the floor, pushing on
their hands in order to stand up
Figure 2. Musculo-skeletal abnormalities in
DMD leading to waddling gait
JPEMS 2015, Szeged
Unlike BMD, in which the life expectancy is only slightly reduced, Duchenne Muscular
Dystrophy is associated with a bad prognosis. Without steroid medication, the lifespan of DMD
patients has a mean of 18 years. The most common cause of death is represented by
cardiorespiratory failure. 1
GENETIC DEFECT
DYSTROPHIN GENE
The dimer protein causing Duchenne Muscular Dystrophy (DMD) is called the dystrophin.
Dystrophin is encoded by dystrophin gene, the larger gene in human genome. Indeed, it is composed
of 2,4 Mb of genomic DNA. It contains 79 exons, corresponding to 14kb, transcripted into mRNA. The
large size of the gene could possibly explain the high mutation rate. The transcription occurs in all
muscle cells (striated, smooth and cardiac cells) as well as in the brain, which explains the mental
disorders patients can sometimes endure. The gene is located on the X-chromosome, which is why
DMD is an X-linked disease. Moreover, it is an X-recessive disease, so mostly males are supposed to
be affected and females are called carriers (figure 3). However, the first detection of the gene and of
its location was guessed because of female affected by DMD. In fact, those females bore an Xautosome translocation, with a common breakpoint at Xp21. In this case, due to the skewedinactivation of the X chromosome, only cells containing the X-autosome survive, because the X
chromosome involved in the translocation survives preferentially so as to maintain functional disomy
of the autosomal genes.
Figure 3. Family tree of a Duchenne Muscular
Dystrophy family with the disorder being
transmitted by carrier females and affecting
males.
TYPE OF MUTATIONS
About 2/3 of all dystrophin gene mutations are deletions. There are hot spots for deletion,
namely, the first 21 exons, as well as around exon 45 to 53, leading to disturbances of the
translational reading frame. Appearance of stop codons, frameshift mutations, altered splicing
signals or promoter mutations can occur, mainly drafting to premature translational termination. A
shorter protein, or often no protein at all is transcripted.
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About 30% of mutations in the dystrophin gene arise as de novo mutations (ie not present in
the mother), which is important to take into account for genetic counselling.
Less frequently, mutations can take the form of duplications or point-mutations. If deletions arise
almost exclusively from maternal meiosis, point-mutations often arise from paternal-meiosis.
PHENOTYPE correlation
The absence or abnormality of dystrophin protein causes the muscle cell degeneration, which
is the onset of DMD. Mutated dystrophin cannot undergo muscle strength anymore. Dystrophin
usually binds to a glycoprotein complex (β- dystroglycan) in the muscle membrane through its Cterminal domain, which makes the link with extracellular laminin, and to intracellular actin through
its N-terminal domain (figure 4). 1
Dystrophin provides mechanical reinforcement of the sarcolemma. It protects it from the
membrane stresses developed during muscle contraction. The sarcolemma can usually undergo both
radial and longitudinal tensions, and since myofilaments are connected to actin, then to dystrophin,
and then to the sarcolemma, dystrophin helps to serve this function as well. Muscle cells cannot
withstand the tension anymore, which leads to an impaired muscle cell function. 3
Figure 4. Dystrophin protein molecule, depicted as a dimer linking
intracellular actin and extracellular laminin.
DIAGNOSIS
As mentioned above, there are some distinctive clinical signs in DMD which may indicate the
diagnosis. However, these features are not enough and further investigations should always be
performed in order to confirm the diagnosis.
Before DNA analysis developed, the diagnosis was based on pedigree information and
evaluation of creatine kinase levels.1 Nowaday, it is possible to have a precise diagnosis by
investigating the dystrophin gene, since the majority of DNA mutations can be detected by DNA
testing. The identification of larger duplications or deletions is obtained by tests such as arrays,
amplification and hybridization. If these large mutations are not to be found, then further tests are
coming for smaller changes, like point mutations. For example, Sanger Gene sequencing or
Resequencing arrays are used for this purpose. 4
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Performing a biopsy might also provide additional information besides the previously
mentioned methods. A sample is taken from the skeletal muscle and examined after the application
of stains or specific antibodies for dystrophin or dystrophin associated molecules
(immunofluorescence). Gamma-sarcoglycan is also a useful marker, since in immunofluorescence
and western blotting in skeletal muscle from the DMD patients showed complete absence of it.5
Since female carriers are usually asymptomatic, it is of an utmost importance to detect the
carriers of the mutant allele. Especially in pregnant female carriers, since their child might inherit
the mutation, thus developing DMD. Prenatal diagnosis is also available by performing
amniocentesis.6
TREATMENT OPTIONS
No treatment to cure Duchenne Muscular Dystrophy have been discovered so far, thus for
now the only possibility is to attempt to improve the life quality of these patients. Physotherapy and
steroids show to be helpful to maintain the mobility for a longer period, but it do not have curable
effects. It is only the gene therapy that might offer a real therapeutic solution for DMD.1
At the moment, several experiments are conducted in different animal models. The most
commonly employed models are mdx mice and canine X-linked muscular dystrophy (CXMD).
Studying the mdx mice model allows us to understand the mechanisms behind the muscle
degeneration and regeneration in DMD. However, it does not provide a precise model of the human
disease: mice show severe muscle weakness, but the lifespan is only reduced with 25%, whereas in
humans it is reduced with 75%. CXMD dogs represent a better alternative, as the pathological
expression of the diseases is more related to the human DMD: progressive skeletal muscle weakness,
associated with degeneration of the myocardium and bad prognosis.7
Currently, several gene therapy strategies are under development. One approach is to
modify the abnormal dystrophin gene (by gene replacement or gene repair) in order to encode a
protein with some residual function. As a consequence, the expression of the disease might be
characterized by less severe manifestations (obtaining Becker Muscular Dystrophy phenotype). In
some mice with dystrophin-negative dystrophy a spontaneous muscle repair can occur and this is
due to the on switching of the gene coding utrophin (protein expressed in the fetus, similar to
dystrophin). Finding a way to reactivate the utrophin gene represents another possible treatment for
DMD disease. The technologies used to accomplish these goals include recombinant DNA, viral
vectors and myoblast implantation.1
In conclusion, finding a cure for Duchenne Muscular Dystrophy represents an important field
of interest for researchers nowadays. Until an effective treatment will be found, the only available
options are genetic counselling for the families with DMD history and offering support for DMD
patients, in order to improve their life quality.
REFERENCES
1
Peter D. Turnpenny, Sian Ellard. Emery’s Elements of Medical Genetics, Philadelphia, PA: Churchill
Livingstone/Elsevier , 2012, 14th Edition
2
http://www.omim.org/entry/310200 Muscular Dystrophy, Duchenne type; DMD, 2014
3
Petrof and Al. 1993, Dystrophin protects the sarcolema from stresses developed during muscle
contraction
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4
http://research.duchenneconnect.org/index.php?option=com_content&view=article&id=155&Itemi
d=241
5
Jung, D., Leturcq, F., Sunada, Y., Duclos, F., Tome, F. M. S., Moomaw, C., Merlini, L., Azibi, K.,
Chaouch, M., Slaughter, C., Fardeau, M., Kaplan, J.-C., Campbell, K. P. Absence of gamma-sarcoglycan
(35 DAG) in autosomal recessive muscular dystrophy linked to chromosome 13q12. FEBS Lett. 381:
15-20, 1996
6
E.J Annexstad, I. Lund-Petersen, M. Rasmussen; “Duchenne muscular dystrophy”; Tidsskr Nor
Legeforen Nr. 14; 5. august 2014
7
http://www.parentprojectmd.org/site/PageServer?pagename=Advance_research_strategies_animal
models
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