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Duchenne muscular dystrophy (DMD) is a severe recessive X-linked form of muscular dystrophy characterized by rapid progression of muscle degeneration, eventually leading to loss of ambulation and death. This affliction affects one in 3500 males, making it the most prevalent of muscular dystrophies. In general, only males are afflicted, though females can be carriers. The disorder is caused by a mutation in the gene DMD, located in humans on the X chromosome (Xp21). The DMD gene codes for the protein dystrophin, an important structural component within muscle tissue. Dystrophin provides structural stability to the dystroglycan complex (DGC), located on the cell membrane. Symptoms usually appear in male children before age 6 and may be visible in early infancy. Progressive proximal muscle weakness of the legs and pelvis associated with a loss of muscle mass is observed first. Eventually this weakness spreads to the arms, neck, and other areas. Early signs may include pseudohypertrophy (enlargement of calf muscles), low endurance, and difficulties in standing unaided or inability to ascend staircases. As the condition progresses, muscle tissue experiences wasting and is eventually replaced by fat and fibrotic tissue (fibrosis). By age 10, braces may be required to aid in walking but most patients are wheelchair dependent by age 12. Later symptoms may include abnormal bone development that lead to skeletal deformities, including curvature of the spine. Due to progressive deterioration of muscle, loss of movement occurs eventually leading to paralysis. Intellectual impairment may or may not be present but if present, does not progressively worsen as the child ages. The average life expectancy for patients afflicted with DMD varies from early teens to age mid 30s. There have been reports of DMD patients surviving past the age of 40 and even 50.
[edit] Incidence/prevalenceDuchenne muscular dystrophy is caused by mutations in the DMD gene, which is located on the X chromosome. Due to this, DMD has an incidence of 1 in 3,500[1] newborn males. Mutations within the DMD gene can either be inherited or occur spontaneously during germline transmission. [edit] EponymDMD is named after the French neurologist Guillaume Benjamin Amand Duchenne (1806-1875), who first described the disease in 1861. [2] [edit] PathogenesisDuchenne muscular dystrophy is caused by a mutation of the dystrophin gene at locus Xp21. Dystrophin is responsible for connecting the cytoskeleton of each muscle fibers to the underlying basal lamina (extracellular matrix) through a protein complex containing many subunits. The absence of dystrophin permits excess calcium to penetrate the sarcolemma (cell membrane). In a complex cascading process that involves several pathways and is not clearly understood, increased oxidative stress within the cell damages the sarcolemma, and eventually results in the death of the cell. Muscle fibers undergo necrosis and are ultimately replaced with adipose and connective tissue. [edit] SymptomsThe main symptom of Duchenne muscular dystrophy, a progressive neuromuscular disorder, is muscle weakness associated with muscle wasting with the voluntary muscles[citation needed] being first affected, especially the pelvis and calf muscles. Muscle weakness also occurs in the arms, neck, and other areas, but not as early as in the lower half of the body. Symptoms usually appear before age 6 and may appear as early as infancy. Generalized weakness and muscle wasting first affecting the muscles of the hips, pelvic area, thighs and shoulders. Calves are often enlarged. The other physical symptoms are:
[edit] Signs and testsMuscle wasting begins in the legs and pelvis, then progresses to the muscles of the shoulders and neck, followed by loss of arm muscles and respiratory muscles. Calf muscle enlargement (pseudohypertrophy) is quite obvious. Cardiomyopathy(DCM) may occur, but the development of congestive heart failure or arrhythmias (irregular heartbeats) is rare.
[edit] Diagnosis[edit] DNA testThe muscle-specific isoform of the dystrophin gene is composed of 79 exons, and DNA testing and analysis can usually identify the specific type of mutation of the exon or exons that are affected. DNA testing confirms the diagnosis in most cases.[3] [edit] Muscle biopsyIf DNA testing fails to find the mutation, a muscle biopsy test may be performed. A small sample of muscle tissue is extracted (usually with a scalpel instead of a needle) and a dye is applied that reveals the presence of dystrophin. Complete absence of the protein indicates the condition. Over the past several years DNA tests have been developed that detect more of the many mutations that cause the condition, and muscle biopsy is not required as often to confirm the presence of Duchenne's. [edit] Prenatal testsIf one or both parents are 'carriers' of a particular condition there is a risk that their unborn child will be affected by that condition. 'Prenatal tests' are carried out during pregnancy, to try to find out if the fetus (unborn child) is affected. The tests are only available for some neuromuscular disorders. Different types of prenatal tests can be carried out after about 11 weeks of pregnancy. Chorion villus sampling (CVS) can be done at 11–14 weeks, and amniocentesis after 15 weeks, while fetal blood sampling can be done at about 18 weeks. Women and/or couples need to consider carefully which test to have and to discuss this with their genetic counselor. Earlier testing would allow early termination, but it carries a slightly higher risk of miscarriage than later testing (about 2%, as opposed to 0.5%). [edit] TreatmentThere is no known cure for Duchenne muscular dystrophy, although recent stem-cell research is showing promising vectors that may replace damaged muscle tissue. Treatment is generally aimed at controlling the onset of symptoms to maximize the quality of life, and include the following.
[edit] PrognosisDuchenne muscular dystrophy eventually affects all voluntary muscles and involves the heart and breathing muscles in later stages. The life expectancy typically ranges from the late teens to the mid-30s.[4] Recent advancements in medicine are extending the lives of those afflicted. [edit] Physical TherapyPhysical therapists are concerned with enabling children to reach their maximum physical potential. Their aim is to:
[edit] Mechanical ventilatory/Respiration AssistanceModern "volume ventilators/respirators," which deliver an adjustable volume (amount) of air to the person with each breath, are valuable in the treatment of people with muscular dystrophy related respiratory problems. Ventilator treatment can begin in the mid to late teens when the respiratory muscles can begin to collapse. However there are people with the disease in their 20's who have no need for a ventilator. If the vital capacity has dropped below 40 percent of normal, a volume ventilator/respirator may be used during sleeping hours, a time when the person is most likely to be under ventilating ("hypoventilating"). Hypoventilation during sleep is determined by a thorough history of sleep disorder with an oximetry study and a capillary blood gas (See Pulmonary Function Testing). The ventilator may require an endotracheal or tracheotomy tube through which air is directly delivered, however, for some people delivery through a face mask is sufficient. If the vital capacity continues to decline to less than 30 percent of normal, a volume ventilator/respirator may also be needed during the day for more assistance. The person gradually will increase the amount of time using the ventilator/respirator during the day as needed. A tracheotomy tube may be used in the daytime and during sleep, however, delivery through a face mask may be sufficient. The machine can easily fit on a ventilator tray on the bottom or back of a power wheelchair with an external battery for portability. [edit] Ongoing research
Promising research is being conducted to find a therapy able to mitigate the detrimental effects of this affliction. There are many avenues currently under intense investigation, including stem cell replacement therapy, exon-skipping, analog up-regulation, and gene replacement. Another avenue is supportive care which is involved in drug development to stave off disease progression. [edit] Stem cell replacementThough stem cells isolated from the muscle (satellite cells) have the ability to differentiate into myotubes when injected directly into the muscle of animals, they lack the ability to spread systemically throughout. To effectively deliver a therapeutic dose to an isolated muscle it would require direct injections to that muscle every 2mm.[5] This problem was circumvented by using another multipotent stem cell, termed pericytes, that are located within the blood vessels of skeletal muscle. These cells have the ability to be delivered systemically and uptaken by crossing the vascular barrier. Once past the vasculature, pericytes have the ability to fuse and form myotubes.[6] This means that they can be injected arterially, crossing through arterial walls into muscle, where they can differentiate into potentially functional muscle. These findings show potential for stem cell therapy of DMD. The pericyte-derived cells would be extracted, grown in culture, and then these cells would be injected into the blood stream where the possibility exists that they might find their way into injured regions of skeletal muscle. [edit] Utrophin upregulationUtrophin regulation is of great interest as it serves as the closest endogenous analog within the human genome. This gene is shorter and located on chromosome 6 in humans. Researchers are currently focusing on understanding the regulation behind its expression within cells. It has been found previously (note Kay Davies) that upregulation of utrophin can partially compensate for muscle cells lacking dystrophin expression. [edit] Exon-skip mediated correctionIn 1990 England et al. noticed that a patient with mild Beckers muscular dystrophy was lacking 46% of his coding region for dystrophin.[7] This functional, yet truncated, form of dystrophin gave rise to the notion that shorter dystrophin can still be therapeutically beneficial. Concurrently, Kole et al. had elucidated modification of splicing by targeting pre-mRNA with antisense oligonucleotides (AONs)[8]. These AON 13-25mers were complementary to their targeting sequence, allowing for high specificity, and were modified to resist degradation by intracellular nucleases. These modifications include oligos with phosphorothioate modified backbones and addition of modifying groups to the 2' of the ribose sugar as well as radically redesigned oligo backbones such as Morpholino and PNA oligos[9]. Splice-targeted AONs were used to correct missplicing in cells removed from beta-thalassemia patients[10][11] and applied to muscular dystrophy[12][13] as well as research in spinal muscular atrophy[14], Hutchinson-Gilford progeria syndrome[15] and other diseases [16]. AONs hold the promise of a potential therapy for patients afflicted with DMD. For instance, DMD may result from mRNA that contain out-of-frame mutations (e.g. deletions, insertions or splice site mutations). It is proposed that disease caused by these frameshift mutations can be relieved by restoring the mRNA sequence downstream of the mutation to its proper reading frame. To do this, AONs are developed targeting regions within the pre-mRNA that can mask spliceosomal recognition of an exon or exons. By targeting these regions with AONs, it is possible to remove exons from the mature mRNA, modifying out-of-frame RNA to produce functional coding dystrophin mRNA (See alternative splicing). Similar AON-based exon excision can be used to remove mutations causing premature stop codons. Morpholino-based multiexon-skipping has been successfully used in a dog model of DMD.[17] Though AONs hold promise, one of their major pitfalls is the need for periodic redelivery into muscles. Systemic delivery on a recurring basis is being tested in humans (http://clinicaltrials.gov/ct2/show/NCT00844597). To circumvent the requirement for periodic oligo delivery, a long-term exon-skip therapy is being explored. This therapy consists on modifying a small nuclear RNA (snRNA) at the 5' end of the non-translated RNA to target regions within pre-mRNA. This has been shown to work in the DMD equivalent mouse, mdx.[18] [edit] Supportive care - drug developmentRecent research shows Losartan, a currently available drug used for treating hypertension, to be effective in halting the progress of the disease in mice that were genetically engineered to have Duchenne's.[19] Human trials are in planning. Some parents of children with Duchenne's are noting reductions of symptomatic severity from a regimen of Protandim, a non-prescription nutritional supplement that increases levels of two specific antioxidant enzymes. Other parents report no benefit. Controlled clinical trials have not yet been conducted, and parent observations may have been influenced by confounding factors such as expectation bias, normal developmental progress, and the common practice of implementing additional nutritional supplements and/or corticosteroids concurrent with the Protandim. However, Protandim is promising on a theoretical level, in that it has the potential to modify the inflammatory/cell death cycle. DMD mouse-model trials of the therapy are in progress, and human trials are planned.[20][21] Research from a group in France led by L. Ségalat has identified a number of drugs that are currently licensed for other applications as halting or reducing dramatically the advance of muscle degeneration in a worm model of DMD.[22] They are now using mouse models to confirm these findings, which so far are looking very promising, confirming the efficacy of these drugs. However, work in mice seems to be moving slowly. The main classes of drugs they identified were SSRI (i.e. antidepressants such as Prozac) and muscle relaxants, such as those used by athletes after heavy training. There is conflicting evidence from animal models suggesting that doing less exercise slows down the rate of degeneration of the muscle; therefore there is a possibility that both these drugs act somewhat as sedatives, although the reality seems to be that the worms and mice are more active overall, as they have less muscle damage and so can remain active for much longer. More recently, a group at the Montreal Heart Institute and McGill University reported that a mouse model of Duchenne's muscular dystrophy demonstrated early metabolic alterations that precede overt cardiomyopathy and may represent an early "subclinical" signature of a defective nitric oxide (NO)/cGMP pathway. Accordingly, they used genetic and pharmacological approaches to test the hypothesis that enhancing cGMP, downstream of NO formation, improves the contractile function, energy metabolism, and sarcolemmal integrity. Treatment with Sildenafil delayed the appearance of symptoms in mouse hearts with Duchenne's and allowed to withstand an acutely increased cardiac workload. [23] [edit] Clinical trialsMore information on the new PTC124 trials, currently nearing the end of Phase II, is available at the www.MDA.org website. This potential treatment would address from 5 to 15 percent of DMD cases where the dystrophin protein cannot be completed due to an incorrect stop codon in the genetic sequence. The PTC124 treatment skips the improper "stop" instruction, allowing reading through of the remaining sequence and completion of the dystrophin protein assembly process. In recent mouse trials, PTC124 was found to repair damaged muscle tissues.[24][25] Safety and efficacy studies of antisense oligonucleotides for exon skipping in Duchenne muscular dystrophy with Morpholino oligos[26] and with 2'-O-methyl phosphorothioate oligos [27] are in progress. [edit] CounselingGenetic counseling is advised for people with a family history of the disorder. Duchenne muscular dystrophy can be detected with about 95% accuracy by genetic studies performed during pregnancy. [edit] Organizations specific to DMDIn addition to charities devoted to muscular dystrophies in general (such as MDA), these charities are devoted exclusively to DMD:
[edit] References
[edit] See also[edit] External links
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