Muscular dystrophy is one of the most difficult disorders to treat. Although, its pathogenesis is well understood there is no known cure available for any of the 9 types of muscular dystrophy. Conventional methods of coping with the disease include exercise and drugs that slow down or eliminate muscle wasting like anabolic steroids and supplementation. Skeletal muscle is the most abundant tissue of the body and is composed of large multinucleated fibers, whose nuclei cannot divide. Consequently, any cell or gene replacement method must restore proper gene expression in hundreds of post-mitotic nuclei, which are embedded in a highly structured cytoplasm and surrounded by a thick basal lamina. Similarly, most pharmacological approaches must invade the complex and partly unknown biochemical mechanism of fiber degeneration.
Historical Aspect of Treatment
Guillaume-Benjamin Duchenne, a French neurologist, who first described Duchenne's muscular dystrophy, used faradic shock forMuscular dystrophy treatment. This was a non-invasive technique of muscle stimulation that used faradic shock on the surface of the skin, which he called "electrisation localisee".
Demonstration of the mechanics of facial expression. Duchenne and an assistant faradize (Faradic shock) the mimetic muscles of "The Old Man.
The current treatment of muscular dystrophy has evolved from electrical simulation to
Gene Therapy and Cell Therapy
Goals of treatment:
1. To correct the genetic defect,
2. To restore functional expression of dystrophin,
3. To slow disease progression, and
4. To improve the quality of life of MD patients
Major therapeutic strategies for Muscular Dystrophy:
1. Gene Therapy:
A. Gene replacement therapy -by
a. Delivery of dystrophin mini-gene contained in vectors like-
• Adenoviral vectors
• AAV (Adeno-Associated Viral Vectors)
• Retroviral Vectors
• Naked plasmid DNA
• Human artificial chromosomes (HACs) (episomal vectors)
(Tedesco et al. described Stem cell-mediated transfer of a human
artificial chromosome which ameliorated muscular dystrophy.)
b. Stem cells
• Satellite cells
• Muscle- or bone marrow-derived SP cells
• Bone marrow-derived stromal cells
B. Gene modification therapy (DNA/RNA Manipulation): changing or
repairing gene mutations.
a. Antisense oligonucleotides ((AONs)) resulting in exon skipping and short forms of dystrophin
• peptide nucleic acids (PNA),
• 2'-O-methyl-phosphorothiate- AONs (2'OMeAO)
(i) GSK2402968- based on the 2'OMeAO chemistry and targeting
• phosphorodiamidate morpholino oligomer (PMO) and
(i) AVI-4658- based on the PMO backbone also targets exon 51
• cell penetrating peptide-conjugated PMO (PPMO).
• locked nucleic acid (LNA)
• ethylene-bridged nucleic acid (ENA)
b. DNA Chimeraplasts to correct point mutations or small deletions
c. Viral-directed exon skipping (U7-snRNA- a non spliceosome small
nuclear RNA (snRNA))
2. Cell Therapy:
Delivery of cells that make new muscle to diseased areas:
A. Muscle precursor cells (Myoblasts)
B. Stem cells that have the ability to differentiate into muscle cells.
a. embryonic stem cells (ESCs),
b. induced pluripotent stem cells (iPSCs) and
c. adult stem cells (ASCs) which include bone marrow derived stem cells, blood- and muscle-derived CD133+ cells, muscle-derived stem cells (MDSC), side population (SP) cells and mesoangioblasts.
3. Pharmacological Therapy:
A. Drugs to turn on the utrophin gene expression
b. SMT C1100
c. okadaic acid
g. New compounds to be screened
B. Drugs to cause read-through of a premature stop codon
a. Aminoglycosides (Gentamicin, Negamicin)
b. Ataluren (PTC124)- a 1, 2, 4-oxadiazole compound, which is a small
molecule that can override nonsense stop translation signals to
produce full-length proteins (dystrophin).
c. More new compounds to be screened
C. Growth factors/Drugs
a. IGF-I - to promote muscle repair
b. Myostatin inhibition (GDF8- TGF-ß superfamily member) - to
decrease muscle damage
c. ACE-031- recombinant fusion protein which promotes muscle
growth by inhibiting ActRIIB signaling.
D. Protease inhibitors to slow muscle breakdown
a. Leupeptin (calpain inhibition)
b. BBIC (serine protease inhibition)
c. MG-132 (proteosome inhibitor)
E. Anti-inflammatory agents
a. Cyclosporine A
e. nNOS (neuronal nitric oxide synthase) upregulation
b. Deflazacort (DFZ)- methyloxazoline derivative of prednisolone
G. Drugs to increase muscle strength
a. Creatine supplements
H. Drugs maintaining calcium Homeostasis
b. omega -3 Fatty acids
All these methods are aimed at slowing down the progression of the disease, or reducing the symptoms and they are also effective in prolonging the lifespan of affected individuals
1. DRUG TREATMENT
Steroids have been demonstrated to be efficacious in slowing the progression of muscular dystrophy especially DMD and in delaying the loss of independent ambulation, stabilize muscle strength and preserve pulmonary functions. (1, 2)
Corticosteroids may enhance myoblast proliferation and promote muscle
regeneration. Alternatively, steroids may inhibit muscle degradation by stabilizing lysosomal-bound proteases or muscle cell membranes. Finally, prednisone could reduce muscle damage and necrosis through its immunosuppressive and anti-inflammatory effects. (3, 4) Multiple randomized trials have found improved function and strength
in children treated with prednisone. (5, 6)
Unfortunately, in these studies prednisone had a great deal of side effects including weight gain, cushingoid features, hypertension, hyperactivity, growth retardation, and cataracts. A methyloxazoline derivative of prednisolone, deflazacort (DFZ), has shown some promise in providing similar effects to prednisone with a less concerning side
effect profile. (7) If both drugs are similarly effective in improving strength and if weight gain is less evident with DFZ then improvements in functional strength may exceed those seen with prednisone.
Reduction in the total amount of steroids with different treatment schedules, such as alternate-day, pulsed, high-dose intermittent or daily low-dose administration, may decrease side effects.
Therapeutic molecules such as ACE-031 are also being developed for the treatment of DMD patients with the goal of improving strength and preserving physical functions.
It is a recombinant fusion protein which promotes muscle growth by inhibiting ActRIIB signaling. (8) In 2010, a phase II study in DMD patients was initiated in Canada but was terminated due to serious safety concerns.
Due to the side effects of the above steroids and therapeutic molecules, many studies have been carried out to study the use of creatine as an effective treatment formuscular dystrophy especially DMD. A study by Tarnopolsky reported the benefits of creatine supplements in patients with DMD. Creatine is a guanidino compound that may confer therapeutic benefit in muscular dystrophy by increasing strength and fatfree
mass (FFM), by its antioxidant properties, by reducing protein breakdown and by enhancing sarcoplasmic reticulum calcium reuptake. (9)
Drugs that have been used to treat myotonia include sodium channel blockers such as procainamide, phenytoin and mexiletine, tricyclic antidepressant drugs such as clomipramine or imipramine, benzodiazepines, calcium antagonists and taurine.
Till 2009 about 10 clinical trials were carried out to test the safety and efficacy of these drugs. Two small studies suggested that clomipramine and imipramine might have a short-term beneficial effect on the myotonia in myotonic dystrophy and one small study suggested that taurine might have a long-term beneficial effect in myotonic dystrophy.
Minor side effects such as dry mouth and dizziness were reported with clomipramine and imipramine, but not with taurine. It was not possible to determine whether drug treatment was safe and effective based on this evidence. Hence, larger, well-designed randomized controlled trials are required. (10)
Few drugs such as Heregulin, L-arginine, TAT-utrophin, okadaic acid and SMT C1100 have shown to turn on the utrophin gene expression.
Heregulin, acts via the N-box motif of the utrophin A promoter (11) and L-arginine, results in an increase in utrophin expression as a result of increased production of nNOS. (12) TAT-utrophin is a recombinant utrophin protein modified with the HIVderived TAT protein transduction domain, improves delivery across the cell membrane. (13) Okadaic acid have also been identified to upregulate utrophin expression in mdx
mice but so far this has not reached therapeutic levels. (14) SMT C1100 targets the primary cause of the disease by reducing the level of muscle membrane damage, as demonstrated by a reduction in force drop following eccentric contractions (15). Serum creatine kinase, muscle fibrosis and necrosis are also reduced indicating that SMT C1100
diminishes the catastrophic secondary pathology associated with the disease. SMT C1100 was taken into Phase I trials, although there were no safety issues, the plasma levels ofthe drug were not high enough for the trials to continue into patients. New formulations of the drug are currently being explored by Summit plc with a view to taking this compound back into the clinic. The therapeutic use of these drugs is promising but
more screens are required for the same.
Nutritional support is often overlooked but is important especially in order to improve quality of life. Antioxidants and anti-inflammatories have known to offer some benefit. Animal studies have shown that diet rich in omega-3-fatty acids prevent skeletal muscle lesions and improves muscle appearance on histological examination. (16)
(Further Details on Nutrition are discussed in Chapter 10)
Management of muscle extensibility and joint contractures is a key part of
rehabilitation management. One goal of physical therapy is to provide regular range of motion exercises to keep the joints as flexible as possible, delaying the progression of contractures, and reducing or delaying curvatures of the spine. Braces are used especially on the ankles and feet to prevent equinus gait. Full-leg braces may be used in DMD to prolong the period of independent walking. Strengthening other muscle
groups to compensate for weakness may be possible if the affected muscles are few and isolated, as in the earlier stages of the milder muscular dystrophies. Regular, nonstrenuous exercise helps maintain general good health. Strenuous exercise is usually not recommended, since it may damage muscles further. Wheelchairs, canes and walkers
are also used to help patients keep their independence and walking capabilities. Treatment programs, especially focusing the shoulder, should be started for the upper extremities. The maintenance of active range of motion and strength results in independence in performance of activities of daily living, such as dressing, oral/facial hygiene, homemaking, and preparation for work.
3. GENETIC COUNSELING
Genetic counseling is advised for people with a family history of the inherited disorder. It helps to identify families at risk, investigate the problem present, interpret information about the disorder, analyze inheritance patterns and risks of recurrence and review available options with the family. For families living with Duchenne or Becker muscular dystrophy, it can offer several benefits. Genetic counseling plays a
major role in DMD; its aim should be to avoid the birth of the affected males. During counseling one can explain the cause of muscular dystrophy, the typical symptoms and course of the disorder, and can discuss and facilitate diagnostic and genetic testing options. Parents often are uncertain about the purpose of genetic counseling and what it entails. In the case of Duchenne muscular dystrophy, the basic purpose of counseling is to help a couple understand the hereditary nature of the disorder and the probable risk for them and other family members of having a dystrophic child. Couples are then able to make informed decisions about future childbearing.
Each time a DMD carrier mother has a child; there are four possible outcomes, each with an equal probability of happening. Thus, the chance of producing an affected son is one in four, or 25 %. Further breakdown of the risk according to the sex of the child, follows that there is a 50% chance that each son will be affected. All daughters will be unaffected, but each has a 50% chance of being a carrier like her mother.
It is important to know that unaffected son of carrier mothers do not have the DMD gene, and therefore, cannot transmit DMD to their offspring. The same is true for those daughters of carriers who have not inherited the DMD gene. If circumstances should allow a male affected with DMD to reproduce, and if his wife was not a carrier of DMD, then all of his sons would be unaffected and free of the gene but all of his daughters would be carriers.
Genetic testing can help tell whether a woman is definitely a carrier or whether she is very unlikely to be a carrier. Carriers have an increased chance of having boys with Duchenne or Becker muscular dystrophy. All women who could be carriers, based on their family history with sons or brothers with DMD/BMD, uncles or cousins on their mother's side of the family who have DMD/BMD, mothers or sisters who are carriers for DMD/BMD, and aunts or cousins in their mother's side of the family who
are carriers for DMD/BMD. If a woman knows she is a carrier, she can make more informed childbearing plans. Identifying carriers in the family can provide information to other family members about their chance of also being carriers and having affected sons.
The method for carrier testing should be determined by the woman's family history, including whether the mutation in the family is known .If the mutation is known; only that mutation needs to be tested. If the mutation in the family is not known because the affected person was not tested, it is best to test him first. If genetic testing was done in the past and no mutation was found, it might be appropriate to test the affected individual again using new and improved tests, which can identify more mutations.
The types of tests that have been used for carrier testing include creatine
phosphokinase (CPK) testing, muscle biopsy, and genetic carrier testing. In most cases, CPK testing and muscle biopsy are not good choices for carrier testing. CPK levels arehigher in child and adolescent carriers than in adult female carriers, who are the ones more likely to have carrier testing. CPK levels also may be increased for reasons other than muscular dystrophy, such as strenuous activity or sickness. A muscle biopsy is an
invasive test, which is less accurate as compared to genetic testing.
If a woman has a child with DMD or BMD and also has other affected male family members, for example an affected brother or nephew, it is extremely likely that she is a carrier. If there are no other affected family members, there is a 66% (or 2 in 3) chance of being a carrier. Approximately 33% (or 1 in 3) of cases of Duchenne muscular dystrophy are caused by what are called new mutations. These are random changes tothe genetic code in the dystrophin gene that happen in only one egg or sperm, that one egg or sperm could create an affected male; rarely, a carrier female child who could later have affected children. The possibility for new mutations is one of the reasons why 1/3 or more of individuals with Duchenne muscular dystrophy will have no family history. Another possibility is that some families have several generations with mostly
or all females (that is, they have no boys to express the disease). These families may not know that there are several generations of carriers. Affected people in the same family almost always have the same mutations in the dystrophin gene and will have the same type of muscular dystrophy.
There are many different reproductive options for carrier families with a higher chance of having a child with Duchenne or Becker muscular dystrophy. There is a 25% chance of having an affected child (25% affected son; 25% unaffected son; 25% carrier daughter; 25% non-carrier daughter). If the child is known to be male, the chance of having an affected son is thus 50%; if it is female, the chance of a carrier daughter is 50%.
X - linked recessive inheritance of Muscular dystrophy
1. Mutation in the family is known: Have a natural pregnancy and pursue testing for sex, followed by testing for the gene mutation in the family.
Chorionic villus sampling (CVS) is generally offered between the 10th and 13th weeks of pregnancy. A small piece of the placenta is tested to determine the sex of the baby. If male, those same cells can be tested for the known mutation in the dystrophin gene in the family. Amniocentesis is generally performed starting at 15 weeks, and can be performed through the end of the pregnancy. Cells from amniotic fluid are tested to determine the sex of the baby. If male, those same cells can be tested for the known
mutation in the dystrophin gene in the family. Prenatal testing is often used to prepare for an affected child, or to make pregnancy termination decisions.
2. Mutation in the family is not known: Have a natural pregnancy and pursue
testing for sex, followed by linkage testing or fetal muscle biopsy.
For families with a confirmed diagnosis of Duchenne or Becker muscular
dystrophy, but where genetic testing has not identified a disease-causing factor, linkage analysis using the genetic material taken from the CVS or amniocentesis may be available. Linkage analysis uses markers along the gene to determine whether the baby has inherited the "at risk" X chromosome. Linkage analysis is usually only available for families that have at least two affected males. This option involves blood draws from
multiple generations, including the affected individuals, so discussing this option prior to a pregnancy is strongly encouraged.
When linkage analysis is not possible, some hospitals offer fetal muscle biopsy (taking a small sample of muscle from the developing baby). This procedure is not offered in very many hospitals and has a higher risk for complications, including death of the fetus, than amniocentesis or CVS. Counseling about the risk and benefits of fetal muscle biopsy is absolutely necessary.
3. Preimplantation Genetic Diagnosis (PGD)
PGD combines in-vitro fertilization (IVF) with genetic testing, with the goal of implanting only unaffected embryos into the uterus. Different women will have different numbers of embryos without the dystrophin gene mutation. IVF and PGD are expensive and invasive technologies that are not available in all medical centers.
4. Egg and sperm donation
Carrier females may consider pregnancy with a donor egg. Egg donation from a non-carrier reduces the chance of having a child with muscular dystrophy to the chance in the general population. Males with DMD or BMD may consider using a donor sperm. Sperm donation from an un-affected male reduces the chance of having carrier daughters to the chance in the general population.
Adoption is another option that may be explored.
In today's world where diagnoses are made earlier, care and management is better, with new therapies on the horizon.
4. GENE THERAPY
Development of gene therapy for muscular dystrophy represents a challenge which requires significant advances in the knowledge of defective genes, muscle promoters, viral vectors, immune system surveillance and methods for systemic delivery of vectors. However, tremendous progress has been made in developing improved viral vectors and avoiding immune reactions against gene transfer. There is a gene therapy method known as targeting repairing or chimeraplast, using a synthetic blend of DNA and the related RNA, which tricks the patient's own cells to repair the mutation. The chimeraplasts match the patients' own DNA except for where the mutation occurs, attach to the DNA, and then activate DNA repair mechanisms. Although this approach initially appeared promising, the repair rate is generally found to be too low to cure. U7, a non spliceosome small nuclear RNA (snRNA), normally involved in the processing of the histone mRNA 3' end, to enhance the delivery of antisense sequences (17). By slightly modifying the binding site for Sm/Lsm proteins, U7 can be converted into a versatile tool for splicing modulation. Delivery of the appropriately modified U7 snRNA using an adeno-associated virus has demonstrated widespread dystrophin restoration in both the mdx mouse and the GRMD (18) models of DMD following only a single dose.
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Copyright © 2018 Association for Muscular Dystrophy