Muscular Dystrophy: Diagnosis, Cause, Signs, and Remedy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive disorder, affecting 1 in 3500 males (Roland, 2000). DMD was first described by French neurologist, Duchenne de Boulogne, and hence, was named after him (Sex-linked Diseases: the Case of Duchenne Muscular Dystrophy (DMD) ½ Learn Science at Scitable, no date). This neuromuscular disease is characterised by the progressive deterioration and weakness of skeletal as well as cardiac muscles, which impacts the patients’ movements. While DMD typically affects males, there are 20-30 women worldwide who are affected by this disease due to translocations in the X chromosome (Strachan and Read, 2010, p. 520). Symptoms of DMD will be first observed at around the age of 3 to 5. An observable symptom of DMD is the increase in size of calf muscles. Several other common symptoms are the weakening of pelvic muscles, the lack of balance and difficulty in climbing stairs (Roland, 2000). By the age of 12, most of the patients would be wheelchair bound. People suffering from Duchenne Muscular Disease would typically die before reaching reproductive age, at around the age of 20, due to cardiac or respiratory difficulties (Fairclough, Bareja and Davies, 2011). This disease is caused by the absence of the dystrophin protein.

The dystrophin gene is the largest human gene located on chromosome Xp21, which is 2.4 kb, containing 79 small exons. The gene encodes for the 427 kDa cytoskeletal dystrophin protein (Michalak and Opas, 2001). The dystrophin gene has at least 7 different promoters, where different cells utilise different promoters for transcription to occur. Along with alternative intron splicing, transcription of the gene is highly complex and varies from tissue to tissue (Sudbery and Sudbery, 2009, pp. 166). As a result, the dystrophin gene is able to encode for several isoforms of dystrophin protein. The predominant isoform of dystrophin is mainly expressed in skeletal muscle cells, with trace amounts present in the brain cells (Michalak and Opas, 2001). Dystrophin has a long and slender, rod-like shape (Thakur, 2015). The protein has four domains, namely the NH2 domain, the central rod domain, the cysteine-rich domain and the COOH terminal domain, as seen in Figure 1 (Blake et al., no date).

Figure 1. Structure of the dystrophin glycoprotein complex found at the sarcolemma of skeletal muscle cells. Dystrophin binds to the actin cytoskeleton at the NH2 terminus while the COOH terminus interacts with other membrane proteins.

The dystrophin protein plays a crucial role in the mechanical as well as a structural function of the muscle membrane, also known as the sarcolemma (Thakur, 2015). Dystrophin does so by stabilising the muscle membrane and maintaining the muscle cell shape. Dystrophin also binds to the cytoplasmic region of the sarcolemma, as part of a complex of glycoproteins. This transmembrane complex is known as the dystrophin-glycoprotein complex (DGC). The DGC is a multimeric protein that bridges the actin components of the cytoskeleton, the basal lamina and the plasma membrane. As a result, force can be conveyed across the cell while providing mechanical stability during contraction of the muscle, producing movement. Membrane stability is provided by increasing membrane stiffness and preventing the sarcolemma from rupturing (Thakur, 2015). The DGC plays a role in cytoskeletal organisation and also provides a signalling pathway between the connective tissue and the cytoskeleton of muscle cells (Michalak and Opas, 2001, Sudbery and Sudbery, 2009, pp. 165-167).

Dystrophin also maintains a regular calcium ion concentration and is used for cell signalling (Thakur, 2015). A regular calcium ion concentration is crucial for the contraction cycle to occur in muscles. Upon the arrival of the action potential at the sarcolemma, the release of calcium ions is triggered. This creates tension at the muscle tendons (Martini, Nath, and Bartholomew, 2014, pp. 329). Calcium ions act as a second messenger to G-protein and receptor tyrosine kinase pathways during cell signalling. The cellular response of these pathways would result in muscle cell contraction (Urry, Cain, and Reece, 2011, pp. 263).

Deletions in the dystrophin gene cause 60-70% Duchenne Muscular Dystrophy cases. As a result of the deletion, one or more of the 79 exons would be lost. Other causes of mutations that result in the absence of dystrophin are duplications and small point mutations. These account for 10% and 15-30% of cases respectively (Characterising Mutations to the Dystrophin Gene, no date). Duplications in the dystrophin gene cause one or more of the 79 exons to be repeated. Exon deletions and duplications in the sequences coding for the -NH2 domain or the –COOH domain would result in a frameshift mutation, producing truncated proteins (Michalak and Opas, 2001, Sudbery and Sudbery, 2009, pp. 165-167). The resulting protein is truncated and unstable. Hence, the protein would not serve its normal function, resulting in the DMD phenotype. However, large deletions that occur in the rod domain of the protein do not produce any serious effect that would result in DMD (Blake et al., 2002).

In the absence of dystrophin, the dystrophin-glycoprotein complex would not form accordingly, affecting the integrity of muscle cells and thus, muscle function (Michalak and Opas, 2001, Sudbery and Sudbery, 2009, pp. 165-167). To produce locomotion, the muscles are repeatedly contracting and relaxing. The damaged muscle fibres weaken, causing the gradual death of muscle fibres. Over time, the weakening of muscles causes the patient to be more susceptible to injury (Duchenne and Becker muscular dystrophy – Genetics Home Reference, 2015). Muscle deterioration then leads to membrane leakage, causing an influx of calcium ions and creatine kinase into the blood (Blake et al., 2002, Michalak and Opas, 2001). Due to the influx of calcium ions, the contraction cycle would be unable to occur. Creatine kinase is an enzyme that catalyses the transfer of energy from ATP to creatine in skeletal muscle cells. The high concentration of creatine kinase indicates muscle damage. (Martini, Nath, and Bartholomew, 2014, pp. 329).

Prospective parents may use genetic testing to understand their carrier status for Duchenne Muscular Dystrophy, if there is a high chance of having an affected child, before making any reproductive decisions (Sudbery and Sudbery, 2009, pp. 297). Genetic testing for DMD may be done through multiplex Polymerase Chain Reaction (PCR) of the dystrophin gene. Deletions in the dystrophin gene mostly affect exons 3-8 or exons 44-60. A mixture of different primer pairs of the DMD gene is used for this reaction. Each primer pair has been designed to amplify a specific exon. After the reaction is completed, a band would be seen for every exon present. A missing band indicates that the exon has been deleted (Sudbery and Sudbery, 2009, pp. 307).

Other ways to test whether a patient suffers from Duchenne Muscular Dystrophy includes the creatine kinase test and a muscle biopsy. Due to muscle membrane leakage, the influx of creatine kinase can be used as a diagnostic test for DMD. Creatine kinase can be found in the blood at high levels when the muscle fibres are damaged. Muscle biopsy is where a small sample of the patient’s muscle is removed through surgery. The sample is then tested for the presence of dystrophin. If no dystrophin is present, the patient is said to suffer from DMD (Diagnostic and Genetic Testing – Parent Project Muscular Dystrophy, no date).

Currently, there is no permanent cure for DMD. Existing treatments of the disease, such as corticosteroids and rehabilitation, only help to delay the progress of the disease, sometimes allowing patients to live till the age of 30. Rehabilitation helps to improve the patient’s movements while corticosteroids delay the occurrence of muscle weakness by maintaining muscle function (Roland, 2000). Through genetic testing methods, such as multiplex PCR, scientists are able to acquire knowledge that aids them in understanding the causes of DMD and find more possible ways to rectify the mutation. Possible treatment methods would include gene therapy and cell therapy. With gene therapy, a functioning cloned dystrophin gene is inserted into muscle cells to supplement the lack of the dystrophin protein product whereas with cell therapy, stem cells, which have the ability to differentiate into muscle cells, are inserted into the patient to allow these new muscle cells to produce the dystrophin required (Fairclough, Bareja and Davies, 2011). However, as Duchenne Muscular Dystrophy is the largest human gene and has a high mutation rate, a long-term, permanent cure for the disease would be difficult to create (Sudbery and Sudbery, 2009, pp. 166).