The dystrophin gene and its abnormalities

What is a dystrophin gene?

What do you think of the dystrophin gene? Some people may think that it is a gene that causes muscular dystrophy. In reality, the dystrophin gene is a gene that is supposed to be present in our body, and if it is defective, our body will have abnormalities. The first thing to do is to have an image of what it is.

Unfortunately, there is currently no effective treatment, and experts are exploring the application of gene therapy. Dystrophin exists inside the cell membrane of muscle cells and plays a role in supporting the cell membrane. When dystrophin is abnormal, it is called dystrophinopathy. It is sometimes called dystrophinopathy. This gene was discovered in 1988. Duchenne muscular dystrophy and Becker muscular dystrophy, which we hear about more and more these days, are thought to be caused by abnormalities in this gene.

Other diseases named dystrophy include congenital muscular dystrophy, limb muscular dystrophy, facioscapulohumeral muscular dystrophy, and muscular rigidity dystrophy, but these are not dystrophin gene abnormalities. Muscular dystrophy is an inherited disease whose main lesion is necrosis and regeneration of skeletal muscle, so many of them are caused by other genetic abnormalities besides the dystrophin gene. Congenital muscular dystrophy is also inherited in an autosomal recessive form and is an abnormality of the basement membrane, which is outside the cell membrane, and the proteins and sugar chains associated with its binding. Limbic muscular dystrophy is caused by abnormalities in various membrane and cytoplasmic proteins such as TTID, LMNA, CAV3, CAPN3, DYSF, and SGCG. Facioscapulohumeral muscular dystrophy is an abnormality of the long arm end of chromosome 4 (4q35). In muscle cells, dystrophin is located inside the plasma membrane, but it is not uniform, and it is spread mostly near the zone I in the costameres. The N-terminus of dystrophin binds to F-actin, and this binding forms part of the structure that allows the contractile protein to bind to the cell membrane.

The C-terminus of dystrophin binds to the dystrophin-associated protein complex. This association creates a dystrophin axis involving transmembrane proteins and basement membrane support proteins. The structure of the dystrophin axis plays an important role in the original function of muscle cells, such as maintaining the shape of the muscle cell surface, protecting it, and generating force through contraction and relaxation, which is efficiently transmitted to bones and joints. So what is the difference between Duchenne muscular dystrophy and Becker muscular dystrophy, which have the same dystrophin gene?

To begin with, genes exist in the form of a double helix of DNA with four types of bases (A, T, G, and C). When a gene is deleted, the amino acid sequence after the deletion is normal as long as the number of missing bases is a multiple of 3. The image of Becker muscular dystrophy can be easily understood if you think of it as Duchenne muscular dystrophy in which the dystrophin protein is shortened at the stop codon due to a deletion other than a multiple of 3.

Genetic mutations include missense and nonsense mutations. A missense mutation is a genetic mutation that results in the substitution, deletion, or duplication of a portion of an amino acid sequence to produce the wrong protein. A nonsense mutation is one in which the synthesis of the protein itself is aborted.

Duchenne muscular dystrophy and Becker muscular dystrophy, mentioned earlier, are each caused by this mutation. Duchenne muscular dystrophy is caused by a nonsense mutation in the dystrophin gene, while Becker muscular dystrophy is caused by a missense mutation. In other words, Duchenne muscular dystrophy is a complete loss of the dystrophin gene, while Becker muscular dystrophy has a portion of the dystrophin gene remaining.

Incidentally, the dystrophin gene is located on the short arm of the X chromosome, Xp21.2. There are various steps in the process of genetic mutation, which can be divided into seven stages: (1) genetic mutation, (2) protein dysfunction, (3) cellular dysfunction, (4) muscle necrosis, (5) muscle loss, fibrosis, fatty degeneration, (6) muscle weakness, (7) various dysfunctions (such as decreased ability to swallow, motor dysfunction, decreased respiratory muscle function, and other systemic muscle weaknesses). (7) Various functional disorders (muscle weakness throughout the body, such as decreased swallowing function, motor dysfunction, and decreased respiratory muscle function).

Next, let's look at the differences between Duchenne muscular dystrophy and Becker muscular dystrophy.

About Duchenne Muscular Dystrophy/Becker Muscular Dystrophy (symptoms, etc.)）

Duchenne muscular dystrophy

Duchenne muscular dystrophy is the most frequent form of muscular dystrophy and occurs in boys because of the X-chromosome recessive form of inheritance. The frequency is approximately 1 in 3,000 to 3,500. The main symptom is muscle weakness. The disease is also characterized by the presence of Gowers' sign.

When the child stands up from a squatting position, he or she puts his or her hands not only on the floor but also on his or her knees, and gradually and slowly gets up using the hands on the knees for support, which is called Gowers' symptom. The disease process begins around the age of 3 years, with walking becoming difficult at the age of 10 years, and the muscles of the respiratory muscles and other organs weakening around the age of 30 years, and the child dies from the disease. Muscle atrophy (thinning of the apparent muscles) is not noticeable at the beginning of the disease.

Rather, the calves are characterized by abnormal thickness, which is called pseudohypertrophy. This muscle hypertrophy is seen in various parts of the body, including the shoulder muscles, buccal muscles, and tongue muscles. Calf hypertrophy is a first observation in most patients with Duchenne or Becker's type, which will be discussed next. Muscle hypertrophy itself is rare in other muscular dystrophies, so physicians first consider Duchenne or Becker type when they see muscle hypertrophy.

As the disease progresses, muscle atrophy becomes more pronounced in the proximal muscles of the trunk (upper arms, thighs, and trunk muscles). When walking, joint contractures (stiffening of the joints and poor joint extension) are only present in the pointed leg caused by shortening of the Achilles tendon. However, muscle atrophy can also spread to the lower body, including the hip and knee joints. Not only the lower body, but also the whole body becomes contractured, including spinal deformity, finger contractures, and temporomandibular joint contractures. Tendon reflexes are weakened or disappear except for the Achilles tendon reflex.

Becker's muscular dystrophy

Becker's muscular dystrophy has similar symptoms, but it is difficult to generalize compared to Duchenne muscular dystrophy because it can vary greatly from case to case, but it has a later age of onset and a slower, more gradual course.

Testing for Duchenne Muscular Dystrophy/Becker Muscular Dystrophy

Genetic testing is essential when Duchenne or Becker muscular dystrophy is suspected, and is performed by the MLPA method, and if this test is positive, a muscle biopsy is performed to confirm the diagnosis. On pathologic images of muscle biopsy, muscle pathology in the early stages of the disease shows nonspecific dystrophic changes, with small and large muscle fibers, local necrosis and regeneration, vitreous-like changes, and fat and connective tissue replacement in the late stages of the disease.

The dystrophin test then describes whether immunohistochemistry shows a loss of dystrophin in the case of Duchenne muscular dystrophy or a decrease in the case of Becker muscular dystrophy. Because the dystrophin gene is a huge gene, it would be ideal to test it with two or three different antibodies against different domains to increase the accuracy of the immunohistochemical test, but this is not essential as a routine test today with the increasing accuracy of genetic analysis.

Under normal conditions, the surface of all muscle fibers stain. However, patients with Duchenne muscular dystrophy do not fluoresce. This is because the dystrophin gene is defective. In carriers, fibers are observed in which dystrophin is irregular and expressed only on a portion of the cell surface. The dystrophin produced by the abnormal X chromosome is partially present.

In the myonuclei in areas of dystrophin deficiency, expression of the X chromosome, which carries the normal dystrophin gene, is often suppressed due to the Lyon phenomenon. However, the degree of deficiency varies from case to case and from site to site, so that almost normal expression does not completely rule out carrier status. Dystrophin-deficient fibers are rare, especially in asymptomatic carriers. On the other hand, when muscle degeneration is strong due to other muscle diseases, dystrophin staining is often lost or reduced, so the interpretation of results should be done with caution, for example by contrasting with other staining results from adjacent sections. When stained with eutrophin, it is often expressed in dystrophin-deficient areas.

Carrier diagnosis involving the mother and female siblings and relatives can be made with considerable accuracy by examination, serum CK activity, electromyography, genetic analysis and dystrophin testing, but ethical considerations are necessary in carrying it out, and it is of course essential to take the patient's privacy into account. As explained earlier, dystrophin gene dysgenesis is an X-linked inheritance. The risk to the originator's siblings depends on the mother, the carrier. In each pregnancy, the carrier woman isDMDThere is a 50% chance of passing on the genetic mutation, with the son who inherits the mutation being the patient and the daughter who inherits the mutation being the carrier. Male patients with Duchenne muscular dystrophy are not fertile, whereas male patients with Becker muscular dystrophy andDuchenne particleMale patients with muscular dystrophy-related dilated cardiomyopathy are fertile. His daughter is a carrier, but if he is a son, his father'sDMDThey do not inherit genetic mutations.

Prenatal diagnosis is possible for at-risk pregnancies if Duchenne muscular dystrophy is confirmed in the family or identified with markers related to genetic information. This makes genetic counseling basically the only symptomatic treatment for a disease that cannot be helped. Gene therapy is not covered by insurance at this stage, but currently being considered is the treatment of the gene called exon skipping or read-through.

In the case of patients who have a number of mutations that is not divisible by 3 due to exon deletions or duplications, it is thought that by making the mutations divisible by 3 by also reading off adjacent exons, it may be possible to express the dystrophin gene in patients who are deficient in the dystrophin gene. Read-through means literally to skip over. Read-through literally means to skip over, and there are a certain percentage of drugs that skip over nonsense mutations. Although it will be very difficult to cure Duchenne muscular dystrophy with these drugs, research is underway to make Becker muscular dystrophy, which is a more slowly progressive disease, so that patients can live as long as possible.

Prenatal Diagnosis

Prenatal diagnosis is possible for carrier pregnancies if a dystrophin gene mutation has been identified in one family member or if linkage has been established. The usual procedure is chorionic villus sampling (CVS), which can be done at about 10-12 weeks gestation, or karyotyping or special tests to identify sex chromosomes from cells obtained from amniocentesis, which can be done at 15-18 weeks gestation, can determine fetal sex. If the karyotype is 46 or XY, DNA extracted from fetal cells can be used to analyze already known etiologic mutations and perform established linkage analysis.

References

• dystrophinopathy(Dystrophinopathies)[Duchenne Muscular Dystrophy (DMD, Pseudohypertrophic Muscular Dystrophy);Becker Muscular Dystrophy (BMD);DMD-Related Dilated Cardiomyopathy]
  Gene Review author:Basil T Darass, MD; Bruce R Korf, MD, PhD;David K Urion, MD