Dystrophin Gene and Its Abnormalities

What is the Dystrophin Gene?

What image do you have when you think of the dystrophin gene? Some people may mistakenly think that it is a gene that causes muscular dystrophy. In reality, the dystrophin gene is a gene that should naturally be present in humans, and its deficiency leads to physical abnormalities. This is an important concept to understand.

Unfortunately, there is currently no effective treatment, and specialists are exploring whether gene therapy can be applied. Dystrophin is located inside the cell membrane of muscle cells and supports the cell membrane. When there is an abnormality in dystrophin, it is called dystrophinopathy. This gene was discovered in 1988. Duchenne muscular dystrophy and Becker muscular dystrophy, which have become more commonly heard recently, are thought to be caused by abnormalities in this gene.

There are other diseases with the name dystrophy, such as congenital muscular dystrophy, limb-girdle muscular dystrophy, facioscapulohumeral muscular dystrophy, and myotonic dystrophy, but these are not due to abnormalities in the dystrophin gene. Muscular dystrophy is a genetic disease where the primary changes are necrosis and regeneration of skeletal muscle, so there are many cases caused by abnormalities in other genes. Congenital muscular dystrophy is an autosomal recessive genetic disorder involving abnormalities in proteins and glycans associated with the basement membrane outside the cell membrane. Limb-girdle muscular dystrophy is caused by abnormalities in various proteins of the cell membrane or cytoplasm such as TTID, LMNA, CAV3, CAPN3, DYSF, SGCG, etc. Facioscapulohumeral muscular dystrophy is caused by abnormalities at the end of the long arm of chromosome 4 (4q35). Dystrophin is located inside the cell membrane of muscle cells, mainly near the I-band in costameres. The N-terminal of dystrophin binds to F-actin, forming part of the structure that binds contracted proteins to the cell membrane.

The C-terminal of dystrophin binds to what is commonly known as the dystrophin-associated protein complex. This binding forms the dystrophin axis, which involves membrane-spanning proteins and basement membrane support proteins. This structure plays a crucial role in maintaining the shape of the muscle cell surface, protecting it, and efficiently transmitting force generated by contraction and relaxation to bones and joints. So, what is the difference between Duchenne muscular dystrophy and Becker muscular dystrophy, which are caused by the same dystrophin gene?

Genes exist as a double helix of DNA with four types of bases: A, T, G, and C. Both involve the frame-shift theory. When a gene deletion occurs, if the number of bases deleted is a multiple of three, the amino acid sequence after the deletion remains normal. Duchenne muscular dystrophy occurs when a non-multiple of three bases are deleted, resulting in a stop codon that prevents the synthesis of dystrophin protein, while Becker muscular dystrophy involves a shorter dystrophin protein. This is a helpful way to understand the difference.

Gene mutations include missense mutations and nonsense mutations. A missense mutation is when part of the amino acid sequence is substituted, deleted, or duplicated due to a gene mutation, resulting in an incorrect protein. A nonsense mutation is when protein synthesis is completely halted due to a mutation.

Earlier mentioned, Duchenne muscular dystrophy and Becker muscular dystrophy are caused by these mutations. Duchenne muscular dystrophy involves a nonsense mutation in the dystrophin gene, while Becker muscular dystrophy involves a missense mutation. This means that Duchenne muscular dystrophy results in a complete deficiency of the dystrophin gene, while Becker muscular dystrophy retains a portion of the dystrophin gene.

Incidentally, the dystrophin gene is located on the short arm of the X chromosome at Xp21.2. The process of gene mutation involves several steps: ① Gene mutation occurs, ② Protein function abnormality, ③ Cell function impairment, ④ Muscle necrosis, ⑤ Muscle mass reduction, fibrosis, fat degeneration, ⑥ Muscle weakness, ⑦ Various functional impairments (swallowing function decline, motor function impairment, respiratory muscle function decline, etc.), leading to a total of seven stages.

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 common type of muscular dystrophy and follows an X-linked recessive inheritance pattern, occurring in boys. The frequency is about 1 in 3000 to 1 in 3500 boys. The main symptom is muscle weakness. A characteristic feature of this disease is Gowers' sign.

Also known as Gowers' maneuver, it is observed when a child stands up from a squatting position, using their hands not only on the floor but also on their knees to gradually push themselves up. The disease typically manifests around the age of three, making walking difficult by age ten, and patients usually die before the age of thirty due to respiratory muscle weakness and other organ involvement. Muscle atrophy (visible thinning of the muscles) is not prominent in the early stages of the disease.

Instead, an abnormal thickness of the calves, known as pseudohypertrophy, is characteristic. This muscle hypertrophy is seen in various parts of the body, including the shoulder muscles, cheek muscles, and tongue muscles. Calf hypertrophy is seen in almost all patients with Duchenne muscular dystrophy or Becker muscular dystrophy. Because muscle hypertrophy is rare in other types of muscular dystrophy, doctors first consider Duchenne muscular dystrophy or Becker muscular dystrophy when they observe muscle hypertrophy.

As the disease progresses, muscle atrophy becomes prominent in the proximal muscles of the trunk (upper arms, thighs, and trunk muscles). During walking, contractures (stiffness of the joints due to shortening of the Achilles tendon) cause tiptoeing. However, muscle atrophy also spreads to the lower body, including the hip and knee joints. Joint deformities occur throughout the body, including the spine, finger joints, and jaw joints. Tendon reflexes are reduced or lost, except for the Achilles tendon reflex.

Becker Muscular Dystrophy

Becker muscular dystrophy presents with similar symptoms but varies greatly among cases compared to Duchenne muscular dystrophy. It generally has a later onset and progresses more slowly.

Tests for Duchenne Muscular Dystrophy/Becker Muscular Dystrophy

When Duchenne muscular dystrophy or Becker muscular dystrophy is suspected, genetic testing is essential. The MLPA method is used, and if this test is positive, a muscle biopsy is performed for a definitive diagnosis. The pathological images of the muscle biopsy show non-specific dystrophic changes, including variability in muscle fiber size, localized necrosis and regeneration, hyaline changes, and replacement with fat and connective tissue in the late stages of the disease.

Subsequently, the dystrophin test is explained immunohistochemically. In Duchenne muscular dystrophy, dystrophin is absent, while in Becker muscular dystrophy, it is reduced. Because the dystrophin gene is large, it is ideal to test with two to three different antibodies against different domains to increase the accuracy of the immunohistochemical test. However, with the advancement in the accuracy of genetic analysis, this is not always required as a routine test.

In normal cases, the surface of all muscle fibers is stained. However, in patients with Duchenne muscular dystrophy, there is no fluorescence because the dystrophin gene is absent. Carriers show dystrophin irregularly expressed on only part of the cell surface fibers. Abnormal X chromosomes produce partial dystrophin.

In muscle nuclei with deficient dystrophin, the Lyon phenomenon often suppresses the expression of the normal dystrophin gene on the X chromosome. However, the extent of the deficiency varies by case and site, so it is currently impossible to completely deny carrier status based on nearly normal expression. In asymptomatic carriers, dystrophin-deficient fibers are rare. In other muscle diseases with severe muscle degeneration, dystrophin staining also disappears or decreases, so careful interpretation of results is necessary by comparing with other staining results of adjacent sections. Utrophin staining often shows expression at dystrophin-deficient sites.

Carrier diagnosis for mothers and female siblings or relatives involves examination, serum CK activity, electromyography, genetic analysis, and dystrophin testing, which are highly accurate. However, ethical considerations are essential, and patient privacy must be respected. As mentioned earlier, dystrophin gene abnormalities are X-linked. The risk for siblings of the proband depends on the carrier status of the mother. Carrier women have a 50% chance of passing the DMD gene mutation to each pregnancy, with sons becoming affected and daughters becoming carriers. While men with Duchenne muscular dystrophy are not reproductively viable, men with Becker muscular dystrophy and DMD-related dilated cardiomyopathy can reproduce. Their daughters will be carriers, but their sons will not inherit the DMD gene mutation.

If Duchenne muscular dystrophy is confirmed in the family or identified by genetic markers, prenatal diagnosis is possible for at-risk pregnancies. Therefore, genetic counseling is essential, although it only provides symptomatic treatment for an incurable disease. Gene therapy is not currently covered by insurance, but exon skipping and readthrough gene therapies are under consideration.

For patients with mutations causing deletions or duplications of exons not divisible by three, exon skipping might allow the dystrophin gene to be expressed by skipping adjacent exons to create a divisible mutation. Readthrough therapy involves skipping nonsense mutations with certain drugs. While it is challenging to cure Duchenne muscular dystrophy, research is ongoing to slow its progression to Becker muscular dystrophy, helping patients live longer.

Prenatal Diagnosis

If a dystrophin gene mutation is confirmed in a family member or linked, prenatal diagnosis is possible for carrier pregnancies. The usual procedure involves chorionic villus sampling (CVS) at 10-12 weeks of pregnancy or amniocentesis at 15-18 weeks to determine fetal sex via karyotyping or special tests. If the karyotype is 46, XY, fetal DNA can be analyzed for the known pathogenic gene mutation or established linkage.

References

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