Duchenne Muscular Dystrophy and Muscle Pathology

Lecture 3: Muscular Dystrophies I

• Dr. Kinga Vojnits is the coordinator of the PDHC CREATE program and a research associate in biomedical microbiome research at the University of British Columbia Okanagan Campus.
• The lecture covers muscular dystrophies.

End Term Group Project

• Groups completed an end-term group assignment.
• Research topics were chosen by the groups to examine biochemical understandings of the selected disease.
• Diseases addressed by students include: genetic diseases, protein misfolding diseases, metabolic diseases, and hematological diseases.
• Proposal submission deadline was March 31, 2025.
• Group presentations ran from April 1st to April 8th, 2025.

Recap from Previous Lecture

• Congenital disorders are present at birth, even if not apparent at birth.
• Congenital disorders can be classified in various ways, including physical anomalies, malformations, and genetic disorders.
• Examples of genetic disorders are spina bifida, cleft lip/cleft palate, cystic fibrosis, and Huntington's disease.

Deletions/Translocations

• DiGeorge syndrome (1 in 4,000 prevalence) symptoms vary depending on deletion location within chromosome 22q11.2.
• 22q11.2 deletion can impact various body parts.
• Congenital heart disease, defects in the palate, immune system issues, learning disabilities, kidney deficiencies and hearing loss are possible with 22q11.2 deletion.

Cri Du Chat Syndrome

• Cri du chat syndrome is identified by a mew-like cry and physical abnormalities observed in patients.
• Abnormalities include low birth weight, failure to thrive, hypotonia, psychomotor retardation, microcephaly, and a round face with anti-mongoloid slant of the eyes.
• This syndrome occurs at a rate of 1 in every 20,000-50,000 births.
• Most cases involve a deletion on the short arm of chromosome 5, with the majority coming from the father.

X-linked Recessive

• Hemophilia A is an X-linked recessive disorder involving coagulation factor VIII deficiency.
• Duchenne muscular dystrophy is characterized by a lack of dystrophin, resulting in abnormal dystrophin.
• Becker's muscular dystrophy is linked to abnormal dystrophin.

Goals of Today's Lecture

• The lecture aims to define muscular dystrophies, exploring Duchenne Muscular Dystrophy (DMD), Mode of inheritance, Pathophysiology.
• The lecture also examines Becker Muscular Dystrophy (BMD), inflammation, calcium overload, fiber type differences, as well as causes and risk factors.

Muscular Dystrophies

• A group of inherited diseases causing degeneration of skeletal and cardiac muscle.
• Causes of muscular dystrophy include: inheritance (dominant and recessive genes), and risk factors include a family history.
• Approximately 50,000 to 250,000 people are affected annually, roughly 1 in every 3500 live male births is affected.
• Different types of muscular dystrophy exist, with Duchenne Muscular Dystrophy (DMD) and Becker Muscular Dystrophy (BMD) being prevalent types.

Muscular Dystrophies (Pie Chart)

• A breakdown of various muscular dystrophy types with percentages. This includes Duchenne Muscular Dystrophy (23%), Becker Muscular Dystrophy (10%), facioscapulohumeral dystrophy (13%), limb-girdle muscular dystrophy (10%), congenital myopathy (7%), Myotonic dystrophy, congenital muscular dystrophy (3%), and hereditary neuropathy (12%).
• The data represents percentages from a pie-chart displaying different types of muscular dystrophy.

Muscular Dystrophies, General Information

• Muscle weakness and loss of muscle tissue, initiating during childhood.
• Can affect muscle strength and action, and involve skeletal muscle and other organs.
• Complications associated with muscular dystrophy include difficulty walking, poor posture, muscle spasms, neurological issues, cardiac problems, and/or functional limitations.

Dystrophin

• A key protein that muscles need to function correctly.
• It acts as a shock absorber/stabilizer for the muscles.
• Protects muscle fibers from damage during muscle contraction/relaxation.

DMD Causes

• Complete absence of dystrophin.
• Large gene deletions in the dystrophin gene
• Point mutations (in exon 45 most common)

The Genetic Cause of DMD

• The dystrophin gene is one of the largest human genes, with ~2.2 million bases, 79 exons, and >3,500 amino acids.
• Over 3,000 human mutations are associated with DMD.

The Dystrophin Gene

• A diagram of the dystrophin gene, showing its exons (numbered 1 to 79) and various sections such as the actin-binding domains, rod domains, WW cysteine-rich regions and hinge regions.

Functional Domains of Dystrophin

• Dystrophin has several functional domains, including actin binding, hinge, rod, and cysteine-rich domains.
• Only four rod domains are necessary for function.
• Internal deletions in Becker's muscular dystrophy affect specific regions of the rod domains.

Types of DMD Mutations

• One point mutation in the mdx mice, and over 3,000 mutations in DMD patients.
• Mutations include exon deletions, point mutations, and duplication.

The Genetic Cause of DMD (diagram)

• Presentation of different mutation types that cause muscular dystrophy (DMD).
• Both types of diagrams present a visual representation of mutations affecting the dystrophin gene, impacting normal dystrophin protein formation.

Mode of Inheritance, X-Linked Recessive

• Mutations occur on the X chromosome within the dystrophin gene—for females, both X chromosomes must be mutated in order for the condition to manifest.
• If a female has one mutated X, she is a carrier, not displaying symptoms. However, this carrier status poses a risk for passing the mutated gene onto her male offspring.
• Males usually manifest the disease if they inherit the mutated X chromosome from a carrier mother.

Dystrophin Diagram

• Diagram depicting the diverse components of the dystrophin protein, including different domains like the actin-binding domain, the central rod domain, and the N-terminal and C-terminal domains.

Dystrophin Stabilizes Muscle Membranes

• Dystrophin's role in stabilizing muscle membranes.

Dystrophin: (Muscle Shock Absorber)

• Dystrophin's function as a shock absorber beneath the muscle membrane.

Loss of Dystrophin Causes DMD

• Shows the difference in appearance between normal and DMD muscle through microscopy. This highlights how the loss of dystrophin negatively impacts muscle health.

Muscle Degeneration

• Lack of dystrophin leads to the fragility and deterioration of muscles.
• Serum creatine kinase serves as a marker for muscle damage.

Inflammation - Healthy Regeneration

• Injury prompts the orchestrated repair of muscle tissue.
• Active satellite cells proliferate, differentiate, and fuse with damaged muscle fibers.
• Inflammation plays a crucial role in the initial stages by clearing debris.

Inflammation - Healthy Regeneration (Diagram)

• Detailed diagram showing the progression of muscle regeneration after injury.

Inflammation - DMD

• In DMD, inflammation persists, leading to secondary necrosis due to persistent inflammation.
• The absence of dystrophin leads to constant degeneration and chronic inflammation in DMD.

Calcium and Muscle

• Diagram depicting the crucial role of calcium within muscle tissue and its impact, accompanied by detailed explanations of each step.

Calcium Influx

• Detailed description of calcium levels within and outside muscle fibers, and the role of calcium in DMD.

Reactive Oxygen Species (ROS)

• ROS production in muscle tissue and its implications for mitochondrial function and homeostasis.
• The impact of ROS on redox homeostasis is explained.

Muscle Physiology - Fiber Types

• Diagram showing different components of muscle physiology associated with fiber types (e.g., myogenic progenitor, myoblast, committed myocyte, nascent myotube, myofibril, Myofiber, Slow Type I, fast Type IIa, Fast Type IIx).
• Explains the role of genes and proteins in fiber types (MYH7, MYH2, MYH1, MyHC).

Another Key Difference Between Fiber Types

• DMD affects fast-glycotic fibers more severely due to factors such as increased muscle stress, mitochondrial biogenesis malfunction related to low PGC-1a, and reduced utrophin levels.

PGC-1 alpha

• Description and function of the PGC-1-alpha protein detailing its function as a transcription factor and master regulator of mitochondrial biogenesis.
• Role of PGC-1a in regulating oxidative metabolism.

Dystrophin and Utrophin – Do They Look the Same?

• Diagram highlighting the structural similarities and differences between dystrophin and utrophin in muscle tissue.
• Utrophin serves as a potential compensation mechanism in the absence of dystrophin.

BMD (Becker Muscular Dystrophy)

• A milder form of DMD, involving truncated dystrophin.

BMD (Becker), Etiology

• In BMD, the cause, or etiology, is a single-gene defect on the short arm of the X chromosome, causing size and amount alterations in dystrophin.

BMD (Becker) Clinical Features

• Less common than DMD, affecting 1 in 30,000 live male births.
• Symptoms are less severe, including delayed symptom onset (above age 7), and an often atypical family history.

BMD (Becker) - Review Paper

• The user is directed to review a specific case report paper regarding a novel splicing mutation in a DMD patient.

Literature

• Provides various research documents, articles, and case studies for further study on Duchenne muscular dystrophy (DMD). This includes biochemical and molecular aspects, relevant case reports, and the discovery of dystrophin within muscular dystrophy.