Neuromuscular Diseases

Neuromuscular diseases involve injury or dysfunction to the peripheral nerve or muscle, or both.
Some are associated with the central nervous system (e.g., lateral sclerosis), but most are restricted to the peripheral nervous system.

Cell bodies, axons, and Schwann cells can be affected.
- Ehlers-Danlos syndrome can cause dysfunction in cell bodies.
- Spinal muscular atrophy affects the axon.
Demyelination syndromes affect Schwann cells.
- Schwann cells insulate axons to increase the speed of impulse propagation, enabling impulses to jump from node to node.
Neuromuscular junction: Injury can occur where the neuron synapses with muscle units or motor units.
Muscle itself: Injury or dysfunction can occur directly in the muscle.

Diseases can be difficult to diagnose due to similarities in presentation but differences in aetiologies and pathogenesis.
Assessments include:
- Physical assessments (observing physical presentation)
- Blood supply assessment
- Blood chemistry and substance analysis
- Muscle biopsies
- Genetic testing
- Electromyography

Example: Duchenne muscular dystrophy vs. other muscular dystrophies.
- Both may show muscle wasting, but Duchenne muscular dystrophy can present with spine curvature and elbow contractures.

Specific enzymes found in skeletal muscle can indicate pathology.
- Normal value: Approximately 200 international units.
- Values between 600-1000: May indicate Duchenne muscular dystrophy or spinal muscular atrophy, requiring further testing.
- 50-fold elevation: Indicates a significant issue, prompting further investigation.

Invasive procedure to obtain a tissue sample for examination.
Procedure:
- Muscle is isolated.
- A scalpel or needle is used to extract a tissue sample.
- The tissue is processed for histological examination.
Histological examination reveals:
- Healthy muscle: Thin fibers.
- Diseased muscle: Greater spaces between muscle cells, large clusters, and a cobblestone appearance.
Nuclei shape and size help identify cell types (e.g., muscle cells).

Used to understand the molecular basis of the disease.
Sequencing a handful of genes may be sufficient based on blood biochemistry and physical presentation.
If unsure, whole exome sequencing can be performed.
Genetic map analysis:
- Compares a program in black (best; non-disease state) with the patient's genetic data.
- Identifies peaks corresponding to specific alleles.
Inheritance:
- Many genetic diseases are inherited from ancestors.
- If the allele is not found in the genetic map, it may be a rapid or de novo mutation.

Evaluates muscle activity in response to involuntary stimulation.
Electrodes stimulate nerves with electrical impulses.
Different traces indicate different conditions.
- Normal trace: Dense activity.
- Lieutenant trace when they fired (reduced firing).
- Amplitude peaks generally indicate the overall output.

Main cause of many neuromuscular diseases.
Example: Duchenne Muscular Dystrophy (DMD).
Mutations in the dystrophin gene.
- The dystrophin gene ( $DMD$ ) is one of the longest genes in the human genome.
- Encodes a structural protein with a molecular mass of 427 kilodaltons.

Mutations can result in:
- Nonsense-mediated decay: Transcription machinery does not recognize the mutation, leading to a truncated or absent protein.
- Synthesis of a truncated protein that is partially functional but not fully effective, leading to later disease onset.
Dystrophin's Role: Connects the inside of muscle cells to the extracellular matrix, enabling mechanical roles for muscle contraction.
- Without dystrophin, muscles lack the ability to effectively pull against each other during contraction.
- Binds to actin filaments inside the cell and to transmembrane proteins that attach to the extracellular matrix.
- Essential for mechanical splicing in muscle cells.
Lack of dystrophin leads to dysfunctional muscle cells and fibrosis.

Caused by mutations in the survival motor neuron 1 ( $SMN1$ ) gene.
Function of SMN protein is not fully understood but is ubiquitously expressed early in development.
Loss of function leads to alpha-motor neuron degeneration.
Different severities:
- Type 0: Diagnosed in utero, limited mobility, rapid respiratory impairment.
- Type 1 (including Type 1A): Onset between 0-6 months.
- Type 2: Onset between 6-18 months, can sit but may not walk indefinitely.
- Type 3: Can walk at some point but not indefinitely.

Caused by mutations in the survival motor neuron genes ( $SMN1$ and $SMN2$ ).
$SMN1$ : Fully functional.
$SMN2$ : Similar but undergoes exon 7 exclusion, leading to only 10-20% functional protein.
Duplications, deletions, and other applications lead to nonsense-mediated decay.
Antisense oligonucleotides are used to encourage exon 7 inclusion.

Addition or replacement of faulty genes.
- Replacement of a faulty gene with a full working copy, packaged inside a virus for delivery.
Surrogate Gene Therapy: Replacing faulty genes with a surrogate copy using a virus.
- Used when the full-length gene is too large to be packaged into a virus.
- Example: Dystrophin gene in DMD.
- Mini-dystrophin: A shortened version of the dystrophin gene is used.
- Contains essential domains for function: actin-binding domain and dystrophin-like domain.
Current Name: The current name for the only licensed gene therapy is lost.

Neuromuscular diseases affect neurons, muscle, or the junction between them.
Diagnosed through physical assessments, muscle biopsies, EMG testing, and genetic testing.
Causes include gene mutations and autoimmune responses.
Spinal muscular atrophy is caused by mutations affecting the survival motor neuron gene.
Molecular gene therapy is at the forefront of genetic therapy, offering promising treatments.
- Examples include antisense oligonucleotides and gene replacement/addition therapies.
Further reading: Suggested research papers, especially on aspects of therapy. Thank you for listening.