L4, Channelopathies of muscle contraction

Question 1

Why is the study of channelopathies useful?

Answer 1

• Useful insights into structure and function relationships / physiological roles
• Many ion channel diseases are genetically heterogenous; same clinical phenotype from mutations in different genes
• Different mutations in same gene can conversely cause very different clinical phenotypes
• However, most channelopathies have been found to be monogenic

Question 2

Types of molecular pathology for channelopathies: (with example for each)

Answer 2

• Mutated genes -> Genetic channelopathies e.g. NM disorders
• Antibodies, toxins -> Autoimmune and toxic channelopathies e.g. Myasthenia Gravis (AChR)
• Abnormal transcription of normal genes -> transcriptional channelopathies (MS; Sodium channel -> technically not a channelopathy)

Question 3

Mutations in skeletal muscle contraction process: Examples linked with channel type

Answer 3

• Multiple sclerosis (loss of motor contractions) -> Nerve voltage gated sodium channel
• Myokymia (involuntary contractions) -> KCNA VG K+ channel
• Familial hemiplegic migraine (FHM) -> Nerve VG Ca2+ channel
• Myasthenia Gravis (chronic muscle weakness) -> Nicotinic AChR
• Myotonia (prolonged muscle contraction) -> Skeletal muscle VG sodium and chloride channels
• Hypokalemic periodic paralysis (flaccid/weak muscles) -> Transverse tubule VG Ca2+ channel
• Human malignant hyperthermia / central core disease -> SR Ca2+ release channel

Question 4

Mechanism for myokymia:

Answer 4

1. Nerve cell is left in a depolarised stae and poised to fire an AP spontaneously due to reduced K+ channel activity
2. AP is prolonged; excessive and uncontrolled release of neurotransmitter

• a.k.a. Episodic Ataxia Type I (AD)
• Also associated with epilepsy
• Genetic mutations in VG K+ channel (Kv1.1) and results in loss of function

Question 5

Myotonia overview and 2 examples:

Answer 5

• Prolonged muscle contraction e.g. Fainting goats
• Animals tend to have attacks of extreme muscle stiffness when attempting quick, forceful movements
• Caused by genetic mutation (Ala885Pro substitution) of skeletal muscle chloride channel gene (CLC1)
• Arresed development of righting response (ADR) mice also have mutated CLC1

Question 6

Mechanism for myotonia:

Answer 6

• Reduced Cl- channel activity
• Loss of function
• Losing dominant conductance required for membrane repolarisation
• Multiple frequent APs -> prolonged muscle contraction
• The mutated channel is open less than wild type -> accumulation of K+ in -tubules with each AP

Question 7

Myotonia in humans:

Answer 7

• Generally resulting from LOF mutations in CLC1; >120 mutations widely distributed across exons (clustered at exon 8)
• 1:100,000 prevalence (rare)
• Muscle stiffness resulting from continued firing of AP in muscle after cessation of voluntary effort
• AR: Generalised myotonia or Becker’s disease -> more severe and progressive
• AD: Gradual development
• Symptoms accentuated by rest and cold, relieved by exercise
• Females have milder phenotype (CLC4 is on X-chromosome)

Question 8

Hypokalemic periodic paralysis:

Answer 8

• Flaccid muscles lasting several hours to days
• LOF mutations to L-type VGCC; 3 most common types are all in S4 domains -> Voltage sensing
• Pathophysiology: Voltage sensor malfunction; VGCC fails to sense action potential delivered by axon; VGCC fails to open
• Can also result from sodium channels

Question 9

Human malignant hyperthermia

Answer 9

• Abnormal reaction to volatile anaesthetics (e.g. halothane
• High fever, skeletal muscle rigidity, hyper-metabolism leading to hyperventilation, hypoxia and lactic acidosis in muscle
• Death within minutes of attack if left untreated (with Dantrolene)
• AD GOF disorder
• Genetic channelopathy arising from mutations in RyR1
• Up to 100s disease-causing point mutations known; affecting regulation of channel gating by Calcium, ryanodine, caffeine and ATP
• Excessive cytosolic Calcium elevations

Question 10

Central core disease:

Answer 10

• Floppy infant disease
• Diagnosed by muscle biopsy
• AD GOF disorder
• In CCD, running through whole length of each fibre the core is unstructured, inactive and devoid of mitochondria due to calcium poisoning
• Many mutations of RYR causing MH also cause CCD (e.g. Arg2435His)

Question 11

Long QT syndrome:

Answer 11

• Abnormal heartbeat which can in cases lead to sudden cardiac arrest and death
• Screening for cardiac action potential….
  1. Sodium influx
  2. Activates Calcium and Potassium channels -> length of QT reflects balance of these types
  3. K+ channel activations and Ca2+ inactivation lead to membrane repolarisation
• Long QT can arise from mutations in LQT1 and 2; VG K+ channels or LQT3; VG Na+ channel

Question 12

3 pathophysiologies for Long QT:

Answer 12

• LQT3: AD; most severe from - mutations in inactivation loop, GOF mutation causing enhanced Na+ influx -> inability to fully inactivate
• LQT1 and 2: LOF mutations in KCNQ1 and HERG K+ channels, respectively -> reduced K+ efflux and prolonged depolarisation

Question 13

What type of channelopathy is Myaesthenia Gravis?

What causes it (three examples)?

Answer 13

• Autoimmune channelopathy of neuromuscular junction
• Fluctuating muscle weakness
• Caused by Abs against AChRs, MuSK, or LRP4 (in post-synaptic membrane at NMJ)
• T cell mediated disease -> impaired signal transduction

Question 14

Two further examples of NMJ autoimmune channelopathies:

Answer 14

• Lambert Eaton syndrome (VGCC Abs)
• Isaacs’ syndrome (VGKC Abs)