Myotonia

Question 1

Name 5 characteristics of the condition

Answer 1

1. hyperexcitablity of skeletal muscle
2. delayed muscle relaxation
3. runs of action potentials
4. myotonic seizures, particularly in response to auditory stimuli
5. muscle stiffness

Question 2

name the 2 types of myotonia, subtypes (if any) and the ion channel that is mutated in each

Answer 2

Myotonia Congenita - caused by mutations in CLCN1 (encodes CLC)

• thomsens (dominant)
• beckers (recessive)

Paramyotonia and K+ aggrevated** **- caused by mutations in SCN4A (encodes Nav1.4)

• fluctuans (mild)
• permenans (severe)
• acetazolomide sensitive

Question 3

NMJ post synaptic terminal

1. which protein sets up the resting Vm?
2. which channel mediates a) depolarisation? b) repolarisation?
3. what does CLC1 contribute to and how?
4. what is the role of L type Ca channels?

Answer 3

1. Na/K ATPase
2. a) Na_v b) K_v
3. contributes to resting Vm, by driving it towards E_cl of -70mV. (sends Vm further away from theshold)
4. mediates Ca induced Ca release from the SR. They are mechanically coupled to ryanodine receptors; conformational changes that open L type channels cause a conformational change in the ryanodine receptor, which consequently opens, enabling the release of Ca

Question 4

THOMSENS MYOTONIA CONGENITA

1. when is the onset of disease?
2. are symptoms mild or severe?
3. name 2 symptoms

BECKERS MYOTONIA CONGENITA

4. when is the onset of disease?
5. are symptoms mild or severe?
6. name 2 symptoms

Answer 4

1. infancy to adulthood
2. mild
3. some muscle weakness and hypertrophy
4. neonatal to infancy (earlier onset)
5. severe
6. some muscle weakness, with more hypertrophy

Question 5

what is the treatment of myotonia congenita? What does this drug do?

Answer 5

1. mexillitine - a use dependent Na_v channel blocker. Only blocks OPEN Na_v channels, so that only the enhanced action potentials are prevented to ensure normal physiology of the patient.

Question 6

1. where are mutations in CLC1 found? (i.e. is there any clustering?)
2. what is the structure of CLC1?
3. How many gates does it have? Therefore how many configurations does it have? What do these different configurations lead to?

Answer 6

1. throughout the channel
2. a double barrelled channel consisting of 4 subunits that come together to form 2 pores
3. 3 - each pore has its own gate that gates the pore independently of the other gate, and a main gate which gates both pores together. Therefore there can be 4 conformations (closed, pore one open, pore two open, both pores open). When only one pore is open, sub-conductance states are produced

Question 7

1. how was the effect of the mutations on CLC1 currents measured?
2. what was the V_1/2 of WT channels? How many WT channels were open at rest?
3. what was the V_1/2 of I90M mutant channels? How many channels were open at rest?
4. What were the effects of adding a 50/50 mix of WT and I290M channels on V_1/2 and Po at rest?
5. how do dominant mutant subunits have a dominant negative effect on WT CLC1 subunits?

Answer 7

1. measuring tail currents at different Vms
2. V_1/2 = -20mV; 15% open at rest
3. V_1/2 = +55mV (shift in Vdep of +75mV); 0% open at rest
4. 0% were open at rest and V_1/2 = +25%. Because this is not half way between the two 100% mixes, and closer to that of all mutant channels, it can be implied that 1290M has a dominant negative effect on WT subunits
5. only one subunit needs to be mutated for a channel to be non-functional. Therefore, in a 50/50 mix, 4/5 of channels produced will contain a mutated subunit thus will be non-functional.

Question 8

1. name a recessive CLC1 mutant
2. what is th effect of this mutant?
3. why are heterozygotes of recessive mutations healthy?

Answer 8

1. E291K
2. most likely trafficking
3. because 50% of their channels are normal. Only 15% functional channels.

Question 9

1. is paramyotonia recessive or dominant?
2. when is the onset of the condition?
3. when do symptoms appear? (2)
4. is hypertrophy present?

Answer 9

1. dominant
2. neonatal to infancy
3. during exercise; cold sensitive
4. no hypertrophy

Question 10

1. where do mutations that cause paramyotonia/K⁺ aggrevated myotonia
2. in the IC loops, S4 domain and the ball and chain

Question 11

1. how was the accumulation of Na visualised using MRI?
2. when was the Na content of the leg measured?
3. how were Vm and IC [Na] determined?
4. what was found in controls with the MRI?
5. what was found in paramyotonia patients with thr MRI?
6. what was found in terms of Vm and IC [Na] in the controls and paramyotonia patients?

Answer 11

1. by using radioactive ²³Na
2. before and after cooling and after exercise
3. by biopsies
4. there was no different in the Na content of the leg after exercise and cooling
5. there was a significant increase in the Na content following exercise and cooling
6. there was no difference between controls and patients before coolingfollowing cooling/exercise there was a small amount of depol in control patients but this wasnt significantfollowing cooling/exercise, there was a significant increase in [Na]_i and significant depolarisation in paramyotonia patients (Vm is closer to threshold, making a AP more likely)

Question 12

1. Name the 2 phases of inactivation of Na_v1.4 and what each is mediated by
2. how was the effect of the F1705I mutation on the kinetics of fast activation measured? What were the findings?
3. how was the effect of the F1705I mutation on the kinetics of fast inactivation measured?
4. what was the V_1/2 for fast inactivation for a) WT channels and b) F1705I mutant channels?
5. what are the physiological implications of this channel?

Answer 12

1. fast (mediated by the ball and chain) and slow (mediated by the closure of the channel)
2. measuring point conductance at various Vms (I/(V-Vrev))/Gmax. No impact on fast activation
3. TAIL CURRENTS. Vm was clamped at the pre-potentials for 300msecs before being clamped at -10mV
4. -71.8mV
5. -63.2
6. In mutant channels, at -71.8mV, where normally half of channels should be closed, less than half of channels are closed. Therfore depolarisation is prolonged, meaning that the Vm is closer to threshold, and lengthening the drive for K+ influx

Question 13

1. How were the mutations that cause paramyotonia found?
2. Name 2 of the mutations found?
3. what were the effects of these mutations on the channels kinetics

Answer 13

1. DHPLC analysis of a 49yo male of polish origin who displayed symptoms of paramyotonia
2. F1705I and T323M
3. F1705I - positive shift in FAST inactivationT323M - positive shift in SLOW inactivation