Hyperkalemic periodic paralysis is characterized by myotonic discharges followed by paralysis. Caused by a mutation in the gene encoding for NaV1.4 channel, patients do not experience symptoms during infancy, but the onset starts between 1-10 years of age. The symptoms severity then increases with age until adolescence. A large increase in gene expression marked by an increase in oxidative capacity of muscles has also been reported in HyperKPP. It is possible that the onset of symptoms is related solely to NaV1.4 channel content/activity reaching a critical level. It is also possible that the onset of some symptoms are due to defective NaV1.4, while other symptoms and the increase in severity with age are related to changes in membrane components as a result of changes in gene expression. To test these possibilities, the progression of paralysis and changes in fiber types were followed with age in HyperKPP mice in relation to changes in NaV1.4 content and activity. Changes in fiber types (index of changes in gene expression), started after the onset of paralysis was observed, which coincided with NaV1.4 channels reaching maximum expression. Therefore, the onset of symptoms was related to defective NaV1.4 channels.

Hyperkalemic periodic paralysis (HyperKPP) is characterized by myotonic discharges and weakness/paralysis. It is a channelopathy that is caused by mutation in the SCN4A gene that encodes for the skeletal muscle Na+ channel isoform (Nav1.4) α-subunit. Limb muscles are severely affected while breathing musculature is rarely affected even though diaphragm expresses the Nav1.4 channel. The objective of this study was to investigate the mechanism(s) that render the HyperKPP diaphragm asymptomatic in order to find a novel long lasting therapeutic approach, to treat HyperKPP symptoms. A HyperKPP mouse model carrying the M1592V mutation was used because it has a similar phenotype to that of patients carrying the same mutation. HyperKPP diaphragm, the limb muscles soleus and EDL all had a higher tetrodotoxin (TTX) sensitive Na+ influx than wild type (WT), but only the soleus and EDL had a depolarized resting potential, lower force and greater K+-induced force loss when compared to WT. The lack of a membrane depolarization in HyperKPP diaphragm was because of greater electrogenic contribution of the Na+ K+ ATPase pump compared to WT while such increase was not observed in EDL and soleus. HyperKPP diaphragm also had greater action potential amplitude than EDL and soleus possibly because of higher Na+ K+ ATPase pump maintaining a low [Na+]i. An inhibition of PKA, but not of PKC, increased the sensitivity of the HyperKPP diaphragm to the K+-induced force depression. So, HyperKPP soleus was exposed to forskolin to increase cAMP levels in order to activate PKA to document whether greater activity of PKA will alleviate HyperKPP symptoms. At 4.7 mM K+, forskolin increased force production, but worsened the decrease in force at 8 and 11 mM K+. Forskolin also did not improve membrane excitability. Pinacidil a KATP channel opener, improved force production at all [K+]e by causing a hyperpolarization of resting EM which then allowed for greater action potential amplitude and more excitable fibers. It is concluded that the development of a better therapeutic approach to treat HyperKPP can include a mechanism which activates Na+ K+ ATPase pumps and KATP channels.

La Paralysie Périodique Hypokaliémique de type 1 (HypoPP1) est une myopathie d'origine génétique, autosomique dominante, caractérisées par des épisodes de paralysies musculaires récurrentes pouvant durer quelques heures à quelques jours. Ces crises de paralysies sont accompagnées d'une hypokaliémie responsable d'arythmies cardiaques entraînant la mort dans les cas les plus graves. Les crises peuvent être déclenchées par un stress, une alimentation riche en glucides ou encore suite à un exercice physique intense. L'HypoPP1 est liée à une mutation dans le gène CACNA1S codant la sous-unité principale du canal calcique musculaire (Cav1.1). A l'exception d'une, toutes les mutations HypoPP1 conduisent au remplacement d'une des arginines les plus externes d'un des segments détecteurs de potentiel du canal, nommés S4, par un acide aminé neutre. Des études réalisées dans des modèles d'expression hétérologue de canaux potassiques ou sodiques, dont les structures sont très proches du Cav1.1, ont montré que des mutations similaires à l'HypoPP1 conduisaient à la création d'une voie de passage ionique dite "accessoire" au travers du domaine détecteur de potentiel générant un courant cationique entrant au potentiel de repos. Si une telle voie de passage ionique existe à travers Cav1.1 dans les cellules musculaires des patients souffrant d'HypoPP1, elle pourrait ainsi induire une dépolarisation des cellules musculaires au point de les rendre inexcitables, conduisant ainsi à la paralysie. Mon travail a consisté à étudier les formes R1239H et V876E du Cav1.1 responsables d'HypoPP1 afin de déterminer si une telle voie de passage accessoire est présente dans les canaux calciques mutés. La mutation R1239H correspond à la substitution d'une histidine à la seconde arginine dans le segment S4 du domaine IV tandis que la mutation V876E présente la particularité de ne pas affecter un S4 mais un segment S3 dans le domaine III. Ce travail s'appuie sur l'expression in vivo du gène codant le Cav1.1 humain sain (WT) ou muté R1239H ou V876E dans les muscles des pattes arrière de souris puis sur l'analyse des mouvements ioniques en combinant des techniques d'électrophysiologie et de mesure des concentrations intracellulaires de H+ ou de Na+ par fluorescence sur fibre musculaire isolée. L'étude de la mutation R1239H a montré qu'un influx de protons générant un courant entrant significativement plus important se développait au potentiel de repos dans les fibres exprimant la mutation en comparaison des cellules exprimant la forme WT. L'étude de la deuxième mutation V876E, sur laquelle aucune donnée fonctionnelle n'était disponible à ce jour, a révélé que cette mutation était elle aussi responsable d'un courant ionique accessoire mais dans ce cas-là porté par les ions Na+. Ce résultat est important car il montre que la formation d'un pore ionique accessoire constitue un mécanisme physiopathologique commun à différentes formes d'HypoPP affectant le canal calcique musculaire, y compris lorsque la mutation n'affecte pas directement un segment S4 / The type 1 Hypokalemic Periodic Paralysis (HypoPP1) is a muscle autosomal dominant genetic disease characterized by episodic attacks of paralysis lasting between a few hours and several days. These attacks are associated with hypokalemia which is responsible of cardiac arrhythmias leading to death in worst cases. Attacks are triggered by stress, high carbohydrate diet or during rest following exercise. HypoPP1 is caused by missense mutations in the gene CACNA1S encoding the main subunit of the voltage-gated calcium channel (Cav1.1) of skeletal muscle. In all but one, HypoPP1 mutations lead to the replacement of an outermost arginine in one of the voltage sensor segment, called S4, with a neutral amino-acid. Experiments realized with the closely structurally related voltage-gated K+ and Na+ channels showed that comparable mutations generate an accessory pathtoway called gating pore through which a depolarizing current flows at rest. If such an accessory pathway also exists in the skeletal muscle Cav1.1, it could initiate depolarization of skeletal muscle to the point of inexcitability and lead to paralysis. My work aimed at investigating the properties of HypoPP1-associated mutant Cav1.1s R1239H and V876E in order to determine if such an accessory pathway could be present in mutated Cav1.1. The R1239H mutation corresponds to the replacement of the second arginine in the S4 segment of domain IV by a histidine and the V876E mutation has the particularity to not affect a S4 segment but the S3 segment in domain III. In this work, we used in vivo gene expression of the wild type (WT) or mutated forms of the human Cav1.1in the hind limb muscle of mice and analyzed ions fluxes by combining voltage-clamp and measurements of intracellular H+ or Na+ by fluorescence. The R1239H mutation was shown to induce a significant larger H+ influx giving rise to a larger inward H+ current at rest as compared to WT. The V876E mutation, which had never been investigated so far, was found to induce an elevated inward current at rest but in this case carried by Na+ ions. These results have relevance because they suggest that the presence of an accessory pathway could be a physiopathological mechanism shared by different HypoPPs, even when the mutation does not directly affect a S4 segment

Les syndromes myotoniques congénitaux atypiques dus à des mutations du canal sodé voltage-dépendant Nav1.4 se distinguent des myotonies congénitales classiques (canal chlore ClC-1) par la présence de traits atypiques incluant des myotonies douloureuses aggravées au froid et à l’ingestion de potassium. La caractérisation clinique et moléculaire de plusieurs familles atteintes de ces conditions rares dans la région du Saguenay-Lac-St-Jean nous a permis de découvrir une nouvelle mutation SCN4A à effet fondateur causant un phénotype de myotonies douloureuses aggravées au froid, parfois accompagné de phénomènes dystrophiques ou paralytiques. L’ampleur de notre cohorte nous permet de commenter sur l’hétérogénité phénotypique observée, sur les traits caractéristiques des syndromes associés au gène SCN4A, sur les implications physiologiques probables d’une telle mutation ainsi que sur les facteurs modulant le phénotype observé. Enfin, notre étude nous permet de souligner l’importance du dépistage familial systématique afin de prévenir les complications anesthésiques potentielles associées à ces conditions. / Congenital myotonic syndromes due to mutations of the voltage-gated sodium channel Nav1.4 differ from those due to mutations of the chloride channel CLC-1 as they tend to present atypical traits including painful myotonias and aggravation of symptoms with cold and potassium ingestion. Indeed, after completing the clinical and molecular characterization of a large cohort of patients affected with these rare conditions in the Saguenay Lac-St-Jean area, we were able to describe a new founder SCN4A mutation presenting with painful cold-induced myotonias and occasional dystrophic and paralytic episodes. Our study illustrates the wide phenotypic variability and the typical traits of SCN4A mutations. In addition, we were able to speculate on the probable physiological consequences of such mutations. Finally, we conclude by stressing the importance of familial screening in order to reduce the incidence of anesthetic complications associated with these conditions.

Les syndromes myotoniques congénitaux atypiques dus à des mutations du canal sodé voltage-dépendant Nav1.4 se distinguent des myotonies congénitales classiques (canal chlore ClC-1) par la présence de traits atypiques incluant des myotonies douloureuses aggravées au froid et à l’ingestion de potassium. La caractérisation clinique et moléculaire de plusieurs familles atteintes de ces conditions rares dans la région du Saguenay-Lac-St-Jean nous a permis de découvrir une nouvelle mutation SCN4A à effet fondateur causant un phénotype de myotonies douloureuses aggravées au froid, parfois accompagné de phénomènes dystrophiques ou paralytiques. L’ampleur de notre cohorte nous permet de commenter sur l’hétérogénité phénotypique observée, sur les traits caractéristiques des syndromes associés au gène SCN4A, sur les implications physiologiques probables d’une telle mutation ainsi que sur les facteurs modulant le phénotype observé. Enfin, notre étude nous permet de souligner l’importance du dépistage familial systématique afin de prévenir les complications anesthésiques potentielles associées à ces conditions. / Congenital myotonic syndromes due to mutations of the voltage-gated sodium channel Nav1.4 differ from those due to mutations of the chloride channel CLC-1 as they tend to present atypical traits including painful myotonias and aggravation of symptoms with cold and potassium ingestion. Indeed, after completing the clinical and molecular characterization of a large cohort of patients affected with these rare conditions in the Saguenay Lac-St-Jean area, we were able to describe a new founder SCN4A mutation presenting with painful cold-induced myotonias and occasional dystrophic and paralytic episodes. Our study illustrates the wide phenotypic variability and the typical traits of SCN4A mutations. In addition, we were able to speculate on the probable physiological consequences of such mutations. Finally, we conclude by stressing the importance of familial screening in order to reduce the incidence of anesthetic complications associated with these conditions.