Periodic paralysis: from molecules to mice

Periodic paralyses (PPs) are a group of muscular disorders with episodes of total or partial muscle paralysis lasting hours to days. Disease onset is often in the first or second decade. The estimated prevalence of PPs is 0.3-1 in 100 000. The episodes of paralysis are often associated with reduced or increased serum potassium levels. In addition to episodes of paralysis, some PP patients present with myotonic stiffness of muscles between the episodes. Treatment of the PPs remains inadequate and patients often experience significant loss of mobility including disabling permanent weakness.

PPs are dominantly inherited genetic disorders. Genetic studies of PP patients have identified mutations affecting genes that code for skeletal muscle ion channels. Ion channels are proteins found in membranes of all living cells that control the passage of ions across the cell membrane. The activity of ion channels controls the electrical signals responsible for many important functions in our bodies. For example, after receiving a signal from neurons, ion channels in skeletal muscles initiate an electrical signalling cascade that controls the muscle contraction. Mutations in the skeletal muscle ion channels disrupt this signal. Myotonic stiffness is caused by inability of the muscle membrane to switch off the electrical signal that controls the contraction. In the case of paralysis, the muscle cell membrane is unable to initiate the electrical signal, resulting in flaccid, non-contracting muscle.

Our overarching aim is to understand the fundamental mechanisms that lead to episodes of paralysis and to permanent weakness, and the mechanisms of therapies to prevent these conditions. In this proposal we aim to find answers to several questions: How does a mutation affect the function of an ion channel? How does a defunct ion channel make the cell unable to send electrical signals that control the muscle contraction? And, what changes in the electrical properties of the muscle cell when the muscle becomes permanently weak? We will evaluate the effects of available and new therapies, and wherever possible, develop personalized therapies in order to intervene in the identified pathophysiological changes caused by a specific mutation.

Periodic paralyses (PPs) are a group of rare muscular disorders with episodes of flaccid muscle weakness. Episodes are associated with low (Hypokalaemic PP (HypoPP)) or high (Hyperkalaemic PP (HyperPP)) serum potassium. HyperPP can present with or without interictal myotonic stiffness. The syndromes overlap with Paramyotonia Congenita (PMC). In addition, many PP patients develop a disabling myopathy later in life. Genetic studies of PP patients have identified mutations affecting genes that code for the skeletal muscle ion channels Nav1.4 and Cav1.1. This proposal is based on our ongoing efforts to study the functional effects of these mutations, and to implement three experimental strategies to significantly improve the characterization of the pathophysiology of the PPs and of the effects of targeted therapies. 1. In order to improve the molecular characterization of the effects of PP mutations we will employ Cut-open voltage clamp (COVC) for measurements of gating pore and gating currents in Xenopus oocytes. 2. In order to express Cav1.1 channels that are difficult to express heterologously, we will use a muscular dysgenesis mouse cell line GLT. This myotube-based expression system will allow us to study the effects of mutant channels on myotube membrane excitability. Molecular and cellular properties of patient derived myotubes will also be investigated. 3. In order to study the mechanisms of permanent muscle weakness we will employ a novel mouse model of PP with myotonia and will characterize changes in electrophysiological properties of muscle fibres in permanently weak muscles. For each aim we will include an investigation of the effects of therapeutic agents to intervene in the detected pathomechanisms in order to develop mutation specific, personalized therapies.

Periodic paralyses (PP) are a group of muscular disorders with episodes of total or partial muscle paralysis lasting hours to days. Disease onset is in the first or second decade. Estimated prevalence of PP is 0.3-1 in 100 000. Some patients present also with interictal myotonic stiffness. In addition, many PP patients develop a disabling myopathy with half of the patients requiring mobility aids later in life. Recent genetic and functional discoveries have aided diagnosis and improved molecular understanding of PPs, but many critical questions remain unanswered in relation to disease mechanisms and pathophysiology. Furthermore, treatment remains inadequate and patients often experience significant loss of ambulation including disabling progressive myopathy.

In addition to academic impact, improved understanding of PP pathomechanisms underlies the applied clinical research. The current proposal also employs translational research with direct clinical impact. Our study will help the clinicians to choose the pharmacotherapy based on understanding of the pathomechanisms of individual mutations and understand the effect of the therapies on alleviating the episodes of paralysis and permanent muscle weakness. As skeletal muscle channelopathies are not a major new target for pharmaceutical industry it is imperative to optimize the use of available pharmacotherapies.

Current treatments remain inadequate. For example, many patients with myotonic symptoms are refractory to mexiletine, the most commonly used drug to treat myotonia. Also, acetazolamide, the most commonly used medication to alleviate episodes of paralysis, has no or negative effects on many PP patients. Our data will have an impact of the patient's life with successful employment of personalized therapies.

Almost 50% of PP patients require mobility aids later in life. This has significant impact on families and immediate surroundings of the patient. Improved therapeutic options will have a significant impact on the quality of life of the patients, their families and surroundings.

The results of our studies have a potential impact on pharmaceutical industry. We aim to identify new targets or endpoints for pharmacological studies to prevent the onset of permanent weakness, to recover the strength after onset of weakness, and to protect the cells from episodes of PP, which may be utilized for research to develop new therapies.

The staff involved in the project will gain advanced skills in electrophysiology (patch clamp, cut-open voltage clamp), myotube cultures and animal models of disease. We have found skilled electrophysiologists to be in high demand, thus we anticipate that the skills honed on this project will place the researchers in high demand for future positions. Graduate and undergraduate students at the IoN and MRC centre will have an opportunity to learn from and participate in the proposed research during lectures and lab projects.