Molecular Physiology of Myotonia and Periodic Paralysis

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DESCRIPTION (provided by applicant): This application is submitted in response to Notice Number NOT-OD-09-058: "NIH Announces the Availability of Recovery Act Funds for Competitive Revision Applications". The proposed studies expand the scope of the original application (R37-AR42703), by extending our studies on the pathomechanism of periodic paralysis to include Ca channel (CaV1.1) mutations responsible for hypokalemic periodic paralysis (HypoPP). The original application was focused exclusively on sodium channel (NaV1.4) defects in myotonia and periodic paralysis. The expanded studies in this competitive revision were not part of the original proposal. More specifically, the new proposed studies are not a line of investigation that failed to meet approval for recommendation of support by prior Scientific Review Group critique. HypoPP is the most common form of inherited periodic paralysis and is a significant cause of morbidity and lost productivity. We are excited to have successfully generated a knock-in CaV1.1 mutant mouse model of HypoPP, which in preliminary studies has a clear phenotype. This is the only animal model of HypoPP created to date and offers a unique opportunity to understand disease mechanism and to test therapeutic interventions. This project is not supported by any active grant to our laboratory, and support from NOT-OD-09-058 will be vital to continue to project in preparation for eventual submission of a new grant (R0-1). The proposed studies will comply with the economic objectives of the Recovery Act by creating 2.5 new positions of employment (postdoc, graduate student, technician) and adding new capital equipment to accelerate the pace of discovery on this project. PUBLIC HEALTH RELEVANCE: The myotonias and periodic paralyses are heritable diseases of skeletal muscle in which mutations of voltage-gated ion channels alter the electrical excitability of the fiber. The long-term goals of the original project (AR42703) were to characterize the functional defects of mutant channels in these disorders and to determine how abnormal channel activity produces symptoms in affected individuals. In these disorders, muscle dysfunction is caused by intermittent derangements in the electrical excitability of the fiber, which may be pathologically enhanced or depressed. Myotonia is a disorder of enhanced excitability wherein a single stimulus elicits a high-frequency burst of action potentials that produces involuntary persistent muscle contraction lasting seconds. Conversely, periodic paralysis results from a depolarization-induced loss of muscle excitability. Mutations of sodium channels (NaV1.4), chloride channels (ClC-1), K channels (Kir2.1), or Ca channels (CaV1.1) are established causes of myotonia and periodic paralysis in humans. The original project was focused exclusively on mutations in the adult skeletal muscle sodium channel (NaV1.4), to address the very interesting mechanistic question of how a point mutation in a single gene may cause myotonia, periodic paralysis, or a combination of both in the same individual. In this revised application, the scope of the project will be expanded to include an investigation of the mechanism by which mutations in the L-type Ca channel (CaV1.1) produce susceptibility to Hypokalemic Periodic Paralysis (HypoPP). The scientific approach is based on a combination of physiological studies in a knock-in mutant mouse model of CaV1.1-HypoPP, expression studies of disease-associated mutant channels, and mathematical modeling of muscle fiber excitability. Impact / Significance of the Revised Studies. While life-expectancy is normal for patients with periodic paralysis, it is a disabling condition that severely impacts performance at school or work, and interferes with activities of daily living. Tremendous advances have been gained in understanding the molecular defects associated with the periodic paralyses, as prototypical ion channelopathies [3, 11, 39]. The mechanistic link between altered channel function and loss of sarcolemmal excitability during an attack of weakness, however, is only partially understood for NaV1.4 mutations [2] and remains a complete mystery for CaV1.1 mutations in HypoPP. Heterologous expression studies have revealed biophysical defects of mutant CaV1.1 channels, but these changes do not readily provide an explanation for depolarization and paralysis. Moreover, the impact of these functional channel defects on sarcolemmal excitability has been difficult to ascertain experimentally, due to the scarcity of human HypoPP muscle suitable for study and the lack of any spontaneous animal model. Our genetically-engineered mouse model with a knock-in R528H mutation provides an outstanding opportunity to study the behavior of mutant CaV1.1 channels expressed in a muscle environment, to experimentally define the mechanism by which Vrest is aberrantly depolarized to cause weakness, and to explore the mode of action by which carbonic anhydrase inhibitors reduce the severity and frequency of attacks. In addition to elucidating disease pathogenesis, these studies may reveal new roles for CaV1.1 channels in maintaining Vrest and may provide a system for the rational development and testing of new therapeutic strategies to alleviate attacks of periodic paralysis. The proposed studies will comply with the economic objectives of the American Recovery and Reinvestment Act and the request for revised applications (NOT-OD-09-058) by creating 2.5 new positions of employment (postdoc, graduate student, technician) and adding new capital equipment to accelerate the pace of discovery on this project.