Mani S. Mahadevan - US grants

Myotonic dystrophy (DM) is the most common inherited neuromuscular disorder in adults with a global incidence of 1 per 8000. The DM mutation was found to be an expansion of an unstable CTG triplet repeat in the 3' untranslated region (3'UTR) of a gene encoding a serine-threonine protein kinase (DMPK). However, the mechanism by which it causes disease is unknown. We and others have found that the mutant DMPK mRNA is trapped within the nucleus of DM cells and forms distinct, stable foci of mRNA. In addition, we have demonstrated that the mutant DMPK 3'UTR mRNAhas significant negative effects on gene expression. Furthermore, we have identified that expression of the mutant DMPK 3'UTR mRNA in myoblasts causes defects in myoblast fusion and differentiation, demonstrating that this RNA work in trans on the expression of other transcripts, and causes a disease relevant cellular phenotype. This study is directed at understanding the molecular biology of DM by addressing the hypothesis that DM is a paradigm for RNA mediated disease processes. The proposed experiments will be aimed at assessing and determining the effect of the DMPK messenger RNA (mRNA) on gene expression. The effects of the normal and mutant DMPK 3'UTR mRNA will be studied initially at the cellular level, secondly from a biochemical persepective and finally in vivo through the creation of a transgenic murine model. The hypothesis to be tested by this proposal is that: Myotonic dystrophy is a disease in which dysregulation of RNA metabolism mediated by the mutant DMPK mRNA contributes to the pathophysiology of DM. The specific aims of this proposal are directed at: 1) studying the effects of the DM mutation in a cell culture model, 2) identifying genes whose expression is altered by the presence of the mutant DMPK 3'UTR mRNA and 3) the establishment of a murine model to study the in vivo effects of the DM mutation on RNA metabolism and their contribution to DM pathogenesis. The long term objectives of this proposal are to understand the molecular mechanisms by which the DM mutation functions in order to provide insight into the pathophysiology of DM, to allow for the development of appropriate animal models, and to eventually lead to a more rational approach to therapeutic intervention in DM.

DESCRIPTION (provided by applicant): Myotonic dystrophy (DM) is the most common inherited neuromuscular disorder in adults. There are two types, DM1 and DM2, both being autosomal dominant disorders caused by expansions of microsatellite repeats within non-coding regions of their respective genes. DM1 is far more common; however both forms of DM are likely to share similar pathogenic mechanisms. The DM1 mutation is an expansion of a CTG triplet repeat in the 3' untranslated region (3'UTR) of the DM protein kinase (DMPK) gene. A prevailing hypothesis in the field is that many aspects of DM are caused by the expression of the mutant mRNA. DM1 and DM2 represent the first examples of toxic RNA mediated disease pathogenesis. We have already developed and characterized extensively, a myoblast cell culture model to clearly demonstrate the toxic effects of the mutant DMPK mRNA on muscle differentiation. To study the hypothesis further, the aims of this proposal are to develop and characterize an inducible transgenic mouse model of RNA toxicity for DM type 1 (DM1) and to develop a siRNA (small interfering RNA) therapeutic approach to get rid of the toxic RNA which can be tested in both our cell culture and transgenic animal models. The development of transgenic mouse models will aid in understanding disease pathogenesis and will also provide a system with which to test out potential therapeutic .strategies. The ability to control gene expression through an inducible system will enable better characterization of and better correlation with the onset and levels of expression of the toxic RNA in DM1 and disease outcomes. Most importantly, because of this property, this model will be one of the first in which we can directly test, at the level of a whole organism, if ablation of expression of the toxic RNA after a period of exposure can reverse its toxic effects. All DM patients have endured exposure to the toxic RNA from birth and thus a model such as this one will be able to provide valuable and relevant insight into this therapeutic strategy.

DESCRIPTION (provided by applicant): Myotonic dystrophy (DM1), the most common form of muscular dystrophy in adults and children, is a multi- systemic, autosomal dominant genetic disorder caused by a mutation that leads to the production of a mutant RNA that is toxic to cells. Currently there are no therapies for DM1. Muscle weakness and wasting are major debilitating factors in DM1 and yet very little is known about the molecular mediators or muscle pathology in DM1. We have developed a mouse model of RNA toxicity in which we have demonstrated reversal of muscle pathology by silencing the toxic RNA. We have used this model to identify new molecules, mechanisms and pathways involved in RNA toxicity in DM1. This particular grant is designed to use genetic analysis and therapeutic trials in our mouse model to address the role of one of the novel molecules that we have identified in order to assess its potential as a viable therapeutic target. This novel finding opens new opportunities for: 1) understanding muscle degeneration, and perhaps cardiac pathology in DM1 and 2) developing new therapies to treat muscular dystrophy in DM1.

Project Summary: Myotonic dystrophy (DM1) is the most common form of muscular dystrophy in adults and children. Though there are a variety of other multi-systemic effects, DM1 is mainly characterized by myotonia and progressive muscle wasting. Muscle weakness, wasting, and fatigue have also been reported as the most impactful adverse outcomes by patients. RNA splicing defects have been identified as key effects of the toxic RNA produced in DM1 patients, and using myoblast cells from mice and DM1 patients, we and others have previously demonstrated the deleterious effects of the toxic RNA on myogenic differentiation. But little is known about the regenerative process in DM1 or the effects of RNA toxicity on this. Addressing this key issue is hampered without a model in which we can develop a thorough understanding of the effects of RNA toxicity on muscle regeneration and one in which to test therapies targeting this process. Satellite cells are key cellular mediators of muscle regeneration in response to damage. Here, we have developed the first RNA toxicity mouse model with demonstrated expression of the toxic RNA in satellite cells. We will use this model to characterize the effects of RNA toxicity on satellite cells and muscle regeneration. We will also study the expression of various key proteins implicated in DM1 such as MBNL1 and CUGBP1 in satellite cells, especially in response to damage. We will also use this model to study the effects of therapeutics on satellite cell function in RNA toxicity. Our goal is to understand the role of RNA toxicity in the process of muscle regeneration in order to provide a platform for developing therapies to treat muscular dystrophy in DM1. .