The myotonic dystrophies (DM) are now collectively recognized as a clinically and genetically heterogeneous group of neuromuscular disorders, characterized by autosomal dominant inheritance, muscular dystrophy, myotonia, and multi-system involvement. Recent work by others and us indicates at least two more DM loci in addition to DM1, the (CTG)n expansion in chromosome 19q13.3. Multiple families with clinically variable presentation from predominantly distal to exclusively proximal muscle involvement show linkage to a locus in 3q21, designated DM2. However, several families with similar presentations have been excluded from this region. Thus, there is at least a third DM locus (DM3), which has yet to be mapped. The long-term goal of this proposal is the identification of DM2 in the families mapping to 3q21, and the mapping and cloning of the remaining gene(s) in the DM2-unlinked families. The characterization of the underlying mutations will be the basis for phenotype/genotype correlations.
Three specific aims are proposed: (1) to clone and characterize DM2 in 3q21; (2) to clone and characterize the gene(s) in DM2-unlinked/DM3 families, and (3) to globally expression profile DM muscle with DNA microarrays. Collaborating with clinical groups from the USA and Europe, we have ascertained 57 families with clinically similar phenotypes, which are negative for the DM1 (CTG)n expansion or any of the other known myotonia loci. Ten of 21 families suitable for linkage analysis show linkage to 3q21, while 11 are unlinked. We have substantially narrowed the DM2 critical region, generated a physical transcript map and started to examine functional-positional candidate genes, using various mutation detection assays. For DM3 we propose the same strategy of positional cloning that has proved successful for DM2. Genome-wide expression profiling of DM muscle will identify dysregulated genes and provide valuable functional clues about potential candidate genes and complex cellular candidate pathways, the overall pathophysiology of DM, and potential molecular therapeutic targets. The identification of these novel genes and the characterization of their mutations and pathophysiological role(s) are the first step in developing potential therapies for patients suffering from these inherited myotonic dystrophies. Moreover, as the cellular pathologies among the different myotonic dystrophies show considerable overlap, the identification of the genes underlying DM2 and DM3 and their corresponding expression profiles may also provide valuable insights into the pathology of DM1, which continues to elude researchers.
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