Mechanisms of Myotonic Dystrophies 1 and 2

Lubov, Timchenko

Abstract

Myotonic Dystrophies type 1 and 2 (DM1/2) are neuromuscular disorders which belong to a group of genetic diseases caused by unstable CTG triplet repeat (DM1) and CCTG tetranucleotide repeat (DM2) expansions. In DM1, CTG repeats are located within the 3' untranslated region of myotonin protein kinase (DMPK) gene on chromosome 19q. DM2 is caused by expansion of CCTG repeats located in the first intron of a gene coding for zinc finger factor 9 on chromosome 3q. The CTG and CCTG expansions are located in untranslated regions and are expressed as pre-mRNAs in nuclei (DM1 and DM2) and as mRNA in cytoplasm (DM1). Investigations of molecular alterations in DM1 discovered a new molecular mechanism responsible for this disease. Expansion of un-translated CUG repeats in the mutant DMPK mRNA disrupts biological functions of two CUG-binding proteins, CUGBP and MNBL. These proteins regulate translation and splicing of mRNAs coding for proteins which play a key role in skeletal muscle function. Expansion of CUG repeats alters these two stages of RNA metabolism in DM1 by titrating CUGBP1 and MNBL into mutant DMPK mRNA-protein complexes. Mouse models, in which levels of CUGBP1 and MNBL were modulated to mimic DM1, showed several symptoms of DM1 disease including muscular dystrophy, cataracts and myotonia. Mis-regulated levels of CUGBP1 in newborn mice cause a delay of muscle development mimicking muscle symptoms of congenital form of DM1 disease. Since expansion of CCTG repeats in DM2 is also located in untranslated region, it is predicted that DM2 mechanisms might be similar to those observed in DM1. However, differences in clinical phenotypes of DM1 and DM2 suggest some specific features in molecular pathways in both diseases. Recent publications suggest that number of pathways affected by RNA CUG and CCUG repeats could be larger than initially thought. Detailed studies of these pathways will help in developing therapy for patients affected with DM1 and DM2.