In vivo co-localisation of MBNL protein with DMPK expanded-repeat transcripts

Myotonic dystrophy (DM1) is the most common form of adult muscular dystroph y and is inherited as an autosomal dominant trait. The genetic basis of DM1 is the expansion of a CTG repeat in the 3 ' untranslated region of a prote in kinase gene (DMPK), The molecular mechanism by which this expanded repea t produces the pathophysiology of DM1 remains unknown. Transcripts from the expanded allele accumulate as foci in the nucleus of DM1 cells and it has been suggested that these transcript foci sequester cellular proteins that are required for normal nuclear function. We have investigated the role of three RNA-binding proteins, CUG-BP, hnRNP C and MBNL, as possible sequester ed factors. Using a combination of indirect immunofluorescence to detect en dogenous proteins and overexpression of proteins with green fluorescent pro tein (GFP) tags we have shown that CUG-BP and hnRNP C do not co-localise wi th expanded repeat foci in DM1 cell lines. However, GFP-tagged MBNL does it self form foci in DM1 cell lines and co-localises with the foci of expanded repeat transcripts. GFP-tagged MBNL does not appear as foci in non-DM1 cel l lines. This work provides further support for the involvement of MBNL in DM1.