Myotonic dystrophy: the role of the CUG triplet repeats in splicing of a novel DMPK exon and altered cytoplasmic DMPK mRNA isoform ratios.

The mechanism by which (CTG) n expansion in the 3 UTR of the DMPK gene causes myotonic dystrophy (DM) is unknown.
We identified four RNA splicing factors--hnRNP C, U2AF (U2 auxiliary factor), PTB (polypyrimidine tract binding protein), and PSF (PTB associated splicing factor) --that bind to two short regions 3 of the (CUG) n, and found a novel 3 DMPK exon resulting in an mRNA lacking the repeats.
We propose that the (CUG) n is an essential cis acting element for this splicing event.
In contrast to (CUG) n containing mRNAs, the novel isoform is not retained in the nucleus in DM cells, resulting in imbalances in relative levels of cytoplasmic DMPK mRNA isoforms and a new dominant effect of the mutation on DMPK.. 
