RNA CUG repeats sequester CUGBP1 and alter protein levels and activity of CUGBP1

An RNA CUG triplet repeat binding protein, CUGBP1, regulates splicing and t ranslation of various RNAs, Expansion of RNA CUG repeats in the 3'-untransl ated repeat of the mutant myotonin protein kinase (DMPK) mRNA in myotonic d ystrophy (DM) is associated with alterations in binding activity of CUGBP1. To investigate whether CUGBP1 is directly affected by expansion of CUG rep eats in DM tissues, we examined the intracellular status of CUGBP1 in DM pa tients as well as in cultured cells over expressing RNA CUG repeats. The an alysis of RNA.protein complexes showed that, in control tissues, the majori ty of CUGBP1 is free of RNA, whereas in DM patients the majority of CUGBP1 is associated with RNA containing CUG repeats. Similarly to DM patients, ov erexpression of RNA CUG repeats in cultured cells results in the re-allocat ion of CUGBP1 from a free state to the RNA.protein complexes containing CUG repeats, CUG repeat-dependent translocation of CUGBP1 into RNA protein com plexes is associated with increased levels of CUGBP1 protein and its bindin g activity. Experiments with cyclohexamide-dependent block of protein synth esis showed that the half-life of CUGBP1 is increased in cells expressing C UG repeats. Alteration of CUGBP1 in DM is accompanied by alteration in tran slation of a transcription factor CCAAT/enhancer-binding protein beta (C/EB P beta), which has been previously described to be a target of CUGBP1. Anal ysis of C/EBP beta isoforms in DM patients with altered levels of CUGBP1 sh owed that translation of a dominant negative isoform, LIP, is induced by CU GBP1. Results of this paper demonstrate that the expansion of CUG repeats i n DM affects RNA-binding proteins and leads to alteration in RNA processing .