Coexpression of the CUG-binding protein reduces DM protein kinase expression in COS cells

Myotonic dystrophy (DM) is the most common form of adult onset muscular dys trophy. Patients have a large CTG repeat expansion in the 3' untranslated r egion of the DMPK gene, which encodes DM protein kinase. RNA trans-dominant models, which hypothesize that the expanded CUG trinucleotide repeat on DM PK mRNA sequesters a factor or disrupts the RNA metabolism of the DMPK mRNA itself and other mRNAs in a trans dominant manner, have been proposed. A c andidate for the sequestered factor, termed CUG-binding protein (CUG-BP), e xists in several alternatively spliced isoforms. We found a human isoform w ith a twelve base insertion (deduced amino acids Leu-Tyr-Leu-Gln) and an is oform with a three base insertion (deduced amino acid Ala) insertion. In or der to elucidate the effects of CUG-BP on DMPK expression, we introduced CU G-BP and DMPK cDNA transiently into COS-7 cells. Cotransfection of CUG-BP d id not significantly affect the expression of either wild type or mutant DM PK at the mRNA level. On the other hand, cotransfection of CUG-BP significa ntly affected the expression of both the wild type and mutant DMPKs at the protein level. This reduction was remarkable when the mutant DMPK construct was used.