EFFECT OF MYOTONIC-DYSTROPHY TRINUCLEOTIDE REPEAT EXPANSION ON DMPK TRANSCRIPTION AND PROCESSING

The myotonic dystrophy (DM) mutation has been identified as an unstabl e, expanded (CTG)(N) repeat in the 3' untranslated region of a gene de signated DM protein kinase (DMPK). Both decreased and increased levels of mutant DMPK mRNA as web as decreased levels of protein have been v ariously reported and invoked to explain disparate molecular bases of this dominantly inherited disease. Most recently, increased nucleosome binding to such expanded repeats has been interpreted as support for transcriptional repression. A quantitative allele-specific RT-PCR proc edure was developed and applied to a spectrum of patient tissue sample s and cell. lines. Equal levels of unprocessed pre-mRNA were produced by the wildtype (+) and disease (DM) alleles in skeletal muscle and ce ll lines of heterozygous DM patients. Thus, any increased nucleosome b inding had no effect at the level of transcriptional initiation and tr anscription of the mutant DMPK locus. in contrast, processed mRNA leve ls from the Dill allele were reduced relative to the + allele as the s ize of the expansion increased. The unstable repeat, therefore, impair s post-transcriptional processing of DM allele transcripts. This pheno menon has profound effects on overall DMPK locus steady-state transcri pt levels in cells missing a wildtype allele and does not appear to be mediated by imprinting, decreased mRNA stability, generation of aberr ant splice forms, or absence of polyadenylation of the mutant allele. (C) 1995 Academic Press, Inc.