Expanded CUG repeat RNAs form hairpins that activate the double-stranded RNA-dependent protein kinase PKR

Myotonic dystrophy is caused by an expanded CTG repeat in the 3' untranslat ed region of the DM protein kinase (DMPK) gene, The expanded repeat trigger s the nuclear retention of mutant DMPK transcripts, but the resulting under expression of DMPK probably does not fully account for the severe phenotype , One proposed disease mechanism is that nuclear accumulation of expanded C UG repeats may interfere with nuclear function. Here we show by thermal mel ting and nuclease digestion studies that CUG repeats form highly stable hai rpins. Furthermore, CUG repeats bind to the dsRNA-binding domain of PKR, th e dsRNA-activated protein kinase. The threshold for binding to PKR is simil ar to 15 CUG repeats, and the affinity increases with longer repeat lengths . Finally, CUG repeats that are pathologically expanded can activate PKR in vitro. These results raise the possibility that the disease mechanism coul d be, in part, a gain of function by mutant DMPK transcripts that involves sequestration or activation of dsRNA binding proteins.