We show here that under physiologically reasonable conditions, CGG repeats in
RNA readily form hairpins. In contrast to its
DNA counterpart that forms a
complex mixture of hairpins and tetraplexes, r(CGG)22 forms a single stable hairpin with no evidence for any other folded structure even at low pH.
RNA with the sequence (CGG)9AGG (CGG)12AGG(CGG)97, found in a
fragile X syndrome pre-mutation allele, forms a number of different hairpins. The most prominent hairpin forms in the 3' part of the repeat and involves the 97 uninterrupted CGG repeats. In contrast to the CUG-
RNA hairpins formed by
myotonic dystrophy type 1 repeats, we found no evidence that CGG-
RNA hairpins activate PKR, the
interferon-inducible
protein kinase that is activated by a wide range of double-stranded RNAs. However, we do show that the CGG-
RNA is digested, albeit inefficiently, by the human
Dicer enzyme, a step central to the RNA interference effect on gene expression. These data provide clues to the basis of the toxic effect of CGG-
RNA that is thought to occur in fragile X pre-mutation carriers. In addition,
RNA hairpins may also account for the stalling of the 40S ribosomal subunit that is thought to contribute to the translation deficit in fragile X pre-mutation and full mutation alleles.