Unfolding of higher DNA structures formed by the d(CGG) triplet repeat by UP1 protein.

Fragile X syndrome is caused by expansion of a d(CGG) triplet repeat in the 5′-untranslated region of the first exon of the FMR1 gene resulting in silencing of the gene. The d(CGG) repeat has been reported to form hairpin and quadruplex structures in vitro, and formation of these higher structures could be responsible for its unstable expansion in the syndrome, although molecular mechanisms underlying the repeat expansion still remain elusive. We have previously proved that UP1, a proteolytic product of hnRNP A1, unfolds the intramolecular quadruplex structures of d(GGCAG)5 and d(TTAGGG)4 and abrogates the arrest of DNA synthesis at d(GGG)n sites. Here, we demonstrate that the d(CGG) repeat forms a peculiar DNA structure, which deviates from the canonical B-form structure. In addition, UP1 was demonstrated by CD spectrum analysis to unfold this characteristic higher structure of the d(CGG) repeat and to abrogate the arrest of DNA synthesis at the site. This ability of UP1 suggests that unfolding of unusual DNA structures of a triplet repeat is required for DNA synthesis processes.