Crystal structures of CGG RNA repeats with implications for fragile X-associated tremor ataxia syndrome

Abstract

The CGG repeats are present in the 5′-untranslated region (5′-UTR) of the fragile X mental retardation gene FMR1 and are associated with two diseases: fragile X-associated tremor ataxia syndrome (FXTAS) and fragile X syndrome (FXS). FXTAS occurs when the number of repeats is 55–200 and FXS develops when the number exceeds 200. FXTAS is an RNA-mediated disease in which the expanded CGG tracts form stable structures and sequester important RNA binding proteins. We obtained and analysed three crystal structures of double-helical CGG repeats involving unmodified and 8-Br modified guanosine residues. Despite the presence of the non-canonical base pairs, the helices retain an A-form. In the G–G pairs one guanosine is always in the syn conformation, the other is anti. There are two hydrogen bonds between the Watson–Crick edge of G(anti) and the Hoogsteen edge of G(syn): O6·N1H and N7·N2H. The G(syn)-G(anti) pair shows affinity for binding ions in the major groove. G(syn) causes local unwinding of the helix, compensated elsewhere along the duplex. CGG helical structures appear relatively stable compared with CAG and CUG tracts. This could be an important factor in the RNA’s ligand binding affinity and specificity.

Watson–Crick-like pairs in CCUG repeats: evidence for tautomeric shifts or protonation

“…A bending of the CCUG helix was also reported by Childs-Disney et al (2014). The bending/coiling effect was not observed in any of the tri-nucleotide repeats analyzed before (Kiliszek et al 2009(Kiliszek et al , 2010(Kiliszek et al , 2011(Kiliszek et al , 2012.…”

The crystal structure of an oligo(U):pre-mRNA duplex from a trypanosome RNA editing substrate

“…8A, below). The trans, transbackbone conformation was previously associated with helix axis bending (Shi et al 1999), was found with guanines in the syn conformation about the glycosidic bond (Pan et al 1999;Kiliszek et al 2011), and was thought to be common in GÁU wobble base pairs in isolation or in tandem (Pan et al 1998). However, none of the three isolated GÁU wobble base pairs in the U-helix had the extended trans, trans-backbone conformation.…”

Triplet repeats in transcripts: structural insights into RNA toxicity

“…In contrast, two opposing guanine residues resolve the accommodation problem by flipping one of the guanine rings from the anti to the syn conformation, which results in a local unwinding of the helix. Two guanine residues present in this structural arrangement form two direct hydrogen bonds between the O6 and N1 atoms and between N7 and N2H (Kiliszek et al , 2011 ). This relatively strong hydrogen bonding system makes the CGG repeat duplexes more stable than the duplexes formed by the CUG and CAG repeats and explains why the CGG repeats do not show the slippage effect .…”

“…In recent years, the crystal structures of short oligoribonucleotides composed of the CUG, CAG and CGG repeats have been determined (Mooers et al , 2005 ;Kiliszek et al , 2009Kiliszek et al , , 2010Kiliszek et al , , 2011Kumar et al , 2011 ). All these repeated sequences form duplex structures that stack onto each other and likely represent the stem portions of hairpins formed by long sequences of this type.…”