FIGURE 2.

Models for FX gene silencing. Potential early steps in the initiation of FX gene silencing are depicted. (A) Two ways in which silencing can be triggered by the FX DNA. The left hand panel depicts a model based on the propensity of DNA methyltransferases to methylate FX hairpins in vitro (Smith et al., 1994; Chen et al., 1995). The right hand panel depicts a silencing scheme in which repeat binding proteins act to recruit silencing complexes based on the silencing mechanism that is thought to be responsible for the silencing of the major satellite repeats in pericentric heterochromatin in mice (Bulut-Karslioglu et al., 2012). In the case of the major satellite repeats recruitment of Suv39h leads to the recruitment of DNA methylases and SUV4-20H which trimethylates H4K20. DNMT: DNA methyltransferase; TF: transcription factor; TSS: transcription start site. (B) Two ways in which RNA-mediated gene silencing might occur in the FX locus. The left hand panel depicts an RNA interference based mechanism for gene silencing based on the model of silencing of centromeric tandem repeats in the fission yeast, Schizosaccharomyces pombe (Volpe et al., 2002). In the FX locus the RNA hairpins formed by the FX repeats in the FMR1 transcript may be the source of the double-stranded Dicer (Dcr) substrates (Handa et al., 2003) as illustrated. In this case the annealing of the small Dicer products to the FMR1 mRNA would occur via the same combination of Hoogsteen and Watson–Crick base pairing that generates the RNA hairpins in the first place. Alternatively, the duplex RNAs could be generated by base pairing of FMR1 mRNA with an antisense transcript generated from this region (Ladd et al., 2007). The RNA-induced transcriptional silencing (RITS) complex, which includes the argonaute family member AGO1, then could mediate heterochromatin formation by associating with nascent transcripts via base pairing with the Dcr products. AGO1 could then recruit other epigenetic modifiers including members of the Polycomb (PcG) Group Complexes including EZH2 as suggested by work in human cells (Kim et al., 2006). The right hand panel depicts a way in which an RNA:DNA hybrid may initiate silencing by tethering the FMR1 transcript to the FMR1 locus while also recruiting silencing complexes (SC) that bind to the repeat, perhaps to the secondary structures formed by the repeat, analogous to what has been reported for the RASSF1A locus (Beckedorff et al., 2013) and for ribosomal DNA repeats and IAP elements (Bierhoff et al., 2014). Dcr1: Dicer 1; Pol II: RNA Polymerase II; RITS: RNA interference (RNAi) effector complex; SC: silencing complex; TSS: transcription start site.