Removal of the oxidized bases by the BER or TCR pathways results in loop formation and expansion. Indeed, loss of OGG1 [ 15••], NEILS 1 [ 46], and XPA [ 47] reduces expansion in mice. Novel mechanisms for enhancing oxidative damage and toxicity are discussed below. Whether RNA–DNA hybrids form at TNRs in other non-coding regions (which generate large expansions) is unknown. In coding regions, the expanded CAG/CTG repeat

GSK1120212 ic50 tracts (n > 35 rpts) overlap in length with those of the FMR-1 ‘normal’ CGG range [ 1, 2••, 3••, 4••, 5•• and 6••] (commonly 30 rpts), which does not form hybrids. Moreover, CAG expansions do not impose transcription silencing of their respective genes [ 1 and 3••]. If a minimum DNA–RNA hybrid causes the transcriptional silencing at a threshold length, then it is unlikely to be a mechanism that is common to all TNR genes. Another consideration in a RNA-dependent hybridization model for threshold is the effect, if any, of bi-directional transcription of the TNR region [48••]. For example, several novel anti-sense FRM1 transcripts exist in the FRM1 locus (ASFMR4-6), and some overlap the CGG repeat region [49]. ASFMR4 transcript MS-275 chemical structure is spliced, polyadenylated and exported to the cytoplasm [42 and 49]. If a bi-directional transcript overlaps with the sense transcript, double stranded RNA is formed as a Dicer substrate. It is not easy to imagine how short

siRNA hybrids within the TNR tract results directly in expansion. Either multiple siRNA binding creates a RNA–DNA hybrid of similar length to that of an mRNA hybrids [40], and are removed by similar mechanisms, or the shorter RNA–DNA hybrid opens the DNA sufficiently to increase Phenylethanolamine N-methyltransferase exposure to oxidative DNA damage at a preferred threshold length (Figure 2a). New models provide insight on how RNA–protein

complexes of threshold length might provoke chemical lesions in DNA, and lead to expansion. TAR-DNA-binding protein 43 (TDP-43) [50] is poised to bind to a RNA–DNA hybrid. TDP-43 is a dimeric protein with two RNA recognition motif (RRM) domains that bind both DNA and RNA [50, 51•• and 52] (Figure 3a–c), and interact with fragile X mental retardation protein (FMRP) in an (FMRP)/Staufen (STAU1) complex [53]. This complex forms aggregates analogous to those of polyglutamine proteins, which induce cellular stress and oxidative DNA damage. The DNA length at which the encoded RNA forms aberrant protein–RNA complexes may be the threshold for the enhanced stress. The mechanisms of RNA aggregate formation are unknown, but it is likely due to the disruption of complex formation at its C-terminus. TDP-43 interacts at its C-terminus with the hnRNP family of translation factors, as well as the splicing factors muscleblind (MBNL) and CUG-BP1 (CUG binding protein 1) [54]. MBNL and CUG-BP1 impart two opposing effects on splicing, and they occur through binding of distinct regions of the target RNA [55].