Cytoplasmic ubiquilin-2-containing inclusions have also been reported in the cerebellar granular and hippocampal molecular layers (Brettschneider et al., 2012). The expanded hexanucleotide repeat in the C9ORF72 gene is reminiscent of multiple prior repeat expansion diseases for which three different prototypes of pathogenic mechanisms have been demonstrated: loss of function of the gene containing the repeat (haploinsufficiency), gain of protein toxicity due to the expression of protein containing the repeat expansion (mutant protein), and gain of RNA toxicity due to the production of RNA containing the repeat (mutant

RNA) ( La Spada and Taylor, 2010). Additional toxic mechanisms can result from complementary repeat-containing RNA produced by bidirectional transcription ( Moseley et al., Histone Acetyltransferase inhibitor 2006) or repeat associated non-ATG (RAN) translation ( Zu et al., 2011), leading to production, respectively, of potentially toxic RNA and protein species. For C9ORF72, because the GGGGCC repeat expansion is located within an alternative noncoding intron 1, the underlying disease pathogenesis may be driven by RNA-mediated or RAN translation-dependent

toxicity or haploinsufficiency or any combination of these ( Figure 4). The location of the repeat expansion in intron 1 of C9ORF72 resembles the CGG repeats of the FMR1 (fragile X mental retardation 1) gene, which, depending on the size of the repeats, yields three different syndromes: fragile X syndrome (>200 repeats), Cell Cycle inhibitor fragile X-associated PDK4 tremor/ataxia syndrome (50–200 repeats), and premature ovarian insufficiency (50–200 repeats) ( Oostra and Willemsen, 2009). Full expansion causes fragile X syndrome (FXS) from loss of FMR1 gene function mediated by hypermethylation of the adjacent FMR1 promoter region and subsequent transcriptional silencing. FMR1 carriers with CGG repeats between 50 and 200 develop fragile X-associated tremor/ataxia syndrome (FXTAS) in which the repeats are unmethylated but produce intention tremor, abnormal gait, peripheral neuropathy,

and cognitive impairment (Oostra and Willemsen, 2009). In contrast to transcriptional silencing in FXS (Tassone et al., 2000), accumulation of FMR1 mRNA in FXTAS is elevated at least 5-fold, presumably because it is stabilized by binding of hnRNP A2/B1, Pur-α (purine-rich binding protein-α), Sam68, hnRNP-G, along with CUG-binding protein 1 (CUG-BP1) and muscleblind (MBNL1), each of which has been shown either to associate biochemically with the rCGG repeats or colocalize with rCGG RNA foci (Jin et al., 2007, Sellier et al., 2010 and Sofola et al., 2007). Furthermore, both Sam68 and hnRNP A2/B1 can be found in the nuclear inclusions of FXTAS patient neurons (Jin et al., 2007 and Sellier et al., 2010).