These complications should be borne in mind in considering whether different repeat sizes within the C9ORF72 gene may provide divergent symptoms/diseases or different severity

of phenotypes. A gain-of-RNA-toxicity mechanism for a repeat expansion disease is best characterized in myotonic dystrophy 1 (DM1), which is caused by up to 2,500 of CTG repeats in the 3′UTR of the myotonic dystrophy protein kinase (DMPK) gene (Lee and Cooper, 2009). Two proteins, CUG-BP1 and muscleblind, were identified selleck chemicals llc to bind to the CUG repeat-containing RNA (Miller et al., 2000 and Timchenko et al., 1996). Of these two proteins, only muscleblind shows repeat-length-dependent association and is selectively sequestered into pathogenic RNA foci (Mankodi et al., 2001). Nevertheless, misregulation of both muscleblind and CUG-BP1 play roles in DM1 pathogenesis. Indeed, CUG repeats lead to activation of protein kinase C (PKC), which in turn phosphorylates CUG-BP1, whose phosphorylated form has increased activity from increased protein stability, thereby activating multiple splicing changes toward fetal isoforms (Kuyumcu-Martinez et al., 2007 and Roberts et al., 1997). The function of the C9ORF72 gene and its predicted protein product are unknown. Recent bioinfomatical analysis implies a potential involvement of the C9ORF72

protein in membrane trafficking and autophagy ( Levine et al., 2013 and Zhang et al., 2012), but this remains to be determined. A 50% reduction of mRNA levels corresponding to both short and long mRNA isoforms of C9ORF72 ( DeJesus-Hernandez et al., 2011 and Gijselinck et al., 2012) has been reported and GSI-IX nmr much is consistent with partial or complete silencing of the expanded allele ( Figure 4A), although it should be noted that the reduction of the corresponding C9ORF72 proteins has not been demonstrated. Antisense oligonucleotide-mediated reduction of C9ORF72

in zebrafish produces reduced axon lengths of motor neurons and locomotion deficit ( Ciura et al., 2013), consistent with the notion that partial loss of the C9ORF72 gene could contribute to disease pathogenesis. Intranuclear RNA foci containing the C9ORF72 hexanucleotide repeat have been reported (DeJesus-Hernandez et al., 2011), which may trap one or more RNA-binding proteins, thereby inhibiting their functions, especially in RNA processing (Figure 4B). While two RNA-binding proteins, hnRNP-A3 (Mori et al., 2013a) and Pur-α (Xu et al., 2013), have been reported to bind GGGGCC repeats in vitro and both were reported to be components of p62-positive TDP-43-negative inclusions in C9ORF72 patients, their role in pathogenesis is unproven. Neither has been demonstrated to localize at RNA foci formed by the hexanucleotide repeat and the predicted loss of RNA processing function that would follow from sequestration of hnRNP-A3 and Pur-α has not been demonstrated in cells and tissues expressing the hexanucleotide repeat-containing RNA.