Toxic expanded GGGGCC repeat transcription is mediated by the PAF1 complex in C9orf72-associated FTD.

An expanded (G4C2)30+ repeat within C9orf72 is the most prominent mutation in familial FTD and ALS. Through an unbiased, large-scale screen in (G4C2)49-expressing Drosophila we identify the CDC73/PAF1 complex (PAF1C), a transcriptional regulator of RNAPII, as a suppressor of G4C2-associated toxicity. Depletion of PAF1C reduces RNA and GR-dipeptide production from (G4C2)30+ transgenes. Interestingly, dPAF1C components, dPaf1 and dLeo1 appear selective for transcription of long, toxic repeat expansions, but not shorter, non-toxic expansions. In yeast, scPAF1C components regulate expression of both sense and anti-sense repeats. PAF1C is upregulated upon expression of (G4C2)30+ in flies and mice. hPaf1 is also upregulated in C9+-derived cells and its heterodimer partner, hLeo1, binds C9+ repeat chromatin. In C9+ FTD, hPAF1 and hLEO1 are upregulated and their expression positively correlates with expression of repeat-containing C9orf72 transcripts. These data indicate that PAF1C activity is an important factor for transcription of the long, toxic repeat in C9+ FTD.