Nuclear RNA foci from C9ORF72 expansion mutation form paraspeckle-like bodies

The GGGGCC (G(4)C(2)) repeat expansion mutation in the C9ORF72 gene is the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Transcription of the repeat and formation of nuclear RNA foci, which sequester specific RNA-binding proteins, is one of the possible pathological mechanisms. Here, we show that (G(4)C(2))(n) repeat RNA predominantly associates with essential paraspeckle proteins SFPQ, NONO, RBM14, FUS and hnRNPH and colocalizes with known paraspeckle-associated RNA hLinc-p21. As formation of paraspeckles in motor neurons has been associated with early phases of ALS, we investigated the extent of similarity between paraspeckles and (G(4)C(2))(n) RNA foci. Overexpression of (G(4)C(2))(72) RNA results in their increased number and colocalization with SFPQ-stained nuclear bodies. These paraspeckle-like (G(4)C(2))(72) RNA foci form independently of the known paraspeckle scaffold, the long non-coding RNA NEAT1. Moreover, the knockdown of SFPQ protein in C9ORF72 expansion mutation-positive fibroblasts significantly reduces the number of (G(4)C(2))(n) RNA foci. In conclusion, (G(4)C(2))(n) RNA foci have characteristics of paraspeckles, which suggests that both RNA foci and paraspeckles play roles in FTD and ALS, and implies approaches for regulation of their formation.