Intranuclear (G4C2)n RNA foci, transcribed from C9ORF72 hexanucleotide expansion mutation, form paraspeckle-like structures

Expansion of GGGGCC hexanucleotide repeat in the C9ORF72 gene is the most common pathogenic mutation in the families with autosomal dominant frontotemporal dementia (FTD), amyotrophic lateral sclerosis (ALS) and FTD/ALS. The expanded repeat is transcribed from the sense and the antisense strand and forms intranuclear RNA foci. Here we show that the core paraspeckle proteins SFPQ, NONO and PSPC1 bind to (G4C2)n RNA in vitro, co-localise with intranuclear RNA foci in cells transfected with the expanded repeats and to a lesser extent in post-mortem brain tissue. Paraspeckles are nuclear structures functioning in nuclear retention of adenosine to inosine edited RNA. They form on a backbone of the long non-coding RNA NEAT1. We demonstrated that the presence of RNA foci increased the number of SFPQ stained subnuclear bodies, however only a small fraction of (G4C2)n RNA foci co-localised with NEAT1. Sense RNA foci lead to nuclear remodelling, as shown by an increased number of SFPQ-stained subnuclear bodies, which form independently of the known paraspeckle platform NEAT1. Thus, our results suggest that (G4C2)n RNA may lead to formation of paraspeckle-like structures, which may compete with NEAT1 for associated proteins and modulate nuclear compartmentalization of paraspeckle- bound RNAs.