TDP-43 dysfunction induces cryptic circular RNAs in ALS/FTD
Nuclear depletion of TDP-43 is a defining pathological feature of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), leading to widespread RNA misprocessing, including the formation of cryptic exons. Here, we identified TDP-43 as a regulator of circular RNA (circRNA) biogenesis in multiple human neuronal cell models, and showed that its dysfunction induces the de novo formation of cryptic circular RNAs (c-circRNAs). Analysis of post-mortem brain transcriptomic data identified a subset of c-circRNAs which are specific for ALS and FTD cases with TDP-43 pathology. Further, we developed highly sensitive rolling-circle amplification-based circRNA detection assays that allow to distinguish TDP-43 pathology in human CNS tissues with a 0.99 AUC. We found that c-circRNAs can co-occur with cryptic linear splicing events, uncovering complex RNA misprocessing hotspots that induce loss of disease-relevant proteins, including RPTOR and EHMT1. Notably, one of these c-circRNAs originates from UNC13A, a gene whose cryptic exon has previously been linked to one of the major GWAS hits in ALS/FTD and that is being pursued as a therapeutic target through splice-switching ASOs. We showed that c-circUNC13A is co-regulated with the linear cryptic transcript and suppression of UNC13A cryptic exon results in c-circUNC13A reduction in cultured neurons and in vivo, highlighting its potential as a target engagement biomarker for emerging UNC13A-directed therapies. Overall, this work identifies a novel molecular mechanism for TDP-43 dysfunction, opening novel avenues for understanding disease pathogenesis and developing much needed pathology biomarkers.