Expanded Detection of Cryptic Exons Through Nonsense-Mediated Decay Inhibition Reveals Broad Landscape of TDP-43 Targets

Transactive response DNA binding protein 43 kDa (TDP-43) is a protein that regulates splicing, the loss of which underlies the pathophysiology of a variety of neurodegenerative disorders, including amyotrophic lateral sclerosis and frontotemporal dementia (ALS-FTD). In disease states, TDP-43 is cleared from the nucleus of neurons, leading to the loss of its splicing repressor function and the aberrant inclusion of cryptic exons. Frequent inclusion of premature termination codons (PTCs) by cryptic exons leads to degradation of cryptic mRNA by nonsense-mediated mRNA decay (NMD). However, detection of cryptic exons relies on inefficient NMD; therefore cryptic exons efficiently targeted and eliminated by NMD remain undetectable through conventional RNA-sequencing. Here we generated a comprehensive set of neuronal targets of TDP-43 in human IPSC-derived i3Neurons (i3N) by combining TDP-43 knockdown with inhibition of several factors regulating NMD, and identified various cryptic exons that were either underestimated or entirely undetected by TDP-43 knockdown alone.