Divergent TDP43-regulated and TDP43-independent cryptic splicing in the cortex and spinal cord

Mislocalization of the nuclear TAR DNA-binding protein 43 (TDP43) is a hallmark of ALS and FTD which leads to de-repression and inclusion of cryptic exons (CEs), promising biomarkers of TDP43 pathology in a spectrum of neurodegenerative diseases. However, most CEs to date have been identified from in vitro models or a single cortical FTD dataset, and little is known about cryptic splicing in the spinal cord, or within different neuronal subtypes. We meta-analyzed published bulk RNAseq datasets representing 1,778 RNAseq profiles of ALS and FTD post-mortem tissue, and in vitro models with experimentally depleted TDP43. We identified 142 cryptic splices, including 68 novel events. We found divergent cryptic splicing primarily between the spinal cord and cortex, validated in an independent ALS cohort by qPCR and supported by in situ hybridization (ISH). We also identified a set of cryptic splices observed in tissue but not in vitro, and, being present in either SOD1-ALS or MAPT-FTD subjects, likely TDP43 independent. Finally, leveraging multiple public single-nucleus RNAseq datasets of ALS and FTD motor and frontal cortex, we confirmed the elevation of cortical-enriched splices in disease and localized them to layer-specific neuronal populations. We provide a web interface to browse the meta-analysis results at https://go.roche.com/CrypticSplicingLandscape. This catalog of cryptic splices will inform efforts to develop biomarkers for tissue-specific and cell type-specific TDP43 pathology.