Deep indel mutagenesis reveals the regulatory and modulatory architecture of alternative exon splicing.

While altered pre-mRNA splicing is a frequent mechanism by which genetic variants cause disease, the regulatory architecture of human exons remains poorly understood. Antisense oligonucleotides (AONs) that target pre-mRNA splicing have been approved as therapeutics for various pathologies including patient-customised treatments for rare diseases, but AON discovery is currently slow and expensive, limiting the wider adoption of the approach. Here we show that deep indel mutagenesis (DIM) -which can be made experimentally at very low cost - provides an efficient strategy to chart the regulatory landscape of human exons and rapidly identify candidate splicing-modulating oligonucleotides. DIM reveals autonomous effects of insertions, while systematic deletion scans delineate the checkerboard architecture of sequential enhancers and silencers in a model alternative exon. The results also suggest a mechanism for repression of transmembrane domain-encoding exons and for the generation of microexons. Leveraging deep learning tools, we provide a resource, DANGO, that predicts the splicing regulatory landscape of all human exons and can help to identify effective splicing-modulating antisense oligonucleotides.