PERSIST: A programmable RNA regulation platform using CRISPR endoRNases

Regulation of transgene expression is becoming an integral component of gene therapies, cell therapies and biomanufacturing. However, transcription factor-based regulation upon which the majority of such applications are based suffers from complications such as epigenetic silencing, which limits the longevity and reliability of these efforts. Genetically engineered mammalian cells used for cell therapies and biomanufacturing as well as newer RNA-based gene therapies would benefit from post-transcriptional methods of gene regulation, but few such platforms exist that enable sophisticated programming of cell behavior. Here we engineer the 5 and 3 untranslated regions of transcripts to enable robust and composable RNA-level regulation through transcript cleavage and, in particular, create modular RNA-level OFF- and ON-switch motifs. We show that genomically introduced transgenes exhibit resistance to silencing when regulated using this platform compared to those that are transcriptionally-regulated. We adapt nine CRISPR-specific endoRNases as RNA-level "activators" and "repressors" and show that these can be easily layered and composed to reconstruct genetic programming topologies previously achieved with transcription factor-based regulation including cascades, all 16 two-input Boolean logic functions, positive feedback, a feed-forward loop and a putative bistable toggle switch. The orthogonal, modular and composable nature of this platform as well as the ease with which robust and predictable gene circuits are constructed holds promise for their application in gene and cell therapies.