Technologies for engineering synthetic transcription factors have enabled many advances in medicine and science. In contrast to existing methods based on engineering of new DNA-binding proteins, we created a Cas9-based transactivator that is targeted to DNA sequences by guide RNA molecules. Co-expression of this transactivator and combinations of guide RNAs in human cells induced specific expression of endogenous target genes, demonstrating a simple and versatile approach for RNA-guided gene activation.

Summary RNA-based regulation and CRISPR/Cas transcription factors (CRISPR-TFs) have yet to be integrated for multiplexed and tunable modulation of gene networks. Moreover, guide RNAs (gRNAs) for CRISPR-TFs have only been expressed from RNA polymerase III promoters in human cells, which limits their use for conditional gene regulation. To overcome these challenges, we combined multiple RNA regulatory strategies, including RNA-triple-helix structures, introns, microRNAs, and ribozymes, with Cas9-based CRISPR-TFs and Cas6/Csy4-based RNA processing in human cells. We describe three strategies for expressing functional gRNAs from RNA polymerase II promoters, which enable multiplexed production of proteins and multiple gRNAs from a single transcript. We used these tools to efficiently modulate endogenous promoters and implement tunable synthetic circuits, including multi-stage cascades and RNA-dependent networks that could be rewired with Csy4 to achieve complex behaviors. This multiplexable toolkit will be valuable for programming scalable gene circuits and perturbing endogenous networks for biology, therapeutic, and synthetic-biology applications.

Mammalian genes are regulated by the cooperative and synergistic actions of many transcription factors. In this study we recapitulate this complex regulation in human cells by targeting endogenous gene promoters, including regions of closed chromatin upstream of silenced genes, with combinations of engineered transcription activator–like effectors (TALEs). These combinations of TALE transcription factors induced substantial gene activation and allowed tuning of gene expression levels that will broadly enable synthetic biology, gene therapy and biotechnology.

TERT promoter mutations reactivate telomerase, allowing for indefinite telomere maintenance and enabling cellular immortalization. These mutations specifically recruit the multimeric ETS factor GABP, which can form two functionally independent transcription factor species: a dimer or a tetramer. We show that genetic disruption of GABPβ1L (β1L), a tetramer-forming isoform of GABP that is dispensable for normal development, results in TERT silencing in a TERT promoter mutation-dependent manner. Reducing TERT expression by disrupting β1L culminates in telomere loss and cell death exclusively in TERT promoter mutant cells. Orthotopic xenografting of β1L-reduced, TERT promoter mutant glioblastoma cells rendered lower tumor burden and longer overall survival in mice. These results highlight the critical role of GABPβ1L in enabling immortality in TERT promoter mutant glioblastoma.

CRISPR gene editing has revolutionized biomedicine and biotechnology by providing a simple means to engineer genes through targeted double-strand breaks in the genomic DNA of living cells. However, given the stochasticity of cellular DNA repair mechanisms and the potential for off-target mutations, technologies capable of introducing targeted changes with increased precision, such as single-base editors, are preferred. We present a versatile method termed CRISPR-SKIP that utilizes cytidine deaminase single-base editors to program exon skipping by mutating target DNA bases within splice acceptor sites. Given its simplicity and precision, CRISPR-SKIP will be broadly applicable in gene therapy and synthetic biology.Electronic supplementary materialThe online version of this article (10.1186/s13059-018-1482-5) contains supplementary material, which is available to authorized users.