CiBER-seq dissects genetic networks by quantitative CRISPRi profiling of expression phenotypes – Science Magazine

Posted: December 12, 2020 at 7:59 pm

CiBER-seq dissects genetic networks

Cells integrate environmental signals and internal states to dynamically control gene expression. Muller et al. developed a technique to dissect this cellular logic by linking targeted, genome-wide genetic perturbations with a deep-sequencing readout that quantitatively measured the expression phenotype induced by each perturbation. The method, dubbed CiBER-seq, was able to recapitulate known regulatory pathways linking protein synthesis with nutrient availability in budding yeast cells. Unexpectedly, the authors found that the cellular logic also appears to consider protein production machinery in this decision. By uncovering additional facets of this deeply conserved pathway, the findings demonstrate the utility of comprehensive and quantitative CiBER-seq profiling in mapping the gene networks underlying cellular decisions.

Science, this issue p. eabb9662

Systematically profiling the effects of genetic perturbations is a powerful approach that has revealed the molecular basis for a wide range of biological phenomena. The simple, programmable DNA recognition of CRISPR-Cas9 enables genome-wide genetic analysis in human cells and many other systems. Cas9 is guided by a short RNA to a complementary sequence in the genome, where it can introduce mutations or alter gene expression. Pooled libraries of guide RNAs (gRNAs) that individually target each gene in the genome allow us to introduce genetic perturbations systematically into a population of cells. A key challenge is measuring the phenotypic effects caused by individual guides in these pooled libraries and linking these phenotypes back to the associated gRNA, thereby finding the gene that is responsible.

Molecular phenotypes such as gene expression changes provide a clear and sensitive measure for many cellular processes. We sought a general approach to profile how the expression of a particular gene of interest changed when other genes were perturbed. We began with a library of gRNAs, each disrupting one gene, and linked these guides with an expression reporter containing a guide-specific nucleotide barcode. gRNAs that alter reporter expression will change the abundance of the expressed RNA barcode specifically associated with that guide. Deep sequencing of these expressed barcodes quantifies each of these guide-specific reporter expression effects individually within a pooled, complex population. We have implemented this strategy by combining CRISPR interference (CRISPRi) with barcoded expression reporter sequencing (CiBER-seq).

We used CiBER-seq to profile the responses of several yeast promoters tied to a range of biological functions. Each promoter yielded a distinct pattern of responses that could be understood in terms of its known function and regulation. For example, we rediscover the control of MET6 expression by regulatory ubiquitylation and connect the bud scar protein Cwp1 to other genes required for budding and cytokinesis. Our analysis of the HIS4 promoter, a well-characterized target of the integrated stress response, yielded a range of genetic perturbations that activate this pathway by causing the accumulation of uncharged transfer RNAs (tRNAs). We also uncovered a notable role for tRNA depletion in this response, as impaired tRNA biogenesis activated HIS4 expression through a distinct pathway. In order to understand this regulation, we carried out genetic interaction analysis and looked for quantitative deviations in CiBER-seq profiles caused by the introduction of a second genetic perturbation. We also developed an indirect CiBER-seq approach to measure translational and posttranslational regulation, which both play roles in the signaling pathways upstream of HIS4.

CiBER-seq produces comprehensive phenotypic profiles that offer insights into gene function and regulation. These high-throughput and quantitative phenotypic measurements are also well suited for the systematic measurement of genetic interactions, which contain rich information about the operation of biological processes. This approach can be applied to study a wide range of transcriptional, translational, and posttranslational regulatory responses, and it has the potential to shed light on many areas of biology.

CRISPR-Cas9 gRNA cassettes are linked with transcriptional reporters containing specific barcodes. The RNA-to-DNA ratio for each barcode, measured by deep sequencing, reveals the reporter expression phenotype induced by each gRNA.

To realize the promise of CRISPR-Cas9based genetics, approaches are needed to quantify a specific, molecular phenotype across genome-wide libraries of genetic perturbations. We addressed this challenge by profiling transcriptional, translational, and posttranslational reporters using CRISPR interference (CRISPRi) with barcoded expression reporter sequencing (CiBER-seq). Our barcoding approach allowed us to connect an entire library of guides to their individual phenotypic consequences using pooled sequencing. CiBER-seq profiling fully recapitulated the integrated stress response (ISR) pathway in yeast. Genetic perturbations causing uncharged transfer RNA (tRNA) accumulation activated ISR reporter transcription. Notably, tRNA insufficiency also activated the reporter, independent of the uncharged tRNA sensor. By uncovering alternate triggers for ISR activation, we illustrate how precise, comprehensive CiBER-seq profiling provides a powerful and broadly applicable tool for dissecting genetic networks.

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CiBER-seq dissects genetic networks by quantitative CRISPRi profiling of expression phenotypes - Science Magazine

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