P07.11.B A NOVEL CRISPR/CAS9 KNOCKOUT LIBRARY UNCOVERS PAN-ESSENTIAL MIRNAS IN HUMAN CANCER CELL LINES

Abstract BACKGROUND MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression that play an important role during cancer initiation and progression. Genome-wide functional screening using the CRISPR/Cas9 system is a powerful tool to uncover genetic dependencies for distinct tumor entities, and has revealed novel insights into pan-cancer fitness genes. Current libraries are focused on single guide RNAs (sgRNAs) targeting protein-coding genes, thereby limiting functional genomics-based investigations of miRNA function. MATERIAL AND METHODS We here designed a novel CRISPR/Cas9 knockout library of 8,107 distinct sgRNAs targeting a total of 1,769 human miRNAs. Using a total of 45 human cancer cell lines, representing 16 different tumor entities, we performed negative selection screens to identify miRNA fitness genes. Screen hits per cell line were scored on the basis of a combination of a supervised Bayesian gene essentiality analysis (BAGEL2) and an unsupervised gene ranking algorithm (MAGeCK). miRNA genes that scored across all cell lines, i.e. potentially pan-essential miRNAs, were identified using a combination of the Adaptive Daisy Model (ADaM) algorithm and the Fitness Percentile (FiPer) method. RESULTS In silico analyses revealed that our library has significantly lower off-target activity for protein-coding genes as compared to previously described miRNA-targeting sgRNA libraries, while providing a higher coverage of miRNAs present in the human genome. In total, only 13% (n=230) of all targeted miRNAs induced a fitness effect on one or more cell lines, and the majority (83%) of these genes induced a dependency in less than 50% of the tested cell lines. Pan-cancer fitness gene detection by ADaM or FiPer differed solely on the level of stringency. As such, ADaM and FiPer identified 34 core fitness or 54 common essential miRNAs, respectively, and almost all core fitness genes (97%) were identified as common essential. Preliminary network analyses revealed that protein-coding genes that are regulated by core fitness miRNAs are significantly enriched for cell cycle and p53 signaling networks, providing a first rationale for their dependency profiles in human cancer cell lines. CONCLUSION We here established a novel method for functional investigation of miRNAs by means of genome-wide CRISPR/Cas9 knockout screens. Our sgRNA library outperforms previously established libraries in terms of specificity and overall miRNA coverage, making it a novel