Highlights d A CRISPR interference platform for genetic screens in human iPSC-derived neurons d Survival screens uncover genes essential for neurons, but not iPSCs or cancer cells d Single-cell RNA-seq screens reveal distinct neuronal roles for ubiquitous genes d Arrayed high-content screens uncover genes controlling neuronal morphology

CRISPR/Cas9-based functional genomics have transformed our ability to elucidate mammalian cell biology. However, most previous CRISPR-based screens were conducted in cancer cell lines, rather than healthy, differentiated cells. Here, we describe a CRISPR interference (CRISPRi)-based platform for genetic screens in human neurons derived from induced pluripotent stem cells (iPSCs). We demonstrate robust and durable knockdown of endogenous genes in such neurons, and present results from three complementary genetic screens. First, a survival-based screen revealed neuron-specific essential genes and genes that improved neuronal survival upon knockdown. Second, a screen with a single-cell transcriptomic readout uncovered several examples of genes whose knockdown had strikingly cell-type specific consequences. Third, a longitudinal imaging screen detected distinct consequences of gene knockdown on neuronal morphology. Our results highlight the power of unbiased genetic screens in iPSCderived differentiated cell types and provide a platform for systematic interrogation of normal and disease states of neurons.2 4 systematic dissection of normal and disease states of neurons, and highlight the potential of interrogating human cell biology and gene function in iPSC-derived differentiated cell types. RESULTS Robust CRISPR interference in human iPSC-derived neuronsAs a first step towards a high-throughput screening platform in neurons, we developed a scalable CRISPRi-based strategy for robust knockdown of endogenous genes in homogeneous populations of human iPSC-derived neurons. We built on our previously described i 3 Neuron (i 3 N) platform, which enables large-scale production of iPSC-derived glutamatergic neurons. Central to this platform is an iPSC line with an inducible Neurogenin 2 (Ngn2) expression cassette in the AAVS1 safe-harbor locus (Fernandopulle et al., 2018;Wang et al., 2017). To enable stable CRISPRi in iPSC-derived neurons, we generated a plasmid (pC13N-dCas9-BFP-KRAB) to insert an expression cassette for CAG promoter-driven dCas9-BFP-KRAB into the CLYBL safe harbor locus, which enables robust transgene expression throughout neuronal differentiation at higher levels than the AAVS1 locus (Cerbini et al., 2015) ( Fig. 1A). We then integrated this cassette into our i 3 N iPSC line, and called the resulting monoclonal line CRISPRi-i 3 N iPSCs. A normal karyotype was confirmed for CRISPRi-i 3 N iPSCs ( Fig. S1A).To validate CRISPRi activity, we transduced these iPSCs with a lentiviral construct expressing an sgRNA targeting the transferrin receptor gene (TFRC). Knockdown of TFRC mRNA was robust in iPSCs and in i 3 Neurons for several weeks after differentiation (Fig. 1B,C). We also validated knockdown of three additional genes, UBQLN2 (Fig. 1D,E), GRN (Fig. 1F,G) and CDH2 (Fig. S1B) by qRT-PCR, Western blot, and/or immunofluorescence. Our platform thus enables potent CRISPRi knockdown of endogenous genes in iPSC-derived neurons.Since CRISPRn-associated DNA damage has been found to be highly toxic to iPSCs (Ihry ...

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Cholesterol is detrimental to human health in excess but is also essential for normal embryogenesis. Hence, enzymes involved in its synthesis possess many layers of regulation to achieve balanced cholesterol levels. 7-Dehydrocholesterol reductase (DHCR7) is the terminal enzyme of cholesterol synthesis in the Kandutsch-Russell pathway, converting 7-dehydrocholesterol (7DHC) to cholesterol. In the absence of functional DHCR7, accumulation of 7DHC and a lack of cholesterol production leads to the devastating developmental disorder, Smith-Lemli-Opitz syndrome. This study identifies that statin treatment can ameliorate the low DHCR7 expression seen with common Smith-Lemli-Opitz syndrome mutations. Furthermore, we show that wild-type DHCR7 is also relatively labile. In an example of end-product inhibition, cholesterol accelerates the proteasomal degradation of DHCR7, resulting in decreased protein levels and activity. The loss of enzymatic activity results in the accumulation of the substrate 7DHC, which leads to an increased production of vitamin D. Thus, these findings highlight DHCR7 as an important regulatory switch between cholesterol and vitamin D synthesis.