Publications by authors named "Mudra Hegde"

10 Publications

  • Page 1 of 1

Massively parallel assessment of human variants with base editor screens.

Cell 2021 Feb;184(4):1064-1080.e20

Genetic Perturbation Platform, Broad Institute, Cambridge, MA 02142, USA. Electronic address:

Understanding the functional consequences of single-nucleotide variants is critical to uncovering the genetic underpinnings of diseases, but technologies to characterize variants are limiting. Here, we leverage CRISPR-Cas9 cytosine base editors in pooled screens to scalably assay variants at endogenous loci in mammalian cells. We benchmark the performance of base editors in positive and negative selection screens, identifying known loss-of-function mutations in BRCA1 and BRCA2 with high precision. To demonstrate the utility of base editor screens to probe small molecule-protein interactions, we screen against BH3 mimetics and PARP inhibitors, identifying point mutations that confer drug sensitivity or resistance. We also create a library of single guide RNAs (sgRNAs) predicted to generate 52,034 ClinVar variants in 3,584 genes and conduct screens in the presence of cellular stressors, identifying loss-of-function variants in numerous DNA damage repair genes. We anticipate that this screening approach will be broadly useful to readily and scalably functionalize genetic variants.
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http://dx.doi.org/10.1016/j.cell.2021.01.012DOI Listing
February 2021

Optimization of AsCas12a for combinatorial genetic screens in human cells.

Nat Biotechnol 2021 01 13;39(1):94-104. Epub 2020 Jul 13.

Genetic Perturbation Platform, Broad Institute of MIT and Harvard, Cambridge, MA, USA.

Cas12a RNA-guided endonucleases are promising tools for multiplexed genetic perturbations because they can process multiple guide RNAs expressed as a single transcript, and subsequently cleave target DNA. However, their widespread adoption has lagged behind Cas9-based strategies due to low activity and the lack of a well-validated pooled screening toolkit. In the present study, we describe the optimization of enhanced Cas12a from Acidaminococcus (enAsCas12a) for pooled, combinatorial genetic screens in human cells. By assaying the activity of thousands of guides, we refine on-target design rules and develop a comprehensive set of off-target rules to predict and exclude promiscuous guides. We also identify 38 direct repeat variants that can substitute for the wild-type sequence. We validate our optimized AsCas12a toolkit by screening for synthetic lethalities in OVCAR8 and A375 cancer cells, discovering an interaction between MARCH5 and WSB2. Finally, we show that enAsCas12a delivers similar performance to Cas9 in genome-wide dropout screens but at greatly reduced library size, which will facilitate screens in challenging models.
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http://dx.doi.org/10.1038/s41587-020-0600-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7854777PMC
January 2021

Genetic screens in isogenic mammalian cell lines without single cell cloning.

Nat Commun 2020 02 6;11(1):752. Epub 2020 Feb 6.

Genetic Perturbation Platform, Broad Institute of MIT and Harvard, 75 Ames Street, Cambridge, MA, 02142, USA.

Isogenic pairs of cell lines, which differ by a single genetic modification, are powerful tools for understanding gene function. Generating such pairs of mammalian cells, however, is labor-intensive, time-consuming, and, in some cell types, essentially impossible. Here, we present an approach to create isogenic pairs of cells that avoids single cell cloning, and screen these pairs with genome-wide CRISPR-Cas9 libraries to generate genetic interaction maps. We query the anti-apoptotic genes BCL2L1 and MCL1, and the DNA damage repair gene PARP1, identifying both expected and uncharacterized buffering and synthetic lethal interactions. Additionally, we compare acute CRISPR-based knockout, single cell clones, and small-molecule inhibition. We observe that, while the approaches provide largely overlapping information, differences emerge, highlighting an important consideration when employing genetic screens to identify and characterize potential drug targets. We anticipate that this methodology will be broadly useful to comprehensively study gene function across many contexts.
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http://dx.doi.org/10.1038/s41467-020-14620-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7005275PMC
February 2020

Genome-wide In Vivo CNS Screening Identifies Genes that Modify CNS Neuronal Survival and mHTT Toxicity.

Neuron 2020 04 30;106(1):76-89.e8. Epub 2020 Jan 30.

Department of Brain and Cognitive Sciences, MIT, Cambridge, MA 02139, USA; Picower Institute for Learning and Memory, Cambridge, MA 02139, USA; Broad Institute of MIT and Harvard, Cambridge, MA 02142, USA. Electronic address:

Unbiased in vivo genome-wide genetic screening is a powerful approach to elucidate new molecular mechanisms, but such screening has not been possible to perform in the mammalian central nervous system (CNS). Here, we report the results of the first genome-wide genetic screens in the CNS using both short hairpin RNA (shRNA) and CRISPR libraries. Our screens identify many classes of CNS neuronal essential genes and demonstrate that CNS neurons are particularly sensitive not only to perturbations to synaptic processes but also autophagy, proteostasis, mRNA processing, and mitochondrial function. These results reveal a molecular logic for the common implication of these pathways across multiple neurodegenerative diseases. To further identify disease-relevant genetic modifiers, we applied our screening approach to two mouse models of Huntington's disease (HD). Top mutant huntingtin toxicity modifier genes included several Nme genes and several genes involved in methylation-dependent chromatin silencing and dopamine signaling, results that reveal new HD therapeutic target pathways.
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http://dx.doi.org/10.1016/j.neuron.2020.01.004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7181458PMC
April 2020

Optimized libraries for CRISPR-Cas9 genetic screens with multiple modalities.

Nat Commun 2018 12 21;9(1):5416. Epub 2018 Dec 21.

Broad Institute of Harvard and MIT, 75 Ames Street, Cambridge, MA, 02142, USA.

The creation of genome-wide libraries for CRISPR knockout (CRISPRko), interference (CRISPRi), and activation (CRISPRa) has enabled the systematic interrogation of gene function. Here, we show that our recently-described CRISPRko library (Brunello) is more effective than previously published libraries at distinguishing essential and non-essential genes, providing approximately the same perturbation-level performance improvement over GeCKO libraries as GeCKO provided over RNAi. Additionally, we present genome-wide libraries for CRISPRi (Dolcetto) and CRISPRa (Calabrese), and show in negative selection screens that Dolcetto, with fewer sgRNAs per gene, outperforms existing CRISPRi libraries and achieves comparable performance to CRISPRko in detecting essential genes. We also perform positive selection CRISPRa screens and demonstrate that Calabrese outperforms the SAM approach at identifying vemurafenib resistance genes. We further compare CRISPRa to genome-scale libraries of open reading frames (ORFs). Together, these libraries represent a suite of genome-wide tools to efficiently interrogate gene function with multiple modalities.
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http://dx.doi.org/10.1038/s41467-018-07901-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6303322PMC
December 2018

Uncoupling of sgRNAs from their associated barcodes during PCR amplification of combinatorial CRISPR screens.

PLoS One 2018 25;13(5):e0197547. Epub 2018 May 25.

Broad Institute of MIT and Harvard, Cambridge, Massachusetts, United States of America.

Many implementations of pooled screens in mammalian cells rely on linking an element of interest to a barcode, with the latter subsequently quantitated by next generation sequencing. However, substantial uncoupling between these paired elements during lentiviral production has been reported, especially as the distance between elements increases. We detail that PCR amplification is another major source of uncoupling, and becomes more pronounced with increased amounts of DNA template molecules and PCR cycles. To lessen uncoupling in systems that use paired elements for detection, we recommend minimizing the distance between elements, using low and equal template DNA inputs for plasmid and genomic DNA during PCR, and minimizing the number of PCR cycles. We also present a vector design for conducting combinatorial CRISPR screens that enables accurate barcode-based detection with a single short sequencing read and minimal uncoupling.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0197547PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5969736PMC
December 2018

Orthologous CRISPR-Cas9 enzymes for combinatorial genetic screens.

Nat Biotechnol 2018 02 18;36(2):179-189. Epub 2017 Dec 18.

Broad Institute of Harvard and MIT, Cambridge, Massachusetts, USA.

Combinatorial genetic screening using CRISPR-Cas9 is a useful approach to uncover redundant genes and to explore complex gene networks. However, current methods suffer from interference between the single-guide RNAs (sgRNAs) and from limited gene targeting activity. To increase the efficiency of combinatorial screening, we employ orthogonal Cas9 enzymes from Staphylococcus aureus and Streptococcus pyogenes. We used machine learning to establish S. aureus Cas9 sgRNA design rules and paired S. aureus Cas9 with S. pyogenes Cas9 to achieve dual targeting in a high fraction of cells. We also developed a lentiviral vector and cloning strategy to generate high-complexity pooled dual-knockout libraries to identify synthetic lethal and buffering gene pairs across multiple cell types, including MAPK pathway genes and apoptotic genes. Our orthologous approach also enabled a screen combining gene knockouts with transcriptional activation, which revealed genetic interactions with TP53. The "Big Papi" (paired aureus and pyogenes for interactions) approach described here will be widely applicable for the study of combinatorial phenotypes.
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http://dx.doi.org/10.1038/nbt.4048DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5800952PMC
February 2018

Creation of Novel Protein Variants with CRISPR/Cas9-Mediated Mutagenesis: Turning a Screening By-Product into a Discovery Tool.

PLoS One 2017 24;12(1):e0170445. Epub 2017 Jan 24.

Broad Institute of MIT and Harvard, Cambridge, MA, United States of America.

CRISPR/Cas9 screening has proven to be a versatile tool for genomics research. Based on unexpected results from a genome-wide screen, we developed a CRISPR/Cas9-mediated approach to mutagenesis, exploiting the allelic diversity generated by error-prone non-homologous end-joining (NHEJ) to identify novel gain-of-function and drug resistant alleles of the MAPK signaling pathway genes MEK1 and BRAF. We define the parameters of a scalable technique to easily generate cell populations containing thousands of endogenous allelic variants to map gene functions. Further, these results highlight an unexpected but important phenomenon, that Cas9-induced gain-of-function alleles are an inherent by-product of normal Cas9 loss-of-function screens and should be investigated during analysis of data from large-scale positive selection screens.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0170445PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5261743PMC
August 2017

Optimized sgRNA design to maximize activity and minimize off-target effects of CRISPR-Cas9.

Nat Biotechnol 2016 Feb 18;34(2):184-191. Epub 2016 Jan 18.

Broad Institute of MIT and Harvard, Cambridge, Massachusetts, USA.

CRISPR-Cas9-based genetic screens are a powerful new tool in biology. By simply altering the sequence of the single-guide RNA (sgRNA), one can reprogram Cas9 to target different sites in the genome with relative ease, but the on-target activity and off-target effects of individual sgRNAs can vary widely. Here, we use recently devised sgRNA design rules to create human and mouse genome-wide libraries, perform positive and negative selection screens and observe that the use of these rules produced improved results. Additionally, we profile the off-target activity of thousands of sgRNAs and develop a metric to predict off-target sites. We incorporate these findings from large-scale, empirical data to improve our computational design rules and create optimized sgRNA libraries that maximize on-target activity and minimize off-target effects to enable more effective and efficient genetic screens and genome engineering.
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http://dx.doi.org/10.1038/nbt.3437DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4744125PMC
February 2016

Rational design of highly active sgRNAs for CRISPR-Cas9-mediated gene inactivation.

Nat Biotechnol 2014 Dec 3;32(12):1262-7. Epub 2014 Sep 3.

Broad Institute of MIT and Harvard, Cambridge, Massachusetts, USA.

Components of the prokaryotic clustered, regularly interspaced, short palindromic repeats (CRISPR) loci have recently been repurposed for use in mammalian cells. The CRISPR-associated (Cas)9 can be programmed with a single guide RNA (sgRNA) to generate site-specific DNA breaks, but there are few known rules governing on-target efficacy of this system. We created a pool of sgRNAs, tiling across all possible target sites of a panel of six endogenous mouse and three endogenous human genes and quantitatively assessed their ability to produce null alleles of their target gene by antibody staining and flow cytometry. We discovered sequence features that improved activity, including a further optimization of the protospacer-adjacent motif (PAM) of Streptococcus pyogenes Cas9. The results from 1,841 sgRNAs were used to construct a predictive model of sgRNA activity to improve sgRNA design for gene editing and genetic screens. We provide an online tool for the design of highly active sgRNAs for any gene of interest.
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http://dx.doi.org/10.1038/nbt.3026DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4262738PMC
December 2014