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NAR Top Articles - Synthetic Biology and Chemistry

Synthetic Biology and Chemistry

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January 2015


Genome engineering in Saccharomyces cerevisiae using CRISPR-Cas systems
J. E. DiCarlo, J. E. Norville, P. Mali, X. Rios, J. Aach and G. M. Church
Nucleic Acids Res. (2013) 41 (7): 4336-4343
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Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-associated (Cas) systems in bacteria and archaea use RNA-guided nuclease activity to provide adaptive immunity against invading foreign nucleic acids. Here, we report the use of type II bacterial CRISPR-Cas system in Saccharomyces cerevisiae for genome engineering. The CRISPR-Cas components, Cas9 gene and a designer genome targeting CRISPR guide RNA (gRNA), show robust and specific RNA-guided endonuclease activity at targeted endogenous genomic loci in yeast. Using constitutive Cas9 expression and a transient gRNA cassette, we show that targeted double-strand breaks can increase homologous recombination rates of single- and double-stranded oligonucleotide donors by 5-fold and 130-fold, respectively. In addition, co-transformation of a gRNA plasmid and a donor DNA in cells constitutively expressing Cas9 resulted in near 100% donor DNA recombination frequency. Our approach provides foundations for a simple and powerful genome engineering tool for site-specific mutagenesis and allelic replacement in yeast.

CRISPR/Cas9 systems have off-target activity with insertions or deletions between target DNA and guide RNA sequences
Y. Lin, T. J. Cradick, M. T. Brown, H. Deshmukh, P. Ranjan, N. Sarode, B. M. Wile, P. M. Vertino, F. J. Stewart and G. Bao
Nucleic Acids Res. (2014) 42 (11): 7473-7485
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CRISPR/Cas9 systems are a versatile tool for genome editing due to the highly efficient targeting of DNA sequences complementary to their RNA guide strands. However, it has been shown that RNA-guided Cas9 nuclease cleaves genomic DNA sequences containing mismatches to the guide strand. A better understanding of the CRISPR/Cas9 specificity is needed to minimize off-target cleavage in large mammalian genomes. Here we show that genomic sites could be cleaved by CRISPR/Cas9 systems when DNA sequences contain insertions ('DNA bulge') or deletions ('RNA bulge') compared to the RNA guide strand, and Cas9 nickases used for paired nicking can also tolerate bulges in one of the guide strands. Variants of single-guide RNAs (sgRNAs) for four endogenous loci were used as model systems, and their cleavage activities were quantified at different positions with 1- to 5-bp bulges. We further investigated 114 putative genomic off-target loci of 27 different sgRNAs and confirmed 15 off-target sites, each harboring a single-base bulge and one to three mismatches to the guide strand...

Large chromosomal deletions and heritable small genetic changes induced by CRISPR/Cas9 in rice
H. Zhou, B. Liu, D. P. Weeks, M. H. Spalding and B. Yang
Nucleic Acids Res. (2014) 42 (17): 10903-10914
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The Cas9/sgRNA of the CRISPR/Cas system has emerged as a robust technology for targeted gene editing in various organisms, including plants, where Cas9/sgRNA-mediated small deletions/insertions at single cleavage sites have been reported in transient and stable transformations, although genetic transmission of edits has been reported only in Arabidopsis and rice. Large chromosomal excision between two remote nuclease-targeted loci has been reported only in a few non-plant species. Here we report in rice Cas9/sgRNA-induced large chromosomal segment deletions, the inheritance of genome edits in multiple generations and construction of a set of facile vectors for high-efficiency, multiplex gene targeting. Four sugar efflux transporter genes were modified in rice at high efficiency; the most efficient system yielding 87-100% editing in T0 transgenic plants, all with di-allelic edits. Furthermore, genetic crosses segregating Cas9/sgRNA transgenes away from edited genes yielded several genome-edited but transgene-free rice plants...

Targeted delivery of antisense oligonucleotides to hepatocytes using triantennary N-acetyl galactosamine improves potency 10-fold in mice
T. P. Prakash, M. J. Graham, J. Yu, R. Carty, A. Low, A. Chappell, K. Schmidt, C. Zhao, M. Aghajan, H. F. Murray, S. Riney, S. L. Booten, S. F. Murray, H. Gaus, J. Crosby, W. F. Lima, S. Guo, B. P. Monia, E. E. Swayze and P. P. Seth
Nucleic Acids Res. (2014) 42 (13): 8796-8807
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Triantennary N-acetyl galactosamine (GalNAc, GN3: ), a high-affinity ligand for the hepatocyte-specific asialoglycoprotein receptor (ASGPR), enhances the potency of second-generation gapmer antisense oligonucleotides (ASOs) 6-10-fold in mouse liver. When combined with next-generation ASO designs comprised of short S-cEt (S-2'-O-Et-2',4'-bridged nucleic acid) gapmer ASOs, approximately 60-fold enhancement in potency relative to the parent MOE (2'-O-methoxyethyl RNA) ASO was observed. GN3: -conjugated ASOs showed high affinity for mouse ASGPR, which results in enhanced ASO delivery to hepatocytes versus non-parenchymal cells. After internalization into cells, the GN3: -ASO conjugate is metabolized to liberate the parent ASO in the liver. No metabolism of the GN3: -ASO conjugate was detected in plasma suggesting that GN3: acts as a hepatocyte targeting prodrug that is detached from the ASO by metabolism after internalization into the liver...

CRISPR/Cas9 systems targeting {beta}-globin and CCR5 genes have substantial off-target activity
T. J. Cradick, E. J. Fine, C. J. Antico and G. Bao
Nucleic Acids Res. (2013) 41 (20): 9584-9592
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The ability to precisely modify endogenous genes can significantly facilitate biological studies and disease treatment, and the clustered regularly interspaced short palindromic repeats (CRISPR) systems have the potential to be powerful tools for genome engineering. However, the target specificity of CRISPR systems is largely unknown. Here we demonstrate that CRISPR/Cas9 systems targeting the human hemoglobin beta and C-C chemokine receptor type 5 genes have substantial off-target cleavage, especially within the hemoglobin delta and C-C chemokine receptor type 2 genes, respectively, causing gross chromosomal deletions. The guide strands of the CRISPR/Cas9 systems were designed to have a range of mismatches with the sequences of potential off-target sites. Off-target analysis was performed using the T7 endonuclease I mutation detection assay and Sanger sequencing. We found that the repair of the on-and off-target cleavage resulted in a wide variety of insertions, deletions and point mutations. Therefore, CRISPR/Cas9 systems need to be carefully designed to avoid potential off-target cleavage sites...

A platform for rapid prototyping of synthetic gene networks in mammalian cells
X. Duportet, L. Wroblewska, P. Guye, Y. Li, J. Eyquem, J. Rieders, T. Rimchala, G. Batt and R. Weiss
Nucleic Acids Res. (2014) 42 (21): 13440-13451
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Mammalian synthetic biology may provide novel therapeutic strategies, help decipher new paths for drug discovery and facilitate synthesis of valuable molecules. Yet, our capacity to genetically program cells is currently hampered by the lack of efficient approaches to streamline the design, construction and screening of synthetic gene networks. To address this problem, here we present a framework for modular and combinatorial assembly of functional (multi)gene expression vectors and their efficient and specific targeted integration into a well-defined chromosomal context in mammalian cells. We demonstrate the potential of this framework by assembling and integrating different functional mammalian regulatory networks including the largest gene circuit built and chromosomally integrated to date (6 transcription units, 27kb) encoding an inducible memory device. Using a library of 18 different circuits as a proof of concept, we also demonstrate that our method enables one-pot/single-flask chromosomal integration and screening of circuit libraries...

Synthesis, biophysical properties and biological activity of second generation antisense oligonucleotides containing chiral phosphorothioate linkages
W. B. Wan, M. T. Migawa, G. Vasquez, H. M. Murray, J. G. Nichols, H. Gaus, A. Berdeja, S. Lee, C. E. Hart, W. F. Lima, E. E. Swayze and P. P. Seth
Nucleic Acids Res. (2014) 42 (22): 13456-13468
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Bicyclic oxazaphospholidine monomers were used to prepare a series of phosphorothioate (PS)-modified gapmer antisense oligonucleotides (ASOs) with control of the chirality of each of the PS linkages within the 10-base gap. The stereoselectivity was determined to be 98% for each coupling. The objective of this work was to study how PS chirality influences biophysical and biological properties of the ASO including binding affinity (Tm), nuclease stability, activity in vitro and in vivo, RNase H activation and cleavage patterns (both human and E. coli) in a gapmer context. Compounds that had nine or more Sp-linkages in the gap were found to be poorly active in vitro, while compounds with uniform Rp-gaps exhibited activity very similar to that of the stereo-random parent ASOs. Conversely, when tested in vivo, the full Rp-gap compound was found to be quickly metabolized resulting in low activity...

On the biophysics and kinetics of toehold-mediated DNA strand displacement
N. Srinivas, T. E. Ouldridge, P. Sulc, J. M. Schaeffer, B. Yurke, A. A. Louis, J. P. Doye and E. Winfree
Nucleic Acids Res. (2013) 41 (22): 10641-10658
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Dynamic DNA nanotechnology often uses toehold-mediated strand displacement for controlling reaction kinetics. Although the dependence of strand displacement kinetics on toehold length has been experimentally characterized and phenomenologically modeled, detailed biophysical understanding has remained elusive. Here, we study strand displacement at multiple levels of detail, using an intuitive model of a random walk on a 1D energy landscape, a secondary structure kinetics model with single base-pair steps and a coarse-grained molecular model that incorporates 3D geometric and steric effects. Further, we experimentally investigate the thermodynamics of three-way branch migration. Two factors explain the dependence of strand displacement kinetics on toehold length: (i) the physical process by which a single step of branch migration occurs is significantly slower than the fraying of a single base pair and (ii) initiating branch migration incurs a thermodynamic penalty, not captured by state-of-the-art nearest neighbor models of DNA, due to the additional overhang it engenders at the junction...

Protein-responsive ribozyme switches in eukaryotic cells
A. B. Kennedy, J. V. Vowles, L. d'Espaux and C. D. Smolke
Nucleic Acids Res. (2014) 42 (19): 12306-12321
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Genetic devices that directly detect and respond to intracellular concentrations of proteins are important synthetic biology tools, supporting the design of biological systems that target, respond to or alter specific cellular states. Here, we develop ribozyme-based devices that respond to protein ligands in two eukaryotic hosts, yeast and mammalian cells, to regulate the expression of a gene of interest. Our devices allow for both gene-ON and gene-OFF response upon sensing the protein ligand. As part of our design process, we describe an in vitro characterization pipeline for prescreening device designs to identify promising candidates for in vivo testing. The in vivo gene-regulatory activities in the two types of eukaryotic cells correlate with in vitro cleavage activities determined at different physiologically relevant magnesium concentrations. Finally, localization studies with the ligand demonstrate that ribozyme switches respond to ligands present in the nucleus and/or cytoplasm, providing new insight into their mechanism of action. By extending the sensing capabilities of this important class of gene-regulatory device, our work supports the implementation of ribozyme-based devices in applications...

In vivo co-localization of enzymes on RNA scaffolds increases metabolic production in a geometrically dependent manner
G. Sachdeva, A. Garg, D. Godding, J. C. Way and P. A. Silver
Nucleic Acids Res. (2014) 42 (14): 9493-9503
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Co-localization of biochemical processes plays a key role in the directional control of metabolic fluxes toward specific products in cells. Here, we employ in vivo scaffolds made of RNA that can bind engineered proteins fused to specific RNA binding domains. This allows proteins to be co-localized on RNA scaffolds inside living Escherichia coli. We assembled a library of eight aptamers and corresponding RNA binding domains fused to partial fragments of fluorescent proteins. New scaffold designs could co-localize split green fluorescent protein fragments to produce activity as measured by cell-based fluorescence. The scaffolds consisted of either single bivalent RNAs or RNAs designed to polymerize in one or two dimensions. The new scaffolds were used to increase metabolic output from a two-enzyme pentadecane production pathway that contains a fatty aldehyde intermediate, as well as three and four enzymes in the succinate production pathway. Pentadecane synthesis depended on the geometry of enzymes on the scaffold, as determined through systematic reorientation of the acyl-ACP reductase fusion by rotation via addition of base pairs to its cognate RNA...

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