Cameron Myhrvold

Nucleic acid detection with CRISPR-Cas13a/C2c2

Sensitive and specific CRISPR diagnostics Methods are needed that can easily detect nucleic acids that signal the presence of pathogens, even at very low levels. Gootenberg et al. combined the allele-specific sensing ability of CRISPR-Cas13a with recombinase polymerase amplification methods to detect specific RNA and DNA sequences. The method successfully detected attomolar levels of Zika virus, as well as the presence of pathogenic bacteria. It could also be used to perform human genotyping from cell-free DNA. Science, this issue p. 438 An ortholog of CRISPR-Cas13a/C2c2 can be used as a highly sensitive detector of specific RNA and DNA sequences. Rapid, inexpensive, and sensitive nucleic acid detection may aid point-of-care pathogen detection, genotyping, and disease monitoring. The RNA-guided, RNA-targeting clustered regularly interspaced short palindromic repeats (CRISPR) effector Cas13a (previously known as C2c2) exhibits a “collateral effect” of promiscuous ribonuclease activity upon target recognition. We combine the collateral effect of Cas13a with isothermal amplification to establish a CRISPR-based diagnostic (CRISPR-Dx), providing rapid DNA or RNA detection with attomolar sensitivity and single-base mismatch specificity. We use this Cas13a-based molecular detection platform, termed Specific High-Sensitivity Enzymatic Reporter UnLOCKing (SHERLOCK), to detect specific strains of Zika and Dengue virus, distinguish pathogenic bacteria, genotype human DNA, and identify mutations in cell-free tumor DNA. Furthermore, SHERLOCK reaction reagents can be lyophilized for cold-chain independence and long-term storage and be readily reconstituted on paper for field applications.

Isothermal Self-Assembly of Complex DNA Structures under Diverse and Biocompatible Conditions

Nucleic acid nanotechnology has enabled researchers to construct a wide range of multidimensional structures in vitro. Until recently, most DNA-based structures were assembled by thermal annealing using high magnesium concentrations and nonphysiological environments. Here, we describe a DNA self-assembly system that can be tuned to form a complex target structure isothermally at any prescribed temperature or homogeneous condition within a wide range. We were able to achieve isothermal assembly between 15 and 69 °C in a predictable fashion by altering the strength of strand-strand interactions in several different ways, for example, domain length, GC content, and linker regions between domains. We also observed the assembly of certain structures under biocompatible conditions, that is, at physiological pH, temperature, and salinity in the presence of the molecular crowding agent polyethylene glycol (PEG) mimicking the cellular environment. This represents an important step toward the self-assembly of geometrically precise DNA or RNA structures in vivo.

Programmable self-assembly of three-dimensional nanostructures from 10,000 unique components

Nucleic acids (DNA and RNA) are widely used to construct nanometre-scale structures with ever increasing complexity, with possible application in fields such as structural biology, biophysics, synthetic biology and photonics. The nanostructures are formed through one-pot self-assembly, with early kilodalton-scale examples containing typically tens of unique DNA strands. The introduction of DNA origami, which uses many staple strands to fold one long scaffold strand into a desired structure, has provided access to megadalton-scale nanostructures that contain hundreds of unique DNA strands. Even larger DNA origami structures are possible, but manufacturing and manipulating an increasingly long scaffold strand remains a challenge. An alternative and more readily scalable approach involves the assembly of DNA bricks, which each consist of four short binding domains arranged so that the bricks can interlock. This approach does not require a scaffold; instead, the short DNA brick strands self-assemble according to specific inter-brick interactions. First-generation bricks used to create three-dimensional structures are 32 nucleotides long, consisting of four eight-nucleotide binding domains. Protocols have been designed to direct the assembly of hundreds of distinct bricks into well formed structures, but attempts to create larger structures have encountered practical challenges and had limited success. Here we show that DNA bricks with longer, 13-nucleotide binding domains make it possible to self-assemble 0.1–1-gigadalton, three-dimensional nanostructures from tens of thousands of unique components, including a 0.5-gigadalton cuboid containing about 30,000 unique bricks and a 1-gigadalton rotationally symmetric tetramer. We also assembled a cuboid that contains around 10,000 bricks and about 20,000 uniquely addressable, 13-base-pair ‘voxels’ that serves as a molecular canvas for three-dimensional sculpting. Complex, user-prescribed, three-dimensional cavities can be produced within this molecular canvas, enabling the creation of shapes such as letters, a helicoid and a teddy bear. We anticipate that with further optimization of structure design, strand synthesis and assembly procedure even larger structures could be accessible, which could be useful for applications such as positioning functional components.

Design Space for Complex DNA Structures

Nucleic acids have emerged as effective materials for assembling complex nanoscale structures. To tailor the structures to function optimally for particular applications, a broad structural design space is desired. Despite the many discrete and extended structures demonstrated in the past few decades, the design space remains to be fully explored. In particular, the complex finite-sized structures produced to date have been typically based on a small number of structural motifs. Here, we perform a comprehensive study of the design space for complex DNA structures, using more than 30 distinct motifs derived from single-stranded tiles. These motifs self-assemble to form structures with diverse strand weaving patterns and specific geometric properties, such as curvature and twist. We performed a systematic study to control and characterize the curvature of the structures, and constructed a flat structure with a corrugated strand pattern. The work here reveals the broadness of the design space for complex DNA nanostructures.

Single-stranded DNA and RNA origami

Large origami from a single strand Nanostructures created by origami-like folding of nucleic acids are usually formed by base-pairing interactions between multiple strands. Han et al. show that large origami (up to 10,000 nucleotides for DNA and 6000 nucleotides for RNA) can be created in simple shapes, such as a rhombus or a heart. A single strand can be folded smoothly into structurally complex but knot-free structures by using partially complemented double-stranded DNA and the cohesion of parallel crossovers. The use of single strands also enables in vitro synthesis of these structures. Science, this issue p. eaao2648 Large nanostructures of up to 10,000 nucleotides can be formed by folding a single nucleic acid strand. INTRODUCTION Self-folding of an information-carrying polymer into a compact particle with defined structure and function (for example, folding of a polypeptide into a protein) is foundational to biology and offers attractive potential as a synthetic strategy. Over the past three decades, nucleic acids have been used to create a variety of complex nanoscale shapes and devices. In particular, multiple DNA strands have been designed to self-assemble into user-specified structures, with or without the help of a long scaffold strand. In recent years, RNA has also emerged as a unique, programmable material, offering distinct advantages for molecular self-assembly. On the other hand, biological macromolecules, such as proteins (or protein domains), typically fold from a single polymer into a well-defined compact structure. The ability to fold de novo designed nucleic acid nanostructures in a similar manner would enable unimolecular folding instead of multistrand assembly and even replication of such structures. However, a general strategy to construct large [>1000 nucleotides (nt)] single-stranded origami (ssOrigami) remains to be demonstrated where a single-stranded nucleic acid folds into a user-specified shape. RATIONALE The key challenge for constructing a compact single-stranded structure is to achieve structural complexity, programmability, and generality while maintaining the topological simplicity of strand routing (to avoid putative kinetic traps imposed by knots) and hence ensuring smooth folding. The key innovation of our study is to use partially complemented double-stranded DNA or RNA and parallel crossover cohesion to construct such a structurally complex yet knot-free structure that can be folded smoothly from a single strand. RESULTS Here, we demonstrate a framework to design and synthesize a single DNA or RNA strand to efficiently self-fold into an unknotted compact ssOrigami structure that approximates an arbitrary user-prescribed target shape. The generality of the method was validated by the construction of 18 multikilobase DNA and 5 RNA ssOrigami, including a ~10,000-nt DNA structure (37 times larger than the previous largest discrete single-stranded DNA nanostructure) and a ~6000-nt RNA structure (10 times larger than the previous largest RNA structure). The raster-filling nature of ssOrigami permitted the experimental construction of programmable patterns of markers (for example, a “smiley” face) and cargoes on its surface, its single-strandedness enabled the demonstration of facile replication of the strand in vitro and in living cells, and its programmability allowed us to codify the design process and develop a web-based automated design tool. CONCLUSION The work here establishes that unimolecular DNA or RNA folding, similar to multicomponent self-assembly, is a fundamental, general, and practically tractable strategy for constructing user-specified and replicable nucleic acid nanostructures, and expands the design space and material scalability for bottom-up nanotechnology. Folding of DNA or RNA ssOrigami structures. (A) Multiple DNA strands have been designed to self-assemble without (left) or with (middle) a long scaffold strand. Here, we fold single long DNA or RNA strands into target shapes (right). (B) Schematics and atomic force microscopy images of single-stranded DNA (top three rows) and RNA (bottom row) nanostructures

Field-deployable viral diagnostics using CRISPR-Cas13

Taking CRISPR technology further CRISPR techniques are allowing the development of technologies for nucleic acid detection (see the Perspective by Chertow). Taking advantages of the distinctive enzymatic properties of CRISPR enzymes, Gootenberg et al. developed an improved nucleic acid detection technology for multiplexed quantitative and highly sensitive detection, combined with lateral flow for visual readout. Myhrvold et al. added a sample preparation protocol to create a field-deployable viral diagnostic platform for rapid detection of specific strains of pathogens in clinical samples. Cas12a (also known as Cpf1), a type V CRISPR protein, cleaves double-stranded DNA and has been adapted for genome editing. Chen et al. discovered that Cas12a also processes single-stranded DNA threading activity. A technology platform based on this activity detected human papillomavirus in patient samples with high sensitivity. Science, this issue p. 439, p. 444, p. 436; see also p. 381 A nucleic acid detection technology identifies viruses with minimal equipment and sample processing requirements. Mitigating global infectious disease requires diagnostic tools that are sensitive, specific, and rapidly field deployable. In this study, we demonstrate that the Cas13-based SHERLOCK (specific high-sensitivity enzymatic reporter unlocking) platform can detect Zika virus (ZIKV) and dengue virus (DENV) in patient samples at concentrations as low as 1 copy per microliter. We developed HUDSON (heating unextracted diagnostic samples to obliterate nucleases), a protocol that pairs with SHERLOCK for viral detection directly from bodily fluids, enabling instrument-free DENV detection directly from patient samples in <2 hours. We further demonstrate that SHERLOCK can distinguish the four DENV serotypes, as well as region-specific strains of ZIKV from the 2015–2016 pandemic. Finally, we report the rapid (<1 week) design and testing of instrument-free assays to detect clinically relevant viral single-nucleotide polymorphisms.

A distributed cell division counter reveals growth dynamics in the gut microbiota

Microbial population growth is typically measured when cells can be directly observed, or when death is rare. However, neither of these conditions hold for the mammalian gut microbiota, and, therefore, standard approaches cannot accurately measure the growth dynamics of this community. Here we introduce a new method (distributed cell division counting, DCDC) that uses the accurate segregation at cell division of genetically encoded fluorescent particles to measure microbial growth rates. Using DCDC, we can measure the growth rate of Escherichia coli for >10 consecutive generations. We demonstrate experimentally and theoretically that DCDC is robust to error across a wide range of temperatures and conditions, including in the mammalian gut. Furthermore, our experimental observations inform a mathematical model of the population dynamics of the gut microbiota. DCDC can enable the study of microbial growth during infection, gut dysbiosis, antibiotic therapy or other situations relevant to human health.

Using synthetic RNAs as scaffolds and regulators

The natural versatility of RNA makes it an ideal substrate for bioengineering. Its structural properties and predictable base-pairing permit its use as molecular scaffold, and its ability to interact with nucleic acids, proteins and small molecules confers a regulatory potential that can be harvested to design RNA regulators in diverse contexts.

Barcode extension for analysis and reconstruction of structures

Collections of DNA sequences can be rationally designed to self-assemble into predictable three-dimensional structures. The geometric and functional diversity of DNA nanostructures created to date has been enhanced by improvements in DNA synthesis and computational design. However, existing methods for structure characterization typically image the final product or laboriously determine the presence of individual, labelled strands using gel electrophoresis. Here we introduce a new method of structure characterization that uses barcode extension and next-generation DNA sequencing to quantitatively measure the incorporation of every strand into a DNA nanostructure. By quantifying the relative abundances of distinct DNA species in product and monomer bands, we can study the influence of geometry and sequence on assembly. We have tested our method using 2D and 3D DNA brick and DNA origami structures. Our method is general and should be extensible to a wide variety of DNA nanostructures.