Multiplex recording of cellular events over time on CRISPR biological tape

Engineering bacteria for cancer therapy

The engineering of living cells and microbes is ushering in a new era of cancer therapy. Due to recent microbiome studies indicating the prevalence of bacteria within the human body and specifically in tumor tissue, bacteria have generated significant interest as potential targets for cancer therapy. Notably, a multitude of empirical studies over the past decades have demonstrated that administered bacteria home and grow in tumors due to reduced immune surveillance of tumor necrotic cores. Given their specificity for tumors, bacteria present a unique opportunity to be engineered as intelligent delivery vehicles for cancer therapy with synthetic biology techniques. In this review, we discuss the history, current state, and future challenges associated with using bacteria as a cancer therapy.

Genetic Circuit-Assisted Smart Microbial Engineering

Rapid advances in DNA synthesis, genetic manipulation, and biosensors have greatly improved the ability to engineer microorganisms with complex functions. By accurately integrating quality biosensors and complex genetic circuits, recently emerged smart microorganisms have enabled exciting opportunities for dissecting complex signaling networks and making responses without artificial intervention. However, because of the lack of design principles, developing such smart microorganisms remains challenging. In this review, we propose the concept of smart microbial engineering (SME) and describe the general features of basic SME, including the circuit architecture, components, and design process. We also summarize the latest SME achievements, remaining challenges, and potential solutions in this growing field.

Recording development with single cell dynamic lineage tracing

Every animal grows from a single fertilized egg into an intricate network of cell types and organ systems. This process is captured in a lineage tree: a diagram of every cell's ancestry back to the founding zygote. Biologists have long sought to trace this cell lineage tree in individual organisms and have developed a variety of technologies to map the progeny of specific cells. However, there are billions to trillions of cells in complex organisms, and conventional approaches can only map a limited number of clonal populations per experiment. A new generation of tools that use molecular recording methods integrated with single cell profiling technologies may provide a solution. Here, we summarize recent breakthroughs in these technologies, outline experimental and computational challenges, and discuss biological questions that can be addressed using single cell dynamic lineage tracing.

Nucleic Acid Databases and Molecular-Scale Computing

DNA outperforms most conventional storage media in terms of information retention time, physical density, and volumetric coding capacity. Advances in synthesis and sequencing technologies have enabled implementations of large synthetic DNA databases with impressive storage capacity and reliable data recovery. Several robust DNA storage architectures featuring random access, error correction, and content rewritability have been constructed with the potential for scalability and cost reduction. We survey these recent achievements and discuss alternative routes for overcoming the hurdles of engineering practical DNA storage systems. We also review recent exciting work on in vivo DNA memory including intracellular recorders constructed by programmable genome editing tools. Besides information storage, DNA could serve as a versatile molecular computing substrate. We highlight several state-of-the-art DNA computing techniques such as strand displacement, localized hybridization chain reactions, and enzymatic reaction networks. We summarize how these simple primitives have facilitated rational designs and implementations of in vitro DNA reaction networks that emulate digital/analog circuits, artificial neural networks, or nonlinear dynamic systems. We envision these modular primitives could be strategically adapted for sophisticated database operations and massively parallel computations on DNA databases. We also highlight in vivo DNA computing modules such as CRISPR logic gates for building scalable genetic circuits in living cells. To conclude, we discuss various implications and challenges of DNA-based storage and computing, and we particularly encourage innovative work on bridging these two areas of research to further explore molecular parallelism and near-data processing. Such integrated molecular systems could lead to far-reaching applications in biocomputing, security, and medicine.

DNA event recorders send past information of cells to the time of observation

While current omics and single cell technologies have enabled measurements of high-resolution molecular snapshots of cells at a large scale, these technologies all require destruction of samples and prevent us from analyzing dynamic changes in molecular profiles, phenotypes, and behaviors of individual cells in a complex system. One possible direction to overcome this issue is the development of a cell-embedded 'event recorder' system, whereby molecular and phenotypic information of a cell(s) can be obtained at the time of observation with their past event information stored in 'heritable polymers' of the same cell. This concept has been demonstrated by many synthetic cellular circuits that monitor and transmit a certain set of environmental and intracellular signals into DNA, and have now been further accelerated by recent CRISPR-related technologies. Notably, the discovery of the RT-Cas1-Cas2 system, which acquires sequences of cellular transcripts into a specific host genomic region, has enabled recording of a broader range of molecular profile histories in the DNA tapes of cells, to understand the dynamics of complex biological processes that cannot be addressed by current technologies.

Development of CRISPR-Mediated Systems in the Study of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe type of X-linked recessive degenerative muscle disease caused by mutations in the dystrophin (DMD) gene on the X chromosome. The DMD gene is complex, consisting of 79 exons, and mutations cause changes in the DMD mRNA so that the reading frame is altered, and the muscle-specific isoform of the dystrophin protein is either absent or truncated with variable residual function. The emerging CRISPR-Cas9-mediated genome editing technique is being developed as a potential therapeutic approach to treat DMD because it can permanently replace the mutated dystrophin gene with the normal gene. Prenatal DNA testing can inform whether the female fetus is a carrier of DMD, and the male fetus has inherited a mutation from his mother (50% chance of both). This article summarizes the present status of current and future treatments for DMD.

Synthetic genetic circuits for programmable biological functionalities

Living organisms evolve complex genetic networks to interact with the environment. Due to the rapid development of synthetic biology, various modularized genetic parts and units have been identified from these networks. They have been employed to construct synthetic genetic circuits, including toggle switches, oscillators, feedback loops and Boolean logic gates. Building on these circuits, complex genetic machines with capabilities in programmable decision-making could be created. Consequently, these accomplishments have led to novel applications, such as dynamic and autonomous modulation of metabolic networks, directed evolution of biological units, remote and targeted diagnostics and therapies, as well as biological containment methods to prevent release of engineered microorganisms and genetic materials. Herein, we outline the principles in genetic circuit design that have initiated a new chapter in transforming concepts to realistic applications. The features of modularized building blocks and circuit architecture that facilitate realization of circuits for a variety of novel applications are discussed. Furthermore, recent advances and challenges in employing genetic circuits to impart microorganisms with distinct and programmable functionalities are highlighted. We envision that this review gives new insights into the design of synthetic genetic circuits and offers a guideline for the implementation of different circuits in various aspects of biotechnology and bioengineering.

Temporal Pattern Recognition through Analog Molecular Computation

Living cells communicate information about physiological conditions by producing signaling molecules in a specific timed manner. Different conditions can result in the same total amount of a signaling molecule, differing only in the pattern of the molecular concentration over time. Such temporally coded information can be completely invisible to even state-of-the-art molecular sensors with high chemical specificity that respond only to the total amount of the signaling molecule. Here, we demonstrate design principles for circuits with temporal specificity, that is, molecular circuits that respond to specific temporal patterns in a molecular concentration. We consider pulsatile patterns in a molecular concentration characterized by three fundamental temporal features: time period, duty fraction, and number of pulses. We develop circuits that respond to each one of these features while being insensitive to the others. We demonstrate our design principles using general chemical reaction networks and with explicit simulations of DNA strand displacement reactions. In this way, our work develops building blocks for temporal pattern recognition through molecular computation.

Progress in the application of CRISPR: From gene to base editing

The system of clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated endonucleases (Cas) has been utilized for genome editing with great accuracy and high efficiency in generating gene knockout, knockin, and point mutations in eukaryotic genomes. However, traditional CRISPR/Cas9 technology introduces double-stranded DNA breaks (DSBs) at a target locus as the first step to make gene corrections, which easily results in undesired mutations. Thus, it is necessary to develop new methods for correcting the unwanted mutations. In this review, we summarize the recent developments and a new approach to genome and base editing by using CRISPR/Cas9. This methodology renders a conversion of one target base into another, for example, C to T (or G to A), and A to G (or T to C) without producing DSBs, requiring a donor DNA template, or generating excessive insertions and deletions. Furthermore, CRISPR/Cas9-derived base editing also improves efficiency in repairing point mutations in the genome.

Riboswitch Signal Amplification by Controlling Plasmid Copy Number

Riboswitches are cis-acting RNA devices in mRNAs that control gene expression in response to chemical inputs. As RNA aptamers that recognize diverse classes of molecules can be isolated by in vitro selection, synthetic riboswitches hold promise for various applications in synthetic biology. One of the major drawbacks of riboswitches, however, is their limited dynamic range. A high level of gene expression in the OFF state (leakage) is also a common problem. To address these challenges, we designed and constructed a dual-riboswitch plasmid in which two genes are controlled by theophylline-activated riboswitches. One riboswitch controls the gene of interest, and another riboswitch controls RepL, a phage-derived replication protein that regulates the plasmid copy number. This single-plasmid system afforded an ON/OFF ratio as high as 3900. Furthermore, we used the system to control CRISPR interference (CRISPRi) targeting endogenous genes, and successfully observed expected phenotypic changes in Escherichia coli.

Cancer diagnosis and immunotherapy in the age of CRISPR

The explosion in genome editing technologies that has occurred in the past decade has revolutionized cancer research and promises to improve cancer diagnosis and therapy. Ongoing efforts include engineering of chimeric antigen receptor-T cells using clustered regularly interspaced short palindromic repeats (CRISPR) to generate a safer, more effective therapy with improved performance in immunologically "cold" tumors, as well as clever adaptations of CRISPR enzymes to allow fast, simple, and sensitive detection of specific nucleotide sequences. While still in their infancy, CRISPR-based cancer therapeutics and diagnostics are developing at an impressive speed and it is likely they will soon impact clinical practice. Here, we summarize their history and the most recent developments.

Applications of CRISPR-Cas in Bioengineering, Biotechnology, and Translational Research

CRISPR technology is rapidly evolving, and the scope of CRISPR applications is constantly expanding. CRISPR was originally employed for genome editing. Its application was then extended to epigenome editing, karyotype engineering, chromatin imaging, transcriptome, and metabolic pathway engineering. Now, CRISPR technology is being harnessed for genetic circuits engineering, cell signaling sensing, cellular events recording, lineage information reconstruction, gene drive, DNA genotyping, miRNA quantification, in vivo cloning, site-directed mutagenesis, genomic diversification, and proteomic analysis in situ. It has also been implemented in the translational research of human diseases such as cancer immunotherapy, antiviral therapy, bacteriophage therapy, cancer diagnosis, pathogen screening, microbiota remodeling, stem-cell reprogramming, immunogenomic engineering, vaccine development, and antibody production. This review aims to summarize the key concepts of these CRISPR applications in order to capture the current state of play in this fast-moving field. The key mechanisms, strategies, and design principles for each technological advance are also highlighted.

DNA-based memory devices for recording cellular events

Measuring biological data across time and space is critical for understanding complex biological processes and for various biosurveillance applications. However, such data are often inaccessible or difficult to directly obtain. Less invasive, more robust and higher-throughput biological recording tools are needed to profile cells and their environments. DNA-based cellular recording is an emerging and powerful framework for tracking intracellular and extracellular biological events over time across living cells and populations. Here, we review and assess DNA recorders that utilize CRISPR nucleases, integrases and base-editing strategies, as well as recombinase and polymerase-based methods. Quantitative characterization, modelling and evaluation of these DNA-recording modalities can guide their design and implementation for specific application areas.

Scaling up genetic circuit design for cellular computing: advances and prospects

Synthetic biology aims to engineer and redesign biological systems for useful real-world applications in biomanufacturing, biosensing and biotherapy following a typical design-build-test cycle. Inspired from computer science and electronics, synthetic gene circuits have been designed to exhibit control over the flow of information in biological systems. Two types are Boolean logic inspired TRUE or FALSE digital logic and graded analog computation. Key principles for gene circuit engineering include modularity, orthogonality, predictability and reliability. Initial circuits in the field were small and hampered by a lack of modular and orthogonal components, however in recent years the library of available parts has increased vastly. New tools for high throughput DNA assembly and characterization have been developed enabling rapid prototyping, systematic in situ characterization, as well as automated design and assembly of circuits. Recently implemented computing paradigms in circuit memory and distributed computing using cell consortia will also be discussed. Finally, we will examine existing challenges in building predictable large-scale circuits including modularity, context dependency and metabolic burden as well as tools and methods used to resolve them. These new trends and techniques have the potential to accelerate design of larger gene circuits and result in an increase in our basic understanding of circuit and host behaviour.

Developmental barcoding of whole mouse via homing CRISPR

In vivo barcoding using nuclease-induced mutations is a powerful approach for recording biological information, including developmental lineages; however, its application in mammalian systems has been limited. We present in vivo barcoding in the mouse with multiple homing guide RNAs that each generate hundreds of mutant alleles and combine to produce an exponential diversity of barcodes. Activation upon conception and continued mutagenesis through gestation resulted in developmentally barcoded mice wherein information is recorded in lineage-specific mutations. We used these recordings for reliable post hoc reconstruction of the earliest lineages and investigation of axis development in the brain. Our results provide an enabling and versatile platform for in vivo barcoding and lineage tracing in a mammalian model system.

Emerging applications for DNA writers and molecular recorders

Natural life is encoded by evolvable, DNA-based memory. Recent advances in dynamic genome-engineering technologies, which we collectively refer to as in vivo DNA writing, have opened new avenues for investigating and engineering biology. This Review surveys these technological advances, outlines their prospects and emerging applications, and discusses the features and current limitations of these technologies for building various genetic circuits for processing and recording information in living cells.

Rewritable multi-event analog recording in bacterial and mammalian cells

We present two CRISPR-mediated analog multi-event recording apparatus (CAMERA) systems that use base editors and Cas9 nucleases to record cellular events in bacteria and mammalian cells. The devices record signal amplitude or duration as changes in the ratio of mutually exclusive DNA sequences (CAMERA 1) or as single-base modifications (CAMERA 2). We achieved recording of multiple stimuli in bacteria or mammalian cells, including exposure to antibiotics, nutrients, viruses, light, and changes in Wnt signaling. When recording to multicopy plasmids, reliable readout requires as few as 10 to 100 cells. The order of stimuli can be recorded through an overlapping guide RNA design, and memories can be erased and re-recorded over multiple cycles. CAMERA systems serve as "cell data recorders" that write a history of endogenous or exogenous signaling events into permanent DNA sequence modifications in living cells.

Harnessing "A Billion Years of Experimentation": The Ongoing Exploration and Exploitation of CRISPR-Cas Immune Systems

The famed physicist-turned-biologist, Max Delbrück, once remarked that, for physicists, "the field of bacterial viruses is a fine playground for serious children who ask ambitious questions." Early discoveries in that playground helped establish molecular genetics, and half a century later, biologists delving into the same field have ushered in the era of precision genome engineering. The focus has of course shifted-from bacterial viruses and their mechanisms of infection to the bacterial hosts and their mechanisms of immunity-but it is the very same evolutionary arms race that continues to awe and inspire researchers worldwide. In this review, we explore the remarkable diversity of CRISPR-Cas adaptive immune systems, describe the molecular components that mediate nucleic acid targeting, and outline the use of these RNA-guided machines for biotechnology applications. CRISPR-Cas research has yielded far more than just Cas9-based genome-editing tools, and the wide-reaching, innovative impacts of this fascinating biological playground are sure to be felt for years to come.

Learning from Noise: How Observing Stochasticity May Aid Microbiology

For many decades, the wedding of quantitative data with mathematical modeling has been fruitful, leading to important biological insights. Here, we review some of the ongoing efforts to gain insights into problems in microbiology - and, in particular, cell-cycle progression and its regulation - through observation and quantitative analysis of the natural fluctuations in the system. We first illustrate this idea by reviewing a classic example in microbiology - the Luria-Delbrück experiment - and discussing how, in that case, useful information was obtained by looking beyond the mean outcome of the experiment, but instead paying attention to the variability between replicates of the experiment. We then switch gears to the contemporary problem of cell cycle progression and discuss in more detail how insights into cell size regulation and, when relevant, coupling between the cell cycle and the circadian clock, can be gained by studying the natural fluctuations in the system and their statistical properties. We end with a more general discussion of how (in this context) the correct level of phenomenological model should be chosen, as well as some of the pitfalls associated with this type of analysis. Throughout this review the emphasis is not on providing details of the experimental setups or technical details of the models used, but rather, in fleshing out the conceptual structure of this particular approach to the problem. For this reason, we choose to illustrate the framework on a rather broad range of problems, and on organisms from all domains of life, to emphasize the commonality of the ideas and analysis used (as well as their differences).

Serine Integrases: Advancing Synthetic Biology

Serine integrases catalyze precise rearrangement of DNA through site-specific recombination of small sequences of DNA called attachment (att) sites. Unlike other site-specific recombinases, the recombination reaction driven by serine integrases is highly directional and can only be reversed in the presence of an accessory protein called a recombination directionality factor (RDF). The ability to control reaction directionality has led to the development of serine integrases as tools for controlled rearrangement and modification of DNA in synthetic biology, gene therapy, and biotechnology. This review discusses recent advances in serine integrase technologies focusing on their applications in genome engineering, DNA assembly, and logic and data storage devices.

Papers of note in Science 358 (6369)

This week’s articles explain why broad-leaved plants are susceptible to a microbial toxin that does not affect monocots; identify the enzymes that detyrosinate tubulin; and present synthetic biological systems for recording environmental signals and for translating a biochemical reaction into dynamic cellular behavior.