The CRISPR gene-editing technology is taking the scientific world by storm, but researchers are still uncovering the platform’s potential and pitfalls.
The “Democratization” of Gene Editing
The idea of modifying human genomes using homology directed repair (HDR) has been around for decades. HDR is a widely-used repair mechanism to fix double-strand breaks in the cell’s DNA. By supplying an exogenous, homologous piece of DNA to the cell and increasing the probability of HDR occurring, changes in the DNA sequence can be introduced to the targeted area.
Some of the first gene editing platforms taking advantage of HDR used engineered endonucleases such as zinc finger nucleases (ZFNs) and transcription activator like effector nucleases (TALENs). Both ZFNs and TALENs required a custom protein to target a specific DNA sequence, making them pricey and very difficult to engineer. The more recently developed CRISPR/Cas9 platform works differently: instead of using a protein, the Cas enzyme uses a small guide RNA to locate the targeted DNA, which is then cut. CRISPR is more low tech and “user-friendly” than other platforms, as users are no longer required to do the onerous chore of protein engineering. Now, the ability to modify genomes can be done without extensive training or expensive equipment. CRISPR can be used for knocking out genes, creating reporter or selection genes, or modifying disease-specific mutations—by practically anyone with basic knowledge of molecular biology.
The CRISPR Revolution
Because of its relative simplicity and accessibility, it is no wonder that life scientists from all over the world are eager to incorporate CRISPR into their research. One of the most remarkable things about CRISPR technology is how quickly its popularity has allowed the platform to evolve. The discovery revealing that CRISPR can be used for RNA-programmable genome editing was first published in 2012 (1). Since then, about 2,000 manuscripts have been published including this technology and millions of dollars have been invested into CRISPR-related research and start-up companies. Scientists have developed a myriad of applications for CRISPR, which can be used in practically any organism under the sun. The popularity and progress of gene editing promises revolutionary advancements in virtually every scientific field: from eradicating disease-carrying mosquitoes to creating hypoallergenic eggs and even curing genetic diseases in humans.
There is no doubt that CRISPR has enormous potential – the widespread interest and rapid progress are evidence of that. The main reason CRISPR has been so widely adapted is because development and customization is way less labor-intensive and time-consuming than previous methods. But material preparation is a miniscule part of the CRISPR platform. Contrary to popular belief, just because CRISPR is easier to use than other methods of gene editing, it is not “easy.”
Hitting the Bull’s-Eye
To understand the limitations of this gene editing technology, it’s important to understand more about how CRISPR works. The CRISPR/Cas9 system functions by inducing double-strand breaks at a specific target and allowing the host DNA repair system to fix the site of interest. Cells have two major repair pathways to fix these types of breaks, Non-Homologous End Joining (NHEJ) and homology directed repair (HDR). NHEJ is faster and more active than HDR and does not require a repair template, so NHEJ is the principle means by which CRISPR/Cas9-induced breaks are repaired. If the editing goal is to induce insertions or deletions through NHEJ to cut out a part of a gene, then using the CRISPR platform is less complicated. But directing CRISPR to correct a gene with exogenous DNA does not work very well, as the rate of HDR occurring at the site of interest is often very low, sometimes less than 1%. For precise genome-editing through CRISPR, it is essential to have HDR while minimizing damaging NHEJ events.
Increasing the rate of HDR is not trivial. Recent research has shown that the conditions for the two repair pathways vary based on the cell type, location of the gene, and the nuclease used (2). Besides, optimizing CRISPR to better control HDR versus NHEJ events is only half the battle. One of the greatest challenges in using CRISPR is to be able to quickly and accurately detect different genome-editing events. Many scientists measure changes by sequencing. A more sophisticated method is to use droplet digital PCR (ddPCR) (3), but many labs have limited accessibility to ddPCR systems. After screening multiple clones and detecting the desired genetic edit comes what is usually the most laborious and time-consuming step: pure clonal isolation. Because each individual cell is affected by CRISPR independently, isogenic cell lines must be established.
CRISPR has become the gold standard for many scientists now that the potential to perform gene editing has become so universal. Top journals expect isogenic lines for characterizing genes and mutations, and many researchers feel pressured to include such experiments to stay competitive in grant proposals. Moreover, a federal biosafety committee has recently approved the first study in patients using this genome-editing technology. To keep up with the pace of this rapidly moving field, significant improvements in CRISPR technology need to be made.
First, a better grasp of the basic principles behind CRISPR is likely to lead to improvements in targeting and efficiency. For example, understanding how the cell type, locus, and genomic landscape affect the targeting and cutting of Cas9 could lead to higher efficiencies of HDR. Similarly, engineering Cas9 and the guide RNA to maximize on-target activity could also accelerate the technology’s success. Screening the ability of the guide RNA to target the intended sequence can be performed in vitro by assessing the Cas9-mediated cuts on a PCR-amplified fragment of DNA; this method often gives a good indication of what to expect in cells. Another hindrance to CRISPR technology is that currently, all methods to detect genome changes require destruction of the cells of interest. Development of an assay to detect chromosomal changes in live cells – reminiscent of live-cell RNA detection – would immensely improve the processes of clonal screening and isolation.
Besides the technical challenges that come with maximizing on-target edits is that Cas9 frequently has off-target effects, producing insertions or deletions at unintended sites. Online algorithms can predict where some of these cuts are likely to occur, but currently, there is no efficient method to identify all possible off-target sites (4). To complicate things further, no two human genomes are identical. Due to genetic variation, predicting off-target effects based on reference genomes remains a challenge. Because so much of the hype around CRISPR surrounds the potential to treat human diseases, it is imperative to make sure that CRISPR does not introduce detrimental changes elsewhere in the genome before therapeutic use in humans. Standard screening methods and biological assays need to be established to robustly assess potential damage done to other sites in the genome and to measure its impact on cell function and mutagenesis.
Despite its shortcomings, the hub of activity surrounding CRISPR has been astounding. And with the force of thousands of scientists working on this technology, the possibilities for the future of CRISPR are boundless.
- Jinek, M., et al., A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science, 2012. 337(6096): p. 816-21.
- Miyaoka, Y., et al., Systematic quantification of HDR and NHEJ reveals effects of locus, nuclease, and cell type on genome-editing. Sci Rep, 2016. 6: p. 23549.
- Hindson, B.J., et al., High-throughput droplet digital PCR system for absolute quantitation of DNA copy number. Anal Chem, 2011. 83(22): p. 8604-10.
- Stella, S. and G. Montoya, The genome editing revolution: A CRISPR-Cas TALE off-target story. Bioessays, 2016. 38 Suppl 1: p. S4-s13.
Last year Allele dedicated a new building space for cleanroom operations to provide a cell banking service for personalized medicine. This facility will be the center of current Good Manufacturing Practice (cGMP) production of human induced pluripotent stem cells (iPSCs) using Allele’s proprietary synthetic mRNA platform. Over the past three months, progress to get the facility up and running has been substantial. Our facility includes four main modules: the reception area and doctors’ offices, a Fibroblast Isolation and Maintenance room, a Reprogramming and iPSC Maintenance room, and a Quality Control room. Air handling, which is a major component of the environmental control system, has been installed and validated. Equipment such as biosafety cabinets, incubators, and refrigerators have been installed and qualified, as well as equipment for performing essential quality control steps. To standardize personnel-related steps of cGMP processing, we have prepared rigorous SOPs and have extensively trained individual manufacturing operators. Overall, we are enthusiastic about the facility’s progress and are committed to delivering the best possible service as the industry leader in iPSC banking.
Three years since our flagship fluorescent protein was published in Nature Methods, mNeonGreen is still shining bright. mNeonGreen is a green-yellow fluorescent protein that has been shown to be highly useful in optical assays from general imaging to FRET to super resolution microscopy applications and more.
What makes this fluorescent protein such a hit among researchers?
Since the cloning of green fluorescent protein (GFP) over 20 years ago, fluorescent proteins have become a standard research tool, enabling labeling and imaging of individual proteins within a cell in real time. To make these probes brighter, faster folding, and to cover a wider range of the visible spectrum, new fluorescent proteins are continually being developed. mNeonGreen was engineered by scientists at Allele Biotech from a protein isolated from Branchiostoma lanceolatum, a marine invertebrate, and is the brightest monomeric green-yellow fluorescent protein ever characterized.
Perhaps the most common use of fluorescent proteins today is genetically fusing them to a protein of interest to image protein localization. Unfortunately, many researchers are unaware that many fluorescent proteins – such as GFP – are prone to forming noncovalent dimers, which can lead to significant artifacts. True monomeric fluorescent proteins such as mNeonGreen are less likely to affect the localization, dynamics, or normal behavior when fused to proteins of interest.
For quantitative imaging and live-cell imaging applications, arguably the most critical parameters to consider when choosing fluorescent proteins are brightness and photostability. mNeonGreen has high fluorescence brightness, so less light can be delivered to the cells to collect ample signal intensity, resulting in less phototoxicity. mNeonGreen also has superior photostability, meaning it can undergo many excitation-emission cycles before photobleaching occurs. Because of these properties, mNeonGreen has been shown to be very effective as a fluorophore in fluorescence resonance energy transfer (FRET) – both as an acceptor and a donor.
The community of biologists taking advantage of super-resolution fluorescence microscopy (Nobel Prize in Chemistry 2014) is rapidly growing. But the ability to resolve cellular structural features depends on the chosen fluorophore’s brightness, labeling density, and the stability of the dark state. mNeonGreen is not only extraordinarily bright, but also can be driven into a temporary dark state by light irradiation, making it a useful tag for single-molecule super-resolution imaging of proteins.
Researchers around the world continue to develop novel applications for mNeonGreen. Recently, mNeonGreen was used to create a genetically encoded voltage sensor that can be used to image subcellular changes in electrical activity. Researchers fused mNeonGreen to a light-sensitive ion channel in neurons, linking mNeonGreen fluorescence with the membrane voltage to create an optical readout for neuronal activity with unprecedented speed and accuracy.
Choosing an appropriate fluorescent protein for an assay is often a source of confusion for researchers. In many cases, the selection of a fluorescent protein is motivated by convenience (e.g., availability of the construct) rather than its performance for a given assay. If mNeonGreen seems like the right fluorescent protein for your assay, we at Allele Biotech have made it easy and painless for researchers to get their hands on it. Laboratories can license mNeonGreen for full use at a low cost.
Questions about licensing or whether mNeonGreen is really right for your lab? Contact firstname.lastname@example.org.
“A bright monomeric green fluorescent protein derived from Branchiostoma lanceolatum.”
Shaner, N. C., Lambert, G. G., Chammas, A., Ni, Y., Cranfill, P. J., Baird, M. A., Sell, B.R., Allen, J.R., Day, R.N., Davidson, M.W., Wang, J.
2013 Nat Methods, 10(5), 407-409. doi:10.1038/nmeth.2413
“High-speed recording of neural spikes in awake mice and flies with a fluorescent voltage sensor.”
Gong, Y., Huang, C., Li, J. Z., Grewe, B. F., Zhang, Y., Eismann, S., & Schnitzer, M. J.
2015 Science, 350(6266), 1361-1366. doi:10.1126/science.aab0810
As the promise for cell-based therapy grows, the interest in making clinically relevant cell lines has skyrocketed for industrial and academic researchers alike. For translation into human therapies, cell-based products must be made following current Good Manufacturing Practice (cGMP). Many groups have already claimed to generate cell lines that are “cGMP-compliant,” “cGMP-ready,” or “certifiable under cGMP.” But what does it take to be truly cGMP-compliant, and what practices can you introduce in your lab to comply with cGMP standards?
A common misconception in the United States is that a facility is granted a ‘cGMP license’ from the government to manufacture cGMP-grade products. Rather, the Food and Drug Administration (FDA) evaluates the manufacturing process for each product to determine if it is compliant with cGMP standards. The primary concern when it comes to deriving cell-based products for therapies is making sure that the product is derived in a safe and reproducible manner. To ensure maximum quality assurance, researchers should
• choose reliable, xenogeneic-free raw materials,
• establish and monitor a clean environment,
• qualify all equipment and software,
• remove variation in laboratory procedures by creating detailed Standard Operating Procedures (SOPs) and by providing rigid process validation at each step.
Nevertheless, even establishing robust quality assurance does not imply that the process is scalable for commercial production. In the world of biologics, “the product is the process.” A requisite step to ensure a smooth transition to cGMP practice is to ensure that the process of manufacturing is not altered due to changes in production scale. For example, depending on the therapy, millions or billions of cells may be required for a single patient. Therefore, it is in the best interest of the researchers to develop a scalable method at the beginning to avoid revamping the entire process (e.g., changing from adherent cells to suspension). Along these lines, the quality control (QC) requirements of cell-based products should be carefully considered and not have to include difficult-to-assay tests. For example, some cell lines have been qualified as cGMP-compliant upon conversion from research-grade conditions to cGMP quality standards. Rigorous tests were performed on the converted lines to ensure that the cells were free of contamination. Even though strict measures were carried out to ensure cGMP compliancy, deriving cell lines in this manner makes scalability and reproducibility a challenge. Ideally, the entire process of deriving cell products for clinical use should be performed under cGMP conditions: from the acquisition of human tissue to the manufacturing, testing, and storage of derivative cell products.
Another important consideration when instituting cGMP-compliance is documentation. Each process must be described with rigorous SOPs, the training of individual manufacturing operators must be well-documented, and the entire established process must be validated and well noted. Failure to document—in the eyes of the FDA—is often equated with failure to perform the underlying activity. It is equally important to remain ‘current.’ The FDA expects manufacturing processes to stay up-to-date with current regulations, even as policies change.
For an academic lab, closely aligning with cGMP standards can ensure that the resulting cell lines are comparable to other truly cGMP-produced products used during clinical trials. It is in the best interest of academic researchers to establish rigorous SOPs and use qualified reagents and equipment, even if it is not possible to carry out all steps in a certified cleanroom. Whenever possible, it is advisable to acquire truly cGMP cell lines from appropriate sources for preclinical projects; if prohibited by costs or other reasons, it is recommended to use a protocol that is as close to cGMP as possible.
Single-domain nano antibodies have a broad range of applications in biochemistry due to their small size, high affinity, and high specificity. Now, a team of researchers from the University of Basel and the University of Zurich has demonstrated that nano antibodies can be used for research in complex living organisms such as Drosophila, uncovering another new and exciting application for nano antibodies.
The team used nano antibodies to develop an assay for studying morphogens, molecules that regulate the pattern of tissue growth and the positions of various cell types within tissue. Morphogens form long-range concentration gradients from a localized source, ultimately determining the fate and arrangement of cells that respond to that gradient. Drosophila is a classic model system for understanding how morphogens regulate organ development. One morphogen called Dpp controls uniform proliferation and growth of the wing imaginal disc. Yet because Dpp is an extracellular, diffusible protein, it is difficult to immobilize in situ. Therefore, despite over 20 years of studying the role of Dpp as a morphogen, the lack of a dynamic system for controlling Dpp gradients has prevented researchers from understanding precisely how Dpp governs development of the wing disc.
By developing a novel synthetic system using nano antibodies, the researchers were able to modulate the concentration gradient of Dpp at the protein level. Their system—coined “morphotrap”—uses a membrane-bound GFP nano antibody to “trap” GFP-tagged Dpp at different locations along the wing imaginal disc. By tethering Dpp in a controlled spatial manner, researchers were able to determine how Dpp gradients affect wing disc development. They discovered that the gradient of Dpp is required for the patterning of the wing disc but not for lateral growth, disproving one of the field’s popular theories that address the role of Dpp. In addition to resolving the controversy with respect to the role of Dpp as a morphogen, this study pioneers a new method for using nano antibodies in situ.
“Dpp spreading is required for medial but not for lateral wing disc growth.”
Harmansa S., Hamaratoglu F., Affolter M., Caussinus E.
Nature. 2015 Nov 19;527(7578):317-22. doi: 10.1038/nature15712. Epub 2015 Nov 9.
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