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1. The Biological Breakthrough of CRISPR-Based Gene Editing

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Samuel H. Sternberg

Columbia University

Samuel H. Sternberg, PhD, runs a research laboratory at Columbia University, where he is an Assistant Professor in the Department of Biochemistry and Molecular Biophysics. He received his BA in Biochemistry from Columbia University in 2007, graduating summa cum laude, and his PhD in Chemistry from the University of California, Berkeley, in 2014. He earned graduate student fellowships from the National Science Foundation and the Department of Defense, and received the Scaringe Award and the Harold Weintraub Graduate Student Award. Sam’s research focuses on the mechanism of DNA targeting by RNA-guided bacterial immune systems (CRISPR-Cas) and on the development of these systems for genome engineering. In addition to publishing his work in leading scientific journals, he recently coauthored a popular science trade book together with Jennifer Doudna, entitled A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution, about the discovery, development, and applications of CRISPR gene-editing technology.

Recommended book: A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution, J. A. Doudna and S. H. Sternberg, Houghton Mifflin Harcourt, 2017.

Few discoveries transform a discipline overnight, but scientists today can manipulate cells in ways hardly imaginable before, thanks to a peculiar technology known as CRISPR (clustered regularly interspaced short palindromic repeats). From elegant studies that deciphered how CRISPRs function in bacteria, researchers quickly uncovered the biological potential of Cas9, an RNA-guided DNA cleaving enzyme, for gene editing. Today, this core capability is being harnessed for a wide variety of ambitious applications, including agricultural improvement, the elimination of infectious diseases, and human therapeutics. CRISPR technology may indeed herald cures to certain genetic diseases and cancer, but so too could it be used to engineer heritable genetic changes in human embryos. What will we choose to do with this awesome power?

The Biological Breakthrough of CRISPR-Based Gene Editing

Introduction

Ever since the discovery of DNA as the genetic information carrier of the cell, scientists have been on an inexorable quest to decrypt the letter-by-letter sequence of the human genome, and to develop tools to manipulate and modify genetic code. The first viral genome was fully sequenced in 1976, and, in the years since, an increasing number of ever-more challenging genomes have been decoded, culminating with the first draft of the human genome published in 2001. Today, interested individuals can purchase genetic tests and learn about their ancestry and disease susceptibility, simply by sending saliva samples in the mail, and a growing number of companies are leveraging huge datasets from millions of individuals to rapidly advance the field of human genomics.

Concurrently, tools to build synthetic DNA molecules in the lab have expanded considerably, beginning with the recombinant DNA revolution in the 1970s. In 2010, researchers at the Venter Institute succeeded in creating the first synthetic cell by manufacturing an entire bacterial genome from scratch, and others have since moved on to build entire chromosomes—the cellular structures that contain DNA—in yeast.

Yet, up until quite recently, tools to modify DNA, and to do so directly in living cells, lagged far behind, particularly in more complex organisms like plants, animals, and humans. In light of our growing awareness of the large number of diseases that are associated with, or caused by, genetic mutations—sickle cell, Huntington’s, and Alzheimer’s, to name just a few—the ideal DNA manipulation tool would enable direct repair of mutations at their source. But the sheer size and complexity of our genome, made up of 3.2 billion letters of DNA contributed by each of the twenty-three pairs of chromosomes we inherit from mom and dad, consigned the dream of precision genome editing to a distant future. Until, that is, CRISPR technology came along.

Today, scientists can use CRISPR to engineer the genome in ways barely imaginable before: repairing genetic mutations, removing pathogenic DNA sequences, inserting therapeutic genes, turning genes on or off, and more. CRISPR democratized genome engineering, unlike the technologies that preceded it, because it is easy to deploy and inexpensive to access. And CRISPR works in an impressive number of different cell types and organisms—everything from maize to mice to monkeys—giving scientists a flexible, diverse, and broadly applicable engineering toolkit to address a wide variety of biological challenges.

Scientists can use CRISPR to engineer the genome in ways barely imaginable before: repairing genetic mutations, removing pathogenic DNA sequences, inserting therapeutic genes, turning genes on or off, and more

What is CRISPR, and how does it work? What is meant by gene editing? Are CRISPR-based treatments to cure genetic disease around the corner? How can CRISPR technology be used to improve agriculture through plant and animal gene editing? Could CRISPR help eradicate pathogenic diseases like malaria? And, perhaps most profoundly, might CRISPR someday be used to rewrite DNA in human embryos, thereby editing genetic code in a way that would be felt for generations to come?

The Origins of CRISPR

As revolutionary as CRISPR has been for biomedical science, its discovery stemmed from basic scientific curiosity about a biological topic about as far removed from medicine as it gets. To understand where CRISPR comes from, we need to delve into one of the longest standing genetic conflicts on Earth: the relentless arms race between bacteria and bacteria-specific viruses (Rohwer et al., 2014).

Everyone knows about bacteria, those pesky microorganisms that can make us sick—think Streptococci, the cause of strep throat and pneumonia, or Salmonella infections that cause food poisoning—but which are also indispensable for normal human function. (We depend on a vast army of bacteria that collectively make up our microbiome and help break down food, produce vitamins, and perform numerous other essential functions.) Few outside the research community, though, may know about the ubiquity of bacterial viruses, also known as bacteriophages (“eaters of bacteria”). In fact, bacteriophages are by far the most prevalent form of life on our planet: at an estimated abundance of ten million trillion trillion, they outnumber even bacteria ten to one. There are approximately one trillion bacterial viruses for every grain of sand in the world, and ten million viruses in every drop of seawater (Keen, 2015)!

Bacterial viruses evolved to infect bacteria, and they do so remarkably well. They exhibit three-dimensional structures that are exquisitely well suited to latch onto the outer surface of bacterial cells, and after attaching themselves in this manner, they inject their genetic material inside the bacterial host using pressures similar to that of an uncorked champagne bottle. After the viral genome makes its way inside the bacteria, it hijacks the host machinery to replicate its genetic code and build more viruses, ultimately destroying the cell in the process. Roughly twenty to forty percent of the ocean’s bacteria are eliminated every day from such viral infections, vastly reshaping the marine ecosystem by causing release of carbon and other nutrients back into the environment.

To understand where CRISPR comes from, we need to delve into one of the longest standing genetic conflicts on Earth: the relentless arms race between bacteria and bacteria-specific viruses

Yet bacteria are not passive bystanders in the face of such an onslaught—quite the contrary. Bacteria possess numerous immune systems to combat viruses at multiple stages during the viral life cycle, which microbiologists have studied for many decades. By the turn of the twenty-first century, the existing paradigm held that, while diverse, these immune systems constituted only a simple innate response to infection. Unlike multicellular vertebrate organisms, which possess innate immune systems together with elaborate adaptive immune systems that can create and store immunological memory, bacteria had no ability to adapt to new threats.

Enter CRISPR, short for Clustered Regularly Interspaced Short Palindromic Repeats. First detected in 1987 in the bacterium Escherichia coli (Ishino et al., 1987), CRISPRs—to put it simply—are bizarre, repeating sections of bacterial DNA that can extend thousands of letters in length. While CRISPRs initially seemed like a rare oddity, a fluke of nature, researchers had detected CRISPRs in dozens of other bacterial species by the early 2000s (Mojica et al., 2000).

Color-scanning electron microscope (SEM) image of bacteria on the surface of a human tongue. Enlarged 10,000 times to a width of 10 cm

These repeating structures were initially described using a number of different and confusing acronyms, and so, in 2002, Dutch researchers simplified their classification with the informative (and catchy) acronym that we still use today (Jansen et al., 2002).

Despite a growing appreciation that CRISPRs were abundant in nature, being found in the genomes of a third of all bacteria and almost all archaea (another domain of single-celled microorganisms), their biological function remained a complete mystery until 2005, when the first clues surfaced linking CRISPR to antiviral immunity (Mojica et al., 2005). Using bioinformatics analyses, researchers were shocked to find viral DNA sequences buried within those repeating sections of DNA, as if the bacteria had somehow stolen the viral genetic code as a form of molecular memory. Might this information allow bacteria to recognize and destroy viral DNA during an infection?

Evidence supporting this hypothesis came from elegant experiments conducted at a yogurt company (Barrangou et al., 2007). Scientists there were hoping to generate virus-resistant strains of the bacterium Streptococcus thermophilus, the major workhouse ingredient used to ferment milk into yogurt and other dairy products, and they noticed that, like E. coli, their S. thermophilus strains also contained CRISPRs. By intentionally infecting their strains with a panel of different viruses and then analyzing the DNA of those bacteria that gained immunity, the researchers proved that CRISPRs indeed conferred adaptive immunity. Almost overnight, the long-standing presumption that bacteria and archaea possessed only comparatively simple defenses against viral pathogens was overturned. Instead, these simple microorganisms employed both innate and adaptive immune systems no less remarkable and versatile than the innate and adaptive systems found in multicellular organisms.

Despite a growing appreciation that CRISPRs were abundant in nature, being found in the genomes of a third of all bacteria and almost all archaea (another domain of single-celled microorganisms), their biological function remained a complete mystery until 2005, when the first clues surfaced linking CRISPR to antiviral immunity

After this breakthrough, it was up to geneticists and biochemists to determine how CRISPR immune systems work. Namely, what enzymes were involved, and how were they able to accurately recognize unique features of viral DNA during an infection? From the work of countless researchers all around the world, a new, unified understanding began to emerge: bacteria and archaea used molecules of ribonucleic acid, or RNA—DNA’s molecular cousin—to identify matching sequences of DNA in the viral genome, along with one or more proteins encoded by CRISPR-associated genes to slice apart the DNA (Klompe & Sternberg, 2018). CRISPR was nothing more than a precision-guided pair of molecular scissors, with the incredible ability to home in on specific sequences of DNA and neutralize them by severing both strands of the double helix. And the star actor in this pathway was a protein enzyme called CRISPR-Cas9 (Gasiunas et al., 2012; Jinek et al., 2012).

The Intersection of CRISPR-CAS9 and Gene-Editing Technology

Resolving the molecular function of CRISPR-Cas9 not only helped solve a key question in bacterial antiviral immune systems. It also immediately revealed immense potential to disrupt a different and seemingly unrelated area of biotechnology: gene editing (Urnov, 2018).

Gene editing refers to a technique in which DNA sequences are modified, or “edited,” directly in the genome of living cells. While effective tools for gene editing in bacteria have been available for decades, the ability to edit DNA in eukaryotic cells, which house the genome in a separate structure called the nucleus, lagged far behind. But in the 1990s, a new strategy for high-efficiency gene editing emerged: if a specific DNA break could be induced at the gene of interest, then the ability to edit that gene was vastly enhanced (Rouet et al., 1994). Somewhat paradoxically, localized DNA damage could serve as a stimulus for DNA repair.

Why might this be the case? Our cells suffer DNA damage constantly, whether from carcinogens or exposure to ionizing radiation, and they have therefore evolved mechanisms for repairing DNA lesions. Detection of a DNA break leads to recruitment of endogenous enzymes to perform this repair, and, over the years, researchers realized that this natural process could be hijacked to install user-defined edits during the repair process. The bottleneck for realizing the full potential of this approach, then, was developing tools to introduce DNA breaks at specific sites in the genome.

The ideal tool would be a “programmable nuclease”—an enzyme that cuts nucleic acids like DNA (hence, “nuclease”), which scientists could easily and rapidly program to recognize and introduce breaks in specific DNA sequences inside the cell (Chandrasegaran & Carroll, 2016). The first such tools were developed in the 1990s and early 2000s, but they were unwieldy, unreliable, and expensive. A researcher might devote months to building a programmable nuclease himself/herself, or spend tens of thousands of dollars outsourcing the work to a company, only to find out that the tool barely worked. In short, gene editing, though validated as a technology, could not realize its full potential because programmable nucleases were simply too hard to engineer.

The discovery of CRISPR-Cas9 offered the perfect solution. Instead of trying to reinvent the wheel, why not harness the programmable nucleases that nature had already sculpted over billions of years of evolution? Whereas bacteria were employing CRISPR-Cas9 to introduce DNA breaks in viral genomes, to prevent infection, perhaps scientists could employ CRISPR-Cas9 to introduce DNA breaks in eukaryotic genomes, to edit genes. The very same property that made CRISPR-Cas9 so effective in adaptive immunity—its ability to precisely home in on DNA targets using an RNA “guide”—might transform researchers’ ability to program nucleases to break specific DNA targets and mark them for repair.

The CRISPR Craze Begins

In June 2012, Doudna, Charpentier, and colleagues published the first article describing CRISPR-Cas9’s essential components and detailing its utility for gene editing (Jinek et al., 2012). Six months later, the first reports surfaced, demonstrating the remarkable effectiveness of CRISPR-Cas9 for gene editing in both mouse and human cells (Cong et al., 2013; Mali et al., 2013). Within months of that, the first gene-edited mice were created with CRISPR, followed in quick succession by proof-of-concept experiments in rice, rats, wheat, and monkeys, and an increasingly dizzying array of other plant and animal model organisms. The “CRISPR craze” was underway (Pennisi, 2013).

Along with a surge in the species whose genomes could now be seamlessly tweaked with CRISPR, 2013 also witnessed an explosion in the kinds of DNA changes that could be accomplished with CRISPR technology. Beyond fixing small typos in the genome, such as the kinds of mutations that cause genetic disease, CRISPR could be leveraged to inactivate or delete entire genes, invert or insert genes, and make changes to multiple genes simultaneously. An entirely distinct category of applications involved the use of a non-cutting version of CRISPR-Cas9, in which the goal was to ferry other payloads to specific genes in order to turn genes on or off, up or down. By altering gene expression without changing the actual sequence of DNA, researchers could begin to control the very same molecular cues that instructed cells to turn into the many different tissues in the body, all using the same underlying genetic code.

2013 witnessed an explosion in the kinds of DNA changes that could be accomplished with CRISPR technology. Beyond fixing small typos in the genome, such as the kinds of mutations that cause genetic disease, CRISPR could be leveraged to inactivate or delete entire genes, invert or insert genes, and make changes to multiple genes simultaneously

Technology development quickly expanded the CRISPR toolkit, which attracted more and more researchers to the budding gene-editing field. Even more important for CRISPR’s widespread adoption than its sheer usefulness, though, was the lowered barrier to entry for novices. First, the ease of engineering CRISPR to target new sites in the genome meant that scientists with a basic understanding of molecular biology could now access what was once an advanced technology requiring years of expertise. (Indeed, some middle- and high-school students now perform CRISPR gene-editing experiments in the classroom [Yu, 2017].) Second, the necessary reagents to perform gene editing could be affordably purchased from nonprofit organizations like Addgene, which distributes CRISPR tools to academic researchers for just $60 (Kamens, 2015). The result has been a swift, worldwide spread of the technology.

Today, CRISPR encompasses a core set of techniques that biomedical scientists must be well versed in, regardless of their particular research focus, model organism, or preexisting skill set. The technology has quickly become indispensable for performing cutting-edge research, and it is safe to say that biology will never be the same again.

Nor will society. Indeed, armed with the power to easily and precisely rewrite genetic code, scientists and nonscientists alike threaten to upend the natural order, reshaping the very process of evolution by substituting random mutation—the aimless, meandering process acted on by natural selection over the eons—with user-defined mutation, introduced at will via CRISPR technology. The result: a newfound mastery over the direction of life itself.

Imminent Impacts on the Planet’s Plants and Animals

Imagine a future world in which you could clone your deceased dog, while also installing DNA mutations that confer hyper-musculature; or in which you could grow super-strains of tomatoes that maintain ripeness long after being picked, mushrooms that do not brown during prolonged storage, and grape vines that are impervious to fungal pests. Out in the countryside, farmers’ pastures accommodate new breeds of dairy cattle, which still retain the same prized genetics resulting from hundreds of years of selective breeding, but no longer develop horns, thanks to gene editing. The nearby pigs possess special mutations that confer viral resistance and also cause them to develop leaner muscles with reduced fat content. In the medical facility one town over, other pigs harbor “humanized” genomes that have been selectively engineered so that the pigs might one day serve as organ donors for humans. Believe it or not, every one of these seemingly fictitious inventions has already been accomplished with the help of CRISPR technology, and the list could go on and on (Doudna & Sternberg, 2017).

Plant breeders are excited by the possibility of engineering new traits into major cash crops with a method that is both safer and more effective than the random mutagenesis methods of the mid- to late-twentieth century, and less invasive than the techniques commonly used to create genetically modified organisms, or GMOs. GMOs are the product of gene splicing, whereby foreign DNA sequences are forcibly integrated into the genetic material of the organism being modified. While no credible evidence exists suggesting that GMOs are any less safe than unmodified plants, they remain the subject of intense public scrutiny and vociferous criticism.

Time will tell whether activist groups or overly restrictive regulation will stunt innovation in this sector. One thing seems clear: different cultures and attitudes in distinct parts of the world will play a major role in shaping the future direction of CRISPR applications

So how will the public then react to gene-edited organisms, which could hit the supermarket in just years (Bunge & Marcus, 2018)? Like GMOs, these products have been engineered in the lab, with the goal of achieving improved yield, enhanced resistance to pests, better taste, or healthier nutritional profile. Unlike GMOs, though, these products do not carry a shred of foreign DNA in the genome, nor any scar from the scientific intervention. In fact, the surgically introduced gene edits are often no different than the DNA mutations that could have arisen by chance. Should we view plant foods any differently if humans introduced a certain mutation, rather than “natural” causes? In spite of the often strident protest against biotechnology products that end up on the dinner table, there are defensible reasons to aggressively pursue gene editing in agricultural improvement if these efforts could address global hunger, nutritional deficiencies, or farming challenges provoked by the future effects of a changing climate.

Time will tell whether activist groups or overly restrictive regulation will stunt innovation in this sector. One thing seems clear: different cultures and attitudes in distinct parts of the world will play a major role in shaping the future direction of CRISPR applications. In the United States, for example, the Department of Agriculture decided in 2018 that plants developed through gene editing will not be specially regulated, as long as they could have been developed through traditional breeding. In stark contrast, the European Court of Justice decided around the same time that gene-edited crops would be subject to the same regulations as GMOs. Meanwhile, the application of CRISPR in agriculture surges ahead in China, which ranks first in worldwide farm output.

Food producers are equally excited by the possibilities afforded by gene-editing technology in animals. Designer DNA mutations can increase muscle content, reduce disease, and make animals, and they also offer biological solutions to problems often solved through more ruthless means. For example, rather than farmers killing off male chicks a day after hatching because female hens are desired, scientists are pursuing gene-editing solutions to bias reproduction, so that only female chicks are born in the first place. Similarly, the remarkable feat by a company called Recombinetics to genetically “dehorn” cattle offers a far more humane alternative to the cruel but widespread practice of cattle dehorning via cauterization. Gene editing could even succeed in producing chickens that lay hypoallergenic eggs, pigs that do not require as many antibiotic treatments, and sheep with naturally altered hair color.

Then there are those applications of CRISPR that verge on science fiction. Rather than harnessing gene-editing technology to create organisms never before seen on Earth, some scientists aim to do exactly the opposite and leverage gene editing to resurrect extinct animals that once existed long ago. Dinosaurs are sadly out of the question, as imagined by Michael Crichton in Jurassic Park—DNA breaks down far too quickly to rebuild the genome of any dinosaur species—but not so with the wooly mammoth. Using extremely well-preserved frozen tissue samples, geneticists have already succeeded in deciphering the letter-by-letter sequence of the wooly mammoth genome, enabling a direct comparison to the genome of the modern-day elephant, its closest relative. Now, George Church and colleagues are using CRISPR to convert specific genes in elephant cells into their wooly mammoth counterparts, prioritizing those genes implicated in functions like temperature sensation, fat tissue production, and skin and hair development. Organizations like the Long Now Foundation hope to bring genetic engineering to bear on many more such de-extinction efforts, with a focus on passenger pigeons, great auks, and gastric-brooding frogs, all of which were directly or indirectly wiped off the planet by human actions. Might we be able to reverse some of the negative impacts that humans have had on biodiversity, using biotechnology? Or should we, instead, be focusing our efforts on preserving the biodiversity that is left, by working harder to curb climate change, restrain poaching, and rein in excessive land development?

There are CRISPR applications that verge on science fiction. Rather than harnessing gene-editing technology to create organisms never before seen on Earth, some scientists aim to do exactly the opposite and leverage gene editing to resurrect extinct animals that once existed long ago

One additional application of CRISPR in animals deserves mention: a potentially revolutionary technology known as a gene drive (Regalado, 2016). The scientific details are complicated, having to do with a clever workaround of those fundamental laws of inheritance first discovered by Gregor Mendel through his work on pea plants. CRISPR-based gene drives allow bioengineers to break those laws, effectively “driving” new genes into wild animal populations at unprecedented speed, along with their associated traits. Perhaps the most promising real-world example involves the mosquito, which causes more human suffering than any other creature on Earth because of its extraordinary ability to serve as a vector for countless viruses (dengue, West Nile, Zika, and so on), as well as the malaria parasite. Imagine if genetically modified mosquitoes, specifically engineered so they can no longer transmit malaria, were released into the wild and allowed to spread their genes. Better yet, what about a line of genetically modified mosquitoes that spread female sterility, thereby hindering reproduction and culling entire wild populations? Proof-of-concept experiments achieving both these feats have already been performed in highly contained laboratory environments, and discussions are underway to determine when the technology is safe enough for field trials. Attempting the eradication of an entire species may rightfully seem like a dangerously blunt instrument to wield, and yet, if mosquito-borne illnesses were to become a thing of the past, saving a million human lives annually, can we justify not taking the risk?

Realizing the Promise of Gene Editing to Treat Disease

Notwithstanding the numerous exciting developments in plant and animal applications, the greatest promise of CRISPR technology is arguably to cure genetic diseases in human patients (Stockton, 2017). Monogenic genetic diseases result from one or more mutations in a single gene, and scientists estimate that there are more than 10,000 such diseases affecting roughly one in every two hundred births. Many genetic diseases like Tay-Sachs disease are fatal at a young age; others like cystic fibrosis can be managed but still lead to a significant reduction in life expectancy; and still others lead to devastating outcomes later in life, such as the physical, mental, and behavioral decline that inevitably occurs for Huntington’s disease patients.

Notwithstanding the numerous exciting developments in plant and animal applications, the greatest promise of CRISPR technology is arguably to cure genetic diseases in human patients

Scientists have been dreaming of a magic bullet cure for genetic diseases ever since DNA mutations were first linked to hereditary illnesses, and there have been incredible strides over the years. For example, after more than twenty-five years of clinical trials, the first gene therapy drug was approved by the United States Food and Drug Administration in 2017, in which patients suffering from a disease of the eye called retinal dystrophy receive healthy genes delivered directly into the eye via a genetically modified virus. Other diseases can be effectively treated using small-molecule drugs, or, in more severe cases, bone marrow transplants. Yet all of these approaches treat the genetic disease indirectly, rather than directly targeting the causative DNA mutations. The ideal treatment would permanently cure the disease by repairing the mutation itself, editing the pathogenic DNA sequence back to its healthy counterpart.

CRISPR offers the possibility of this ideal treatment. In dozens of proof-of-concept studies already published, scientists have successfully leveraged CRISPR in cultured human cells to eradicate the mutations that cause sickle cell disease, beta-thalassemia, hemophilia, Duchenne muscular dystrophy, blindness, and countless other genetic disorders. CRISPR has been injected into mouse and canine models of human disease, and achieved lasting and effective reversal of disease symptoms. And physicians have already tested the first gene-editing-based treatments in patients, though it is too early to say whether or not the treatments were efficacious.

A researcher initiates a CRISPR-Cas9 process at the Max- Delbrueck Molecular Medicine Center in Berlin, Germany. May 22, 2018

In a parallel and equally exciting avenue of research, CRISPR is being combined with a promising (and Nobel Prize-winning) new avenue of cancer treatment, known as cancer immunotherapy. Here, human immune cells are enhanced with genetic engineering, endowing them with specialized molecules that can hunt down markers specific to cancer, and then potently eliminate cancerous cells from the body. In a remarkable first, Layla Richards, a one-year-old patient from London who was suffering from acute lymphoblastic leukemia, the most common type of childhood cancer, was cured in 2015 using a combination of gene-edited immune cells and bone marrow transplant. Chinese scientists have since initiated clinical trials in dozens of other patients using gene-edited immune cells to treat cancer, and additional trials are imminent in the US and Europe.

To be sure, many challenges remain before the full potential of CRISPR-based disease cures can be realized. For one, the tricky problem of delivery remains: how to deliver CRISPR into the body and edit enough of an adult patient’s forty trillion cells to have a lasting effect, and to do so safely without any adverse effects. Additionally, the edits need to be introduced with an extreme level of accuracy, so that other genes are not inadvertently perturbed while the disease-associated mutation is being repaired. Early reports highlighted the risk of so-called off-target effects, in which CRISPR induced unintended mutations, and, given the permanent nature of DNA changes, the bar must be set very high for a gene-editing therapy to be proven safe.

CRISPR is being combined with a new avenue of cancer treatment, known as cancer immunotherapy, in which human immune cells are enhanced with genetic engineering, endowing them with specialized molecules that can hunt down markers specific to cancer, and then potently eliminate cancerous cells from the body

In spite of the risks and remaining hurdles, the possibilities seem limitless. New studies now surface at a rate of more than five per day, on average, and investors have poured billions of dollars into the various companies that are now pursuing CRISPR-based therapeutics. Someday soon, we may find ourselves in a new era in which genetic diseases and cancer are no longer incurable maladies to be endured, but tractable medical problems to be solved.

The Looming Ethical Controversy over Embryo Editing

When should that solution begin, though? While most researchers are focused on harnessing CRISPR to treat patients living with disease, a small but growing number of scientists are, instead, exploring the use of CRISPR to prevent disease, not in living patients but in future individuals. By repairing DNA mutations directly in human embryos conceived in the laboratory, from the merger of egg and sperm through in vitro fertilization (IVF), these scientists hope to create heritable gene edits that would be copied into every cell of the developing human and passed on to all future offspring.

Introducing gene edits into embryos constitutes a form of germline editing, in which the germline refers to any germ cells whose genome can be inherited by subsequent generations. Germline editing is routinely practiced by animal breeders because it is the most effective way of creating genetically modified animals, and, indeed, methods for injecting CRISPR into mouse embryos have been all but perfected over the last five years. Yet the notion of performing similar experiments in human embryos is cause for alarm, not just because of heightened safety concerns when introducing heritable mutations, but because of the ethical and societal ramifications of a technology that could forever alter the human genome for generations to come.

In 2015, when it became clear that CRISPR would make human germline editing a distinct possibility, numerous white papers were published by scientists and nonscientists alike, calling attention to this troubling area of technology development. In almost perfect synchrony, though, the first research article was published by a group of Chinese scientists in which, for the first time ever, human embryos were subjected to precision gene editing. The resulting embryos were not implanted to establish pregnancies, and those initial experiments were not particularly successful at achieving the desired edits, but, nevertheless, the red line had been crossed. In the years since, additional studies have continued to surface, and in one of the most recent articles from a group in the US, the technique was shown to be far safer than before, and far more effective. Many fear that the first humans to be born with engineered DNA mutations may be just around the corner.

The media is rife with doomsday scenarios auguring a future of designer babies with superhuman intelligence, strength, or beauty, and it is important to realize the flaws in such alarmist reactions. The vast majority of human traits can only partially be ascribed to genetics, and they typically result from thousands and thousands of gene variants, each one of which has only a vanishingly small impact on determining the trait. It is hard enough to introduce a single mutation precisely with CRISPR, and no amount of gene editing would be able to achieve the thousands of edits required for attempting to alter these traits. The sci-fi futuristic scenarios depicted by movies like Gattaca and books like A Brave New World are bound to remain just that: science fiction.

In 2015, for the first time ever, human embryos were subjected to precision gene editing. The resulting embryos were not implanted to establish pregnancies, and those initial experiments were not particularly successful at achieving the desired edits, but, nevertheless, the red line had been crossed

Nevertheless, the emergence of facile, powerful, gene-editing technology may change the way we think about reproduction, particularly when it comes to highly penetrant, disease-associated mutations. If it becomes possible to eradicate a mutation before birth, eliminating the chance that a child could ever develop a particular disease, or pass on the disease-associated mutation to his/her children, should we not pursue such an intervention? But how might such interventions change the way society perceives individuals already living with disease? Who would have access to such interventions, and would they be offered equitably? And might the use of CRISPR to eliminate disease-associated mutations simply be the first step down the slippery slope of harnessing gene editing for genetic enhancements?

These are hard questions, questions that must be discussed and debated, and not just by scientists but by the many other stakeholders who will be affected by gene-editing technology: patients and patient advocacy groups, bioethicists, philosophers, religious leaders, disability rights advocates, regulators, and members of the public. Furthermore, we must endeavor to reach across cultural divides and seek international consensus, thereby avoiding a potential genetic arms race in which countries compete to innovate faster than others.

There are real risks associated with the unfettered pursuit of human germline editing, but this must not be allowed to stifle the development of CRISPR for improving our society in other ways. Few technologies are inherently good or bad: what is critical is how we use them. The power to control our species’ genetic future is both terrifying and awesome, and we must rise to the challenge of deciding how best to harness it.

Concluding Thoughts

Ten years ago, the term CRISPR was familiar to just a few dozen scientists around the world. Today, hardly a day passes without a feature story touting the amazing possibilities of CRISPR technology, and gene editing is quickly becoming a household term. CRISPR has starred in a Hollywood blockbuster movie, appeared in countless TV series, been discussed by governmental agencies worldwide, and become available online for purchase as a do-it-yourself kit. Ten years hence, CRISPR will impact the food we eat and the medicine we take, and it will undoubtedly continue to prove instrumental in our understanding of the natural world all around us.

So, too, will our understanding of CRISPR itself continue to evolve. For the field of bacterial adaptive immunity is anything but stagnant, and new discoveries abound as researchers continue to dig deeper and deeper into the billion-year-old genetic conflict between bacteria and viruses. We now know that CRISPR-Cas9 is just one of many remarkable molecular machines that microbes have evolved to counteract the perpetual assault from foreign pathogens, and scientists continue to invent innovative applications of this biological treasure trove. Who would have thought that scientific curiosity and basic research investigations could unveil such a promising area of biotechnological exploration?

The American physicist, Leonard Susskind, once wrote: “Unforeseen surprises are the rule in science, not the exception.” Let us see where the next big breakthrough comes from.

Acknowledgments

I thank Abigail Fisher for assistance with early drafts of the article outline. I regret that space constraints prevented me from more thoroughly discussing the research being led by my many colleagues, and I encourage interested readers to consult the included references for a more thorough discussion of related work.

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La revolución biológica de la edición genética con tecnología CRISPR

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Sternberg, Samuel H.. "The Biological Breakthrough of CRISPR-Based Gene Editing." In Towards a New Enlightenment? A Transcendent Decade. Madrid: BBVA, 2018.

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