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This process is far simpler and more effective than any other gene-manipulation technology in existence.
In recent experiments, CRISPR has been used to “humanize” the DNA of pigs, giving rise to hopes that these animals might someday serve as organ donors for humans.
Scientists hope to eventually eradicate mosquito-borne illnesses, such as malaria and Zika, or perhaps even wipe out the disease-carrying mosquitoes
CRISPR enables scientists to accomplish such feats by finding and fixing single incorrect letters of DNA out of the 3.2 billion letters that make up the human genome, but
Physicians have already begun treating some cancers with souped-up immune cells whose genomes have been fortified with edited genes to help them hunt down cancerous cells.
Now, for the first time ever, we possess the ability to edit not only the DNA of every living human but also the DNA of future generations—in essence, to direct the evolution of our own species.
Gene editing forces us to grapple with the tricky issue of where to draw the line when manipulating human genetics.
“Someday we may consider it unethical not to use germline editing to alleviate human suffering.”
They concluded that a single cell in her body must have experienced an uncommon and usually catastrophic event called chromothripsis—a recently discovered phenomenon in which a chromosome suddenly shatters and is then repaired, leading to a massive rearrangement of the genes within it.
The NIH scientists determined that the fortunate cell must have been a hematopoietic stem cell, a type of stem cell from which every kind of blood cell in the body originates and that has an almost unlimited potential to proliferate and self-renew. That cell had passed along its rearranged chromosome to all its daughter cells, eventually repopulating Kim’s entire immune system with healthy new white blood cells that were free of the CXCR4 mutation.
This chain of events—so unlikely that I had a hard time even conceiving
the scientific literature is peppered with other examples of patients who were partially or completely cured of a genetic disease through accidental, spontaneous “editing” of the genome.
Yet the two SCID patients in New York were the exceptions to this terrible rule; they remained surprisingly healthy into adolescence and adulthood. The reason in both cases, scientists determined, was that the patients’ cells had spontaneously corrected the disease-causing mutation in a gene called ADA, and they’d done it without disturbing the remainder of the gene or the chromosome.
Overall, however, the odds of being spontaneously cured of a genetic disease are minuscule.
The human genome comprises about 3.2 billion letters of DNA, with around 21,000 protein-coding genes.
Interestingly, a genome’s size is not an accurate predictor of an organism’s complexity; the human genome is roughly the same length as a mouse or frog genome, about ten times smaller than the salamander genome, and more than one hundred times smaller than some plant genomes.
The consequences of this tiny change in the protein—a difference of just ten atoms out of more than eight thousand total—are
Sickle cell disease is an example of a recessive genetic disease. This means that both copies of an individual’s HBB gene must carry the mutation for that person to be affected; if only one copy has the alteration, the nonmutated gene can produce enough normal hemoglobin to overcome the negative effects of the mutated hemoglobin. People with only one mutated copy of the HBB gene are still carriers of the sickle cell trait, however, and while they’re usually unaffected, they can still pass the mutated gene on to their offspring.
Other genetic diseases exhibit dominant inheritance, meaning that just a single copy of the mutated gene is enough to cause the disease. One example is WHIM syndrome, in which the one thousandth letter of the CXCR4 gene is mutated from a C to a T; the mutant gene creates a hyperactive protein that dominates the functioning of the healthy gene.
Scientists have precisely identified well over four thousand different kinds of DNA mutations that can cause genetic disease.
As a tool, viral vectors are astoundingly reliable; researchers working with viral vectors can get genes into target cells with nearly 100 percent efficiency. For the scientists who pioneered their therapeutic use, viral vectors were the ultimate Trojan horse.
Viruses know not only how to get their DNA inside a cell but also how to make the new genetic code stick.
expected. Instead of the gene copies being distributed haphazardly throughout the different chromosomes of the genome, Capecchi found that the genes were always clustered together in one or a few regions, with many copies overlapping one another, as if they’d been deliberately assembled. In fact, he determined, that’s exactly what had happened.
Cells, it seemed, could do most of the hard work of modifying their genomes all by themselves. This meant that scientists could deliver genes more gently, without using viruses to ram new DNA into the genome. By tricking a cell into thinking that the recombinant DNA was simply an extra chromosome that needed to be paired with a matching gene already in its genome, scientists could ensure that the new DNA was combined with the existing, native genetic code through homologous recombination.
Scientists estimate that there are somewhere on the order of 1031 bacteriophages on earth; that’s ten million trillion trillion, or a one with thirty-one zeros after it. A single teaspoon of
seawater contains five times more phages than there are people in New York City.
At the time I spoke with Jill, four major bacterial defense systems had been identified. In the most prominent one, bacteria decorate their own genomes with unique markings that subtly
change the DNA’s chemical appearance without affecting how genetic information is expressed; in
Could a strain of S. thermophilus actually make itself more resistant to a particular bacteriophage by splicing new DNA into its own CRISPR region that matched the sequences of DNA found in the phage?
we now knew, bacteria had in CRISPR a remarkably effective form of adaptive immunity, one that allowed the bacterial genome to steal snippets of phage DNA during an infection and use it to mount a future immune response. As Blake put it, CRISPR functioned like a molecular vaccination card: by storing memories of past phage infections in the form of spacer DNA sequences buried within the repeat-spacer arrays, bacteria could use this information to recognize and destroy those same invading phages during future infections.
Caribou Biosciences to commercialize the Cas proteins. At
we imagined creating simple kits that scientists, or even clinicians, could use to detect the presence of viral or bacterial RNA in body fluids.
In the warfare waged between bacteria and viruses, Cas9’s function made perfect sense. Armed with a cache of RNA molecules derived from the CRISPR array, where snippets of phage DNA had been stored, Cas9 could readily be programmed to slice up corresponding sites within viral genomes. It was the perfect bacterial weapon: a virus-seeking missile that could strike quickly and with incredible precision.
we had built the means to rewrite the code of life.
I imagined that it would democratize a technology that had once been the privilege of the few. In the days before CRISPR, gene editing required sophisticated protocols, formidable scientific expertise, and substantial financial resources, and it could be performed on only a few model organisms.
In just a few simple and routine steps, Martin and I had selected an arbitrary DNA sequence within the 3.2-billion-letter human genome, designed a version of CRISPR to edit it, and watched as the tiny molecular machinery followed through with its new programming—all inside living human cells. With that success, we had validated our new technology that offered scientists the remarkable ability to rewrite the code of life with surgical precision and astonishing simplicity. In what felt like no time at all, CRISPR had already caught up to almost twenty years of research and development in other
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Genes are, after all, just carriers of information, like the blueprints of a house; the goal of gene editing is not merely to alter the blueprints, but to change the form of the structure that gets built. In many cases, this means altering the proteins that the genes encode and that cells produce during gene expression.
First, a temporary copy of DNA, called messenger RNA (or mRNA), is made in the cell’s nucleus. Like a strand of DNA, the mRNA is a chain of letters, and its sequence matches the sequence of the DNA it copied (the only major exception being that T gets replaced by U). The mRNA is exported out of the cell’s nucleus and delivered to a protein-synthesizing factory called a ribosome, which translates the four-letter language of RNA (A, G, C, and U) into the twenty-letter
language of proteins (the twenty amino acids). This translation proceeds according to the genetic code, a cipher in which every three-letter RNA combination, called a codon, instructs the ribosome to add one specific amino acid. (With sixty-four possible codons but only twenty amino acids, many codons code for the same amino acid, and three codons serve as stop signs to terminate protein synthesis.) The ribosome begins at one end of the mRNA and reads one consecutive codon after another, adding the corresponding amino acids to the growing protein chain until it reaches the other end of the
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When animal geneticists began using CRISPR, they sought to create gene knockouts that would manifest themselves in obvious ways. One of the favorite targets was a gene called TYR. Having arisen more than half a billion years ago, the TYR gene is widely distributed among animals, plants, and fungi; it produces a protein called tyrosinase that is involved in synthesizing melanin, an important pigment. TYR mutations in humans lead to a deficiency in tyrosinase and cause type I albinism, a genetic condition associated with vision defects, pale skin lacking pigmentation, and red eyes. If CRISPR was
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Barely a year later, a Japanese research team repeated the same experiment but with a few minor tweaks and achieved efficiencies of 97 percent, with thirty-nine out of forty pups displaying a radiant, homogeneous albino exterior. In a matter of weeks, the team had permanently and precisely altered the genetic composition of an entire generation of animals (and all their future progeny) in a way that nature had never intended.
Gene knockouts are just one of the many gene-editing strategies that researchers have perfected with CRISPR.
Rather than introducing permanent genetic changes by editing DNA, the deactivated CRISPR allowed scientists to make temporary changes that would not alter the underlying genetic information of a cell but nevertheless affected how genetic information was expressed. In particular, he transformed CRISPR into a gene-expression
But the real reason that CRISPR exploded onto the biotech scene with such force and vitality was its low cost and ease of use. CRISPR finally made gene editing available to all scientists.
With CRISPR, however, scientists can easily design a version to target their gene or genes of interest, prepare the requisite Cas9 protein and guide RNA, and execute the experiments themselves using standard techniques, all within mere days and without requiring any outside assistance. The only thing necessary to get started is a copy of the basic CRISPR-containing artificial chromosome, or plasmid. This need has been conveniently met on a massive scale by the nonprofit organization Addgene,
Addgene is like Netflix, only for plasmids. Once Martin and I had submitted our CRISPR article, we sent his plasmids to Addgene for safekeeping, much as film studios license their movies to Netflix. Many other research laboratories that produce CRISPR plasmids do the same. Addgene keeps careful track of the plasmids it has on file, advertises the plasmids and their exact specifications on a website, and generates thousands of duplicate copies that can be distributed to eager customers.
It’s become something of an old saw in our young field: what used to require years of work in a sophisticated biology laboratory can now be performed in days by a high school student. Some experts have suggested that, with today’s tools, anyone can set up a CRISPR lab for just $2,000. Others predict a rise in do-it-yourself biohackers,
And in a page taken straight out of a famous book-to-film sci-fi franchise, some laboratories are pursuing a venture known as de-extinction, which is nothing less than the resurrection of extinct species through cloning or genetic engineering.
Yes, someday soon, CRISPR might be employed to destroy entire species—an application I never could have imagined when my lab first entered the fledgling field of bacterial adaptive immune systems just ten years ago.
Gene-edited organisms, by contrast, contain tiny alterations to existing genes that give the organism a beneficial trait by tweaking the levels of proteins that were already there to begin with—without adding any foreign DNA. In this respect, gene-edited organisms are often no different than those organisms produced by mutation-inducing chemicals and radiation.
