CRISPR is the basis of a revolutionary gene editing system. One day, it could make it possible to do everything from resurrect extinct species to develop cures for chronic disease.
It’s built on a natural adaptation found in the DNA of bacteria and single-celled organisms.
CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats.
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They’re really just bits of genetic code with a specific, recognizable format. They contain a sequence that shows up over and over again, though it’s often reversed each time.
That’s what makes it "Palindromic:” palindromes are words that can be read the same backwards as forwards. Palindromes are common in DNA. Some serve as backups for damage to our genetic code, while others are common in cancer mutations.
With CRISPR, a group of enzymes recognize certain repeats, and break the DNA there to insert important information in the middle. These insertions are called “spacers,” and they contain the genetic code of different viruses that have invaded in the past.
Such previous invasions served a very important evolutionary purpose: immunizing against foreign threats.
Researchers first discovered CRISPR in E. Coli in the 1980s. When E. coli survives viral attacks, it incorporates some of the virus DNA into its own genetic code.
E. Coli isn’t unique in using this strategy. Between the 1980s and 2000, scientists found that lots of bacteria and single-celled organisms incorporate viral DNA this way.
Cells use these sequences as templates for transcribing complementary strands of RNA.
When viruses matching the template sequence enter the cell, the complementary RNA binds to them, and directs a series of CRISPR-associated enzymes, or “Cas” enzymes, to attack—cutting invader DNA at the binding site. That neutralizes the viral threat.
The CRISPR-Cas system is incredibly effective. It’s also easy to manipulate, letting us alter a cell’s genetic code however we want.
In 2012, French microbiologist Emmanuelle Charpentier and American biochemist Jennifer Doudna discovered that Cas enzymes—specifically Cas9—can be reprogrammed to cut nearly any part of the genome, using RNA sequences made in a lab. Those “guide RNA” molecules tell Cas9 where to cut DNA in a cell.
For their discovery, Charpentier and Doudna won the Nobel Prize in Chemistry in 2020.
And the use of CRISPR has taken off in science since their breakthrough.
But scientists are still far from realizing CRISPR’s potential.
Cas9 is great at suppressing or knocking out unwanted genes. But for most medical purposes, it’s not enough to cut unwanted DNA out. Scientists need to control how the DNA repairs itself.
Left to their own devices, cells tend to repair broken DNA using a method that introduces lots of random errors. Researchers can provide cells with templates to guide the repair process, but they’re still working on making that more reliable.
Researchers have found lots of applications for CRISPR in animals, like making disease-resistant chickensand pigs, and mosquitos that can’t bite or lay eggs. But they’ve got many projects underway, like making disease-resistant crops—including wine grapes. More ambitiously, they’re working to genetically alter pigs so their organs could be transplanted into humans. And bring extinct species such as the passenger pigeon back to life, by tweaking the genomes of similar birds.
When it comes to the human genome, though, scientists have been more hesitant. Editing our own DNA could easily end up causing more problems than it solves.
While Cas9 reliably cuts DNA where we want it to, recent experiments have shown it can also affect genes far off-target. And even if we could get it to work reliably, many experts have flagged ethical concerns about using the technology for eugenics and “designer babies.” If parents can one day pay scientists to edit their babies’ DNA, making them stronger and smarter, CRISPR could make the world even more unequal and prejudiced.
In 2018, Chinese researcher HEH JEE'-an-qway claimed he’d used CRISPR to make HIV-resistant children. Whether or not he succeeded, his work violated China’s National Health Commission rules, and he was sentenced to three years in prison.
Using CRISPR on babies is widely illegal. But there are some cases where using CRISPR on humans may be worth the risk.
In 2020, American researchers began the first clinical trials injecting CRISPR directly into living humans, aiming to repair a genetic mutation that causes blindness.
Many researchers hope CRISPR-based therapies could eventually cure hereditary diseases; they’ve already seen promising results in various animal studies. Though given the risks of editing the human genome, we’re still a ways off from widespread use of CRISPR in medicine.
CRISPR has given science a tool to reliably tinker with the code of life. But the question remains: can we do so safely and ethically, while avoiding the unintended consequences of such power?