What Is CRISPR and What Can It Really Do?

Muscle cells, edited with CRISPR to fix the mutation that causes muscular dystrophy. These cells are in a lab dish, though, not in a person.
Photo: National Institutes of Health

A DNA-editing technique called CRISPR keeps popping up in the news, in one medical breakthrough after another. In theory, CRISPR can find any problem area in DNA, snip it out, and replace it with a fresh set of nucleotides. But in real life, that cut-and-paste job isn’t always straightforward.


Like many biotech techniques, CRISPR was invented by bacteria and creatively repurposed by scientists. Bacterial cells can contain stretches of DNA with a weird repetitive pattern (“clustered regularly interspaced short palindromic repeats”) that turn out to be a scrapbook of viruses and other foes that the bacterium has already fought and vanquished. With this information, proteins like Cas9 (“CRISPR associated [protein] 9") can find those intruders the next time they show up. The protein carries out a search-and-destroy mission guided by a snippet of DNA from the scrapbook.

In the lab, scientists can tweak this system to search for any DNA sequence they like, including those in our own genes. What if we could repair a defective gene in somebody with a genetic disorder, for example? CRISPR’s potential is huge.


But CRISPR is also a new technology, and it’s not magic. Researchers have to work out the kinks in CRISPR-based approaches. We can’t do CRISPR routinely in humans until we know that it’s safe, for example. And it may not be easy to get the CRISPR molecules into all the cells where their target DNA actually lives. So here’s your reality check on what the new tech can actually do.

Criminals aren’t using it to evade DNA detection

If criminals can be found through DNA databases, then perhaps an enterprising murderer can change the DNA in their body so that it no longer matches the samples they’ve left at crime scenes. (The Daily Mail reported that one scientist had speculated this might be possible.) Nice idea, but CRISPR’s big challenge is actually delivering the DNA-editing technology to all of the cells it’s supposed to reach. We currently don’t know if it’s possible to CRISPR all the cells in a person’s body.

This Twitter thread by Layla Katiraee outlines some of the other problems. Forensic DNA tests look at many different places in the genome, so you’d need to do many different CRISPR edits. The DNA tracked in forensic databases is also tricky to work with, biochemically. And even if you could alter your genome, you’d want to test whether you’d succeeded (not an easy or cheap job) before offering up some blood to the police.


Bottom line: not happening anytime soon.

You probably can’t pump up your muscles, either

What if you don’t want to change all your DNA, but just tweak a little something for cosmetic purposes? Biohacker Josiah Zayner injected his arm with a CRISPR recipe to block a gene called myostatin. Babies who naturally have a defect in this gene are born looking like tiny bodybuilders, because they have little to no myostatin to put the brakes on their muscle growth. Altering the gene in an adult’s cells, however, won’t necessarily do the same thing.


Even when researchers did a myostatin-blocking CRISPR experiment on 35 dog embryos, only two puppies were born with the desired mutation. And those two ... well, they don’t exactly look like doggie hulks.

Bottom line: seems very unlikely.

There’s hope for curing disease

CRISPR on adult humans faces serious challenges, but it’s plausible enough that trials are beginning this year. Teams of researchers in Europe and the US are planning to treat the blood disorders beta-thalassemia and sickle-cell disease this way. They plan to extract patients’ own blood-producing stem cells, use CRISPR to edit out a defect in those cells’ DNA, and then re-introduce the cells into patients’ bodies. It’s conceptually similar to a bone marrow transplant, except each person is their own donor.


Bottom line: promising, but too early to tell if it will work.


CRISPR crops could be the new GMOs

Plenty of the crops we grow today have been changed from their original, wild versions. In some cases, that’s because of a natural mutation that occurred millennia ago. In others, people directed the destiny of their favorite plants by choosing which plants or animals to breed with each other. And in recent decades, we’ve been able to tinker with living things’ DNA directly.


Certain techniques have come to be known as genetic engineering, and plants produced this way are known as GMOs, or genetically modified organisms. (Despite some bad press, GMOs aren’t any better or worse for you than their non-GMO counterparts.)


GMO crops have extra layers of government regulation beyond what “conventionally” bred plants have to deal with. But this year, the USDA decreed that CRISPR’d crops will not count as GMOs if they don’t contain foreign DNA. So, for example, if you take a corn plant and add a gene from another living thing, that could be considered a GMO. But if you just use CRISPR to delete a gene, without adding anything new, the law considers that equivalent to breeding a new corn variety the old-fashioned way.

But the FDA announced that they view CRISPR in animals as a form of gene therapy, which means it’s regulated as a veterinary drug. So if you want to use CRISPR to produce a hornless cow, or a purebred dog that’s missing one of the harmful mutations that often come along with inbreeding, you would have to go through the extremely expensive process of getting it approved as a new drug.


Bottom line: CRISPR plants could be in a grocery store near you in a few years, but CRISPR-edited animals face more barriers.


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Can’t wait for CRISPR bacon.