CRISPR is a specific, efficient and versatile gene-editing technology we can harness to modify, delete or correct precise regions of our DNA. CRISPR stands for Clustered Regularly Interspaced Short Palindromic repeats.
As a gene-editing tool, CRISPR/Cas9 has revolutionized biomedical research and may soon enable medical breakthroughs in a way few biological innovations have before. CRISPR edits genes by precisely cutting DNA and then letting natural DNA repair processes to take over.
How it works
The system consists of two parts: the Cas9 enzyme and a guide RNA.
Cas9 is a CRISPR-associated (i.e. "Cas") enzyme, that acts as “molecular scissors” to cut DNA at a location specified by a guide RNA. It can cut apart DNA. Bacteria fight off viruses by sending the Cas9 enzyme to chop up viruses. In the lab, researchers use a similar approach to turn the microbe’s virus-fighting system into the coolest new biotechnology.
Scientists start with RNA. RNA is a molecule that can read the genetic information in DNA. The RNA finds the spot in the nucleus of a cell where some editing activity should take place. This guide RNA points Cas9 to the precise spot on the DNA where a cut is called for. Cas9 then locks onto the double-stranded DNA and unzips it.
This allows the guide RNA to pair up with some region of the DNA it has targeted. Cas9 snips the DNA at this spot. This creates a break in both strands of the DNA molecule. The cell, sensing a problem, repairs the break.
Fixing the break might disable a gene (the easiest thing to do). Alternatively, this repair might fix a mistake or even insert a new gene (a much more difficult process).
Cells usually repair a break in their DNA by gluing the loose ends back together. This is a sloppy process. It often results in a mistake that disables some gene. That may not sound useful — but sometimes it is.
Scientists cut DNA with CRISPR/Cas9 to make gene changes, or mutations. By comparing cells with and without the mutation, scientists can sometimes figure out what a protein’s normal role is. Or a new mutation may help them understand genetic diseases. CRISPR/Cas9 also can be useful in human cells by disabling certain genes - ones, for instance, that play a role in inherited diseases.
CRISPR/Cas9 and related tools can now be used in new ways, such as changing a single nucleotide base - a single letter in the genetic code - or adding a fluorescent protein to tag a spot in the DNA that scientists want to track. Scientists also can use this genetic cut-and-paste technology to turn genes on or off.
The potential applications of CRISPR are endless.
Just some of the major uses of CRISPR are:
- Curing diseases - such as cancer, blindness, and Alzheimer's disease.
- De-extinction - Using CRISPR technology, researchers plan to introduce genes from the passenger pigeons into its modern-day relative - the band tail pigeon. The hybrids will be bred for several generations until the offspring DNA exactly matches that of the extinct species.
- Eradicating pests - CRISPR could help us control the numbers of animal species that transmit infectious diseases or that are invasive in a particular ecosystem.
- Immortality - This might be a bit over-the-line, but CRISPR could, in theory, be used to cure death.
Why aren't people using it yet?
So far CRISPR’s biggest impact has been felt in basic biology labs. This low-cost gene editor is easy to use. That has made it possible for researchers to delve into the basic mysteries of life. And they can do it in ways that used to be difficult if not impossible.
Conventional CRISPR-Cas9 only has about a 1% success rate for DNA insertion. That's really low! CRISPR still has a long way to go before it can be used safely and effectively to repair - not just disrupt - genes in people.
This is particularly true for most diseases, such as muscular dystrophy and cystic fibrosis, which require correcting genes in a living person. This is because if the cells were first removed and repaired then put back, too few would survive. And the need to treat cells inside the body means gene editing faces many of the same delivery challenges as gene transfer - researchers must devise efficient ways to get a working CRISPR into specific tissues in a person, for example.
Using CRISPR to cut out part of a gene - not correct the sequence - is relatively easy to do. In fact, this strategy is already being tested in a clinical effort to stop HIV infection.
But when CRISPR is used to correct a gene using a strand of DNA that scientists supply to cells, not just to snip out some DNA, it doesn’t work very well. That’s because the cells must edit the DNA using a process called homology-directed repair, or HDR, that is only active in dividing cells.
Unfortunately, most cells in the body - liver, neuron, muscle, eye, blood stem cells - are not normally dividing. For this reason, knocking out a gene is a lot simpler than knocking in a gene and correcting a mutation.
The most-discussed safety risk with CRISPR is that the Cas9 enzyme, which is supposed to slice a specific DNA sequence, might also make cuts in other parts of the genome that could result in mutations that raise cancer risk.