Jennifer Doudna: We can now edit our DNA. But let's do it wisely (1)
A few years ago, with my colleague, Emmanuelle Charpentier, I invented a new technology for editing genomes.
It's called CRISPR-Cas9. The CRISPR technology allows scientists to make changes to the DNA in cells that could allow us to cure genetic disease. You might be interested to know that the CRISPR technology came about through a basic research project that was aimed at discovering how bacteria fight viral infections.
Bacteria have to deal with viruses in their environment, and we can think about a viral infection like a ticking time bomb -- a bacterium has only a few minutes to defuse the bomb before it gets destroyed. So, many bacteria have in their cells an adaptive immune system called CRISPR, that allows them to detect viral DNA and destroy it. Part of the CRISPR system is a protein called Cas9, that's able to seek out, cut and eventually degrade viral DNA in a specific way.
And it was through our research to understand the activity of this protein, Cas9, that we realized that we could harness its function as a genetic engineering technology -- a way for scientists to delete or insert specific bits of DNA into cells with incredible precision -- that would offer opportunities to do things that really haven't been possible in the past. The CRISPR technology has already been used to change the DNA in the cells of mice and monkeys, other organisms as well.
Chinese scientists showed recently that they could even use the CRISPR technology to change genes in human embryos. And scientists in Philadelphia showed they could use CRISPR to remove the DNA of an integrated HIV virus from infected human cells. The opportunity to do this kind of genome editing also raises various ethical issues that we have to consider, because this technology can be employed not only in adult cells, but also in the embryos of organisms, including our own species.
And so, together with my colleagues, I've called for a global conversation about the technology that I co-invented, so that we can consider all of the ethical and societal implications of a technology like this. What I want to do now is tell you what the CRISPR technology is, what it can do, where we are today and why I think we need to take a prudent path forward in the way that we employ this technology.
When viruses infect a cell, they inject their DNA.
And in a bacterium, the CRISPR system allows that DNA to be plucked out of the virus, and inserted in little bits into the chromosome -- the DNA of the bacterium. And these integrated bits of viral DNA get inserted at a site called CRISPR. CRISPR stands for clustered regularly interspaced short palindromic repeats. (Laughter) A big mouthful -- you can see why we use the acronym CRISPR.
It's a mechanism that allows cells to record, over time, the viruses they have been exposed to. And importantly, those bits of DNA are passed on to the cells' progeny, so cells are protected from viruses not only in one generation, but over many generations of cells. This allows the cells to keep a record of infection, and as my colleague, Blake Wiedenheft, likes to say, the CRISPR locus is effectively a genetic vaccination card in cells. Once those bits of DNA have been inserted into the bacterial chromosome, the cell then makes a little copy of a molecule called RNA, which is orange in this picture, that is an exact replicate of the viral DNA. RNA is a chemical cousin of DNA, and it allows interaction with DNA molecules that have a matching sequence. So those little bits of RNA from the CRISPR locus associate -- they bind -- to protein called Cas9, which is white in the picture, and form a complex that functions like a sentinel in the cell.
It searches through all of the DNA in the cell, to find sites that match the sequences in the bound RNAs. And when those sites are found -- as you can see here, the blue molecule is DNA -- this complex associates with that DNA and allows the Cas9 cleaver to cut up the viral DNA. It makes a very precise break. So we can think of the Cas9 RNA sentinel complex like a pair of scissors that can cut DNA -- it makes a double-stranded break in the DNA helix. And importantly, this complex is programmable, so it can be programmed to recognize particular DNA sequences, and make a break in the DNA at that site. As I'm going to tell you now, we recognized that that activity could be harnessed for genome engineering, to allow cells to make a very precise change to the DNA at the site where this break was introduced.
That's sort of analogous to the way that we use a word-processing program to fix a typo in a document. The reason we envisioned using the CRISPR system for genome engineering is because cells have the ability to detect broken DNA and repair it.
So when a plant or an animal cell detects a double-stranded break in its DNA, it can fix that break, either by pasting together the ends of the broken DNA with a little, tiny change in the sequence of that position, or it can repair the break by integrating a new piece of DNA at the site of the cut. So if we have a way to introduce double-stranded breaks into DNA at precise places, we can trigger cells to repair those breaks, by either the disruption or incorporation of new genetic information. So if we were able to program the CRISPR technology to make a break in DNA at the position at or near a mutation causing cystic fibrosis, for example, we could trigger cells to repair that mutation.