Seeding a Genetic Engineering RevolutionApr 18, 2014
Curing diseases, and feeding the hungry are just a few applications of a gene targeting technology that was developed by Dr. Dana Carroll, Professor of Biochemistry. He discusses how he brought the technology to where it is today, and his hopes for where it will lead. In honor of his achievements, he will be receiving the Herbert Sober career award from the American Society for Biochemistry and Molecular Biology later this month.
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Host: Curing diseases and feeding the hungry are just a few of the applications of a gene targeting technology that was developed by my guest, Dr. Dana Carroll. In honor of his achievements, he will be receiving the Herbert Sober Lecture Award from the American Society for Biochemistry and Molecular Biology later this month.
Gene targeting is a way to change a piece of DNA in a living organism. This technology already existed, but you made it much more useful. When you started your work developing this gene targeting technology, what problems were you trying to address?
Dr. Carroll: Here's an analogy. Let's say you're a book editor, and you've got a manuscript in front of you. The author has used the wrong word in one place, and you really want to correct that one word. Twenty years ago, the only tool you had if you were a gene targeter, the analogy of this editor, was to change every word in the whole text at random at some frequency and hope that one of these changes would hit the word you were interested in.
Host: I see.
Dr. Carroll: What the new technology allows you to do is to go to that specific word. You've got a recognition principle that's going to take you to that specific word and allow you to change it uniquely.
Host: Right. The beauty of this technology is that, theoretically, you should be able to change any trait in any animal or any plant that you know the sequence for.
Dr. Carroll: That's right. You have to know the genetics behind it. You have to know what genes are responsible for a particular trait.
Host: Well, maybe we should talk about the technology. What is it that you've developed?
Dr. Carroll: We actually started from the perspective of Mario Capecchi's technology for which he won a Nobel Prize. Mario's big contributions were figuring out how to find the very, very rare recombination products in cells that were the result of gene targeting. But the frequency of the gene targeting events was really low, on the order of 1 in every 1 million cells. My perspective on the problem was that the target wasn't interested in what you wanted to do, and somehow you had to damage the target so that it would be interested in joining with a piece of DNA that you had put in from the laboratory.
We started with the zinc finger nucleases. The key to all of these nuclease base technology of which there are now three main ones, is to have the ability to bind a specific sequence in DNA and then link that capability to a DNA cleavage capability. What we knew about the zinc fingers, at that time, was that they could be manipulated to redirect binding and cleavage to different DNA sequences, so you could choose what sequence you wanted to hit. That's actually how things panned out. I couldn't have imagined how things have developed since then, but that was what we were looking to do.
Host: You started working on this technology almost 20 years ago, right?
Dr. Carroll: Well, it's 18.
Host: Not quite. Eighteen?
Dr. Carroll: Eighteen years ago.
Host: What did you think it would be used for?
Dr. Carroll: At that time, I would say it was sort of a pipe dream. It was something we could envision, but we couldn't be certain we'd ever get there.
Dr. Carroll: So, yes, it should be used in a sort of human gene therapy. We know where a lot of disease mutations are. What we'd like to be able to do is go in and correct those mutations back to the normal sequence, and this technology allows you to do that. Actually delivering the reagents to the right cells at the right time is a challenge, still. But there are examples where this is actually being done.
There are applications to crop plants that people have been working on for quite a long time now, trying to either introduce greater nutritional value into standard crop plants, introduce drought tolerance or herbicide tolerance or herbicide resistance, things like this, into crop plants.
One of the applications that I absolutely love is to livestock. I have a friend in Minnesota named Scott Fahrenkrug who's started a company based on this sort of application. Dairy farmers around the country saw the horns off their cows because they don't want the cows to injure other cows or the farmers. You can actually go directly into the good milk producers and make a unique change in their genome that eliminates the horn.
Host: I bet you never thought of that application 18 years ago.
Dr. Carroll: I did not.
Host: That's amazing.
Dr. Carroll: Right.
Host: What about in human gene therapy? I think it's being used in clinical trials now.
Dr. Carroll: Yes. The initial results from that have just been published in the New England Journal of Medicine. This was a collaboration between clinicians at the University of Pennsylvania School of Medicine and Sangamo BioSciences, a company in the Bay Area that has licensed our technology, and they have absolutely used our technology in this clinical trial.
The trial is for AIDS. Here's how the trial goes. A patient comes into the clinic. They are HIV-infected. They still have some functional T cells, so these T cells are taken from the circulation, and they're treated in the laboratory with zinc finger nucleases that disable a gene called CCR5. The gene produces a protein that is a code receptor that HIV depends on for infection. The cells are put back into the same patient, and the clinical trial has given these patients HIV-resistant T cells so they no longer go into this immune crisis that we call AIDS.
Host: Wow. That's exciting. There are many companies now who are running with maybe this technology or the next generation of this technology, and it's become very useful.
Dr. Carroll: Right. We licensed this to Sangamo. Sangamo subliicensed it to Dow AgroSciences, and they've been using it to modify crop plants.
The other nuclease technology that's emerged just in the last couple of years is called CRISPRs, and people are very excited about this CRISPR system. People all over the world have jumped on it with both feet, and it's being used in a lot of different applications.
Host: So these kind of newer developments are really based on the principles that you developed?
Dr. Carroll: Right. Right. The CRISPR system, although it doesn't have any shared components, the whole basis for all of this technology is double strand breaks allow you to make specific changes in genomic DNA.
Host: So what are some of the most fulfilling aspects of how this has come around for you?
Dr. Carroll: It's been a lot of fun to do this. These tools are so logical. There's nothing mysterious about how these work. These are just designer DNA cleavage reagents that are based on very simple principles. The fact that they work as well as they do is amazing and gratifying.
One of the things that's been the most fun for me has been to watch how other people have taken the basic technology and developed it going onto the next generations and applied it to lots of different organisms. I have a slide that I show when I give talks that lists all the organisms I know about where these various nucleases have been used for genomic targeting, and there are more than 40 now. Plants, animals, microorganisms, experimental organisms. It's really a long list and it's some pretty amazing things.
Announcer: Interesting, informative, and all in the name of better health. This is The Scope Health Sciences Radio.