Almost as soon as you've met Holden Thorp, you sense that he lives in that charmed time when the work is enormously fun and the possibilities are infinite. He probes microscopically deep, into the chemistry of DNA. He thinks universally wide, to every blood donor, to every child and parent in the world.
Consider, for instance, this story-a fiction, for now: Your child has been sick for a week. At your pediatrician's office, the nurse draws a little blood. The blood trickles into a disposable plastic vial, which rests in an instrument wired to a microcomputer. The nurse touches the keyboard, and the computer compares the sample's genetic profile to one of a hundred profiles stored on a microchip. In a few minutes, your doctor knows exactly what virus or bacterium is causing the trouble, and prescribes a treatment specifically for that bug. No guesswork. No unnecessary antibiotics.
For Holden Thorp, associate professor of chemistry, this story is no fairy tale. In a decade or so, he says, the technologies he and his colleagues are developing may well be key parts of a reliable, low-cost, universally available method for detecting viruses or almost anything else possessing DNA.
Not that DNA testing is new. Many laboratories already use assays to identify viral agents, including those which cause AIDS and hepatitis. But for the vast majority of viral infections, including cold and flu, there are no assays and no specific antiviral drugs.
As Thorp sees it, a primary roadblock has been the method commonly used for detecting genetic traits.
"Most gene analysis requires the use of fluorescence," he says. "That means you need a laser and a CCD [charge-coupled device] camera, and in a lot of cases you have to extract the DNA and use an enzyme and put a fluorescent label on that. It's very hard to maintain purity, and the technology is expensive."
If you want a fast, accurate, and economical test, Thorp says, you have to get rid of enzymatic labeling and detect genetic characteristics without dismantling the strand of DNA. And that's what his work is all about.
"Our invention does two things," he says. "First, we use guanine from DNA as the label. Second, we detect it electrochemically. Nobody has done this because the guanine is inside the DNA structure where it's hard to reach."
To detect the guanine, Thorp's method looks for a mismatch, the incomplete hybridization of a probe strand of DNA with its complement. Where the mismatch occurs, guanine oxidizes at an accelerated rate. But detecting a mismatch is one thing, locating it is another. Identifying a particular genetic trait requires knowing where the mismatch occurs.
To distinguish one mismatch from another, Thorp and his colleagues use "inorganic mediators," metallic compounds that seek out the guanine and try to plug themselves into the DNA. Because the compounds never fit precisely into the double helix, they extract electrons from the guanine across a gap, the length of which depends upon the type of mismatch. The varying rates of electron transfer as the guanine oxidizes can be captured using electrodes and computers. The technique is so precise, Thorp says, that it can detect the mutation of a single base.
Because detecting genetic markers is a challenge on the frontiers of virtually all of the life sciences, the potential applications for his invention, Thorp says, are "endless." The technology will develop most rapidly, he predicts, for use in blood screening, and in areas such as oncology and pediatrics, where it could help many millions of patients.
"The market is huge," Thorp says. "Half the doctors' visits in the world are for infection. And there are so many rhinoviruses, for example, that nobody can identify which one is causing the infection, so no one can make any money developing a drug for any one virus."
Despite the well-publicized use of DNA tests in criminal trials, Thorp thinks forensic uses for his invention probably will develop later, as tag-alongs to medical applications.
"Forensic uses are not really lucrative," Thorp says. "If you have a forensic test for matching DNA, you're only talking about testing a few criminals here and there. But if you can screen for hepatitis and HIV, then you might screen the whole world's blood supply."
But first, Thorp says, there are several technical hurdles to cross. And he is careful to explain that his invention represents one part of an enormous scientific endeavor. As he has been perfecting his detection method, a number of companies have been working on other necessary pieces of the equation-the isolation of nucleic acids from samples, for instance. Another massive effort will be required to establish genetic profiles for each organism or substance that scientists and physicians wish to detect.
But the long-awaited test for DNA markers is well on its way, Thorp says. "People are definitely going to get there. We just hope to have a part."
The "we" he refers to includes Dean Johnston, now at Otterbein College, Katherine Glasgow of UNC-CH, and a number of other colleagues and graduate students. Thorp notes especially the work of Mark Wightman in the Department of Chemistry, who is, Thorp says, "a world leader" in the kind of miniaturization involved in the detection technology. And basic research on metallic compounds, pioneered at UNC-CH by Tom Meyer and Royce Murray of the Department of Chemistry, gave Thorp much of the necessary knowledge about electrochemistry and inorganic mediators.
Fortunately, Thorp says, these basic studies of metallic compounds converged with his own fundamental interest in the chemical reactivity of DNA, which he had studied for its role in cancer. From this work, he had become more and more impressed with the importance of guanine.
"Guanine is really the criminal in mutagenesis because it's so easily oxidized," Thorp says. Because the reactions important in mutagenesis are also important in chemotherapy or radiation therapy, a method for detecting changes in guanine would have implications for both diagnosis and treatment.
So it was this convergence, the intersecting paths of basic studies, that led to Thorp's invention. And while it is exciting to watch his work make its way into the marketplace, the real satisfaction, he says, is the basic research, the intellectual challenge of understanding the chemistry of DNA.
"Even after the applications are come and gone," he says, "I'll still be interested in this kind of research."
Last modified: 5/20/96