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Researchers at the Johns Hopkins University have developed a new method for finding specific sequences of DNA by making them light up beneath a microscope. The researchers, who say the technique will have important uses in medical research, demonstrated its potential in their laboratory by detecting a sample of DNA containing a mutation linked to ovarian cancer. As reported in the November 2005 issue of Nature Materials, when a laser shines on a quantum dot, the QD can pass the energy on to a nearby molecule, which in turn emits a fluorescent glow that is visible under a microscope. But quantum dots alone cannot find and identify specific DNA strands. For that, the research team, led by J.T.-H. Wang, used two biological probes made of synthetic DNA. Each of these probes is a complement to the DNA sequence the researchers are searching for. Therefore, the probes seek out and bind to the target DNA. Each DNA probe also has an important partner. Attached to one probe is a Cy5 molecule that glows when it receives energy. Attached to the second probe is a biotin molecule, which sticks to the streptavidin coating the surface of the quantum dot. To create their nanosensor, the researchers mixed the two DNA probes, plus a quantum dot, in a laboratory dish containing the DNA they were trying to detect. The two DNA probes linked up to the target DNA strand, holding it in a sandwichlike embrace. Then, the biotin on one of the probes caused the DNA “sandwich” to stick to the surface of the quantum dot. When the researchers shined a laser on the mix, some of the laser light was absorbed in the quantum dot, which passed the energy on to the Cy5 molecule attached to the second probe. The Cy5 released this energy as a fluorescent glow (see Figure 1). If the target DNA had not been present in the solution, the four components would not have joined together, and the distinctive glow would not have appeared. Each quantum dot can connect to up to about 60 DNA sequences, making the combined glow even brighter and easier to see. “Conventional methods of finding and identifying samples of DNA are cumbersome and time-consuming,” said Wang, an assistant professor in the Department of Mechanical Engineering and the Whitaker Biomedical Engineering Institute at Johns Hopkins. “This new technique is ultrasensitive, quick, and relatively simple. It can be used to look for a particular part of a DNA sequence as well as for genetic defects and mutations.” To test their technique, the researchers obtained DNA samples from patients with ovarian cancer and detected DNA sequences containing a critical mutation. “This method may help us identify people at risk of developing cancer, so that treatment can begin at a very early stage,” Wang said. RESEARCH/RESEARCHERS

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