Quantum dots used for cancer tests

August 18, 2009: Researchers at Johns Hopkins U. have developed a test for early-warning cancer signs using quantum dots, with potentially “huge clinical implications.”

The system, which detects both the presence and quantity of certain DNA markers in the sputum of lung cancer patients, and was found to be more sensitive and faster than conventional methods. More real-world testing is needed, but “if we continue to see exciting progress, this testing method could easily be in wide use within the next five years,” according to study co-author Stephen B. Baylin, deputy director of the Johns Hopkins Kimmel Cancer Center.

The specific target of the test is DNA methylation which occurs when methyl attaches to cytosine, a DNA building block. When this happens at specific gene locations it can stop the release of tumor-suppressing proteins; cancer cells then more easily form and multiply. Finding this gene DNA methylation, are thus seen as having a higher risk of developing cancer. It’s also seen as a warning sign of cancer-precursor genetic mutations.

The researchers’ detection method involves singling out the DNA strands with methyl attachments through “bisulfite conversion,” whereby all non-methyl segments are converted into another nucleotide. Copies of the remaining DNA strands are made, two molecules (a biotin protein and a fluorescent dye) are attached at either end, and the strands are mixed with quantum dots that are coated with a biotin-attractive chemical. Up to 60 DNA strands are attracted to a single quantum dot. A UV light or blue laser activates the dots, which pass the energy to the fluorescent molecules on the DNA strands which then light up and are identifiable via a spectrophotometer, which both identifies and can count the DNAN methylation.

In this illustration by Yi Zhang, quantum dots are depicted as gold spheres that attract DNA strands linked to cancer risks. When the quantum dots are exposed to certain types of light, they transfer the energy to fluorescent molecules, shown as pink globes, that emit a glow. This enables researchers to detect and count the DNA strands linked to cancer. (Source: Johns Hopkins)

Results, described in the paper published in the August issue of the journal Genome Research:

Key features of MS-qFRET include its low intrinsic background noise, high resolution, and high sensitivity. This approach detects as little as 15 pg of methylated DNA in the presence of a 10,000-fold excess of unmethylated alleles, enables reduced use of PCR (as low as eight cycles), and allows for multiplexed analyses.

Implications for the procedure include the ability to more frequently screen for cancer, replacing “traditionally more invasive” methods with a simple blood test, noted one of the study’s lead authors, doctoral student Vasudev J. Bailey. They could also help determine whether a cancer treatment is working, paving the way toward “personalized chemotherapy,” he added. Moreover, since different cancer types possess different genetic markers (lung cancer markers differ from leukemia, for example), the test should identify which cancer a patient is at risk of developing, the researchers note.

Johns Hopkins has applied for international patent protection covering the testing technique, and staff are “in talks” with an unnamed biotechnology company to license the application. The work is supported by grants from the National Cancer Institute, the National Science Foundation, the Hodson Foundation, and the Flight Attendant Medical Research Institute.


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