Detecting specific DNA sequences that are markers for disease could be a boon for early diagnosis of cancer and identifying which treatment would be more effective. Researchers at Johns Hopkins have developed such a method using quantum dots and have now tested its effectiveness on the sputum of lung cancer patients. The results of the study so far have shown that the new technology is quite promising:
The target of this test is a biochemical change called DNA methylation, which occurs when a chemical group called methyl attaches itself to cytosine, one of the four nucleotides or base building blocks of DNA. When methylation occurs at critical gene locations, it can halt the release of proteins that suppress tumors. When this occurs, it is easier for cancer cells to form and multiply. As a result, a person whose DNA has this abnormal gene DNA methylation may have a higher risk of developing cancer. Furthermore, these methylation changes appear to be an early event that precedes the appearance of genetic mutations, another precursor to cancer.
To detect this DNA methylation, the Johns Hopkins team found a way to single out the troublesome DNA strands that have a methyl group attached to them. Through a chemical process called bisulfite conversion, all segments that lack a methyl group are transformed into another nucleotide.
Then, another lab process is used to make additional copies of the remaining target DNA strands that are linked to cancer. During this process, two molecules are attached to opposite ends of each DNA strand. One of these molecules is a protein called biotin. The other is a fluorescent dye. These partner molecules are attached to help researchers detect and count the DNA strands that are associated with cancer.
To do this, these customized DNA strands are mixed with quantum dots, which are crystals of semiconductor material whose sizes are in the range of only few nanometers across.
These dots are usually employed in electronic circuitry, but they have recently proved to be helpful in biological applications as well. Quantum dots are useful because they possess an important property: They easily transfer energy. When light shines on a quantum dot, the dot quickly passes this energy along to a nearby molecule, which can use the energy to emit a fluorescent glow. This behavior makes the cancer-related DNA strands light up and identify themselves.
In the Johns Hopkins cancer test, the quantum dots have been coated with a chemical that is attracted to biotin–one of the two molecules that were attached to the DNA strands. As a result, up to 60 of the targeted DNA strands can stick themselves to a single quantum dot, like arms extending from an octopus. Then, an ultraviolet light or a blue laser is aimed at the sample. The quantum dots grab this energy and immediately transfer it to the fluorescent dyes that were attached earlier to the targeted DNA strands. These dye molecules use the energy to light up.
These signals, also called fluorescence, can be detected by a machine called a spectrophotometer. By analyzing these signals, the researchers can discover not only whether the sample contains the cancer-linked DNA but how much of the DNA methylation is present. Larger amounts can be associated with a higher cancer risk.
Press release: New DNA Test Uses Nanotechnology to Find Early Signs of Cancer …
Abstract in Genome Research: MS-qFRET: A quantum dot-based method for analysis of DNA methylation…
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