Quantum dots catch cancer early

JOHNS HOPKINS (US)—Using tiny crystals called quantum dots, researchers have developed a highly sensitive test to look for DNA attachments that often are early signs of cancer.

“If it leads to early detection of cancer, this test could have huge clinical implications,” says Jeff Tza-Huei Wang, an associate professor of mechanical engineering at Johns Hopkins University. “Doctors usually have the greatest success in fighting cancer if they can treat it in its early stage.”

The target of the test was 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, making  it 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.

The 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.

The scientists mixed the DNA strands with quantum dots, crystals of semiconductor material only a few nanometers across in size typically used in electronic circuitry. A nanometer is one-billionth of a meter.

When light shines on a quantum dot, the dot quickly passes the energy along to a nearby molecule that can use the energy to emit a fluorescent glow. This behavior makes the cancer-related DNA strands light up and identify themselves.

The quantum dots were 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.

The test, which detects both the presence and the quantity of certain DNA changes could alert people who are at risk of developing the disease and could also tell doctors how well a particular cancer treatment is working, the researchers say.

The test represents a “very promising platform” to help doctors detect cancer at an early stage and to predict which patients are most likely to benefit from a particular therapy, says study coauthor Stephen B. Baylin, deputy director of the Johns Hopkins Kimmel Cancer Center.

The study, which included the detection of DNA markers in sputum from lung cancer patients, appeared to be more sensitive and able to deliver results more quickly than current methods.

“The technique looks terrific, but it still needs to be tested in many real-world scenarios,” Baylin says. “Some of these studies are already under way here. If we continue to see exciting progress, this testing method could easily be in wide use within the next five years.”

“This kind of information could allow a patient with positive methylation to undergo more frequent cancer screening tests and could replace the traditionally more invasive ways for obtaining patient samples with a simple blood test,” adds study coauthor Vasudev J. Bailey, a biomedical engineering doctoral student  “It’s also important because these test results could possibly help a doctor determine whether a particular cancer treatment is working. It could pave the way for personalized chemotherapy.”

In addition, because different types of cancer exhibit distinctive genetic markers, the researchers say the test should be able to identify which specific cancer a patient may be at risk of developing. Markers for lung cancer, for example, are different from markers for leukemia.

The study appears in the August issue of the journal Genome Research. The research was 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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