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Liquid lasers detect cancer genes

BioPhotonics
Apr 2012

ANN ARBOR, Mich. — Genes linked to cancer and other diseases may now be simpler to detect with a new liquid laser technique that clearly distinguishes between mutated and healthy DNA.

Researchers at the University of Michigan say their method works better than the current approach, which uses fluorescent dye and other biological molecules to find and bind to mutated DNA strands. Their findings were reported in Angewandte Communications (doi: 10.1002/anie.201107381).


Researchers have developed a highly sensitive technique based on laser emission for differentiating a target DNA strand from strands that contain single-base mismatches. Laser emission is used to amplify the small difference in signals that are generated by the different strands after they bind with a molecular beacon. The conversion is similar to analog-to-digital. Courtesy of Christopher Burke.


“Conventional fluorescence works in a sort of analog mode,” said Xudong Fan, an associate professor in the department of biomedical engineering and principal investigator of the project. “[A] small binding difference between the target DNA and the single-base-mismatched DNA for the probe would yield a small difference in the fluorescence signal.”

In contrast to the conventional method, the optofluidic laser performs digital detection to amplify the fluorescent signals. The ratio between laser emission and fluorescence can be as large as 10,000:1 (experimentally, the team achieved 200:1), which allows the researchers to pick up the target DNA from a mixture of single-base-mismatched DNA with 50 times higher concentration, Fan explained. “We found a clever way to amplify the intrinsic difference in the signals.”

To amplify the difference, he did a bit of backtracking.


Using this liquid laser, University of Michigan professor Xudong Fan developed a highly sensitive technique for identifying mutated DNA that differs from healthy DNA by a single base. The fine white horizontal line is the capillary cavity that enables the laser to amplify the intrinsic difference in the light signals from healthy and mutated DNA. Courtesy of Nicole Casal Moore.


Liquid lasers, discovered in the late 1960s, amplify light by passing it through a dye, rather than a crystal, as do solid-state lasers. Fan has worked to develop these for the past five years. In his setup, the signal is amplified in a glass capillary called a ring resonator cavity.

Typically, conventional detection is performed in a vial, but in Fan's work, the detection was performed in a liquid laser. The laser acts as a sample holder and, more importantly, provides strong optical feedback to amplify the small difference in fluorescence, he said. Last year, his team discovered that it could use DNA to turn a liquid laser on and off. At the time, it had no practical applications in mind.

The group began to investigate what was causing the different laser outputs, using its findings to detect differences in the DNA.

“I had an intuition, and it turns out the output difference was huge,” Fan said.

The discovery could advance the understanding of the genetic basis of diseases and also prove useful for applications in personalized medicine, which aims to target drugs and other therapies to individual patients based on a thorough knowledge of their genetic information.

“We are now using linear DNA probes in conjunction with saturation dye,” Fan said. The detection is much more straightforward and cheaper than when they used molecular beacon probes in their experiments.

“In addition, we are also including on-chip PCR [polymerase chain reaction] and DNA melting analysis to analyze longer DNA (hopefully hundreds of bases) more precisely,” he added.


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