A quick, inexpensive, yet sensitive technique for finding cancer markers brings researchers closer to a "liquid biopsy" - a test that uses a small blood or serum sample to find cancer, rather than the invasive tissue that is routinely used for diagnosis.
Researchers at the University of Illinois developed a method for capturing and counting cancer-associated microRNAs or tiny pieces of messenger substances that are secreted from cells and can be detected in blood or serum with single-molecule resolution. The team published its results in the Proceedings of the National Academy of Sciences.
"Cancer cells contain gene mutations that allow them to multiply uncontrollably and avoid the immune system. Some of these mutations occur in microRNAs," said study leader Brian Cunningham, professor of electrical and computer engineering in Illinois. Cunningham also heads the Holonyak Micro and Nanotechnology Lab in Illinois.
"There are specific microRNA molecules that are known to be present and concentrated to be related to the presence and aggressiveness of certain types of cancer, so they are referred to as biomarkers that can be the target for a diagnostic test," he said.
Cunningham's group developed a technique called Photonic Resonator Absorption Microscopy to capture and count microRNA biomarkers. In collaboration with Professor Manish Kohli of the Moffitt Cancer Center in Florida, they tested PRAM on two microRNAs known as markers for prostate cancer.
They found that it was sensitive enough to detect small amounts in a patient's serum, but also selective enough to detect the marker under a cocktail of molecules that would also be present in the serum.
"One of the main challenges in biosensor technology is to maintain sensitivity and selectivity at the same time," said Nantao Li, PhD student and co-lead author. "You want it to be sensitive enough to detect very small amounts, but you don't want it to take every RNA in the blood. You want that specific sequence to be your goal."
PRAM achieves both qualities through the combination of a molecular probe and a photonic crystal sensor. The probe is very specifically linked to a specific microRNA and has a protective cap that is released when it finds and binds to the target biomarker. The exposed end of the probe can then bind to the sensor and generate a signal visible through a microscope.
Each individual binding probe sends a separate signal that the researchers can count. This means that researchers can detect much smaller amounts than conventional methods such as fluorescence, which have to cross a certain threshold in order to emit a measurable signal. The ability to count each biomarker also has the added benefit that researchers can monitor changes in the concentration of the biomarker over time.
"With PRAM, we squeeze a sample into a solution and get an ad within two hours," said postdoctoral researcher Taylor Canady, co-author of the study. "Other technologies that produce single-molecule readings require additional processing and steps, and they take a day or more to wait. PRAM seems to be clinically much more practical. In addition, using an optical signal instead of fluorescence we could one day build a miniaturized device that doesn't need a trained laboratory technician. "
The PRAM approach can be adapted to different microRNAs or other biomarkers and is compatible with existing microscope platforms.
"This approach takes the idea of doing a" liquid biopsy "for low-concentration cancer-relevant molecules one step closer to reality," said Cunningham. "This progress shows that it is possible to have an inexpensive and routine method that is sensitive enough to require just a drop of blood. The results of the test can give a doctor information about whether chemotherapy is working or whether it is getting better Cancer is developing a new mutation that would make it resistant to a drug, or whether a person who had previously been treated for cancer may have a remission. "
Each individual binding probe sends a separate signal that the researchers can count. This means that researchers can detect much smaller amounts than conventional methods such as fluorescence, which have to cross a certain threshold in order to emit a measurable signal. The ability to count each biomarker also has the added benefit that researchers can monitor changes in the concentration of the biomarker over time.
Journal Reference:
Taylor D. Canady et al, Digital-resolution detection of microRNA with single-base selectivity by photonic resonator absorption microscopy, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1904770116