March 20, 2006
Gold Nanoparticles Speed Gene Detection Techniques
Cancer is a disease caused by genetic mutations, so it should be no surprise that cancer researchers and clinical oncologists alike are eager for new, inexpensive methods that would enable them to rapidly and accurately identify mutations associated with cancer. Using gold nanoparticles, two groups of investigators have each developed gene-identification methods that may fill this need.
Research teams headed by Lloyd Smith, Ph.D., at the University of Wisconsin-Madison, and Robert Corn, of the University of California-Irvine, each published papers in the journal Analytical Chemistry that detail the two new approaches to rapid, quantitative gene identification. Both of the new techniques identify single-nucleotide polymorphisms (SNPs), the most frequent type of human genetic variation, with the help of DNA microarrays. In each case, sample DNA is added to a gene chip containing known sequences of DNA. Each of these known sequences corresponds to a known SNP associated with a disease. After any DNA in the sample binds to its chip-immobilized complement, the chip is washed to remove any unbound DNA.
In the Wisconsin team’s method, the DNA chip is then treated with an enzyme that degrades the ends of the bound DNA, leaving a free, reactive phosphate group that over the course of two subsequent biochemical reactions is labeled with a gold nanoparticle. Because of the techniques the researchers used, only those places on the DNA chip where a piece of sample DNA had been bound to a SNP-identifying DNA probe become labeled with gold nanoparticles. When the chip is then viewed under an electron microscope, the gold nanoparticles appear as white pinpoints on a black background. Commercial software can readily analyze the electron micrograph, providing an accurate tally of any SNPs that were present in the original sample.
The investigators note that their methodology improves labeling efficiency by as much as 15-fold over conventional analytical methods. In addition, the signal-to-noise ratio – a measure of how strong the analytical signal is compared to background signal – is some 4-fold higher using gold nanoparticles as the detection reagent.
The UC-Irvine group took advantage of the extreme optical brightness of gold nanoparticles to generate its highly sensitive SNP detection scheme. When irradiated with light, gold nanoparticles generate bright light of a different frequency by a process called plasmon surface resonance. In their method, Corn and his colleagues take the washed DNA chip and use an enzyme that attaches a short piece of DNA linked to a gold nanoparticle to the ends of the sample probe wherever it has bound to complementary pieces of DNA from the sample. Again, gold nanoparticles end up only where the DNA chip has identified the presence of a specific SNP in the original sample.
Using a plasmon resonance scanner, the investigators can easily spot which of the chip-based probes bound to sample DNA. To test their method, the UC-Irvine team successfully identified DNA containing point mutations, or SNPs, in the BRCA1 gene that are associated with breast cancer.
The Smith group’s work was detailed in a paper titled, “Quantitative detection of individual cleaved DNA molecules on surfaces using gold nanoparticles and scanning electron microscope imaging.” An abstract of this paper is available through PubMed.
The work from the Corn group was detailed in a paper titled, “Single-nucleotide polymorphism genotyping by nanoparticle-enhanced surface plasmon resonance imaging measurements of surface ligation reactions.” This paper was published online in advance of print publication. An abstract is available at the journal’s website.