Credit: © 2009 NPG

Various methods exist to detect and quantify DNA and RNA molecules in biological samples — for example, binding of target molecules to single-stranded DNA probes on different surfaces have been measured by fluorescence or changes in surface stress. However, many of these techniques require nanomolar concentrations of the target molecules, and amplification schemes are necessary to detect femtomolar concentrations. Researchers at the Rowland Institute at Harvard University and Stanford Genome Technology Centre in California now show that attomolar levels of microRNA can be detected in tumour samples by measuring the changes in the stiffness of the probe molecules before and after binding to their microRNA targets.

Ozgur Sahin and colleagues1 immobilized single-stranded DNA on a gold substrate and allowed complementary target DNA molecules to bind before measuring the changes in stiffness with an atomic force microscope. The resulting stiffness maps showed that unbound DNA molecules were stiffer than those bound with target molecules, and computer analysis of the maps allowed the number of bound molecules to be counted. Down to one attomole of DNA — which is eight orders of magnitude better than current methods — could be detected with this approach. Moreover, only 200 pg of RNA (compared with 1 mg in conventional microarray experiments) is required when analysing samples extracted from bladder and colon tumour tissues.

This nanomechanical approach to detecting DNA and RNA is advantageous in that it requires only a small amount of genetic material without the need for further manipulations. Furthermore, it is compatible with multiplexed analysis in microarray platforms for high throughput.