US scientists are catching tiny amounts of DNA in a drive to develop a test for enzyme activity linked to tumour growth.

Jaromir Ruzicka and co-workers from the University of Washington in Seattle, US, are looking to develop an automated assay for telomerase activity, which occurs in 90 per cent of human tumours. In what they call 'an essential step toward the realisation of this goal,' they have developed an assay that can detect as little as 111pg of DNA without having to amplify it.

Telomerase is an enzyme that acts on telomeres, TTAGGG nucleotide repeats at the ends of chromosomes. Each time DNA is copied before cell division, telomeres shorten because polymerases cannot copy all the way to the ends of the DNA strands. Once the telomeres reach a certain length, cells are signalled to stop dividing. But, cancer cells are essentially immortal because telomerase adds back TTAGGG repeats after each cell division, meaning that no stop signal is ever sent. The enzyme is therefore a potential target for antitumour therapies and so practical tests for its activity are needed.

Enlarged view of the flow cell port of the lab-on-valve

One way to test for telomerase activity is to monitor elongation of short DNA primers. The telomere repeat amplification protocol (TRAP) assay is currently the most widespread assay that does this, but it relies on the polymerase chain reaction (PCR) to amplify the DNA before detection. This makes it susceptible to contamination due to prolonged sample handling and false negative results due to PCR failure.

Ruzicka's assay captures biotin-bound DNA on polysaccharide beads coated in the protein streptavidin, taking advantage of biotin's high affinity for the protein. Using the beads allowed the team to pre-concentrate the DNA for very sensitive detection using OliGreen - a compound that fluoresces upon complexing DNA. 

Ruzicka explained that, by combining the bead method with instrumentation already developed by the team, their new DNA assay is not only sensitive, but can also be automated, bringing them a step closer to a practical, non-PCR alternative to the TRAP assay.

Freya Mearns