Low template DNA
Commercially available kits used in the analysis of DNA are optimised to produce DNA profiles from 0.5 to 2ng of sample when amplified for the standard PCR cycles. Samples containing less than 250pg could not previously have been analysed as they would not be sufficiently amplified to produce a good quality profile.
It is possible to analyse samples that are as small as 100pg by increasing the number of PCR cycles from the standard 28-30 to 34, as this provides the additional amplification required to produce a full DNA profile from small amounts of template DNA.
What is low template DNA?
Low template DNA or low copy number DNA (LCN) refers to samples that contain less than the 250pg (>100pg) required to produce a complete profile using the standard 28-30 cycles. LCN was launched into casework in the UK in 1999. The ability to gain profiles from such small samples has vastly widened the range of sample types that can be analysed.
Items or areas where no discreet stain is visible can be swabbed for 'touch' DNA meaning DNA that has been transferred from a person simply by them touching an item. It is also of use in degraded samples.
The Omagh controversy
Low template DNA hit the headlines in December 2007 due to the concerns voiced by Mr Justice Weir during the Omagh bombing trial and the investigation into the validity of LCN DNA that followed as a result.
Mr Justice Weir criticised the use of LCN DNA after a profile of a completely unconnected person was identified on some bomb casing. He voiced concerns about the amount of research into the technique and the safeguards present to prevent such occurrences.
The Association of Chief Police Officers subsequently suspended the use of 34 cycle DNA analysis in criminal investigations pending a review by the Crown Prosecution Service. An investigation into the 34 cycle PCR technique was then instigated.
The report that resulted, headed by Professor Brian Caddy, deemed LCN DNA fit for purpose and so following a brief, month long suspension the technique was reinstated in January 2008. The investigation did however conclude that, despite there being nothing to suggest that there were any problems with low template DNA evidence, the strength and weight of the evidence is still an area to be considered and should be presented to the jury with care and in light of all other evidence in that case.
Analysis of low template DNA
Low template DNA samples are analysed in much the same way as normal DNA samples. Due to the lower amounts of DNA being analysed, contamination is of even more concern than in traditional analysis. This has lead to laboratories having specific LCN DNA working areas to try to keep these areas as 'clean' as possible. As with regular DNA analysis, separate laboratories for pre and post PCR handling are required, as is full personal protection equipment usually comprising of gloves, lab coats, face masks and hair nets.
Low template DNA is extracted in exactly the same way as regular analysis using a variety of commercially available kits. Once extracted, samples are quantified to see how much DNA has been extracted from a swab/sample. This is particularly important to highlight samples that will require the extra cycles of PCR. Any samples that contain 0.5-2.0ng of DNA should produce full profiles without any additional cycles.
Those that contain less than 0.5ng of DNA or fail to produce a profile when amplified for 28-30 cycles are amplified for 34 cycles. The method only differs to the standard PCR cycle method in that samples are amplified for a further six cycles and are amplified and profiled in duplicate. To reduce spurious alleles - and to some extent certain stochastic effects - only alleles that appear in both duplicates can be included in a profile.