Low Copy Number DNA and the Appeal Court Ruling in December 2009

Appeal Court Ruling December 2009

The use of DNA profiling has revolutionised the use of science in legal cases. DNA profiles produced from manufactured kits such as SGMPlus and Identifilier are routinely used as evidence in criminal cases. The kits are designed and validated by the manufacturer to operate within a specific range of amounts of DNA, typically 0.5 – 2.5ng (a nanogram is a 1,000th of a millionth of a gram). The kits work by copying (amplifying) the DNA molecules contained in a sample a number of times to produce enough to be detected in the analyser. The chemistry used by the kits is capable of amplifying just one molecule of DNA. By varying the conditions under which the kit is used some claim to be able to produce profiles from much lower amounts of starting (template) DNA. The general term for these techniques is variously referred to as Low Copy Number (LCN) or Low Template DNA (LTDNA) analysis, although some restrict the term LCN to the process used by the FSS Ltd which uses 34 cycles of amplification instead of the routine 28-cycle process recommended by the manufacturer. However, with very low numbers of template DNA molecules the process may fail to amplify the template. This leads to a number of problems in the interpretation of the resulting profiles. These are caused mostly by sampling, or stochastic, errors caused by the failure of the chemistry to work effectively with such low numbers leading to poor reproducibility of the results.

There has been a long-standing debate within the scientific community regarding the reliability of the LCN technique because of the presence of these stochastic effects. In the trial of Sean Hoey in Northern Ireland (the Omagh Bomb trial) The Forensic Institute, with Professor Allan Jamieson as the expert witness, are the first and only UK organisation to have seriously challenged the use of the technique in British Courts. The LCN technique was specifically designed to analyse amounts of DNA below 100pg and to produce reliable profiles even in the presence of stochastic effects (allelic dropout in particular). Professor Jamieson (in line with other eminent international experts) argued that the presence of these effects, which increase as the amount of DNA decreases, made the interpretation of these profiles unreliable.

The recent Appeal Court decision in Reed vindicates this view of the technique in endorsing the hallmark of reliability as the absence of stochastic effects and explicitly enabling challenges to what the Court would expect to be the ‘rare’ circumstances of evidence being led from cases involving between 100 and 200pg. By extension, although the Court expressed no explicit view, it may be expected that cases in the sub-100pg range will now be extremely rare or absent; in any event, they will invite challenge.

The Court considers, as stated in evidence by Professor Jamieson, that profiles produced with more than 200pg of template DNA, “can produce electrophoretograms which are capable of reliable interpretation”. However, according to the User’s Manual for the SGMPLus kit it is also a fact that the standard method produces reliable profiles with amounts as low as 250pg. At best, that would seem to leave a ‘useable range’ for LCN of about 50pg.

Other complex, and as yet unanswered, questions remain. Many LCN profiles are mixtures, containing small amounts of DNA from two or more people. Does the 200pg limit apply to the total template DNA or to one or other of the contributors which, if the total is 200pg must by definition be less than that? Presumably the answers will only emerge as we take on other cases to present the appropriate challenges yet again.

(The same Court expressed some concern regarding the expertise of Professor Jamieson. That topic is dealt with here)