Elements of Forensic Science – Low copy number DNA

Operation Liberal in the late 1990s was a West Midlands police operation involving multiple forces targeting doorstep crime, particularly aimed at fake official callers preying on the elderly.

At this time, Staffordshire police were piloting the use of Low Copy Number DNA, (LCN DNA), which was emerging as a scientific technique. One of the cases dealt with by Operation Liberal concerned that of an old lady who had money stolen whilst being distracted by a fake Water Company official.

The conman stated there was a problem with the water supply and he would need to turn off her supply for a period. Whilst she showed the bogus official where her water stopcock was, an accomplice entered her bedroom and stole her savings.

As part of the pilot study, the police decided to swab the stopcock for DNA. It was felt that a rarely used stopcock might be free from cross-contamination. In the event, a DNA sample from the stopcock was matched to one of the records on the national DNA database (NDNAD). A suspect was arrested, found guilty and received a substantial prison sentence. All happy? Perhaps not!

DNA profiling has revolutionised the use of science in criminal cases. In the UK, the standard profile is obtained using a system known as AmpFISTR®SGMPlus™, I kid you not, or SGMPlus. Without making the process overly complicated, DNA is separated from a sample, repeatedly copied (amplified) by a process known as a Polymerase Chain Reaction (PCR), then analysed using gel electrophoresis (that is the simple explanation). DNA profiles obtained from manufactured kits such as SGMPlus or Identifiler are routinely used as probative evidence in criminal cases.

Blood, semen, hair with roots, sweat, nasal mucus (snot) are good sources for DNA sampling. Skin cells, dandruff, urine & faeces are usually a poor source of nuclear DNA. To enhance the chances of a DNA match in tiny or poor quality samples of source DNA, the Forensic Science Service (FSS) came up with a controversial solution that uses 34-cycles of amplification instead of the customary 28-cycle process recommended by manufacturers of the kits above. In the simplest terms possible, a tiny sample (template) of DNA is bulked up (amplified) by repeated cycles until a sufficient amount for analyses is available. This process is known as Low Template DNA (LT DNA) or Low Copy Number DNA (LCN DNA).

The controversy surrounding LOW Copy DNA is due to the microscopic amount of a DNA sample required to obtain a profile. Theoretically, a full DNA profile can be built from a single cell, but usually, a sample of 0.5-2.5 of a nanogram is required for amplification of the Polymerase Chain Reaction (PCR) method. A nanogram is one thousand millionth of a gram (0.000000001g or 1x 10-9g) about 150 cells. LCN DNA has been specifically designed to analyse amounts of DNA below 100 pico-grams. A pico-gram is one million, millionth of a gram (0.000000000001g or 1 x 10-12g).

With such very low numbers of template DNA molecules, the amplification process may fail, leading to problems when interpretation a profile. Furthermore, sample cross contamination is a common, ever-present problem. Shake hands with someone and your DNA is transferred to their hand. If they then open a door, your DNA is now on the doorknob. Brush past someone in a pub, and your DNA is transferred to their clothing and vice versa. If that person is then the victim of a mugging, explain how your DNA came to be on their clothing.

The FSS has enthusiastically praised the claimed benefits of LCN DNA, but the process has significantly failed to be internationally accepted. Currently, Britain, Australia and the Netherlands are the only countries to consent to it being used as courtroom evidence.

Professor Dan Krane from the USA and professor Jamieson from an independent consultancy, the Forensic Institute, summarised the position as follows. ‘In scientific analyses it is essential that we can assess how much we can trust any result. This trust is based on knowledge of how often a technique gives the correct (i.e. true) value when used to measure something. The error rate for the LCN DNA method is unknown, but the lack of consistency in results is demonstrably high enough to cause serious concerns as to the reliability of this method’.

Professor Jamieson then went on to suggest a method of checking for error rates, by diluting a sample of a known profile in ever-smaller amounts over 1,000 test samples. To then run the LCN DNA system, and chart the results. Repeat the process with different samples until a statistical error rate is obtained.

The FSS initially claimed in high profile criminal cases such as the Omagh bomb trial that such dilution tests were unnecessary. Subsequently, the Forensic Institute discovered that the FSS had conducted dilution tests in secret, and requested the data under the Freedom of Information Act. The FSS would only release the data under strict confidentiality agreements preventing any publication or illustration of their results. You have to wonder why, if there are no significant errors in the process?

Professor Jamieson, together with other eminent international experts, concluded that the 28-cycle SGMPlus PCR procedure is validated satisfactorily. However, the extension 34-cycle LCN technique has not undergone any essential scientific testing and scrutiny to enable it to be considered a reliable evidential-quality process.

So what does this all mean? In essence, LCN DNA evidence given in court may not be as bulletproof as claimed. Can someone have an identical DNA profile (genotype) as you? Yes, if you have a twin. In criminal cases, all DNA profiles given as evidence are based on only 10 loci (points) of a genotype, with an additional sex-identifying locus, a protein, Amelogenin, giving a male XX or female XY indication. Yet, there are billions of loci in a full genotype. It is the immense quality of loci in a full human genotype that is repeatedly referred to in courtroom evidence in terms of odds of many billions to one. With a routine DNA profile based on a very limited number of loci, quoting enormous numbers is simply disingenuous.

Keith Rose is currently resident at HMP Whitemoor