Whodunnit?

John Humble escaped justice for nearly 30 years. But in October 2005, his game was up. Police, working with scientists at the UK Forensic Science Service (FSS), identified him as the hoaxer who sent letters claiming to be from the Yorkshire Ripper during the late 1970s. In March 2006 he was convicted of perverting the course of justice and jailed for eight years.

DNA evidence played a key role in the conviction, and the result ‘is largely due to advances in the technology used for DNA profiling,’ says one of the scientists who worked on the case. ‘It’s also a testament both to the power of techniques available to us, and to the huge investigative tool that is the National DNA Database.’

According to the UK Home Office web site, in 2004/05 the crime detection rate rose from 26% to 40% in cases where DNA was recovered. Home Office statistics show that the annual number of crimes detected where DNA scene-to-suspect links were made quadrupled from 8612 in 1999/2000 to 35 605 in 2004/05 and the number of DNA suspect-to-scene matches increased by 75% from 23 021 to 40 169 over the same period.

These figures don’t surprise chemists like Chris Pickford, who works in the Culham laboratory of LGC Forensics, a division of LGC, previously known as the Laboratory of the Government Chemist and formerly known as Forensic Alliance. ‘DNA technology has evolved very rapidly,’ he says. ‘This has led to much greater sensitivity when looking at small or difficult-to-extract samples.’

In the past much larger samples were required, but now full profiles can be obtained from minute spots of fresh or dried blood that are barely visible to the naked eye or from a single hair root. The most common method used involves examining how often base pairs in short tandem repeats (STRs), regions of DNA that vary greatly from individual to individual repeat in specific loci, or locations, on the DNA strand.

In the UK, the STRs at, typically, ten loci are targeted using short stretches of DNA called primers which earmark the particular DNA region for copying or amplification. After amplification by the polymerase chain reaction (PCR), the resulting DNA fragments are then separated using electrophoresis. This produces a profile or ‘fingerprint’ -- three ‘electrophoretograms’ often displayed vertically one above the other -- analogous to a bar code.

Roadside DNA testing
Currently, DNA profiles take two to three days to produce. But some forensic chemists believe ‘we are only a few years away from a laptop system that could produce a profile in half an hour or so’. The key to this roadside DNA testing, they say, lies partly in developing new on-chip technologies and solid state detectors that respond to changes in the code at certain DNA loci, and partly in optimising processes that are already in use.

New methods of generating and analysing DNA profiles are being developed all the time. Many of these techniques make it possible to obtain profiles from what might seem to be very unpromising material. Where only tiny amounts of DNA are recovered, for example, low copy number (LCN) analysis, which enhances the amplification of the DNA by increasing the number of cycles used in PCR to obtain a profile, can be used. LCN played an important role in helping to find the killer of the British backpacker, Peter Falconio in Australia when, in December 2005, scientists from the Forensic Science Service (FSS) used LCN to obtain a DNA profile from the hand ties used to restrain Falconio’s girlfriend. The profile matched that of Bradley Murdoch, who was later convicted of the murder.

Use of single nucleotide polymorphisms, or SNPs, to assign different personal characteristics is another recent development that is rapidly gaining in importance. A SNP for red hair has been identified within the last few years, and the use of SNPs to identify ethnic origins is also being explored. But some chemists working in this area urge caution. ‘Whether society would allow you to use SNPs in this way is another question. There is always the need to balance what is scientifically possible against individual freedoms, and the ethics of such uses have yet to be fully examined and accepted by society.’

Although SNPs have not yet been shown to be capable of giving good results on mixed profiles – samples where the DNA comes from two or more individuals – primers developed to target STRs on the Y, or male, chromosome make it possible to home in on the DNA profiles of males. This is particularly helpful in rape cases. For example, in 2005, scientists at Forensic Alliance were able to use this technique to provide the necessary evidence to convict a rapist in Lancashire, UK.

Even where full profiles from nuclear DNA cannot be obtained, forensic chemists can still extract useful evidence using alternative mitochondrial DNA. Mitochondria are the powerhouses of living cells and are present in much higher numbers than cell nuclei, improving the chance of obtaining a profile.

But reliance on mitochondrial DNA does have some disadvantages. Mitochondrial cells are only inherited from the mother, and because the mitochondrial profile is common to all female descendents it will be much more common in a population, so less discriminating. And because the UK National DNA Database is based on profiles from nuclear DNA, it does not include mitochondrial DNA profiles, so mitochondrial DNA evidence cannot be used with the DNA database profiles to identify individuals. However, these profiles are useful when it comes to eliminating or including someone in the inquiry. There are currently a number of cases going through the UK courts where mitochondrial DNA evidence gathered by LGC Forensics is playing an important role.

Techniques such as familial searching, meanwhile, can pinpoint individuals who have similar profiles. In 2003 familial searching played a crucial role in the UK conviction of Jeffrey Gafoor for the murder of Lynette White in 1988. To obtain the necessary evidence, Forensic Alliance scientists obtained a profile from a tiny spot of blood that had been missed in the original investigation. This matched a profile on the database belonging to a boy who would have been too young at the time to commit the offence. But, by using familial searching, the scientists were able to reveal a line of enquiry that pointed towards the boy’s uncle, Gafoor. He then confessed to the killing.

Protein power
Even when no DNA match – familial or otherwise – can be established, tell-tale traces of proteins left in blood samples and saliva found at crime scenes can provide useful clues about the lifestyle and health status of a perpetrator. They can even provide clues about certain individual characteristics. In common with other biological fluids, the composition of blood changes as the proteins degrade when subjected to different environmental conditions. The results from traditional protein affinity assays, such as ELISAs, are affected by protein degradation.

However, a team led by Mikhail Soloviev at Royal Holloway College, London, is attempting to overcome the problem by basing their assay on peptides, obtained by digesting the original protein sample material using proteolytic enzymes. The sample sizes required are very small -- nanomol amounts are all that is required for analysis using protein arrays. Peptides themselves are less susceptible to degradation than the overall protein molecule, so the group is able to sidestep many problems related to sample collection or storage. The technique -- which is still under development -- should be useful where fresh samples are not available.

The group expects the assay to be useful in routine diagnostics and medical testing. But in forensic work the information from protein assays could provide vital clues about whether the sample was made up, say, of menstrual blood, or whether it resulted from a wound. The technique also offers other advantages, as ‘by relying on arrays, rather than techniques such as mass spectrometry, peptidomics assays can be carried out cheaply and in the field’, says Soloviev.

Crime watch
Staying ahead of the criminals requires continuous development, forensic scientists say. ‘Thanks to television programmes about crime, criminals are more forensically aware than ever before. And they, too, can take advantage of their increasing knowledge about DNA profiling. As forensic scientists, we always need to keep one step ahead.’