It may be the gold standard of forensic science, but questions are now being raised about DNA identification from ever-smaller human traces. Natasha Gilbert asks how low can you go?When Peter Hoe was found stabbed to death in his home in North Yorkshire, UK, in the afternoon of 13 October 2006, investigators were able to connect the murder to brothers Terence and David Reed on the basis of a small amount of DNA lifted from shards of plastic found near the body. The men were convicted the next year.But an appeal to the ruling heard in 2009 raised questions about the reliability and interpretation of DNA profiles drawn from very small amounts of genetic material, a technique known as low-copy-number analysis. In the appeal, the Reeds’ lawyers argued that Valerie Tomlinson, an officer involved in the analysis at the Forensic Science Service (FSS) based in Birmingham, UK, had overstepped her bounds by speculating how the men’s DNA came to be on the pieces of plastic x97 thought to have broken off two knife handles. The appeal failed last December, but a larger question looms about how suspects can be fingered from such a small amount of DNA.The case is one of the most recent public airings of a highly charged debate in the science and law-enforcement communities about low-copy-number analysis. Some argue that profiles are not reproducible, are prone to contamination and lack a scientifically validated means for deciding on their accuracy. Moreover, the methods and procedures employed are shrouded in secrecy x97 leading some forensics researchers to demand that the interpretation of such profiles, if not the practice itself, be re-evaluated.x93DNA evidence is not flawless.x94Low-copy-number analysis is accepted in just a handful of countries including Britain and New Zealand, but it is being applied in many cases. The FSS says that it has used the technique in more than 21,000 serious criminal cases since its development in the late 1990s. Recent appeals affirming the validity of low-copy-number typing suggest that law enforcement may increasingly embrace the technique. Some suggest that its wider acceptance could threaten DNA profiling’s 20-year reputation as the gold standard of forensic science. Contrary to the public’s perception, “DNA evidence is not flawless”, says Peter Neufeld, co-director of the Innocence Project, an advocacy group in New York that has campaigned for broader access to DNA evidence to overturn wrongful convictions.British geneticist Alec Jeffreys developed DNA profiling in the 1980s. Profiles are drawn from short, repeating sequences of DNA scattered throughout the genome, called short tandem repeats (STRs). Because the number of repeats varies widely from person to person, the length of these STRs is also highly variable, meaning that by measuring several STRs x97 between 10 and 17 x97 forensic scientists can declare with calculable probability whether DNA left at a crime scene belongs to a suspect.A soupxe7on of cellsScientists employ the polymerase chain reaction (PCR) to amplify the STRs to detectable levels. The copies are then separated according to size by electrophoresis. The different-sized STRs show up as a pattern of peaks on an electropherogram that can be compared against a database or an individual.Standard DNA analysis requires about 200 picograms of DNA, 33 cells’ worth, or twice that for haploid sperm cells. Technicians can generally get more than enough DNA from visible samples such as blood or semen. But to produce profiles from just a few cells, scientists have developed ways of increasing the sensitivity of the analysis, including running more PCR cycles to copy more DNA or purifying the sample after PCR to remove unused reagents. In many cases the amount of starting material for low-copy-number analysis is unclear. The quantitation methods, also based on PCR, can suggest that no DNA is present, but the technicians will still be able to produce at least a partial profile. This sensitivity, although remarkable, has a downside.During analysis of any sample, large or small, random fluctuations occur that distort the results. In low-copy-number profiling, forensic scientists generally split their limited amount of DNA into two or three samples and run analyses on two of them. The third, if available, is reserved for the defence. The results of analyses aren’t completely reproducible, profiles often won’t match and the scientists generally accept as true those STR signals that show up in both runs. The practice is worrying, says Budowle: “The logic of this approach makes some sense, but the confidence in it has not been assessed.”Inherent bias?Common forensics practices can also lead to biasing errors, says Dan Krane, a molecular evolutionist at Wright State University in Dayton, Ohio. “Forensic scientists tell me that it is easier for them to distinguish between noise and what is really coming from the DNA if they have a reference sample to work with,” he says. Low copy number or not, that reference sample is often the suspect’s DNA x97 and faint peaks in a crime-scene sample might seem more convincing when viewed side by side with a strong peak from the suspect. Only a couple of labs in the United States, he says, require blind testing in their protocols.The courtroom is not the best place for the science of low-copy-number analysis to be debated, says Allan Jamieson, director of the Forensic Institute in Glasgow, which provides forensic-science services to UK police forces.Source: NatureFull article:http://www.nature.com/news/2010/100317/full/464347a.html