本文探讨了在法国一起刑事案件中，传统DNA测试无法区分同卵双胞胎的挑战，并介绍了全基因组测序、线粒体DNA分析和DNA甲基化分析等新兴科学方法如何帮助解决这一问题。
Identical twins on trial: can DNA testing tell them apart?
In a French criminal trial, conventional DNA analysis couldn’t distinguish between twin brothers, but emerging scientific methods could help in such cases
Conventional DNA testing failed to distinguish between twin brothers during a French criminal trial.
An identical twin commits a crime and leaves their DNA at the scene. Can DNA testing reveal which twin is the culprit?
This question reportedly came up in a case in France last month, in which the DNA of twin brothers was found on a gun, but conventional DNA testing could not determine which of the brothers it belonged to. The men are monozygotic twins — the result of a single egg splitting in two after being fertilized by a single sperm — meaning they have the same DNA.
When DNA is found at a crime scene, forensic scientists typically attempt to identify the person it belongs to using a technique called short tandem repeat (STR) analysis. The technique uses the polymerase chain reaction (PCR) method to amplify up to 30 specific regions of the genome that have a lot of genetic variation, says Brendan Chapman, a forensic scientist and researcher at Murdoch University in Perth, Australia.
These amplified regions are sequenced to determine the number of STRs and their base-pair patterns, which can then be compared against genealogy databases or STR profiles of suspects to identify potential matches. Ordinarily, Chapman says, “we can differentiate one person from the next very easily”. But with monozygotic twins, “there just is no difference” in those regions of the genome.
Whole-genome sequencing can make it possible to tell identical twins apart. By analysing the entirety of a person’s genome, scientists can identify differences caused by mutations that occurred after an egg split. These changes are rare, says Chapman. For instance, a 2014 study identified just five genetic changes in a pair of adult twins.
There have been rare cases in which sequencing of the entire genome has helped courts to distinguish between twins, says Xanthé Weston, a criminologist and researcher at Central Queensland University in Mackay, Australia. But using whole-genome sequencing requires obtaining a sufficient amount of DNA to analyse, adds Weston.
Other researchers have reported success in distinguishing between twins by sequencing DNA found in the mitochondria, the powerhouses of cells. Compared with nuclear DNA, which is used in genome sequencing and STR analysis, mitochondrial DNA (mtDNA) mutates more often, meaning it is more likely to differ between twins.
US courts have admitted mtDNA analysis as evidence since the mid-1990s. But it has not been used in cases involving twins, says Weston.
Another promising technique looks at the addition of methyl groups to DNA, a process called methylation, which can change how genes work. These ‘epigenetic’ changes can be caused by factors including a person’s behaviours — such as their diet and drinking or smoking habits — and their environment.
Last year, a team of scientists in South Korea sequenced the genomes of 54 pairs of newborn monozygotic twins. After identifying 711 sites at which methylation commonly differed, they selected and tested various combinations and 5 that were particularly variable. By comparing how much methylation had occurred at these five sites, the team was able to differentiate between 50 of the 54 pairs of twins. When they repeated the experiment in two adult cohorts, they succeeded in differentiating between 41 of 47 pairs and 105 of 118 pairs of twins.
DNA-methylation analysis has not yet been used to distinguish between twins in court. And Chapman notes that this kind of analysis might not be able to distinguish between twins who have similar variations caused by leading similar lifestyles or being exposed to similar environmental factors such as pollution.
The new techniques are promising, Chapman adds, but there are challenges to overcome before they can be used in forensic investigations. For instance, they all require relatively large amounts of DNA, but samples from crime scenes often comprise only small amounts or are in poor condition.
Analyses can also be expensive and time-consuming, partly because of the work needed to demonstrate to a courtroom that the results are conclusive enough to be counted as evidence. Last year, whole-genome sequencing was admitted in a US murder trial for the first time.
Weston says that it is important to combine DNA evidence with other types of evidence, such as fingerprints, when prosecuting someone, because there are many reasons why someone’s DNA might be present at a crime scene. “I would never want to see somebody prosecuted on the basis of DNA alone,” she adds.