Chimerism, and How It Can Skew Criminal Investigations

Meet Chris Long, a leukemia patient who received a life-saving bone marrow transplant from an organ donor. Years after his procedure, however, it was found that swabs of his lips and cheeks contained both his DNA and that of his donor. Perhaps even more surprisingly, all of the DNA in his semen and blood belonged to his donor.

Long is a chimera: someone with two sets of DNA. There has only been roughly 100 confirmed cases of chimerism, with the most common cause being pregnant women absorbing cells from their fetuses or vice versa. Chimerism has been observed in twins and triplets as well, presumably because of their fetal proximity. As in Long’s case, blood transfusions can also confer chimerism onto a previously unaffected individual. Chimerism isn't very harmful and usually does not affect the individual’s day-to-day life. The condition’s relatively low profile has led to insufficient research on the subject, especially on its prevalence in criminal investigations and forensic science.

In recent decades, forensic investigation of crime scenes has enabled investigators to catch many elusive criminals and exonerate many wrongly accused suspects. The crux of many of these investigations lies in DNA and in the long-held belief that a single perpetrator leaves behind only a single genetic code.

But what if a chimera committed the crime? They would have also left behind the DNA of the innocent donor, possibly living thousands of miles away. Scientists call these individuals chimeric criminals: criminals who leave two sets of DNA in a crime scene. This brings about many questions but there are two that must be addressed: “Should the police worry that a suspect whose DNA does not match a crime-scene sample is a chimera?” and “Should we rethink some of the hundreds of DNA exonerations that have proved so important in pinpointing sources of error in the criminal justice system?” The situation proves more tricky and ambiguous when factoring in several studies that suggest chimerism being way more prevalent than we think it is. In their book “Genetic Justice,” authors Sheldon Krimsky and Tania Simoncelli, who are both heavily involved in cutting-edge scientific developments, maintain how “chimerism could be commonplace in the general population.” Widespread prevalence of chimerism could very well flip the well-established forensic scene upside down.

David H. Kaye’s article on chimeric criminals provides much-needed commentary on the mechanisms of chimerism and its implications on the field of forensics. He argues that the conclusion that chimerism “could undermine the very basis of forensic DNA system” is “as unfounded as it is unnerving.” His principal argument is that chimerism does not actually skew criminal investigations as much as we may think it does. He says that we should not consider all cases of chimerism because many of them can be categorized as “microchimerism,” with insignificant amounts of foreign DNA present in a crime scene. This cracks down on Krimsky and Simoncelli’s suggestion that most, if not all, of us could be chimeras. Microchimerism is mainly caused by the transfer of blood between mother and fetus or between fraternal twins. Blood transfusions cause temporary chimerism in the blood but do not deter the investigation because no other person will have that unique dual DNA type, and other samples like cheek and lip swabs will still overlap with the blood evidence enough to not exclude the suspect at hand.

Complete bone marrow transplants, as in Long’s case, cause functionally permanent chimerism and are impossible to identify in a simple physical check. In many cases, investigators have only circumstantial evidence to deduce which of the two people the DNA points to is the criminal. These confusions have already presented themselves in select cases in the past. For example, in 2008, a car accident in Seoul, South Korea led the country’s National Forensic Service on a weeks-long investigation to uncover the identity of the crash victim. However, blood examinations showed that they were female while the body and kidney appeared to be male. Spleen and lung examinations contained both types of DNA. It was a while before the investigators discovered the victim was indeed male, and that the female DNA was accounted for by the fact that he had received a bone marrow transplant from his daughter. Similarly, Kaye presents the “tetragametric chimerism,” caused by the fusion of twins (four sex cells) very early during development. Sex cells, even from the same parent, are virtually never the same, causing the resulting chimeric offspring to have tissues that do not match one another which may lead to false exclusion.

These chimerisms culminate into an overarching question for forensics. How common would it be for an individual to have a single cell line in a forensic tissue of interest (e.g. blood or semen) but not have the same cell line in cheek and lip swabs that provide reference DNA? Answering this would allow investigators to discern whether “chimeras are a rule rather than a rare exception.” Kaye has an answer, concluding that though chimerism is real and observable, it does not warrant an overhaul on established forensic procedures since it is only responsible for a small number of false accusations and exonerations. Nevertheless, the medical and legal communities must be equipped and ready to grapple with the questions surrounding chimerism even as forensic technologies improve and our understanding of chimerism advances.