Advances in Touch DNA Forensics: Where Are We Now and What Does the Future Hold?

Most recently seen in the news to identify the perpetrator behind the murder of four University of Idaho students, the now widely used touch DNA forensics revolutionized the forensics industry when it was first introduced two decades ago.

To understand how far the technique has come, and how much more there is to go, Lab Manager sat down to chat with world-renowned Australian forensic researcher Adrian Linacre.

What is touch DNA?

First used in the early 2000s, touch DNA is essentially a trace of DNA left behind after human tissue or bodily fluid comes into contact with objects in a crime scene. Prior to touch DNA, forensic professionals relied on sufficient amounts of human blood, semen, or other human fluids (“about the size of a quarter,” according to an article published by Scientific American) to identify individuals connected to a crime scene.

“Now, we can generate DNA profiles from only a few cells,” emphasizes Adrian Linacre, PhD, professor of touch DNA forensics and chair in forensic DNA technology at the College of Science and Engineering at Flinders University in Adelaide, South Australia.

Cases that had previously stumped police departments for decades could finally reach closure; perhaps, the most famous examples include the cases of JonBenet Ramsey, and that of Adnan Syed, popularized through the podcast Serial.

“Everything has become more sensitive. That’s good in some ways, such as finding remaining material on a knife you have not used for a long time,” states Linacre, alluding to the recent break in the 2022 University of Idaho killings, where DNA traces from a knife sheath found at the scene of the crime matched the perpetrator’s DNA.

The downside, however, explains Linacre, is that when sampling a crime scene, the generated DNA profiles can be attributed to multiple people, many of whom might have been absent from the scene when the crime was committed.

Advances in touch DNA forensics

Linacre started his journey in forensics in Glasgow, Scotland. The city has experienced a considerable decline in crime in the past decade, according to an October 2022 Glasgow Times article.

Linacre worked in Glasgow from the 1990s to the early 2000s when the World Health Organization labelled Glasgow “the murder capital of Europe,” according to the Washington Post. To say Linacre had plenty of work would have been an understatement, so sampling was probably not a problem.

And yet, Linacre explains, “The question I often asked myself was, ‘Where do you sample?’”

In the absence of obvious DNA sources, it may be hard to know which area to target for collection—according to Linacre, forensic professionals were figuratively working in the dark.

Without high-quantity samples like blood traces, forensic operators would blindly swab high-likelihood areas such as light switches, doorknobs, or the odd can of diet cola. Such samples might not contain DNA because the area targeted for collection was not recent enough for DNA to be collected intact, or they may never have been a source of DNA at all.

This challenge could prove costly, as crime scene operators would only know whether there was genetic material once they got to the lab and finished processing the sample. But Linacre’s group had an idea: what if they could visualize the DNA with a dye?

Since the 1950s, scientists have been imaging DNA via nucleic acid-binding dyes such as ethidium bromide. There is only one problem: these dyes are known to cause genetic mutations.

In the absence of obvious DNA sources, it may be hard to know which area to target for collection—according to Linacre, forensic professionals were figuratively working in the dark.

Linacre’s group needed something safe to use because they wanted the new forensic tool to be easily adopted through the use of a spray, so the group turned to a Promega product called Diamond Nucleic Acid Dye (DD), a non-intercalating dye. In their 2018 Forensic Science International: Genetics paper, Linacre’s team demonstrated that a massively (at least 500 times) diluted DD spray could be safely utilized to image a fingerprint under fluorescent digital microscopy.

Within seconds of spraying a fingerprint on a slide, fluorescent green globs—skin cells—were visible under the microscope. Similarly, they also confirmed the presence of skin cells on a rope and then used collection tape to gather the cells.

Looking through the microscope, the rope no longer fluoresced, but the collection tape did—now peppered with little green dots indicative of DD-stained corneocytes, ready for DNA extraction and downstream sequencing. Perhaps for the first time in DNA forensics, the transfer of trace cells could be visualized from sample to collection material as a way of checking the thoroughness of sample collection almost in real time.

Now touch DNA was less elusive, but how could one be sure that DNA collected was enough to generate a profile? In 2020, Linacre’s team published a study on the limit of detection of touch (skin) and non-touch (saliva) deposits to answer that very question.

Studying the detection limits of touch and non-touch deposits

There are many variables that could affect sensitivity, including the natural state of touch DNA. Skin contact leaves behind the outermost layer containing cells known as corneocytes, which are anucleated, with mostly broken-down DNA. Much of corneocyte-derived touch DNA may not be suitable for amplification and downstream profile generation at the lab, according to their paper.

There are also many other factors like environmental exposure that can impact the persistence of touch DNA for forensic analysis. But what about at the lab? Would sampling and extraction lead to further loss of nucleic material?

Linacre’s team realized that such questions had not been properly assessed in the field, and they were vital—and perhaps most importantly, manageable—variables to unearthing the limit of detection of touch DNA.

Linacre’s group, led by PhD student Piyamas Kanokwongnuwut, set up experiments with saliva and touch samples and two different collection methods (tape and swab), followed by either direct PCR or DNA extraction and then PCR. Their data showed that the use of DNA extraction for both saliva and touch samples yielded highly variable DNA quantities, or significant loss.

With direct PCR, however, Kanokwongnuwut reproducibly amplified full DNA profiles from as little as 40 buccal cells after either swab or tape collection. Despite the benefits of direct PCR, more touch DNA samples were needed. —Sample collection did not matter here, she found. With swabbing, five times less corneocytes (about 800 cells) were needed to generate full DNA profiles; through tape collection, the minimum amount of corneocytes needed to output a full DNA profile was at least 4000.

These investigations showed that there are ways to increase the chances of success of generating full DNA profiles at the lab, despite the challenging nature of touch DNA, but of course the data have seeded even more questions for Linacre’s team. One such question is, how many times would a person have to touch something for their profile to be captured?

What challenges remain in touch DNA forensics?

It turns out that for some people, touching something once may be enough; this is because humans differ on how fast they lose the outermost surface of their skin due to sex hormones’ influence on epidermal differentiation, and other factors like infections (e.g., human papillomavirus impacts keratinocyte proliferation).

These people for whom touching a surface once may be enough to leave DNA traces worthy of a full DNA profile have been given the moniker “shedders” in the forensic community. In a study published in 2022 by Linacre’s group, “heavy shedders consistently generated informative profiles,” up to 80 percent of the time in some cases.

This, Linacre’s team has observed, is both due to shedding status and touch DNA loss associated with repetitive surface contact. Even for super shedders (those who shed more skin than the average person), constant contact results in cells coming off the touched area, which Linacre notes is useful information in cases of firearm offences where repetitive loading of bullets may take place.

Linacre’s team has also demonstrated that light shedders leave very little DNA behind, and their DNA traces may be often unreadable; however, this finding does not mean light shedder perpetrators can walk away undetected.

Scientists are working to extract information from the more abundant DNA of the microbiome, which varies individual to individual. According to Linacre, though still at the experimental stage, research into microbiomes has already been utilized in sexual assault cases, where transfer of specific microbe signatures may provide important clues on assailants.

One thing is for certain: As sample analysis has become more sensitive than decades prior, sample preservation and optimal collection will need to be continuously optimized. The field may also benefit from integration by adding information from microbes or external DNA sources to correctly identify each unique signature present at a crime scene as relevant or irrelevant to the crime committed.