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With modern technology, the simple act of picking up an object or touching a surface can lead to the identification and apprehension of a criminal. In the past few years, not only have the number of touch DNA evidence items being submitted to the lab for analysis skyrocketed, but the number of journal articles regarding touch DNA and DNA transfer (both primary and secondary) has also increased greatly. This article is intended to update the reader on the latest touch and transfer DNA research and attempts to answer some of the most common questions that are asked regarding the topic.
What is “touch” DNA?
First, a review of what touch DNA is and how it arrives on an object. Touch DNA is simply DNA that is transferred via skin cells when an object is handled or touched. The average human sheds roughly 400,000 skin cells per day;1 however, since it is known that the top-most layers of skin are basically “dead”, being keratinized and having lost their nuclei,2 where does the touch DNA come from? Kita, et al,2 performed experiments which showed that small amounts of fragmented DNA are present on the surface of the skin and they theorized that these fragments of DNA may be constantly sloughed off the keratinized cornified layer of skin and that sweat may also contain fragmented DNA. Later research verified that the presence of sweat helps to contribute to the DNA profile obtained from touch DNA samples.3 This research also showed that cell free nucleic acids, or CNAs, (basically free-floating DNA fragments not encapsulated in the cell nucleus) contribute greatly to the total amount of DNA present in a sample with CNAs being detected in the sweat of 80% of healthy individuals tested. It was also found that, along with CNAs, nucleated cells were present in sweat samples taken from volunteers. Interestingly, most DNA extraction methods do not utilize the portion of the sample where CNAs are found—the aqueous portion of the extract—and after centrifugation to collect the cellular material, the supernatant (containing the CNAs) is generally discarded.3
How much DNA is left behind when an object is touched?
One of the most common questions asked regarding touch DNA is, “how much DNA is expected to be transferred” given a certain set of circumstances. Unfortunately, this is a difficult question to answer as there are so many variables involved. However, we can use the information from various studies to provide an idea of how much DNA might be recovered from touched objects (Table 1).
Is DNA always left on an object via touch?
It is important to note that not every contact leaves enough DNA behind to yield a DNA profile. Often the question is asked, “If a person touched this item, would they necessarily have left DNA behind?” The short answer is no, not always. Journal articles regarding the transfer of DNA have shown that DNA is not always transferred through contact alone. Lowe, et al10 found that 12 of 30 subjects transferred little to no DNA to a sterile tube after handling it for 10 seconds. A research article by Rutty11 showed that of 29 simulated strangulation samples, only 19 yielded DNA results. Of these, only 7 showed signs of any DNA not belonging to the victim. Phipps and Petricevic12 discovered that 51%–70% of individuals (depending on which hand was tested) failed to leave behind their DNA on a sterile tube that was held for 10 seconds. In addition, a study by Raymond, et al7 on trace DNA success rates noted that of 252 trace casework samples (all from surfaces touched by hands), 111 (44%) did not produce a profile.
It is important to take into account, however, the date the study was published and what methodologies were used at that time, as improvements in the technology and methodology of DNA analysis have arisen. For example, many of the earlier touch and transfer DNA articles report results from samples amplified with Profiler Plus and/or COfiler which are older amplification kits that often used a larger amplification volume (50 μl, as compared to the standard 25 μl reaction volume of today’s kits) and required a split of the sample extract into two amplification reactions. In addition, the majority of the earlier papers list extraction volumes of anywhere from 50 μl to 200 μl. It was not as common for the DNA extract to be concentrated to a smaller volume (as little as 10 to 12 μl) as it is today, so it may be that additional DNA profiles would have been developed in these earlier studies if this concentration step had been performed. In addition, many labs today have the option of using specialized techniques designed to maximize their DNA results. For example, in addition to concentrating one’s DNA extract, the analyst may also have the option to increase the injection time on the capillary electrophoresis instrument thus increasing the amount of DNA entering the capillary for detection. Additional Taq enzyme and the protein BSA (which helps overcome PCR inhibition) can be added to the amplification reaction. Some labs have the option to reduce the amplification volume even further (from 25 μl to 12.5 μl, for example), which has been shown to increase the sensitivity of the reaction. In addition, the amplified DNA can be subjected to post-amplification clean-up steps and additional amplified product can be added to the sample tube for injection. Each of these steps have the possibility of increasing the amount of DNA observed on the resultant electropherogram, but it is important to be aware of possible interpretational difficulties and stochastic effects that may arise from using these techniques.
What factors tend to increase the amount of DNA available for transfer?
Researchers aren’t exactly sure why sometimes DNA is transferred to an object via touch and sometimes it is not. However, some known factors that can affect the amount of DNA available for transfer include:
• Shedder status. Several studies have indicated that some individuals may be considered “good shedders,” someone who tends to slough or shed skin cells at a greater rate than others.10 However it should be noted that the use of the terms “good” and “bad” shedders has been debated as other studies have found that it is nearly impossible to determine someone’s shedder status as repeat tests on the same person on different days can give very different results.12
• Hand washing. Hand washing will remove many of the shed cells leaving little DNA available for transfer.
• Personal Habits. Some individuals touch their face, eyes, nose, hair, etc. more often than others, thus picking up DNA from those areas to be transferred onto the next thing that is touched. Wickenheiser1 described this as “loading” the fingers with DNA.
• Type of Contact. Factors such as pressure and friction can also relate to how much DNA is transferred onto a touched object. An increase in the amount of pressure applied tends to lead to an increase in the amount of DNA transferred and the application of friction to the contact increases the amount of DNA transferred even further.9
• Substrate. Rough surfaces (wood, concrete, grooved surfaces) tend to collect and retain skin cell DNA better than smooth surfaces.
• Perspiration. It is thought that sweat can increase the amount of DNA transfered for two reasons. First, as the sweat passes through the pores and makes its way to the skin surface, it can collect cells along the way and wash them to the surface.1 Second, it has been shown that sweat also contains both epithelial cells and cell-free nucleic acids.3
Once the DNA gets there, how long will it stay?
Another common question when dealing with touch DNA is, “how long does it last?” As with most questions relating to touch DNA, there is no easy answer. Very little study has been done to assess the persistence of touch DNA, but this is becoming an increasingly important area of research, particularly as defense attorneys begin presenting arguments to suggest that touch DNA found at a crime scene can be explained by the defendant’s presence at the scene at an earlier time that is unrelated to the crime at hand.
Consider the following scenario: A DNA profile matching a male suspect is located on a brick wall outside the home of a murder victim. The DNA appears to be from skin cells as it is negative for blood, semen, and saliva. How did the DNA get there? The prosecution’s theory is that the suspect fled the crime scene by climbing over the brick wall. Considering that blood evidence matching the victim is found nearby on the wall, this is a viable possibility. However, upon investigation, it is revealed that the suspect previously lived at the home in question and is, in fact, a relative of the murder victim. Could it then be possible that the suspect left his skin cell DNA at an earlier date when he either lived at the home or visited his relative? How can we determine which of the scenarios are accurate?
Only one study this author is aware of directly addresses the persistence of touch DNA. Raymond, et al,13 conducted an investigation into the persistence of DNA at crime scenes. They applied known quantities of “buffy coat” DNA (the white blood cell and platelet layer of whole blood) to gloss-painted wooden window frames, pieces of vinyl, and control samples. Not surprisingly, they found that the chance of recovering DNA from an outdoor crime scene decreases significantly over time with two weeks showing a significant drop in alleles detected for most of the samples. The control samples fared much better, with full profiles able to be developed even after six weeks (the longest time period tested). This study also provided a table of results obtained from actual touch DNA casework samples and included the time between the offense and the collection of the evidence. Great variability existed; however, it is interesting to note that two of the three evidence items with greater than 50 days between offense and collection yielded DNA profiles. One additional study dealing with direct amplification of touch DNA samples briefly discusses the stability of touch DNA on fabrics. Volunteers rubbed their thumb and forefinger between a sample of fabric for 5 seconds. The fabric samples were left exposed to light on a window ledge and then subjected to direct amplification. Nearly complete PowerPlex 16 profiles were able to be generated from touch DNA on acrylic, nylon, and polyester for up to 36 days after transfer.14
To assess whether it is possible that a touch DNA profile could have been left behind at a time prior to the alleged incident, it is important to gather as much information as possible. What is the item of evidence? Rough objects would be expected to collect more skin cell DNA as compared to smooth items. Does the item have cracks, crevices, or grooves where skin cell DNA might collect and be somewhat protected? An example might be a gun grip, buttons on a cell phone, or a computer keyboard. Was the item indoors (DNA expected to last longer), or was it outside exposed to the elements (heat, humidity, water, UV light, and bacterial growth all degrade DNA)? Was the item an object that may have been touched by many people (bank door handles, etc.) or only by very few (a weapon or a car steering wheel, for example)? It is impossible, however, to put an exact timeline on how long touch DNA evidence, or any biological evidence for that matter, might last. It is clear that additional studies are needed in order to help answer the questions regarding the persistence of touch DNA evidence that are often asked by investigators and attorneys alike.
Is it possible to tell who handled an item last or most?
Due to the variable nature of DNA transfer, it is generally not possible to determine who might have handled an item last or most. For example, just because one person’s DNA profile is more prevalent on an item like a cell phone, does not necessarily mean that they must have handled the phone last. In addition, it does not mean that the phone must be theirs because more of their DNA is present. It could simply mean that one of the factors that tend to increase the amount of DNA transferred (as discussed above) is in play. In fact, three studies have discussed instances wherein the person who touched an object transferred someone else’s DNA to the object.4,10,16
What sort of items might possibly contain touch DNA?
The sky is the limit as almost any item might conceivably contain touch DNA evidence. The key is to focus on those items that were most likely to be touched given a particular crime event. In addition, it is important to select items that have not been handled by multiple individuals as the DNA results may be uninterpretable due to the complex mixtures of DNA likely to be obtained. Investigators and analysts must use their experience and their common sense in order to choose the most useful and probative items to test for touch DNA.
- Wickenheiser, RA. Trace DNA: A review, Discussion of Theory, and Application of the Transfer of Trace Quantities of DNA Through Skin Contact. J. Forensic Sci. 47 (3) (2002) 442-450.
- Kita T, et al. Morphological study of fragmented DNA on touched objects. Forensic Sci. Int. Genet. 3 (2008) 32-36.
- Quinones I and Daniel B. Cell free DNA as a component of forensic evidence recovered from touched surfaces. Forensic Sci. Int. Genet. 6 (2012) 26-30.
- Daly DJ, et al. The transfer of touch DNA from hands to glass, fabric, and wood. Forensic Sci. Int. Genet. 6 (2012), 41-46.
- Bright, J and Petricevic SF. Recovery of trace DNA and its application to DNA profiling of shoe insoles. Forensic Sci. Int. 145 (2004) 7-12.
- Allesandrini, et al. Fingerprints as evidence for a genetic profile: Morphological study on fingerprints and analysis of exogenous and individual factors affecting DNA typing. J. Forensic Sci. 48 (3) (2003).
- Raymond JJ, et al. Trace DNA success rates relating to volume crime offences. Forensic Sci. Int.: Gen. Suppl. Ser. (2009) 136-137.
- van Oorschot RAH and Jones MK. DNA fingerprints from fingerprints. Nature. Vol 387 (1997) 767.
- Goray, M et al. Investigation of secondary DNA transfer of skin cells under controlled test conditions. Legal Medicine 12 (2010) 117-120.
- Lowe A, et al. The propensity of individuals to deposit DNA and secondary transfer of low level DNA from individuals to inert surfaces. Forensic Sci. Int. 129 (2002) 25-34.
- Rutty GN. An investigation into the transference and survivability of human DNA following simulated manual strangulation with consideration of the problem of third party contamination. Int. J. Legal Med. (2002) 116: 170-173.
- Phipps M and Petricevic S. The tendency of individuals to transfer DNA to handled items. Forensic Sci. Int. 168 (2007) 162-168.
- Raymond, JJ et al. Trace evidence characteristics of DNA: A preliminary investigation of the persistence of DNA at crime scenes. Forensic Sci. Int. Genet. 4 (2009) 26-33
- Linacre, A, et al. Generation of DNA profiles from fabrics without DNA extraction. Forensic Sci. Int. Genet. 4 (2010) 137-141.
- Ladd C, et al. A systematic analysis of secondary DNA transfer. J. Forensic Sci. 44 (6) (1999) 1270-1272.
- Djuric M, et al. DNA typing from handled items. Forensic Sci. Int.: Gen. Suppl. Ser. (2008) 411-412
Suzanna R. Ryan, MS, D-ABC is an owner/consultant with Ryan Forensic DNA Consulting in Carlsbad, CA. Ms. Ryan is a former DNA analyst and technical leader with over 13 years of experience in both public and private DNA laboratories. firstname.lastname@example.org