Source for feature photo: “Airman 1st Class Atiba Timley, 6th Medical Support Squadron laboratory technician” by Airman 1st Class David Tracy
As our technological savvy in DNA typing advances, researchers look to more difficult samples than DNA-rich blood and hair follicles to link a suspect to a crime scene, like “touch” samples. “Touch” samples are so-called because they originate from the skin cells that our bodies shed and deposit onto the things we touch.
The quantity of “touch” DNA depends on how often we interact with the item or surface. For example, items that we use everyday have more of our skin cells than items touched occassionally. Items touched by many people will have different amount of skin cells depending on how much it is touched by an individual and for how long. The amount of DNA left behind can also depend on the amount of skin cells that a person sheds; individuals range from poor shedders to high shedders, determined by the quality of the DNA profile produced; poor shedders tend to have low quality DNA profiles in comparison to the high quality DNA profiles obtained from high shedders. These characteristics create challenges when obtaining a DNA profile from the trace, or incredibly low, amount of DNA left behind from just a touch.
Normally, scientists identify who skin donors are by first extracting DNA from the sample, such as a bloodstain or hair follicle. However, extraction procedures may lose what little DNA there is. The authors of this study proposed skipping the DNA extraction step completely and directly amplifying the DNA. This is done through direct polymerase chain reaction (PCR).
In forensics, scientists are interested in short tandem repeats (STR) to generate an identity profile for an individual, like a barcode. This barcode, comprised of 20 genetic markers on the human genome, can be obtained from a sample at a crime scene and either include or exclude a suspect as the source of said sample. The genetic markers, known as loci, correspond to different locations on chromosomal DNA and can differ in length between individuals in a population. For example, one of these loci is called vWA, located on chromosome 12. vWA can be all the way to 390 nucleotides long. Since all humans inherit two sets of chromosomes, one from each parent, everyone has 2 different (or 2 of the same) sets of each gene. You can see that for a set of 20 genes, with a combination of two and upwards of 15 different nucleotide lengths, there are many different barcodes that humans can get – making DNA profiles a unique and very selective way to identify an individual. To identify how often someone has a certain locus repeat in a population, known as the allele frequency, scientists use PCR to amplify each loci’s nucleotide sequence millions of times. This allows the STR DNA nucleotides to produce strong and reliable signals during sequencing analysis, and scientists can correctly identify the number of repeats in each genome location.
DNA technologists use capillary electrophoresis (CE) to sequence the STR fragments i.e. identify the number and order of AGTC bases linked together that correspond to the loci. This is possible because CE can separate the different DNA fragment lengths based on how they move through the capillary; larger or longer fragments will move slower than smaller or shorter fragments.
Martin et al. tested common sources where touch DNA can be found in criminal cases, including evidence common at bombing cases such as fuses, insulated wires, and circuit boards. They collected “touch” DNA samples by swabbing the surfaces of these items and amplified the trace amount of DNA using direct PCR, analyzing via CE sequencing to generate a STR DNA profile. The authors also considered donors with different shedding ranges. Martin et al. compared the percentage of cases positively identified between their direct PCR method and the standard protocol (extraction first), using the same protocol and the most common commercially-available STR kits for CE sequencing, Identifiler and Globalfiler. Identifiler and Globalfiler are both used in forensic DNA flow-through with Identifiler able to identify 16 loci compared to Globalfiler which can identify 24 loci. The commercial developers believe that more loci increases the CE’s ability (and the forensic technologist running the CE) to recognize different lengths of STR fragments – a smaller percent of the profile is needed to make the profile informative.
They found that from just a touch, these kits could identify enough of the 20 required loci to generate an accurate and informative DNA profile without DNA extraction. To generate an accurate DNA profile, a minimum of 12 STRs need to be present. The authors demonstrated this for “touch” DNA samples in addition to proving that using Identifiler kits resulted in STR DNA profiles 100% of the time with more accuracy (more loci identified) than Globalfiler. (Fig 2) This means that when touch DNA is found at the crime scene, Identifiler has a higher probability of obtaining a usable profile than Globalfiler.
Another complication for obtaining profiles from miniscule amounts of DNA are when there are multiple DNA sources all mixed together. Mixtures create challenges when analyzing who touched an object and how that person is relevant to the case – as sometimes skin donors are not relevant at all! This is where major and minor contributor status comes in. The amount of skin cells allows scientists to determine whether a person was a major contributor (their DNA is the primary source found in the sample) or a minor contributor (their DNA is a secondary source found in the sample or in trace amounts). This can get tricky with touch DNA since it is most commonly found in trace amounts, and heavy shedders are mistaken as major contributors.
This study’s results indicate that DNA can be lost during the extraction process, leaving little to none that can be used for generating a DNA profile. Skipping the extraction step can produce more DNA for the amplification procedure, ensuring higher accuracy i.e. greater loci coverage in producing a profile. Touch DNA can play a major role in linking a suspect to a crime and can also specifically link a suspect to an object of interest, like the murder weapon, if no fingerprints are found. Though “touch” DNA normally means a low chance of successfully identifying a person, crucially, by using this new method, case analysts can obtain the most information possible with the small amount of DNA.
|Title||DNA profiles generated from a range of touched sample types|
|Authors||Belinda Martin, Renee Blackie, Duncan Taylor and Adrian Linacre|
|Journal||Forensic Science International: Genetics|