Following the transfer of DNA: How far can it go?

The issue of DNA transfer is becoming increasingly important in crime scene situations, as DNA analytical techniques now detect tiny amounts. Whereas primary and secondary DNA transfers have been well studied, subsequent transfer steps have received much less focus. This study aimed to measure the detectability of a DNA source after multiple transfer events. Transfer of wet blood gave a full genetic profile well beyond the secondary transfer events on both cotton and glass substrates. Dry blood gave a full profile well beyond the secondary transfer events on glass only, but to a lesser extent than wet blood. Touch DNA only produced a full profile on the primary substrate on both cotton and glass, and detectable quantities beyond the secondary transfer event on glass only. Our results will contribute to a better understanding of the tertiary and subsequent transfer of DNA, which will allow for improved evaluation of the likelihood of alternative scenarios explaining why an individual's DNA was found at a crime scene.     

Recovery of human DNA profiles from poached deer remains: A feasibility study

Poaching is a crime that occurs worldwide and can be extremely difficult to investigate and prosecute due to the nature of the evidence available. If a species is protected by international legislation such as the Convention on International Trade in Endangered Species of Wild Fauna and Flora then simply possessing any part of that species is illegal. Previous studies have focused on the identification of endangered species in cases of potential poaching. Difficulties arise if the poached animal is not endangered. Species such as deer have hunting seasons whereby they can legally be hunted however poaching is the illegal take of deer, irrespective of season. Therefore, identification of deer alone has little probative value as samples could have originated from legal hunting activities in season. After a deer is hunted it is usual to remove the innards, head and lower limbs. The limbs are removed through manual force and represent a potential source of human touch DNA.We investigate the potential to recover and profile human autosomal DNA from poached deer remains. Samples from the legs of ten culled deer were obtained (40 in total) using minitapes. DNA from samples was extracted, quantified and amplified to determine if it would be possible to recover human STR profiles.Low quantification data led to the use of an extended PCR cycling protocol of 34 cycles. Samples from seven deer amplified, however some samples were excluded from further analysis due to ‘drop in’ alleles or the low level of successfully amplified loci. Samples from five deer could be further analysed and gave match probabilities ranging from 6.37 × 10^{− 3} to 9.53 × 10^{− 11}.This study demonstrates the potential of recovering human touch DNA from poached animal remains. There is the potential for this test to be used in relation to other species of poached remains or other types of wildlife crimes. This is the first time, to our knowledge, that human STR profiling has been successfully applied to touch DNA in regards to simulated wildlife crime.     

车辆内接触DNA检测方法的分析比较

目的探讨汽车内接触DNA的分离方法及遗传标记分型效率。方法收集单人长期驾驶的11辆小型轿车,采用粘取法和擦拭法富集方向盘、变速杆和手刹三个部位的脱落细胞,采用磁珠法和硅胶膜法提取基因组DNA,采用GoldenEye^TM 20A和PowerPlex■Fusion进行扩增,并对检验结果进行比较分析。结果方向盘在基因座分型正确率、等位基因drop-in和drop-out基因座比率、单基因座正确率以及单基因座等位基因drop-in和drop-out率六个方面均表现最好,其次为变速杆,最差为手刹;擦拭法和粘取法之间DNA提取在获得的DNA总量和STR检测正确成功率方面无统计学差异;PPFusion与20A的总体基因座分型比较正确率无差异,但单基因座正确率优于20A,drop-out发生率低于20A,drop-in发生率高于20A。结论汽车内脱落细胞的检测可优先采集方向盘部位,根据载体质地选择擦拭法或粘取法采集脱落细胞,选用硅胶膜法或磁珠法提取DNA,PPFusion和20A两个试剂盒均可,分析结果时需特别注意drop-in和drop-out。     

The use of the M-Vac® wet-vacuum system as a method for DNA recovery

Collecting sufficient template DNA from a crime scene sample is often challenging, especially with low quantity samples such as touch DNA (tDNA). Traditional DNA collection methods such as double swabbing have limitations, in particular when used on certain substrates which can be found at crime scenes, thus a better collection method is advantageous. Here, the effectiveness of the M-Vac® Wet-Vacuum System is evaluated as a method for DNA recovery on tiles and bricks. It was found that the M-Vac® recovered 75% more DNA than double swabbing on bricks. However, double swabbing collected significantly more DNA than the M-Vac® on tiles. Additionally, it was found that cell-free DNA is lost in the filtration step of M-Vac® collection. In terms of peak height and number of true alleles detected, no significant difference was found between the DNA profiles obtained through M-Vac® collection versus double swabbing of tDNA depositions from 12 volunteers on bricks. The results demonstrate that the M-Vac® has potential for DNA collection from porous surfaces such as bricks, but that alterations to the filter apparatus would be beneficial to increase the amount of genetic material collected for subsequent DNA profiling. These results are anticipated to be a starting point to validate the M-Vac® as a DNA collection device, providing an alternative method when DNA is present on a difficult substrate, or if traditional DNA collection methods have failed.     

Cell free DNA as a component of forensic evidence recovered from touched surfaces

In the course of a criminal investigation, DNA is often recovered from items that have been handled by an individual. Whilst there have been studies investigating the propensity of different individuals to deposit DNA, little is known about the factors involved in the transference of DNA through touch. This investigation seeks to clarify some of the underlying processes involved in DNA transfer, as to better understand the significance of so-called “touch DNA” evidence (tDNA). It was shown that an average yield of 11.5 ng of DNA could be recovered from 1 mL cell-free sweat samples leading to the hypothesis that cell-free nucleic acids (CNAs) of a suitable length for standard DNA profiling are transferred during handling/touching items. A method of standardization of tDNA deposition was developed to overcome the significant sample to sample variability in DNA levels characteristic of tDNA samples. The glass bead method allowed the creation of identical tDNA sample sets, thus permitting direct comparisons to be made in the efficiency of various extraction methods. Extraction methods designed to optimize CNA recovery from touched articles resulted in comparable yields in a general population study, however the methods resulted in a twofold increase in DNA yields from touched items touched by individuals with sweaty hands. These results suggest that the CNA component of touched surfaces should be included to maximize profiling success of tDNA.     

纸张上潜在手印三种前处理方法对DNA检验的影响

目的通过比较常见纸张上潜在手印盲提法与显现后精准提取法的接触DNA检出率,探讨常见纸张上接触DNA前处理的优选方案。方法比较五种常见纸张上使用盲提法和显现潜在手印(茚二酮显现法、茚三酮熏显法)后精准提取法采集的接触DNA样本检出率。结果粗糙日历纸盲提的接触DNA检出率为17.8%,通过茚二酮法和茚三酮法显现的潜在手印所提取的DNA检出率分别为75.6%、77.8%;光滑日历纸三种方法所提取的接触DNA检出率为4.4%、11.1%、11.1%;A4复印纸三种方法接触DNA检出率为20%、37.8%、66.7%;牛皮纸三种方法接触DNA检出率为20%、68.9%、64.4%;快递纸袋的三种方法接触DNA检出率为2.2%、6.7%、46.7%。结论不同纸张上潜在手印经显现后接触DNA检出率不同,通过茚二酮或茚三酮显示潜在手印后精准提取DNA的前处理方法相较于盲提法的接触DNA检出率高。实战中可应用此类方法同时获得手印与DNA分型,以有效提高证据力。     

Single source DNA profile recovery from single cells isolated from skin and fabric from touch DNA mixtures in mock physical assaults

The ability to obtain DNA profiles from trace biological evidence is routinely demonstrated with so-called ‘touch DNA evidence’, which is generally perceived to be the result of DNA obtained from shed skin cells transferred from a donor's hands to an object or person during direct physical contact. Current methods for the recovery of trace DNA employ swabs or adhesive tape to sample an area of interest. While of practical utility, such ‘blind-swabbing’ approaches will necessarily co-sample cellular material from the different individuals whose cells are present on the item, even though the individuals' cells are principally located in topographically dispersed, but distinct, locations on the item. Thus the act of swabbing itself artifactually creates some of the DNA mixtures encountered in touch DNA samples. In some instances involving transient contact between an assailant and victim, the victim's DNA may be found in such significant excess as to preclude the detection and typing of the perpetrator's DNA. In order to circumvent the challenges with standard recovery and analysis methods for touch DNA evidence, we reported previously the development of a ‘smart analysis’ single cell recovery and DNA analysis method that results in enhanced genetic analysis of touch DNA evidence. Here we use the smart single cell analysis method to recover probative single source profiles from individual and agglomerated cells from various touched objects and clothing items belonging to known donors. We then use the same approach for the detection of single source male donor DNA in simulated physical contact/assault mixture samples (i.e. male ‘assailant’ grabbing the wrist, neck or clothing from the female ‘victim’, or being in transient contact with bedding from the ‘victim’). DNA profiles attributable to the male or female known donors were obtained from 31% and 35% of the single and agglomerated bio-particles (putative cells) tested. The known male donor ‘assailant’ DNA profile was identified in the cell sampling from every mixture type tested. The results of this work demonstrate the efficacy of an alternative strategy to recover single source perpetrator DNA profiles in physical contact/assault cases involving trace perpetrator/victim cellular admixtures.     

Shedder status—An analysis over time and assessment of various contributing factors

The shedder status of a person is an important consideration when evaluating probabilities of DNA transfer during activity-level assessments. As an extension of our previously published study, the shedder statuses of 38 individuals were reassessed 1 year later. The study found that shedder status may change over time for some individuals and was associated with one's gender, number of items touched, and mobile phone usage. In 29% of touch events, no DNA allele was detected and in 99% of touch events, the amount of DNA deposited was <2 ng. The study also found that in 0.6% of touch events, the participant could be excluded as a contributor of the observed DNA profile, with another person being included. Additionally, our investigations suggest that the current three-category system for shedder status classification may require further refinement to better represent the individuals' shedder status in a population.