Contamination of forensic DNA evidence in the light of Hungarian court decisions – A review of 25 years

The evaluation of forensic DNA expert opinions (in some countries expert witness testimonies) and the way it affects criminal judgement is of paramount importance. We have selected one of the largest challenges when it comes to the evaluation of forensic DNA evidence, contamination of DNA samples, and examined how it influences the decisions judges make about the credibility of DNA evidence in Hungary.     

The problem of DNA contamination in forensic case work—How to get rid of unwanted DNA?

The PCR technique has become a powerful and very sensitive tool in a broad field of research, that is, molecular biology, medical diagnostics, population genetics, ancient DNA analysis and forensic casework.However, the high sensitivity down to single molecules can easily cause false-positive PCR results due to different types of contamination. In this study, artificial DNA contaminations (saliva and pure DNA) were treated with UV irradiation and other decontamination procedures. A satisfactory DNA removal could not be achieved, emphasizing the necessity of contamination avoidance.     

Trace DNA success rates relating to volume crime offences

In this study, 252 trace DNA samples (from handled surfaces) from 201 burglary, robbery and drugs cases were compiled to assess success rates and to interpret the value of trace DNA evidence in volume crime investigations. The average amount of DNA recovered from the trace DNA samples collected was 1.7 ng. Full or major (12 or more alleles) profiles were recovered from 14% of samples. Samples from firearms and burglary points of entry were the least successful. Mixtures were recovered from 21% of samples, presenting a case for the collection of more elimination profiles to enable more samples to be used for database purposes. The research highlighted the difficulties in collecting data relating to the success rates of samples. Computerised automation of this process would be extremely beneficial in the assistance of policy development, method application, training, and investigative usefulness.     

Evaluation of the use of freezer mill to improve DNA retrieval from dried cotton swabs

This investigation evaluated the use of a freezer mill to improve retrieval of DNA from dried cotton swabs (using 100 μl saliva) compared to uncrushed swabs and whole saliva by measuring DNA yield and profile average peak height. Three treatments were tested; short, medium (as for a bone sample) and extended. The samples subjected to the freezer mill had the powder that remained on the freezer mill components collected (using a water and agitation method). All freezer mill samples returned a lower average DNA yield than either uncrushed swabs or whole saliva. The powder from the crushed swabs comprised an average of 35% of the total DNA yield, whereas the powder from the components comprised 65%. Allele drop-out was observed in samples exposed to extended treatments. Both short and medium treatments provided significantly higher peak heights than uncrushed cotton swabs with equal quantities of DNA (P < 0.05). Using a freezer mill on dried cotton swabs does not increase the DNA yield. This investigation suggests collecting powder from the freezer mill components will increase DNA yield, especially from trace samples.     

The effect of NucleoSpin® Forensic Filters on DNA recovery from trace DNA swabs

Forensic DNA profiling is a globally accepted method for human identification, however, obtaining full DNA profiles from trace DNA can be challenging. The optimal recovery of DNA from trace DNA swabs is therefore crucial. Methods for extracting DNA from swabs often make use of a spin basket combined with a centrifugation step, to enhance the release of cells from the swab prior to DNA extraction. The NucleoSpin® Forensic Filter (Macherey-Nagel, Düren) is a type of spin basket, but it has not been thoroughly assessed on trace DNA samples. This study aimed to assess if the inclusion of the NucleoSpin® Forensic Filter significantly improved DNA recovery and DNA profiling success from cotton and flocked swabs used to collect trace DNA and buccal cells (control). Buccal cells and trace DNA samples were collected from 25 volunteers using each swab type (cotton and flocked) in duplicate. DNA was extracted from the samples using the NucleoSpin® DNA Forensic kit, one set with, and the other set without, NucleoSpin® Forensic Filters. DNA concentration was assessed using real time PCR, and DNA profiling was done using the PowerPlex® ESX 16 system. The inclusion of the NucleoSpin® Forensic Filters significantly improved DNA concentration for buccal cells that were collected using flocked swabs (p = 0.035). However, no significant differences were noted for trace DNA samples for either swab type. There was also no significant difference in DNA profiling success when NucleoSpin® Forensic Filters were used, regardless of swab and sample type. These results may be helpful for laboratories that are considering the NucleoSpin® Forensic Filters in the DNA extraction workflow, particularly for trace DNA samples.     

The transfer of touch DNA from hands to glass, fabric and wood

The transfer of DNA from hands to objects by holding or touching has been examined in the past. The main purpose of this study was to examine the variation in the amount of DNA transferred from hands to glass, fabric and wood. The study involved 300 volunteers (100 for glass, 100 for fabric and 100 for wood) 50% of which were male and 50% female. The volunteers held the material for 60 s. The DNA was recovered from the objects using a minitape lift, quantified using the Quantifiler kit assay, extracted using a ‘Qiagen^® QIAamp DNA mini kit’ and amplified using the AmpFlSTR^® SGM Plus™ Amplification Kit at 28 cycles. The results show that using ANOVA there was a significant difference (F = 8.2, p < 0.05) between the three object types in the amount of DNA recovered. In terms of DNA transfer and recovery, wood gave the best yield, followed by fabric and then glass. The likelihood of success of obtaining a profile indicative of the holder was approximately 9% for glass samples, 23% for fabric and 36% for wood. There was no significant difference between the amount of DNA transferred by male or female volunteers. In this study good shedder status, as defined by obtaining useful profiles of 6 or more alleles, is estimated at approximately 22% of the population. The phenomenon of secondary transfer was observed when mixed DNA profiles were obtained but the incidence was low at approximately 10% of the total number of samples. DNA profiles corresponding to more than one person were found on objects which had been touched by only one volunteer. Although secondary transfer is possible the profiles obtained from touched objects are more likely to be as a result of primary transfer rather than a secondary source.     

Investigation into the usefulness of DNA profiling of earprints

DNA profiling of biological trace evidence has been used for many years. The application of this technique specifically to the DNA profiling of earprints has not to date been thoroughly investigated. This report presents the results of 60 earprints collected from three healthy adult volunteers under controlled laboratory conditions. DNA profile analysis revealed that high levels of non-donor alleles are observed when earprints are collected for DNA profiling. The source of these non-donor alleles is investigated and the impact that their presence within the profile may have on the use of this technique is discussed.     

Simulating transfer and persistence of a chemical marker powder for Lycopodium clavatum spores

In this research a chemical marker powder, based on Lycopodium clavatum spores, was studied to determine its transfer and persistence on a T-shirt. Such chemical marker powders are used to provide evidence that a person has handled a covertly marked object, such as a drug package. The powder was found to transfer readily between a marked item and the person handling it. The powder was found to persist on a T-shirt for up to 13 h; however, there was only a very small amount of powder remaining at this time. The rate of loss of the L. clavatum spores was found to follow a decay curve. The largest decrease in spores from the T-shirt was seen in the first 2 h after the marked item had been handled.     

改良IPEP法用于痕量DNA检测的实验研究

目的探讨改良扩增前引物延伸(IPEP)法对痕量DNA样本STR检测分型的效果。方法用改良IPEP法对痕量样品DNA进行全基因组扩增(WGA),扩增产物用实时荧光定量PCR技术定量、用AmpFLSTR~ Indentifiler~试剂盒作基因型检测。结果该方法可增加模板DNA约200~1100倍。基因组DNA不低于0.025ng时,可获得15个STR基因座和Amelogenin性别基因座的分型结果。基因组DNA0.01~0.025ng时,可获得9个以上基因座的分型结果。结论改良IPEP法可有效提高痕量DNA样本STR分型检验的灵敏度,有较好的实用价值。