The influence of substrate on DNA transfer and extraction efficiency

doi:10.1016/j.fsigen.2012.09.004

Forensic Science International: Genetics

Volume 7, Issue 1, January 2013, Pages 167-175

https://doi.org/10.1016/j.fsigen.2012.09.004 Get rights and content

Abstract

The circumstances surrounding deposition of DNA profiles are increasingly becoming an issue in court proceedings, especially whether or not the deposit was made by primary transfer. In order to improve the currently problematic evaluation of transfer scenarios in court proceedings, we examined the influence a variety of nine substrate types (six varieties of fabric, plywood, tarpaulin, and plastic sheets) has on DNA transfer involving blood. DNA transfer percentages were significantly higher (p = 0.03) when the primary substrate was of non-porous material (such as tarpaulin, plastic or, to a lesser degree, wood) and the secondary substrate porous (such as fabrics). These findings on transfer percentages confirm the results of previous studies. Fabric composition was also shown to have a significant (p = 0.03) effect on DNA transfer; when experiments were performed with friction from a variety of fabrics to a specific weave of cotton, transfer percentages ranged from 4% (flannelette) to 94% (acetate).

The propensity for the same nine substrates to impact upon the efficiency of DNA extraction procedures was also examined. Significant (p = 0.03) differences were found among the extraction efficiencies from different materials. When 15 μL of blood was deposited on each of the substrates, the lowest quantity of DNA was extracted from plastic (20 ng) and the highest quantities extracted from calico and flannelette (650 ng). Significant (p < 0.05) differences also exist among the DNA extraction yield from different initial blood volumes from all substrates. Also, significantly greater (p < 0.05) loss of DNA was seen during concentration of extracts with higher compared to lower initial quantities of DNA. These findings suggest that the efficiency of extraction and concentration impacts upon the final amount of DNA available for analysis and that consideration of these effects should not be ignored. The application of correction factors to adjust for any variation among extraction and concentration efficiencies among substrates is proposed.

Introduction

Secondary transfer of DNA can have an important bearing on criminal investigations with its potential to cast doubt over the origin, and, as a consequence, the validity of DNA evidence present at a crime scene [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. This potential has led to requests during court proceedings for assessment of the likelihood of DNA transfer under certain proposed situations [14], [15], [16], [17]. Such assessments require an understanding of the variables which can affect transfer. However, to date, only limited research in this area has been published [8], [9].

Recent reports of DNA transfer [8], [9] focussed on transfer associated with three substrates: cotton (representing a soft and porous material), plastic (representing hard and non-porous surfaces) and, to a lesser extent, wool. Significant differences in transfer could be attributed to substrate types; transfer from a non-porous substrate to a non-porous secondary substrate reached a maximum average of 64% whilst transfer from the same substrate to a porous surface attained a maximum average of 100% [8]. There are, however, many other types of substrate which can be present at crime scenes and it is currently unknown how these, and their characteristics, such as porosity, composition and density, may influence DNA transfer. Other factors which may influence the assessment of DNA transfer, such as differences in the loss of DNA during extraction and concentration processes, were also not examined in the earlier studies [8], [9].

A substrate class commonly associated with DNA evidence is fabric as it forms many items of clothing worn by persons involved in or affected by crime, as well as a variety of common household items. Fabrics consist of natural and/or synthetic fibres woven into a matrix, and there are many different fibre compositions of varying weave matrices in common use [18]. It is impractical to analyse all fabrics that could be found at a crime scene. We investigated undyed fabrics comprising common fibres, such as cotton, polyester and acetate, woven to form a common, simple weave type known as a plain weave. Flannelette and twill woven cotton were included in order to compare and contrast the effects of more complex weave types on transfer. The set of fabrics we chose allowed examination of the effect of common fibre types, and, to a limited extent, weave types, on transfer.

The substrate on which biological material is deposited may have an effect on the efficiency of DNA extraction. In any controlled study of transfer variables, transfer percentages are calculated by using quantities of DNA collected from two substrates: the primary substrate on which the biological material was deposited, and the secondary substrate to which some of it was transferred. If the efficiencies of the techniques for sampling and/or extraction from one substrate differed from the efficiencies of the same techniques used on a different substrate, the DNA quantities used in calculations of transfer would be inaccurate.

Differences in efficiency may also be present among samples extracted from different initial volumes of blood deposited on the same substrate, leading to greater inaccuracy when calculating transfer between substrates which contain considerably different blood volumes. We hypothesised that different quantities of DNA are obtained from the same volume of biological material deposited on different substrates due to differences in extraction efficiency. Further, that if this is so, then a correction to the values is required.

Although not directly related to substrate characteristics, the efficiency of concentrating trace DNA samples after extraction may vary according to the initial concentration of DNA, and lead to further inaccuracies in the final quantity of DNA determined.

In this study we investigate a range of substrates commonly associated with crime scenes, but hitherto not examined, to determine if, and to what extent, the variables identified by Goray et al. [8], [9] affect transfer. The efficiency of the extraction technique used to process DNA from each of these substrates is also examined. Our intention is that the findings will provide greater accuracy in any assessment of the likelihood of specific transfer events which may be considered in criminal investigations and/or court proceedings.

Section snippets

Substrates

Six fabrics were selected as substrates, each with a different composition of cotton, polyester and/or acetate. Three additional substrates included a slightly porous hardwood variety of plywood and two completely non-porous substrates; polyethylene tarpaulin and plastic transparency. Two of the substrates, cotton drill and plastic transparency (OfficeMax^® Australia, catalogue #1219839), were the same as those examined by Goray et al. [8], [9], and were included for comparison purposes. Table 1

Transfer percentages for test substrates

Transfer percentages between the various substrate combinations are presented in Fig. 2, Fig. 3 from which a number of general trends can be identified. Firstly, transfer was highest when the primary substrate was non-porous and the secondary substrate porous. Secondly, transfer was significantly higher when friction was applied, and thirdly, transfer was highest for wet rather than dried biological material.

Comparisons of DNA transfer from different fabrics under optimum transfer conditions

DNA transfer

Our experiments confirm the findings of Goray et al. [8], [9] that (i) the quantity of DNA decreases if samples have dried prior to transfer, (ii) that friction increases the amount transferred, and (iii) in general, a highly porous surface acquires more DNA than a non-porous surface, but transfers less as a primary substrate. However, fabrics, other than cotton drill, transferred significantly less DNA than non-porous substrates, and that less DNA was transferred to non-porous than porous

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The influence of substrate on DNA transfer and extraction efficiency

Abstract

Introduction

Section snippets

Substrates

Transfer percentages for test substrates

DNA transfer

Forensic Sci. Int.

Forensic Sci. Int.

Forensic Sci. Int. Genet.

Leg. Med.

Forensic Sci. Int. Genet. Suppl. Ser.

Forensic Sci. Int.

Forensic Sci. Int. Genet. Suppl. Ser.

Forensic Sci. Int. Genet.

Forensic Sci. Int. Genet.

Forensic Sci. Int. Genet. Suppl. Ser.

Forensic Sci. Int. Genet.

Leg. Med.

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.