The Impact of Pressure and Its Effects on The Amount and Quality of DNA Deposited by Touch

Abstract

DNA can be transferred by touch. This touch DNA is used to gain information to help with criminal investigations, disaster victim identification or eve missing persons (Sankhla and Kumar, 2017). The significance value of DNA profiling had increased in forensic investigations over time and this has helped develop technology to extract DNA and gain information via DNA profiling from small amounts. There are many variables that affect the deposition of DNA by touch such as surface type, nature of contact and shedder status. Variables such as pressure have shown to increase the transfer of DNA between two surfaces. Can pressure impact the DNA deposition directly from skin on an object?

An experiment that investigated the impact of pressure and its effects on the amount and quality of DNA deposited by touch. The experiment included participants to expert pressure from the fingertips onto a DNA-free polycarbonate placed in top of a balance for one minute. All five finger marks were swabbed as one sample and DNA was extracted, quantified and profiled (Tobias et al., 2017). To exert the pressure of 4, 21 or 37kPa, the area of the fingertips combined was used and the weight at which to push the balance was determined that would result in the desired pressure for each hand. Both left and right hands were used by the volunteers in random order for three non-consecutive days. Results showed a correlation between the pressure between skin and surface and the amount of DNA deposited. As the pressure increased, the DNA deposited increased. This resulted in detection for more alleles from the volunteer as well as unknown sources. The days or the hands for which (left or right) the DNA sample from the polycarbonate was collected for showed no significant difference in the amount of DNA, although the pressure at 21 and 37 kPa showed significant difference in the amount of DNA between individuals. These findings provide an insight on the impact of pressure on DNA deposition and shows that pressure is a kay variable and should be considered during a criminal investigation while prioritising surfaces or objects.

Shredding capacity is different for each individual. An experiment investigated the factors that influence the transfer of DNA onto objects handled and a process called ‘shredding’ consisted of volunteers holding sterile plastic tubes. Any material transferred onto the tube was swabbed and amplified using STR profiles generated using the AmpFISTR SGM PlusTM. These volunteers were asked to hold tubed with each hand and has to participate in a series of handwashing experiments. The profiling results were compared in an attempt to characterise the type of shredding. The main aim was to check whether a person is a ‘good shredder’ or a ‘bad shredder’. The results showed many variables affecting the shredding, some variables affected the shredding significantly such as the hand the tube was held in, also the time elapsed since the hand was last washed. Hence, they concluded it was complicated to characterise an individual as a ‘good’ or a ‘bad’ shredder. In this experiment 60 volunteers were used and none of them were truly ‘good’ shedders and that if there were any ‘good’ shedders they would be rare to find. In terms of shredding, it would be difficult to apply this variable at each case individually and therefore should be used as general background data for the interpretation of trace DNA evidence.

Secondary transfer is one of the forefronts in the research for forensic DNA. Secondary transfer is a when one person’s DNA is transferred passively onto an object by someone or something else. Sources such as skin cells are responsible for the deposition of small amounts of trace DNA on an object after it’s been touched or used. The chances of placing an individual who might be innocent at a crime scene increases if an object with such trace DNA is found and taken in as evidence for a crime investigation. This issue of contamination by secondary transfer is now an issue in the courts of law and its significance is increasing. Experiments are being done to detect secondary DNA sample. The transfer of DNA through an intermediary source had not been tested using the technologies currently in use that are implemented to for increase the chances of obtaining results from low template and low-quality samples. An experiment was DNA to see if these sensitive current technologies were capable of detecting interpretable secondary DNA. This experiment included hand to hand contact where the participants held hands for two minutes and then immediately handled knives. The DNA collected from swabbing the knives were amplified. The DNA typing results showed that 85% of the samples have a secondary DNA contributor who has not touched or handled the knife directly. In 5 of the samples the secondary DNA contributor was either the major or the only contributor despite not being in contact with the knife (Kokshoorn et al., 2016). This showed that the assumption that DNA on an object is due to direct contact can be risky and can put someone wrongly at a place.

DNA Fingerprinting

Alec Jefferys discovered variable number tandem repeats (VNTR) in 1984. In 1985 he applied the restriction fragment length polymorphism (RFLP) technique to create a “DNA fingerprint”. RFLP is a technique where the DNA was cut at where a specific sequence of bases occurred using restriction enzymes. This gave small pieces of DNA that varied in length. This was a big innovation for forensic science as it allowed to create profiles that were specific to an individual. The use of this technology and this technique required a large amount of DNA to be present to create a fingerprint. This caused a serious issue for the forensic samples, one of the other major drawbacks was that it was a time-consuming process which is not easily available during a criminal investigation and it also required intensive labour work.

Kerry Mullis invented the Polymerase Chain Reaction (PCR) in 1986 which improved the efficiency of DNA testing. The VNTR analysis improved with the introduction of PCR as it enabled the analysis of small amounts of DNA. The use of VNTR with PCR cause problems such as the shorter alleles were amplified but the longer alleles were undetected and were not amplified. This led to the discovery of Short Tandem Repeats (STRs). These were much smaller than VNTRs and hence were all detected and amplified efficiently. The transition from VNTRs to STRs for DNA analysis made it possible to successfully analyse degraded DNA and trace DNA significantly quicker.

The increased sensitivity of PCR made it susceptible to contamination and therefore strict protocols were followed while using PCR. Due to fewer alleles in STRs ore loci were required to produce information about the likelihood of two people sharing a profile and hence STR multiplex system was introduced. This system allowed many STR loci to be simultaneously analysed. The use of fluorescent labels, automated sequencing technology and commercial STR kits, the PCR-STR technology became the preferred technology for the use in forensic laboratories.

Trace DNA

Trace DNA sample is a sample that falls below the recommended threshold at any stage during its analysis, this is from the sample collection, through to profile interpretation as defined by Van Oorschot et al. (2010). Terms such as touch DNA, low template DNA (LT-DNA), low copy number DNA (LCN-DNA) and low-level profile are used interchangeably. All the terms given above although used interchangeably have different relevance at different stages during DNA analysis (Van Oorschot et al., 2010). The terms Touch DNA refers to the minute amount of DNA collected and/or extracted whereas low template defines the minute mount od DNA material used during the amplification stage; low copy number relates to the increased cycle number during amplification rather than the amount of DNA material, a profile is referred to as low level when the peak of heights is below validated threshold level.

To enhance the sensitivity of the standard PCR method an easy technique is used. This is where the number of cycles is increased from 28 to 34. This increased sensitivity of the LCN technique of increasing the cycles has enabled the recovery of DNA from touched surfaces. The implication of the use of this technique has increased the evidentiary value of items collected from crime scenes. Previously, DNA testing was mainly used to help solve crimes such as homicide and rape but with the ability to detect ‘touch DNA’ from evidence collected from crime scenes such as robberies, break-ins, and hijackings has allowed for recovery and typing of DNA which in turn has allowed more DNA recovery from more types of evidence such as from masks worn on the face during robbery to bite marks left on victims of rape or homicide.

The LCN-DNA technique although provides increased sensitivity has its own drawbacks. The increased number of cycles during PCR gives larger stutter peaks, allele drop in and/or out, heterozygote imbalance, locus drop outs and the occurrence of unknown allele peaks or contamination. Contamination can occur at any point during and within the chain of custody, it is the transfer of DNA after the crime event. Small amount of DNA contamination isn’t a major problem when dealing with high amount of DNA input but plays a major role and has a major impact when dealing with low template DNA analysis. LCN DNA analysis is in practice by majority of the forensic laboratories, the strength of evidence derived from this type of DNA analysis has decreased compared to the conventional DNA analysis methods. This is due to the uncertainties that relate to the method of transfer of the DNA and how and when the DNA was transferred along with the interpretation and reporting of the results obtained (Linacre, 2009). Foster et al. (2008) investigated methods other methods that can be used to enhance the 28 cycle PCR so that the problems that were the result of increased number of PCR cycles can be reduced. From their study they concluded that by combining PCR product clean-up, concentration, increased sample loading along with increased injection parameters the same or even a better quality STR profiles were produced from 28 cycle PCR as those that were generated using the LCN method of 34 cycle PCR.

DNA testing and the law

Forensic application of DNA analysis is wide and cover a large spectrum of cases from criminal cases to missing persons’ case including wildlife cases making DNA testing and analysis an undisputed asset in the law enforcement. The fist legal application of DNA profiling in the United Kingdom dates back to 1985 by Alec Jeffery’s for an immigration case. DNA profiling has been used to solve serious crimes since a long time but in recent years is being used to solve volume crimes. The first case to use DNA profiling solved by Alec Jefferys was also the first case where the DNA analysis and profiling technology helped exonerated an innocent person. In 1985 Alec Jeffery was called to assist the police with a double rape and murder case in Leicestershire. The case was that a person who had confessed to one of the crimes, three years apart. DNA profiling liked both the crimes but excluded the suspect from both these scenes. This success prompted the use of DNA profiling in criminal investigations including cold cases missing persons’ cases, mass disaster cases, parentage cases and exonerations of those who were wrongfully convicted. There are many reasons for wrongful convictions as suggested by research, DNA is one of the most important tools that is utilized to prevent and uncover wrongful convictions. It has also been determined that with access to DNA testing within the criminal justice system the likelihood of exoneration for murder and sexual assaults increase by 6.93 times.

As the technology advances new problems emerge although while acknowledging the benefits of DNA testing. From the beginning it was presumed that if DNA was found at or near the crime scene that individual was present at or near the crime scene. Due to the existence and understanding of secondary or multiple DNA transfer, it can no longer be taken that one’s DNA at any place means an interaction with the environment or presence at the scene. Further studies and experiment showed that secondary DNA transfer is possible, and this cannot be ignored. To determine the validity of the theory of multiple DNA transfer many studies and experiments were conducted. Different conclusions were drawn regarding the phenomenon to criminal investigations based on the method of analysis. Some concluded that secondary DNA has an impact on the routine analysis of DNA whereas some concluded to the contrary. This theory of secondary transfer has created issues in court as lawyers are increasingly proposing scenarios where multiple transfers may have occurred as an explanation for the presence of a person’s DNA at or around the crime scene. This has thrown a curve-ball to the application of DNA in criminal investigations.