Examination of Firearms Review: 2007 to 2010 - Interpol

6.4 Alternative DNA amplification methods
Currently the PCR amplification method is used in forensic DNA analysis. However
methods based on PCR are considered to be complex because it requires
sophisticated and expensive equipment and skilled operators to perform the assay 36 .
PCR may introduce false positive results from nonspecific amplification or allelic
drop-in (due to slippage, following a random genetic drift model 43 ) because of the
thermal cycling steps 36 . Furthermore PCR does not have true multiplexing
properties 60 . Therefore the forensic community has to be aware of alternative DNA
amplification methods that are being developed. For example whole genome
amplification (WGA) can be used to amplify all the DNA in a sample 34 . An alternative
to PCR is rolling circle amplification (RCA). Hatch et al 61 proposed to use RCA in
order to identify single nucleotide polymorphisms (SNPs). RCA provides highly
specific DNA sequence recognition and intense signal amplification from small
starting probes, and can be used to perform parallel screening of DNA for SNPs.
Recently Su et al 36 described the use of RCA in combination with magnetic beads
and electrochemiluminescence to detect point mutations (e.g. SNPs). They
demonstrated that RCA could be used to discriminate between two alleles and can
be applicable for the detection of a variety of biorelated markers.
6.5 Mobile technologies
The above mentioned future developments are noteworthy, nevertheless there is one
development expected to revolutionize current DNA casework: the application of
mobile DNA profiling technologies. Such an instrument is described by Hopwood et
al 62 , which uses an integrated microfluidic system for the analysis of multiplex STR
DNA profiles from reference buccal samples. Until now routine analysis of DNA
reference samples (e.g. buccal swabs) have to undergo laboratory testing, which
means that the samples have to be transported from the police force to a forensic
laboratory. To assure this transportation process is efficient police forces store the
samples until they have enough samples to dispatch. This step is time consuming
therefore an advantage of a portable instrument would be the ability to process
reference samples in for example a police custody suite, assuring immediate analysis
of samples. The development of such mobile technologies has been discussed for
many years. Integrated microfluidic systems, also called micro total analysis systems
(microTAS) have demonstrated to be successful modules for DNA extraction, PCR
amplification, and capillary electrophoresis (CE), offering a tremendous potential for
miniaturizing and integrating DNA analysis in forensic STR analysis. However some
improvements are required if the systems are to be used for probative means,
nevertheless they can be used as a tool to provide some probable cause in the
apprehension of potential perpetrators whose profiles have been previously entered
into a DNA database 63 . The biggest challenge of microTAS is the extraction of DNA
from a crude cell lysate and processing it into a full DNA profile without operator
intervention (which would make it relatively simple to use). To ensure that a full DNA
profile is obtained a number of variables were identified as critical: sample collection,
DNA extraction, PCR efficiency, PCR recovery, and robustness of the CE detection
instrumentation 62 . The major cause of variation in the system’s output is the input, for
different samples taken from different individuals have different quantities of cells,
releasing different quantities of DNA upon lysis.
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