Commentary on: Foran DR, Gehring ME, Stallworth SE. The recovery and analysis of mitochondrial DNA from exploded pipe bombs. J Forensic Sci 54;1:90-4.

摘要

Sir, Foran et al. (JFS 54:90-94) describe mitochondrial DNA (mtDNA) analysis of handled pipe bomb components. The practices used in their experiments are inappropriate in forensic casework. Ninety-four of 114 PCR amplifications of the swab samples in this experiment yielded insufficient or no amplification product for DNA sequence analysis after 38 cycles of mtDNA amplification. These samples were subsequently processed further: a 1 muL subsample of first-round PCR product, whether it was visible on a yield gel or not, was placed into a second nested amplification of 24 cycles, giving a total of 62 cycles of amplification. With this approach, every swab produced an amplification product. Due to tight controls applied to handling of materials prior to sampling, and because the investigators knew the profiles of all contributors, every amplification product should have been assigned to a donor. However, the investigators recovered 10 profiles (28% of the samples) that could not be assigned. Therein lies the problem of nested PCR. The use of nested PCR can yield amplification product from contaminant molecules rather than the target DNA, particularly when the amount of target DNA is minimal. If a contaminant does become detectable through nested PCR, it is impossible to discriminate between the contaminant, a handler, or someone who may have handled the materials some time well before bomb preparation. In addition, PCR artifacts may result due to stochastic effects that occur when amplifying low levels of DNA.