High sensitive DNA typing approaches for the analysis of forensic evidence: comparison of nested variable number of tandem repeats (VNTR) amplification and a short tandem repeats (STR) polymorphism.

The approach of using nested primers for the APO B variable number of tandem repeats (VNTR) increases the sensitivity of the polymerase chain reaction (PCR) to single cell level. Different experiments and a comparison to the short tandem repeats (STR) system VWA were carried out, to determine the applicability of this method to forensic samples. Nested amplification of the Apo B VNTR was affected by a strong tendency towards preferential amplification of the shorter alleles. This phenomenon was observed for DNA quantities as low as 100 pg and impaired, depending on the allele length, the results for mixed samples. As expected, VWA polymorphism showed less preferential amplification. The high sensitivity of both PCR systems is accompanied by an increased susceptibility to contamination. Using artificially contaminated bloodstains, the bloodstain genotype, the contamination or both genotypes could be found on one piece of evidence. Here a single analysis can lead to an incorrect result. Therefore a strategy for obtaining reliable results should consist of multiple stain extractions and the amplification of different stepped dilutions of the DNA solution.