Improved analysis of long STR amplicons from degraded single source and mixed DNA

DNA profiles from degraded samples often suffer from information loss at the longer short tandem repeat (STR) loci. Sensitising the reactions, either by performing additional PCR cycles or increasing the capillary electrophoresis injection settings, carries the risk of over-amplifying or overloading the shorter fragments. We explored whether profiling of degraded DNA can be improved by preferential capturing of the longer amplified fragments. To this aim, a post-PCR purification protocol was developed that is based on AMPure XP beads that have size-selective properties. A comparison was made with an unselective post-PCR purification system (DTR gel filtration) and no purification of the PCR products. Besides a set of differently and serially degraded single source samples, unequal mixtures of degraded DNAs were analysed, in order to extract more genotyping information for the minor contributor without overloading the major component at the shorter amplicons. Purification by the AMPure protocol resulted in higher peak heights especially for the longer amplicons, while DTR gel filtration gave higher peaks for all amplicon sizes. Both purification methods presented more detected alleles, with the AMPure protocol performing slightly better, on average. In conclusion, the in-house developed AMPure protocol can be employed to improve STR profile analysis of degraded single source and (unequally) mixed DNA samples.