Increasing the discrimination power of forensic STR testing by employing high-performance mass spectrometry, as illustrated in indigenous South African and Central Asian populations

Abstract

Short tandem repeat (STR) typing has become the standard technique in forensic methodology for the identification of unknown samples. National DNA databases have been established that contain STR genotypes for intelligence purposes. Due to their success, national DNA databases have been growing so fast that the number of advantageous matches may become a logistic problem for the analysts. This is especially true for partial STR profiles as they display reduced discrimination power. To overcome this drawback, modified versions (so-called mini-STRs) of existing loci were introduced as well as new loci to improve the information content of (partial) STR profiles. We pursue an alternative approach that makes use of nucleotide variation within the amplified STR fragments, which can be discerned by mass spectrometry. We have developed an assay that determines molecular masses from crude STR amplicons which were purified and separated by a liquid chromatographic system directly hyphenated to an electrospray ionization mass spectrometer. We present here new population data of forensically relevant STRs in Khoisan and Yakut populations. These autochthonous groups were selected as they may harbor additional STR alleles that are rare or unobserved in modern humans from cosmopolitan areas, especially for the Khoisan, which are known to represent a very ancient human population. The analysis of the molecular mass of STRs offered a widened spectrum of allele variability escorted by enhanced forensic use. Thus, established STR data derived from fragment size analysis can still be used in casework or in the context of intelligence databasing.