Two-person DNA mixture interpretation based on a novel set of SNP-STR markers

Abstract

Autosomal short tandem repeats (STR) markers analysed by PCR and capillary electrophoresis (CE) represent the gold-standard for forensic DNA analysis. With the improved sensitivity of detection equipment, a larger number of mixed DNA profiles can be obtained from trace amounts of DNA that conventionally used to appear as a single source. More specifically, two-source DNA mixtures, comprising the victim’s and the perpetrator’s DNAs, are often encountered in forensic casework, where the victim’s DNA represents a major component of the mixture. Unfortunately, unbalanced two-person DNA mixtures with a ratio larger than 20:1 (here we have named this kind of mixture extremely unbalanced DNA mixture) provide limited information on the minor component. Although the development of probabilistic software has made interpretation of results from mixed DNA easier, high mixture ratios lead to an uninformative likelihood ratio (LR), considering the minor component. Therefore, a technique that can be performed on the conventional CE platform, while enhancing the ability to detect minor DNA in extremely unbalanced DNA mixtures, may be very useful in forensic casework. Our previous research has reported that SNP-STRs, in conjunction with a PCR technique based on amplification refractory mutation system (ARMS), can be used to resolve extremely unbalanced two-person DNA mixtures. To further explore the capacity of SNP-STR markers to help analyse such DNA mixtures, we developed 11 novel SNP-STR markers. The ARMS-based PCR was then used to design allele-specific primers, where each primer targeted one SNP allele located in the flanking region of the tandem repeats. This method allowed primers to specifically and selectively amplify minor DNA without interference from DNA of the major component because the selected SNP allele was not shared with the major contributor. A survey of the selected 11 SNP-STRs in a southwest Chinese Han population showed high levels of polymorphism. Assays on two-person DNA mixtures showed that all the allele-specific primers could target minor DNA even when the amount of major DNA was a 100-fold higher. Therefore, this novel set of SNP-STR markers provides an alternative method for the analysis of extremely unbalanced two-person DNA mixtures.