Recombinase polymerase amplification of forensic short tandem repeat loci

Short tandem repeats (STRs) are highly polymorphic repetitive DNA sequences extensively used in forensic science for identification of individuals. STR genotyping is usually performed by capillary electrophoresis (CE) or next-generation sequencing (NGS) in centralized laboratories. However, there is an increasing need for a low cost, portable and rapid STR genotyping method. Multiple methods for miniaturization have been explored, all relying on polymerase chain reaction (PCR) for generating amplicons. PCR requires precise thermal cycling, which complicates the design of the STR genotyping microfluidic device. Recombinase Polymerase Amplification (RPA) is an isothermal DNA amplification method that operates between 37 °C and 42 °C and completes within 40 min. This, along with the robustness of reagents and reduced stutter rate compared to PCR makes RPA a suitable candidate for implementation in an STR genotyping microfluidics device as well as a part of the established STR genotyping work flows. In this proof-of-concept study, we evaluate RPA assay for amplification of forensically relevant STR loci. Thirteen core STR loci of the Combined DNA Index System (CODIS) were amplified using RPA in both singleplex and multiplex formats. The amplicons were then analyzed using three different methods: CE, Illumina and Oxford Nanopore Technologies (ONT). A subset of 5 loci was used for CE analysis. CE, Illumina and ONT sequencing of singleplex RPA each resulted in complete and correct STR profiles across all samples for the standard input of 1 ng. Sensitivity assessment demonstrated that complete and correct genotypes were achieved with DNA inputs of 62 pg and above for all but locus D8S1179. Attempts at multiplex RPA amplification resulted in incomplete or incorrect STR profiles. This outcome highlights a challenge in adapting RPA for simultaneous amplification of multiple STR loci, which is a standard requirement in forensic DNA profiling.