The effects of dithiothreitol (DTT) on fluorescent qPCR dyes

DNA extractions of semen samples commonly utilize dithiothreitol (DTT) to reduce and disrupt disulfide bonds. Although traditional extraction techniques remove DTT before downstream analyses, the forensic DNA community has recently explored Y-screening, direct amplification, and direct cell lysis assays that omit purification but employ reducing agents to lyse spermatozoa. This study examined the impact of residual DTT on downstream processes involving fluorescent dyes. Quantification using Investigator^® Quantiplex HYres revealed a significant increase in the male DNA yield (p = 0.00056) and a >150,000,000-fold increase in the male:human DNA ratio when DTT remained in extracts versus when it was filtered out using a traditional purification method. When DTT was present with Quantifiler™ Trio, the true mean DNA yield for the large autosomal target significantly increased (p = 0.038) and the average reported DNA yields increased 1.1-fold, >9.5-fold, and 1.3-fold for the small autosomal, large autosomal, and male targets, respectively. DTT-spiked DNA standards from both kits were impacted similarly to samples with residual DTT, demonstrating that observed effects were related to DTT and not the extraction method. This study corroborates other reports that DTT adversely affects multiple dyes (e.g., Cy5, Quasar 670, SYBR Green I, TMR, and Mustang Purple^® ). Overall, DTT causes inaccurate quantities and, consequently, inaccurate calculated male:female ratios when used in conjunction with these kits. Thus, implementation of newer direct-to-PCR assays incorporating DTT should either be avoided or used only with carefully evaluated, compatible dyes.