Human identification of single hair shaft using a mass spectrometry mRNA typing system

Hair is one of the most common forms of forensic biological material at various crime scenes. So far, human identification cannot be effectively accomplished with a single telogen hair encountered in forensic casework due to the detection limit. Emerging studies have revealed RNA as a promising biomarker in hair shafts, while the single telogen hair could not be successfully genotyped even after being examined with the recently developed mRNA typing system. MALDI-TOF MS, the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, enables sensitive and accurate measurement of DNA products. To address this problem and further develop the analysis technology of hairs, we established a mass spectrometry system for human identification based on a single hair shaft using 25 polymorphic SNPs located on 18 mRNA molecules (KRT31, RFK, KRT86, KRT35, PABPC1, KMT2D, LEMD2, TBC1D4, CTC1, PPP1R15A, RBM33, LRRC15, KRT33A, KRTAP12-2, KRT81, AHNAK, KRTAP4-8, FLG2). The forensic application of the detection system was evaluated, and all hair samples used were collected from individuals in Shanxi province. Firstly, we demonstrated that the RNA typing results of a single hair shaft were in perfect concordance with DNA typing results and confirmed the consistency between hairs from different body parts. To assess the potential influence of positions along the hair shaft, 6 cm long hair shafts from the distal end were examined by the MALDI-TOF MS system, whose genotype could be successfully detected. The system was capable of detecting aged samples stored for 390 days and could also be employed on various types of hair samples, such as white hair and permed or dyed hair. Finally, 50 unrelated individuals from Shanxi province were genotyped for the population study, and the CDP of the system in the Shanxi population is 0.998928. In this study, we established a mass spectrometry system for human identification based on a single hair shaft. We used a single hair shaft, rather than multiple hair shafts reported in our previous report, to get a full typing profile. The system sensitivity was substantially enhanced, which provided a valuable strategy for forensic practice to perform human identification using hairs.