DNA methylation-based organ tissue identification: Marker identification, SNaPshot multiplex assay development, and interlaboratory comparison

Identifying body fluids and organ tissues is highly significant as they can offer crucial evidence in criminal investigations and aid the court in making informed decisions, primarily through evaluating the biological source and possibly at the activity level up to death or fatal damage. In this study, organ tissue-specific CpG markers were identified from Illumina's methylation EPIC array data of nine organ tissues, including epidermis, dermis, heart, skeletal muscle, blood, kidney, brain, lung, and liver, from autopsies of 10 Koreans. Through the validation test using 43 samples, 18 hypomethylation markers, with two markers for each organ tissue type, were selected to construct a SNaPshot assay. Two multiplex assays involving forward and reverse SBE primers were designed to help investigators accurately determine the organ origin of the analyzed tissue samples through repeated analysis of the same PCR products for markers. The developed multiplex demonstrated high accuracy, achieving 100.0 % correct detection of the presence of nine organ tissue types in 88 samples from autopsies of 10 Asians. However, two lung samples showed additional positive indications of the presence of blood. An interlaboratory comparison using 80 autopsy samples (heart, skeletal muscle, blood, kidney cortex, kidney medulla, brain, lung, and liver) from 10 individuals in Germany revealed overall comparable results with correct detection of the presence of eight organ tissue types in 92.5 % samples (74 of 80 samples). In the case of six samples, it was impossible to determine the correct tissue successfully due to drop-outs of unmethylation signals at target tissue marker loci. One of these lung samples revealed only non-intended off-target signals for blood. The observed differences might be due to differences in sample collection during routine autopsy, technical differences due to the PCR cycler, and the threshold used for signal calling. Indicating the presence of additional tissue type and off-target unmethylation signals seems alleviated by applying more stringent hypomethylation thresholds. Therefore, the developed SNaPshot multiplex assays will be valuable for forensic investigators dealing with organ tissue identification, as well as for prosecutors and defense aiming to establish the circumstances that occurred at the crime scene.