Variation in Mitochondrial DNA Heteroplasmy from Blood, Buccal, and Hair Samples

Forensic samples may experience suboptimal environmental conditions wherein nuclear DNA is susceptible to degradation. The relatively higher copy number per cell typical of mitochondrial DNA (mtDNA) makes it more persistent under these conditions and, thus, forensically relevant. Furthermore, mtDNA has been found to express point heteroplasmy (PHP), or single base substitutions, that impact lineage identification. The aim of the current research is to determine if variation in expression of PHP exists when comparing blood, buccal, and head hair samples from five subjects previously determined to have PHPs. Five hairs were collected from five discrete scalp locations (25 total) per subject. One scalp site from a single subject was used to further study variation down the length of the hair shaft. Extracted mtDNA was processed and sequenced using the Ion Chef and Ion S5 platform, followed by data analysis using the Torrent Suite and Converge Software. Results were evaluated using performance metrics such as concordance across sampling sites, strand balance, noise, and read depth. Current SWGDAM guidelines state a sequence comparison results in a judgement of exclusion if the samples differ at two or more nucleotide positions and is inconclusive if the samples differ at one nucleotide position. As it is possible for individuals to express two heteroplasmic sites or present as homoplasmic in one tissue type and heteroplasmic in another, the results of this study may suggest revision of sample collection methodology from known individuals when interpreting hair evidence.