MitoMetrics: Incorporation of mtDNA profile discrepancies in likelihood ratio calculations

Interpretation of mitochondrial DNA (mtDNA) evidence in a forensic context faces challenges, particularly when evaluating mtDNA profiles from different tissues. In hair, for example, the segregation of mtDNA shows tight bottlenecks that can result in different mtDNA profiles between and along hair shafts, and between hair and other reference tissues of the same donor. Current forensic interpretation guidelines for mtDNA are based on conventions on the number of discrepant positions observed between the two compared profiles. Most legislations consider two discrepancies between samples as an exclusion, and one discrepancy as an inclusion or as an inconclusive result. More data are needed to understand the variation and occurrence of discrepancies in samples from the same donor, to be able to incorporate this knowledge into a mathematical approach that effectively quantifies the probability of these events. This study reports the first project of the MitoMetrics collaborative initiative. In this study, data were generated from several participating laboratories, and previously published data, along with data generated from casework. mtDNA profiles from blood/buccal reference samples were compared to those from hair shafts from the same individuals. Results report the levels of heteroplasmy detected in the different tissues, the number of differences observed between the tissue comparisons, and the number of discrepant positions observed. We suggest a preliminary model for calculating the evidential value of mtDNA-based evidence using a likelihood ratio approach, that takes into consideration the occurrence of discrepancies between profiles from the same donor. This work represents the first attempt to quantify the probability of finding discrepant events between tissues and to incorporate these when reporting mtDNA in a forensic context.