We have sequenced the entire mtDNA genome (mtGenome) of 241 individuals who match 1 of 18 common European Caucasian HV1/HV2 types, to identify sites that permit additional forensic discrimination. We found that over the entire mtGenome even individuals with the same HV1/HV2 type rarely match. Restricting attention to sites that are neutral with respect to phenotypic expression, we have selected eight panels of single nucleotide polymorphism (SNP) sites that are useful for additional discrimination. These panels were selected to be suitable for multiplex SNP typing assays, with 7-11 sites per panel. The panels are specific for one or more of the common HV1/HV2 types (or closely related types), permitting a directed approach that conserves limiting case specimen extracts while providing a maximal chance for additional discrimination. Discrimination provided by the panels reduces the frequency of the most common type in the European Caucasian population from approximately 7% to approximately 2%, and the 18 common types we analyzed are resolved to 105 different types, 55 of which are seen only once.

Though investigations into the use of massively parallel sequencing technologies for the generation of complete mitochondrial genome (mtGenome) profiles from difficult forensic specimens are well underway in multiple laboratories, the high quality population reference data necessary to support full mtGenome typing in the forensic context are lacking. To address this deficiency, we have developed 588 complete mtGenome haplotypes, spanning three U.S. population groups (African American, Caucasian and Hispanic) from anonymized, randomly-sampled specimens. Data production utilized an 8-amplicon, 135 sequencing reaction Sanger-based protocol, performed in semi-automated fashion on robotic instrumentation. Data review followed an intensive multi-step strategy that included a minimum of three independent reviews of the raw data at two laboratories; repeat screenings of all insertions, deletions, heteroplasmies, transversions and any additional private mutations; and a check for phylogenetic feasibility. For all three populations, nearly complete resolution of the haplotypes was achieved with full mtGenome sequences: 90.3-98.8% of haplotypes were unique per population, an improvement of 7.7-29.2% over control region sequencing alone, and zero haplotypes overlapped between populations. Inferred maternal biogeographic ancestry frequencies for each population and heteroplasmy rates in the control region were generally consistent with published datasets. In the coding region, nearly 90% of individuals exhibited length heteroplasmy in the 12418-12425 adenine homopolymer; and despite a relatively high rate of point heteroplasmy (23.8% of individuals across the entire molecule), coding region point heteroplasmies shared by more than one individual were notably absent, and transversion-type heteroplasmies were extremely rare. The ratio of nonsynonymous to synonymous changes among point heteroplasmies in the protein-coding genes (1:1.3) and average pathogenicity scores in comparison to data reported for complete substitutions in previous studies seem to provide some additional support for the role of purifying selection in the evolution of the human mtGenome. Overall, these thoroughly vetted full mtGenome population reference data can serve as a standard against which the quality and features of future mtGenome datasets (especially those developed via massively parallel sequencing) may be evaluated, and will provide a solid foundation for the generation of complete mtGenome haplotype frequency estimates for forensic applications.

HighlightsWe provide an update on developments in mtDNA forensic genetics.We review the recent literature for the detection of heteroplasmy using next generation sequencing techniques.We highlight artefacts and problems in datasets.We discuss the implications for forensic practice.

The typing of single nucleotide polymorphisms (SNPs) located throughout the mitochondrial genome (mtGenome) can help resolve individuals with an identical HV1/HV2 mitotype. A set of 11 SNPs selected for distinguishing individuals of the most common Caucasian HV1/HV2 mitotype were incorporated in an allele specific primer extension assay. The assay was optimized for multiplex detection of SNPs at positions 3010, 4793, 10211, 5004, 7028, 7202, 16519, 12858, 4580, 477 and 14470 in the mtGenome. Primers were designed to allow for simultaneous PCR amplification of 11 unique regions in the mtGenome and subsequent primer extension. By enzymatically incorporating fluorescently labeled dideoxynucleotides (ddNTPs) onto the 3' end of the extension primer, detection can be accomplished with a capillary-based electrophoresis (CE) platform common in most forensic laboratories. The electrophoretic mobility for the extension primers was compared in denaturing POP4 and POP6 CE running buffers. Empirical adjustment of extension primer concentrations resulted in even signal intensity for the 11 loci probed. We demonstrate that the assay performs well for heteroplasmy and mixture detection, and for typical mtDNA casework samples with highly degraded DNA.