Measuring the Rate of mtDNA Heteroplasmy in Different Population Groups

NIJ 2014-DN-BX-K022 and 2016-DN-BX-0171

Forensic laboratories have not taken advantage of intrinsic levels of mitochondrial (mt) DNA heteroplasmy that exists in humans.  A next generation DNA sequencing (NGS) approach using massively parallel sequencing (MPS) methods will allow the community to achieve this goal.  Prior to implementation, validation studies must be conducted, and a better understanding of the rates of heteroplasmy will be required in order to effectively report casework findings.  The rate of heteroplasmy in the control region (CR) of the mtgenome has been measured in a European population (NIJ 2014 DN-BX-K022), and is in the process of being measured in individuals of African, East Asian and Latino decent (2016 DN-BX-0171).

We have recently measured the rate of heteroplasmy in the control region (CR) of the mtgenome using an MPS approach on the MiSeq from Illumina.  Rates were assessed on an individual and per nucleotide basis for ~550 individuals of European ancestry, across different age groups and for each gender.  To address a previous limitation, we have worked with a local company (SoftGenetics, Inc., State College, PA) to develop a software tool for researchers and practitioners to better align MPS data associated with mtDNA sequence, and to provide reporting practices consistent with forensic interests (Holland et al., 2017).  Given our understanding of the diverse nature of mtDNA haplotypes across different population groups, and that mutations resulting in low-level heteroplasmy may be associated with local sequence and their effects on the replication process, the goals of our expanded research study are to measure the rate of heteroplasmy for individuals of African, East Asian and Hispanic ancestry, and compare the rates to those for individuals of European ancestry.  Findings will be used to help guide the refinement of best practices regarding the reporting of mtDNA heteroplasmy; including statistical analysis.  Below is a figure reflecting the rate of heteroplasmy in a European population group for three different age groups.  A weak association of rates to age was observed, and no association was observed for gender.