SFR Guidance > Human Mitochondrial DNA (mtDNA) Sequencing

Guidance Notes: Human Mitochondrial DNA (mtDNA) Sequencing

Mitochondrial DNA (mtDNA) sequencing analyses the base pair sequence of the DNA found in intra cellular objects called mitochondria.

Mitochondria are small organelles present inside most human cell types. They are the site of energy production within cells and each cell can contain several thousand mitochondria. Each mitochondrion contains numerous copies of its own genetic material consisting of a small circular strand of DNA 16,569 base pairs long. Due to a relatively large degree of DNA sequence variability compared to the rest of the mitochondrial genome, two regions known as hypervariable region 1 (HV1) and hypervariable region 2 (HV2) are useful for the identification of individuals.

DNA sequence data is generated from HV1 (mtDNA bases 16024 to 16365) and HV2 (mtDNA bases 73 to 340). The sequence obtained is compared to a standard reference sequence known as the ‘revised Cambridge Reference Sequence’ (rCRS). Any base positions in which the sample sequence differs from the rCRS are tabulated to produce the “mitochondrial haplotype” of the sample tested.

The high number of mitochondria present in cells and the small size of the mitochondrial genome mean that that it is often possible to detect DNA from very small, old or degraded samples. It is also possible to obtain sequence data from samples such as hair shafts and old or burnt bone when there may not be sufficient nuclear DNA present to obtain a conventional STR DNA profile.

The mitochondrial genome is inherited unchanged via the maternal line and consequently all the offspring of a mother will have the same mtDNA sequence unless a mutation event has occurred. It is possible to search databases of mitochondrial sequences to obtain an estimate of the relative frequency of a particular sequence in the particular population(s) of interest.

On some occasions an individual may have two slightly different mitochondrial DNA sequences present in their mitochondria. This is known as heteroplasmy. As a result it is not unusual to detect two slightly different sequences of mitochondrial DNA in samples such as hair taken from the same individual.

The results of mitochondrial DNA sequencing are not suitable for direct comparison with standard STR DNA profiling results. There is no equivalent to the National DNA Database for mitochondrial DNA sequences. Instead, mitochondrial DNA sequencing results are almost exclusively used for direct comparison between samples and individuals.