Analysis of heteroplasmy in hypervariable region II of mitochondrial DNA in maternally related individuals

Mitochondrial DNA sequences have been widely employed for identity investigation. However, the presence of a heteroplasmic site may complicate sequence analysis for forensic purposes when two samples are compared. To study this potential problem, we analyzed the hypervariable region of the displacement loop in five maternally related individuals, that is, grandmother, mother, one son, and two daughters. The results showed that three of them had a heteroplasmic site at nucleotide position (np) 204, located in the hypervariable region II. By using Bayesian inference to assess the significance of the mother-offspring pairs, a likelihood ratio of 1.78 × 10⁵ was obtained. Therefore, Bayesian inference does not place the prior odds into the context of the two different likelihood ratios derived from the DNA evidence. On the other hand, the chromatogram of the denaturing high-performance liquid chromatography system proved that the single peak in a sequencing chromatogram at np 204 was, in fact, heteroplasmic in nature. This study demonstrated that heteroplasmy is a common occurrence in tissue from normal individuals and should be taken into account in forensic investigation when samples appear to differ at a single nucleotide position by direct sequencing.