Evaluating length heteroplasmy in the human mitochondrial DNA control region

We present allelic data for three known and one new C-tract in the human mitochondrial DNA (mtDNA) control region, and we measure intergenerational mutation rates at such C-tracts. In detail, in a sample of 1,172 mtDNA sequences, we demonstrate the existence of an instability threshold of eight consecutive cytosines, at and above which the phenomenon of length heteroplasmy arises. To determine mutation rates, we draw on mtDNA sequences in up to four generations of 248 pedigrees for families living in high or low-radiation environmental conditions. The high-radiation sample gives the most conservative (fastest) mutation rate likely to be encountered in any forensic context. We find that the C-tract mutation rate is up to 6% per generation, and we observe an excess of cytosine gains over losses. Case studies and guidelines for evaluating mtDNA heteroplasmy are provided.