Heteroplasmic mitochondrial DNA mutations in normal and tumor cells

The presence of hundreds of copies of mitochondrial (mt) DNA in each human cell poses a challenge for complete characterization of mtDNA genomes by conventional sequencing technologies(1). Here, we describe digital sequencing of mtDNA genomes using massively parallel sequencing-by-synthesis. Though the mtDNA of human cells is considered to be homogeneous, we found widespread heterogeneity (heteroplasmy) in the mtDNA of normal human cells. Moreover, the frequency of heteroplasmic variants among different tissues of the same individual varied considerably. In addition to the variants identified in normal tissues, cancer cells harbored additional homoplasmic and heteroplasmic mutations that could also be detected in patient plasma. These studies provide new insights into the nature and variability of mtDNA sequences and have intriguing implications for mitochondrial processes during embryogenesis, cancer biomarker development, and forensic analysis. In particular, they demonstrate that individual humans are characterized by a complex mixture of related mitochondrial genotypes rather than a single genotype.