Pervasive within-Mitochondrion Single-Nucleotide Variant Heteroplasmy as Revealed by Single Mitochondrion Sequencing

http://www.cell.com/cell-reports/references/S2211-1247(17)31668-6

Morris J, Na YJ, Zhu H, Lee JH, Giang H, Ulyanova AV, Baltuch GH, Brem S, Chen HI, Kung DK, Lucas TH, O'Rourke DM, Wolf JA, Grady MS, Sul JY, Kim J, Eberwine J

This study looks at the prevalence of mutations in mitochondrial DNA within single mitochondria. The authors do this by collecting single mitochondria from cells with a micropipette, then perform PCR to amplify the copy number of DNA and finally illumina deep sequencing.

The authors collected 118 samples from the brains of lab mice (C57BL/6N strain), and found on average 3.9 single-nucleotide variants per mitochondrion with a standard deviation of 5.71 (although the mtDNA copy number per mitochondrion was not quantified). Some of the mutations observed are thought to be deleterious: for instance, a mutation found at position 9027 (G>A) encoding MT-CO3 (complex III of the respiratory chain) is a missense mutation, annotated to have moderate pathophysiological impact. The authors found 59 samples with this mutation. The intra-mitochondrial heteroplasmy was > 90% for 39 of these mitochondrial samples.

The authors also collected 21 samples of mitochondria from 8 different neurons from the brain of a 63-year-old female using residual tissue removed after surgery. From these samples, the authors found that within-mitochondrion heteroplasmy was ~50% less common in their human sample than in lab mice.  The authors also found that the within-mitochondrion heteroplasmy of different mitochondria in the same cell, and the inter-cellular heteroplasmy between cells, tended to be similar in their human sample but different in mouse.

The authors suggest that the differences between humans and mice are most likely due to the effect of only observing a single individual for their human experiment, but many individuals for mice. The authors found a large effect from the identity of the mother in determining the extent of within-mitochondrion heteroplasmy.