Clonal Expansion of Early to Mid-Life Mitochondrial DNA Point Mutations Drives Mitochondrial Dysfunction during Human Ageing

Abstract

Age-related decline in the integrity of mitochondria is an important contributor to the human ageing process. In a number of ageing stem cell populations, this decline in mitochondrial function is due to clonal expansion of individual mitochondrial DNA (mtDNA) point mutations within single cells. However the dynamics of this process and when these mtDNA mutations occur initially are poorly understood. Using human colorectal epithelium as an exemplar tissue with a well-defined stem cell population, we analysed samples from 207 healthy participants aged 17–78 years using a combination of techniques (Random Mutation Capture, Next Generation Sequencing and mitochondrial enzyme histochemistry), and show that: 1) non-pathogenic mtDNA mutations are present from early embryogenesis or may be transmitted through the germline, whereas pathogenic mtDNA mutations are detected in the somatic cells, providing evidence for purifying selection in humans, 2) pathogenic mtDNA mutations are present from early adulthood (<20 years of age), at both low levels and as clonal expansions, 3) low level mtDNA mutation frequency does not change significantly with age, suggesting that mtDNA mutation rate does not increase significantly with age, and 4) clonally expanded mtDNA mutations increase dramatically with age. These data confirm that clonal expansion of mtDNA mutations, some of which are generated very early in life, is the major driving force behind the mitochondrial dysfunction associated with ageing of the human colorectal epithelium.

Replication Errors Made During Oogenesis Lead to Detectable De Novo mtDNA Mutations in Zebrafish Oocytes with a Low mtDNA Copy Number

“…As we found de novo mutations in 20% of the oocytes, this implies that 1% of oocytes will carry a pathogenic de novo mutation (with a heteroplasmy level $1.5%). The presence of low-level mtDNA mutations in the oocyte could, after fertilization, lead to mtDNA disease later in life due to genetic drift, which could lead to fixation of the mutation (Greaves et al 2014;Yin et al 2015), or in the offspring of the following generation, as inheritance through the mtDNA bottleneck can cause shifts in the heteroplasmy level between mother and child, also leading to fixation of the mutant mtDNA (Blok et al 1997).…”

Mitochondrial depolarization in yeast zygotes inhibits clonal expansion of selfish mtDNA

“…The displacement of the wild-type mtDNA, which encodes a full set of RNAs and proteins, by mutant mtDNA with deleterious mutations can be detrimental for the cells (Aanen and Maas, 2012;Ye et al, 2014). Such displacement plays an important role in the development of mitochondrial genetic diseases and aging (Greaves et al, 2014; for a review, see also Taylor and Turnbull, 2005). Why does the displacement occur?…”

mtDNA Heteroplasmy in Monozygotic Twins Discordant for Schizophrenia

“…However, to quantify the mtDNA heteroplasmy in a cell is not easy [11]. Recent studies have made great efforts to improve the sensitivity of heteroplasmy detection, and nextgeneration sequencing (NGS) has been established as being an effective method for the identification of low levels of mtDNA heteroplasmy [15][16][17][18][19][20][21].…”