Mitochondrial DNA copy number in human disease: the more the better?

Abstract

Most of the genetic information has been lost or transferred to the nucleus during the evolution of mitochondria. Nevertheless, mitochondria have retained their own genome that is essential for oxidative phosphorylation (OXPHOS). In mammals, a gene-dense circular mitochondrial DNA (mtDNA) of about 16.5 kb encodes 13 proteins, which constitute only 1% of the mitochondrial proteome. Mammalian mtDNA is present in thousands of copies per cell and mutations often affect only a fraction of them. Most pathogenic human mtDNA mutations are recessive and only cause OXPHOS defects if present above a certain critical threshold. However, emerging evidence strongly suggests that the proportion of mutated mtDNA copies is not the only determinant of disease but that also the absolute copy number matters. In this review, we critically discuss current knowledge of the role of mtDNA copy number regulation in various types of human diseases, including mitochondrial disorders, neurodegenerative disorders and cancer, and during ageing. We also provide an overview of new exciting therapeutic strategies to directly manipulate mtDNA to restore OXPHOS in mitochondrial diseases.

Keywords: Alzheimer’s disease; Parkinson’s disease; TFAM; ageing; cancer; mitochondria; mitochondrial diseases; mtDNA; mtDNA copy number; neurodegenerative disorders.

Fig. 1

Schematic representation of mtDNA maintenance, packaging and genetics. (A) mtDNA nucleoids and the replisome. mtDNA is present in multiple copies within the cell. It is compacted by TFAM into structures known as nucleoids. Mitochondrial nucleoids can be found in a compacted or relaxed state depending on the local TFAM concentration. In the relaxed state, mtDNA is accessible for replication by the mitochondrial replisome which is formed by the mitochondrial RNA polymerase POLRMT, the hexameric DNA helicase TWINKLE, the tetrameric mtSSB and the mtDNA polymerase gamma POLγ. POLγ is a heterotrimer formed by a catalytic subunit with DNA polymerase and 3′‐5′ exonuclease activities (encoded by POLGA), and by two accessory subunits (encoded by POLGB) required for the tight DNA‐binding and processive DNA synthesis. During replication, TWINKLE unwinds and proceeds on the DNA in a 5’ to 3’ direction, and the single‐stranded DNA generated by TWINKLE activity is protected by the mtSSB binding. The RNA primers for replication initiation are generated by POLRMT. (B) The concept of heteroplasmy. Mutations affecting the mtDNA can coexist with the wild‐type molecules, a condition known as heteroplasmy. mtDNA mutations are tolerated until they exceed a certain level (threshold). Therefore, defects in the OXPHOS system will manifest only when the proportion of mutated mtDNA molecules exceeds the biochemical threshold, which is known to be tissue‐ and mutation‐specific.

Fig. 2

Therapeutic strategies to manipulate mtDNA and disease severity. (A) Boosting mitochondrial biogenesis represents an unspecific approach to decrease disease severity. With this approach, a general increase in all mitochondrial components, as well as mtDNA levels, is accomplished in order to rescue biochemical OXPHOS defects. (B) Mitochondrially directed ZNFs and TALENs selectively cut mtDNA in a sequence‐specific way. Mutant mtDNA is cleaved and the resulting linear molecules are quickly degraded generating a transient mtDNA reduction in the cell, which is restored by replication of the residual (wild‐type) mtDNA. (C) A moderate and selective increase in mtDNA copy number can be achieved by modulating TFAM levels. By doing so, mitochondrial function can be partially restored due to absolute increase of the functional mtDNA copies. Schematic representations of mtDNA levels (black lines) and their correlation with disease severity (green lines) are represented for each approach on the right‐hand side panels.