Sometimes more is more: raising mitochondrial DNA copy number could treat disease

Mitochondria are the power plants of our cells and are descendants of cooperating bacteria. A study in mice shows that increasing the copies of mitochondria’s own DNA could be used as therapy for certain human diseases.

Isabel Hofman

Mitochondria are the power plants of eukaryotic cells, as the mitochondrial respiratory electron transport chain serves as the main cellular energy source. These organelles are generally accepted to be descendants of primitive bacteria that started to cooperate with single-celled organisms that became our primordial ancestors (BOX 1). As a relic of these ancient times, mitochondria still carry their own genetic material1.

BOX 1: Evolution of mitochondria
The exact identities of both the ancestral mitochondrion, and the cell that engulfed it to become the ancestor of all eukaryotes, are unclear. Several theories speculate on the conformation of the ancient tandem as the metabolic nature of the pre-mitochondrion and the evolutionary stage of the eukaryotic ancestor, are unknown. The latter might have been a rather primitive archaea-like single-celled organism or a more evolved primitive eukaryote with extensive intracellular structure and organization. The pre-mitochondrion likely had quite a different metabolism compared to modern human mitochondria, as supported by the diverse nature of mitochondria found in the different classes of eukaryotes today. What is certain, is that the close cooperation of both cells endowed the pre-eukaryote with a tremendous advantage and thus the ability to evolve into all different classes we know today, including all mammals1.

Just like defects in nuclear DNA, mutations in mitochondrial DNA (mtDNA) can cause disease. It is estimated that more than 1/5000 people have a hereditary mitochondrial disease which can affect all organs. Treatment options for these conditions are limited and often mostly focus on treating symptoms. Studies have shown that the amount of mutant DNA present is important for the appearance of the phenotypes, which mostly present themselves only at high mutant DNA levels (BOX 2)2-4. Recent promising approaches aim to lower the amount of mutant DNA using targeted endonucleases5. A contrasting strategy seeks to raise the copy number of the total mtDNA, an approach that can partially reverse phenotypes of infertility in mice linked to mitochondrial dysfunction6.  A new study confirms these earlier hopes and explores the underlying mechanisms by which a high copy number of mitochondrial DNA can protect against mitochondrial disease7.

BOX 2: The principle of heteroplasmy of mitochondrial mutations
Each cell has several mitochondria that can have several copies of mitochondrial DNA (mtDNA), either wildtype or mutant. As cells divide, the mitochondria are segregated randomly to the daughter cells. This causes a random drift in the number of mutant mtDNA copies present in the cell. Homoplasmic cells have copies of mtDNA with the same, generally wildtype, genotype while heteroplasmic cells have a certain proportion of mutant mtDNA copies. The degree of heteroplasmy is tightly linked to disease phenotypes as most mitochondrial disease symptoms require high copy numbers of haploinsufficient or recessive mutations2.

The study focuses on a mouse strain carrying a mutation in the mitochondrial transfer RNA for the amino acid alanine (tRNAAla). At 20 weeks of age these mice are free of symptoms, but at 50 weeks they have defects in their hearts and colons related to dysfunction of the respiratory electron transport chain. Interestingly, these animals have elevated copy numbers of mitochondrial DNA when symptomatic at older age. The authors of this study hypothesized that this could be a compensatory mechanism of the tissue itself in response to the mutation. They decided to mimic the cells’ own strategy by overexpressing mitochondrial protein TFAM (mitochondrial transcription factor A) which leads to a raised copy number of mtDNA.

Phenotypic studies of these mice show a less severe phenotype when TFAM is overexpressed in both heart and colon tissues while knock-out of the gene has the opposite effect, causing phenotypes already at 20 weeks. Surprisingly, signs of disease in the colon at 50 weeks were markedly reduced again. This, the researchers propose, is the result of rapid turnover of cells in the colon and selection of the heathier cells. To test this, they looked at changes in the proportion of healthy and mutant mitochondrial DNA in specific places in the colon that either displayed the phenotype or appeared normal. This analysis revealed that in TFAM knock-out animals, there is a higher selection for cells without the mutation than in cells that overexpress TFAM, pointing to a protective effect of a higher copy number.

This study demonstrates that in addition to lowering the number of mutant copies of mitochondrial DNA, raising the total copy number can have protecting effects in mitochondrial disease in mice. This could thus prove to become a promising strategy for the human mitochondrial diseases that are caused by high mutation load and are currently untreatable.

The proportion of mutant mitochondrial DNA (mtDNA) generally needs to reach high levels to cause a phenotype (shown here as a blue line). This study shows that raising total mtDNA copy number can raise this threshold and thus protect against mitochondrial diseases.
This figure was compiled using images from the SMART medical image depository at smart.servier.com

References:

1) Martin, W. & Mentel, M. (2010) The Origin of Mitochondria. Nature Education 3(9):58

2) The dynamics of mitochondrial DNA heteroplasmy: implications for human health and disease James B. Stewart & Patrick F. Chinnery Nature Reviews Genetics volume 16, pages 530–542 (2015)

3) N.-G. Larsson, M. H. Tulinius, E. Holme, A. Oldfors, O. Andersen, J. Wahlström, J. Aasly, Segregation and manifestations of the mtDNA tRNALys A→G(8344) mutation of myoclonus epilepsy and ragged-red fibers (MERRF) syndrome. Am. J. Hum. Genet. 51, 1201–1212 (1992).

4) J. P. Grady, S. J. Pickett, Y. S. Ng, C. L. Alston, E. L. Blakely, S. A. Hardy, C. L. Feeney, A. A. Bright, A. M. Schaefer, G. S. Gorman, R. J. Q. McNally, R. W. Taylor, D. M. Turnbull, R. McFarland, mtDNA heteroplasmy level and copy number indicate disease burden in m.3243A>G mitochondrial disease. EMBO Mol. Med. 10, e8262 (2018).

5) S. R. Bacman, J. H. K. Kauppila, C. V. Pereira, N. Nissanka, M. Miranda, M. Pinto, S. L. Williams, N.-G. Larsson, J. B. Stewart, C. T. Moraes, MitoTALEN reduces mutant mtDNA load and restores tRNAAla levels in a mouse model of heteroplasmic mtDNA mutation. Nat. Med. 24, 1696–1700 (2018).

6) M. Jiang, T. E. S. Kauppila, E. Motori, X. Li, I. Atanassov, K. Folz-Donahue, N. A. Bonekamp, S. Albarran-Gutierrez, J. B. Stewart, N.-G. Larsson, Increased total mtDNA copy number cures male infertility despite unaltered mtDNA mutation load. Cell Metab. 26, 429–436.e4 (2017).

7) Filograna R, Koolmeister C, Upadhyay M, et al. Modulation of mtDNA copy number ameliorates the pathological consequences of a heteroplasmic mtDNA mutation in the mouse. Sci Adv. 2019;5(4):eaav9824. Published 2019 Apr 3. doi:10.1126/sciadv.aav9824

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