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.

Friedreich ataxia is a neurodegenerative disease with an autosomal recessive inheritance. In most patients, the disease is caused by the presence of trinucleotide GAA expansions in the first intron of the frataxin gene. These expansions cause the decreased expression of this mitochondrial protein. Many evidences indicate that frataxin deficiency causes the deregulation of cellular iron homeostasis. In this review, we will discuss several hypotheses proposed for frataxin function, their caveats, and how they could provide an explanation for the deregulation of iron homeostasis found in frataxin-deficient cells. We will also focus on the potential mechanisms causing cellular dysfunction in Friedreich Ataxia and on the potential use of the iron chelator deferiprone as a therapeutic agent for this disease.

Lung adenocarcinoma is a common aggressive cancer and a leading cause of mortality worldwide. Here, we report an important in vivo role for mitochondrial DNA (mtDNA) copy number during lung adenocarcinoma progression in the mouse. We found that lung tumors induced by KRAS ^G12D expression have increased mtDNA levels and enhanced mitochondrial respiration. To experimentally assess a possible causative role in tumor progression, we induced lung cancer in transgenic mice with a general increase in mtDNA copy number and found that they developed a larger tumor burden, whereas mtDNA depletion in tumor cells reduced tumor growth. Immune cell populations in the lung and cytokine levels in plasma were not affected by increased mtDNA levels. Analyses of large cancer databases indicate that mtDNA copy number is also important in human lung cancer. Our study thus reports experimental evidence for a tumor-intrinsic causative role for mtDNA in lung cancer progression, which could be exploited for development of future cancer therapies.

Yeast frataxin homolog (Yfh1) is the orthologue of human frataxin, a mitochondrial protein whose deficiency causes Friedreich Ataxia. Yfh1 deficiency activates Aft1, a transcription factor governing iron homeostasis in yeast cells. Although the mechanisms causing this activation are not completely understood, it is assumed that it may be caused by iron-sulfur deficiency. However, several evidences indicate that activation of Aft1 occurs in the absence of iron-sulfur deficiency. Besides, Yfh1 deficiency also leads to metabolic remodeling (mainly consisting in a shift from respiratory to fermentative metabolism) and to induction of Yhb1, a nitric oxide (NO) detoxifying enzyme. In this work, we have used conditional Yfh1 mutant yeast strains to investigate the relationship between NO, Aft1 activation and metabolic remodeling. We have observed that NO prevents Aft1 activation caused by Yfh1 deficiency. This phenomenon is not observed when Aft1 is activated by iron scarcity or impaired iron-sulfur biogenesis. In addition, analyzing key metabolic proteins by a targeted proteomics approach, we have observed that NO prevents the metabolic remodeling caused by Yfh1 deficiency. We conclude that Aft1 activation in Yfh1-deficient yeasts is not caused by iron-sulfur deficiency or iron scarcity. Our hypothesis is that Yfh1 deficiency leads to the presence of anomalous iron species that can compromise iron bioavailability and activate a signaling cascade that results in Aft1 activation and metabolic remodeling.