Mitochondrial DNA damage as driver of cellular outcomes

Abstract

Mitochondria are primarily involved in energy production through the process of oxidative phosphorylation (OXPHOS). Increasing evidence has shown that mitochondrial function impacts a plethora of different cellular activities, including metabolism, epigenetics, and innate immunity. Like the nucleus, mitochondria own their genetic material, but this organellar genome is circular, present in multiple copies, and maternally inherited. The mitochondrial DNA (mtDNA) encodes 37 genes that are solely involved in OXPHOS. Maintenance of mtDNA, through replication and repair, requires the import of nuclear DNA-encoded proteins. Thus, mitochondria completely rely on the nucleus to prevent mitochondrial genetic alterations. As most cells contain hundreds to thousands of mitochondria, it follows that the shear number of organelles allows for the buffering of dysfunction-at least to some extent-before tissue homeostasis becomes impaired. Only red blood cells lack mitochondria entirely. Impaired mitochondrial function is a hallmark of aging and is involved in a number of different disorders, including neurodegenerative diseases, diabetes, cancer, and autoimmunity. Although alterations in mitochondrial processes unrelated to OXPHOS, such as fusion and fission, contribute to aging and disease, maintenance of mtDNA integrity is critical for proper organellar function. Here, we focus on how mtDNA damage contributes to cellular dysfunction and health outcomes.

Keywords: DNA repair; cellular outcomes; mitochondria; mitochondrial genome; mtDNA damage.

Figure 1.

Mitochondrial DNA damage and repair. Exogenous and endogenous sources can produce different lesions in mitochondrial DNA (mtDNA), like single-strand (SSB) and double-strand DNA breaks (DSB), complex bulky lesions, e.g., DNA protein crosslinks (DPCs), and DNA inter- and intrastrand crosslinks. Specialized mitochondrial DNA repair pathways and quality control systems safeguard mtDNA genome stability, as mitochondrial base excision repair (mtBER), mitophagy, degradation, or recombination. The lesion types and their respective means of removal are shown on the right.

Figure 2.

Fates of mitochondrial DNA damage. Mitochondrial DNA (mtDNA) damage can lead to different outcomes, depending on the type of lesion present on the genome. The three main outcomes associated with the presence of damage on the mtDNA are shown.

Figure 3.

H₂O₂ causes a reduction of NADH:ubiquinone oxidoreductase core subunit I (ND1) and cytb mRNA levels and mRNA half-life in human fibroblasts. SV-40 transformed human fibroblasts were treated with 200 µM H₂O₂ for 15 or 60 min and harvested immediately after treatment (time 0), or allowed to recover for 1.5 and 3 h. Total RNA was isolated and analyzed by Northern blot. Signals are expressed as percentage of the controls after normalization to 18S ribosomal RNA; ND1 (A) and cytb (B). Data are presented as means ± SE of n = 3 independent biological replicates. Statistical analysis was performed by one-way ANOVA. *P < 0.05, **P < 0.01, and ***P < 0.001 versus control. Total RNA was isolated from cells cultured in the presence or absence of 200 µM H₂O₂ at 0, 15, 30, 45, and 60 min after incubation with 1 µg/mL actinomycin D. Linear regression analysis of the relative amounts of ND1 (C) and cytb (D) mRNA after normalization to 18S ribosomal RNA signal. Differences between the slopes are at the P < 0.05 (ND1) and P < 0.01 (cytb).

Figure 4.

Cross talk between mitochondrial DNA (mtDNA) and immune response activation. Inflammation can be triggered by several stimuli that include mitochondrial nucleic acids release or low levels of mitochondrial transcription factor A (TFAM). Specific sensing mechanisms engage different immune responses, like the nucleotide-binding oligomerization domain leucine-rich repeat and pyrin domain-containing protein 3 (NLRP3) inflammasomes and the type I interferon response.