Elimination of rNMPs from mitochondrial DNA has no effect on its stability

Area of Science:

• Mitochondrial biology
• Genome stability
• Molecular genetics

Background:

• Ribonucleotides (rNMPs) pose a threat to nuclear genome stability and can cause DNA strand breaks.
• While tolerated in mitochondrial DNA (mtDNA), aberrant rNMP levels are implicated in disease models.
• The impact of rNMPs on mtDNA stability throughout an organism's lifespan remains unclear.

Purpose of the Study:

• To investigate the effect of incorporated rNMPs on mtDNA stability across the lifespan of mice.
• To determine if physiological rNMP loads contribute to age-related mtDNA quality decline.

Main Methods:

• Mice lifespan study to assess mtDNA rNMP content.
• Tissue-specific analysis of rNMP levels correlated with the rNTP/dNTP ratio.
• Analysis of mtDNA copy number, deletions, and strand breaks in aged mice, including SAMHD1 knockout models with altered dNTP pools.

Main Results:

• mtDNA rNMP content increased in early life and varied by tissue, correlating with the rNTP/dNTP ratio.
• SAMHD1 knockout mice exhibited near-absent mtDNA rNMPs.
• The absence of mtDNA rNMPs did not significantly alter mtDNA copy number or the prevalence of mtDNA damage (deletions, strand breaks) in aged mice.

Conclusions:

• Physiological levels of rNMPs in mtDNA do not appear to compromise mtDNA stability or quality during aging.
• The age-related decline in mtDNA quality is not driven by the accumulation of physiological rNMP loads.
• mtDNA tolerates a certain level of rNMPs without detrimental effects on genome integrity over time.