Origins of human mitochondrial point mutations as DNA polymerase gamma-mediated errors

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Mitochondrial DNA (mtDNA) mutational spectra in human cells, tissues, and tumors are remarkably similar.
• This similarity suggests that endogenous processes, rather than exogenous factors, are the primary drivers of mtDNA mutations.

Purpose of the Study:

• To investigate the hypothesis that errors by mitochondrial DNA polymerase gamma (Pol γ) are the predominant source of mtDNA point mutations.
• To compare in vitro mutations generated by human Pol γ with known in vivo mutational hotspots in human tissues.

Main Methods:

• In vitro point mutation generation using human DNA Pol γ in a specific mtDNA sequence.
• Comparison of in vitro generated mutations with eighteen previously identified in vivo mutational hotspots in human tissues.
• Analysis of mutation behavior during Polymerase Chain Reaction (PCR) with varying heating times at 98°C.

Main Results:

• Mutations generated by human Pol γ in vitro showed significant concordance with in vivo mutational hotspots, accounting for 83% of observed events.
• Approximately half of the mutations were resistant to prolonged PCR heating, while the other half increased proportionally with heating time.
• This suggests that both primary misincorporation errors and miscopying errors past thermal degradation products (like uracil from deaminated cytosines) contribute to the mutational spectrum.

Conclusions:

• Endogenous errors mediated by DNA Pol γ are the primary source of mitochondrial point mutations in the studied human tissue sequence.
• Both direct misincorporation and misrepair/miscopied errors contribute approximately equally to the observed mitochondrial mutational spectrum.