Nuclear DNA repair mutants exhibit progeroid symptoms. There are many types of DNA damage, and, accordingly, we have evolved mechanisms to deal with each type of damage. Nucleotide excision repair removes bulky adducts, and base excision repair removes damaged bases. Mismatch repair fixes nucleotides that aren’t matched in their correct A:T/G:C configuration. Lastly, non-homologous end joining and recombination can fix double stranded breaks. Deficiencies in several of these repair mechanisms have been implicated in aging, and they may play a role in age-related disease.
Repair mechanisms also exist for mitochondrial DNA. Mitochondria have robust base excision repair, and there is new evidence for mismatch repair. But do deficiencies in mtDNA repair play a similar role in aging? We’ve already seen that mitochondrial DNA damage accumulates with age. And calorie restriction, the gold standard of lifespan extension, prevents this increase in damage. A new report in Nucleic Acids Research looks at the controversy over when mitochondria DNA deletions occur: is it during replication or repair? Using a proofreading-deficient mitochondrial DNA polymerase (POLG), which causes premature aging phenotypes and early death, Bailey et al provide supporting evidence that the majority of damage comes from replication pausing and breakage at fragile sites.
This particular mutation in POLG results in high levels of point mutations and linear DNA (mtDNA is normally circular). Most of the linear fragments came from one particular region, and POLG is known to initiate and pause replication at specific mtDNA regions. This indicates a common location of breakage. The authors showed that the POLG mutant mouse had increased replication intermediates compared to wild type. Because of the high number of point mutations, POLG might be stalling at sites it is attempting to repair. In this manner, DNA damage is acting as a checkpoint for replication. POLG mutant mice mtDNA is also more sensitive to the single stranded nuclease S1, indicating chromosomal breakage. These single stranded ends can give rise to deletions through recombination.
The authors argue that the increased level of chromosomal breakage and the replicative pausing in the mutant mouse are responsible for the progeroid symptoms of the POLG mouse. In their view, mitochondrial DNA replication is actually upregulated in order to compensate for the reduction in replication capacity. Because of the high levels in point mutations, ox/phos activity would be decreased, which might lead to an even greater need for mitochondrial DNA replication.
And because DNA processing resources – nucleotide precursors as well as enzymes such as RNase H1, Flap endonuclease, and Brca1 – are shared by mitochondria and the nucleus, it is possible that there is a connection between the POLG mutator mouse and mutations in nuclear DNA repair proteins. The phenotype of DNA repair mutants could be caused not by mutations themselves, but by the effort it takes to prevent DNA mutation from occurring past some threshold which would cause cellular catastrophe. The authors note the similarities between POLG and WRN, a helicase in the nucleus. Like POLG, WRN is involved with both DNA replication and DNA repair. Mutations in WRN cause similar DNA breakage and lead to the human progeroid Werner syndrome.
What do you think? Is it possible that the problem in progeroid models is not due to the DNA damage itself, but to the energy required to prevent a catastrophic collapse of DNA integrity?
Bailey, L., Cluett, T., Reyes, A., Prolla, T., Poulton, J., Leeuwenburgh, C., & Holt, I. (2009). Mice expressing an error-prone DNA polymerase in mitochondria display elevated replication pausing and chromosomal breakage at fragile sites of mitochondrial DNA Nucleic Acids Research DOI: 10.1093/nar/gkp091