Repair of mitochondrial DNA after various types of DNA damage in Chinese hamster ovary cells

TLDR

Mitochondrial DNA repair has long been assumed absent, although a base‑excision–like process may exist but no nucleotide‑excision pathway has been identified. The study aimed to determine which DNA repair mechanisms operate in mitochondria by examining repair of specific DNA lesions. Using a recently developed gene‑specific DNA repair assay, the authors investigated repair of cyclobutane dimers, cisplatin intrastrand and interstrand crosslinks, and alkali‑labile sites in mitochondrial DNA. They found that mitochondrial DNA does not repair pyrimidine dimers or complex alkylation damage, repairs cisplatin intrastrand crosslinks minimally, but efficiently removes cisplatin interstrand crosslinks and N‑methylpurines (~70 % by 24 h), showing that repair capacity depends on lesion type.

Abstract

Using methodology recently developed to assess gene-specific DNA repair, we have demonstrated that it is possible not only to study mitochondrial DNA repair, but also directly to compare mitochondrial and nuclear DNA repair in the same biological sample. Complex enzymatic mechanisms recognize and repair nuclear DNA damage, but it has long been thought that there was no DNA repair in mitochondria. Therefore, in an attempt to delineate more clearly which DNA repair mechanisms, if any, are functioning in mitochondria, we have investigated the repair of several specific DNA lesions in mitochondrial DNA. They include cyclobutane dimers, cisplatin intrastrand adducts, cisplatin interstrand crosslinks and alkali-labile sites. We find that pyrimidine dimers and complex alkylation damage are not repaired in mitochondrial DNA, and that there is minimal repair of cisplatin intrastrand crosslinks. In contrast, there is efficient repair of cisplatin interstrand crosslinks as evidenced by ∼ 70% of the lesions being removed by 24 h. Additionally, there is efficient repair of N-methylpurines following exposure to methylnitrosourea with ∼ 70% of the lesions being removed by 24 h. The results of these studies reveal that repair capacity of mitochondrial DNA damage depends upon the type of lesion produced by the damaging agent. We speculate that a process similar to the base excision mechanism for nuclear DNA exists for mitochondrial DNA but that there is no nucleotide excision repair mechanism to remove more bulky lesions in this organelle.