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

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 approximately 70% of the lesions being removed by 24 h. Additionally, there is efficient repair of N-methylpurines following exposure to methylnitrosourea with approximately 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.

Mitochondria, oxidative DNA damage, and aging

“…In contrast to the above lesions, mitochondria cannot repair complex alkylation damage generated by Nnitroso-N-butylurea (134), or by bis(2-chloroethylmethyl)-amine (127), nor can they repair 6-4 photoproducts (126) or psoralen induced interstrand crosslinks (138), and there is only minimal repair of cisplatin intrastrand crosslinks (127). The latter finding is consistent with their inability to repair pyrimidine dimers.…”

“…It was assumed that mitochondria completely lacked the ability to repair DNA. Although it has been confirmed that mitochondria cannot repair pyrimidine dimers (127,126), it is now known that DNA repair is indeed present in mitochondria, and that many other types of damage are repaired quite efficiently.…”

“…This assay and its variations have been used to measure DNA repair of mtDNA in human and hamster cells without the need for mitochondrial isolation, and a variety of mtDNA repair activities have been observed. Alkali-labile sites induced by nitrosourea streptozotocin (136), N-methylpurines following exposure to methylnitrosourea (127), and N7 methylguanine adducts induced bytreatment with methyl methanesulfonate (MMS) (137) are all lesions for which removal from the mitochondrial genome has been observed.…”

Mitochondrial DNA repair in aging and disease

“…However, subsequent studies have demonstrated that certain types of damage to DNA bases resulting from deamination, simple alkylation and oxidation can be efficiently repaired in mitochondria (reviewed in 29-31). Thus, the major DNA repair mechanism acting in mitochondria is base excision repair (BER) (29)(30)(31)(32)(33)(34). In general, BER starts with recognition and removal of a damaged or inappropriate base by a DNA glycosylase that cleaves the Nglycosylic bond between the base and the sugar.…”

“…Mitochondrial BER has been demonstrated by numerous groups and shown to target oxidatively modified DNA bases, such as 8-oxoG and thymine glycol (29)(30)(31)(32)(33)(34)36,37). DNA repair enzymes isolated from mitochondria include several types of damage-specific DNA glycosylases (36)(37)(38)(39), AP endonuclease (40), and DNA ligase (41,42).…”