Potential role of NER proteins and mitochondrial nucleases Our model assumes that mitochondria in C. elegans, like other metazoans studied to date, lack NER such that UVC-induced mtDNA damage is irreparable and persist- ent (12–15). Recently, however, NER proteins CSA and CSB have been shown to localize to mitochondria and to have a direct role in sensing and responding to oxidative mtDNA damage (117,118), as well as inducing mitochon- drial turnover via autophagy in response to cell stress (119). Based on these studies, an interesting future research direction will be to determine if CSA/CSB aid in the response to helix-distorting mtDNA damage and facilitate the removal of these lesions via autophagy. There is evidence mtDNA harboring double strand breaks resulting from high levels of oxidative lesions can be degraded in vitro by nucleases residing in mitochondria (120–123) of which endonuclease G, involved in nDNA degradation during apoptosis and EXOG, involved in mitochondrial long patch BER (123), remain the primary candidates. Additionally, ﬁssion yeast possesses a UV-damaged DNA endonuclease-dependent excision repair that drives repair of UV photodimers in mtDNA by nicking the phosphodiester bond 50 to the UV-induced photodimers to initiate BER (81). This repair mechanism has not been observed in higher organisms. Still, it is possible that degradation of mtDNA harboring irrepar- able UVC-induced photodimers may in part involve nuclease-dependent degradation. We considered this less likely than our current model for two reasons. Firstly, the degradation rate reported for mtDNA with strand breaks is signiﬁcantly faster than that reported here, with signiﬁ- cant degradation reported within an hour of oxidant exposure (122). Secondly, when we screened viable C. elegans mutants of known endonucleases including cps-6, the C. elegans homolog of endonuclease G using the larval arrest protocol, we found that mutations in cps-6, crn-6 and nuc-1 did not exacerbate larval arrest (Supplementary Figure S3). This indicates that these nu- cleases are not necessary for recovery from UVC-induced mtDNA damage in larval C. elegans and suggests that they are not involved in removal of UVC-induced mtDNA damage. However, because of the potential redundancy in mitochondrial nucleases, we cannot exclude a role of nucleases in recovery from and removal of UV-induced mtDNA damage. Broader implications of persistent mtDNA damage removal In summary, our results suggest a model whereby mitochondrial dynamics and autophagy play a role both speciﬁcally in removal of mtDNA damage, and more broadly in the response to mtDNA damage and mitochon- drial dysfunction (Figure 8). This research shows that UVC-induced mtDNA damage can be removed, and furthermore demonstrates that mitochondrial dynamics and autophagy are part of a novel pathway for the removal of otherwise irreparable mtDNA damage. Based on this and published literature, we hypothesize that initial recovery of mitochondrial function from such Nucleic Acids Research, 2012, Vol. 40, No. 16 7927 mtDNA damage requires mitochondrial fusion and mtDNA replication, and is assisted by removal of this damage via autophagy. Upon recovery from mtDNA damage, mitochondrial ﬁssion is restored creating a heterogeneous population of mitochondria, some with a high proportion of damaged mtDNA, which are removed by autophagy, potentially in a selective fashion involving PINK1. This pathway is of potentially great signiﬁcance because of the fact that many common environmental contaminants cause high levels of mtDNA damage that, like UVC-induced damage, is not repaired in the mito- chondrial genome. This pathway may also be important for removal of a subset of oxidative mtDNA damage (124) and damage induced by certain biological aldehydes (125) that are repaired in the nucleus by NER, extending the importance of our ﬁndings to damage caused by normal metabolism. Finally, mutations in genes in these pathways are associated with human disease states, demonstrating the potential for important gene–environment interactions affecting mitochondrial health after genotoxin exposure. SUPPLEMENTARY DATA Supplementary Data are available at NAR Supplementary Figures 1–3. Online: ACKNOWLEDGEMENTS We would like to thank Dr Alexander van der Bliek, Dr Ding Xue, Adam Knight, Dr Guy Caldwell and Dr Christina Lagido for providing mutant and transgenic C. elegans strains, Dr Windy Boyd for assistance with RNAi, Alex Ji for assistance with oxygen consumption experiments and Dr Alicia Melendez for advice on LGG-1::GFP analysis. FUNDING National Institute of Environmental Health Sciences [1 P30 ES-011961-01A1, 1R01-ES017540-01A2]; National Institute of Neurological Disorders and Stroke [1 R21-NS065468-01]. Funding for open access charge: National Institute of Environmental Health Sciences [1 P30 ES-011961-01A1, 1R01-ES017540-01A2]. Conﬂict of interest statement. None declared. REFERENCES 1. Greaves,L.C., Reeve,A.K., Taylor,R.W. and Turnbull,D.M. (2012) Mitochondrial DNA and disease. J. Pathol., 226, 274–286. 2. Bossy-Wetzel,E., Barsoum,M.J., Godzik,A., Schwarzenbacher,R. and Lipton,S.A. (2003) Mitochondrial ﬁssion in apoptosis, neurodegeneration and aging. Curr. Opin. Cell Biol., 15, 706–716. 3. Lin,M.T. and Beal,M.F. (2006) Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases. Nature, 443, 787–795. 4. Lowell,B.B. and Shulman,G.I. (2005) Mitochondrial dysfunction and type 2 diabetes. Science, 307, 384–387. 5. Brandon,M., Baldi,P. and Wallace,D.C. (2006) Mitochondrial mutations in cancer. Oncogene, 25, 4647–4662. 6. Backer,J.M. and Weinstein,I.B. (1982) Interaction of Benzo(a)pyrene and its dihydrodiol-epoxide derivative with