An additional epigenetic–mitochondrial interaction could be the alteration of mtDNA methylation by environmental stressors, although it is currently unclear whether mtDNA transcription is linked to altered mtDNA methylation in the same manner as is nuclear DNA (nDNA) methylation. Furthermore, effects of exposures on putative mitochondrial epigenetic states will generally occur in the context of direct effects on both mitochondrial and nuclear epigenetics; these effects may or may not be mechanistically linked. For example, what are the steps involved in response to air pollution? Is the mitochondrial response an early step in cellular reprogramming (., an increase in mitochondrial content or biogenesis, followed by alterations in methylation of nuclear-encoded mitochondrial genes)? Recent human studies have demonstrated effects of air pollution exposure on mtDNA copy number, a marker that can be applied in large population studies and may reflect both mtDNA damage and dysfunction ( Carugno et al. 2012 ; Hou et al. 2010 , 2013 ; Janssen et al. 2012 ; Pavanello et al. 2013 ). There is growing evidence suggesting that air pollution exposure modifies methylation not only in the nDNA but also in the mtDNA ( Baccarelli et al. 2009 ). Although this finding might help to identify individuals at higher risk of air pollution effects, including acute and long-term cardiorespiratory disease, lung cancer, and neurological effects, there are conflicting reports in the literature regarding the function of mtDNA methylation ( Dzitoyeva et al. 2012 ; Hong et al. 2013 ; Iacobazzi et al. 2013 ). A fundamental question is whether cytosine methylation takes place in mtDNA, particularly in sequences that are rich in CpG dinucleotides. Intriguingly, recent evidence appears to suggest that methylation can occur in cytosines both in a CpG context and in cytosines that are not in CpG sites. Specifically, increased cytosine methylation has been observed in promoter regions of the mitochondria heavy strand located at the 5´-end of the D-loop (involved in DNA synthesis), suggesting a role in regulating mtDNA replication. Moreover, the observation of 5-hydroxymethylcytosine in mtDNA provides additional evidence that mtDNA may be epigenetically regulated. That this base has been established without the action of TET dioxygenases, which do not contain a mitochondrial targeting sequence, suggests other modes for demethylation and ultimately metabolic reprogramming that could be mediated via cross-talk with the nucleus ( Bellizzi et al. 2013 ; Shock et al. 2011 ).
Professor Chan is an executive editor of Advanced Drug Delivery Reviews and on the editorial advisory boards of various pharmaceutical journals, including PharmaceuticalResearch and International Journal of Pharmaceutics . He was appointed to the Subcommittee on Pharmaceutical Aerosol Standards for the Australian Therapeutic Goods Administration in 1996–97, and served as a member (2007–08) on the in vitro – in vivo correlation subgroup of the International Pharmaceutical Aerosol Consortium on Regulation and Science in Washington DC. He is a Fellow of the American Association of Pharmaceutical Scientists (AAPS), Fellow of Royal Australian Chemical Institute (RACI), Chair of the NSW Pharmaceutical Science Group of the RACI, and Vice President of the Asian Federation for Pharmaceutical Sciences.