BioMed Central Page 1 of 12 (page number not for citation purposes) Epigenetics & Chromatin Open Access Research Both telomeric and non-telomeric DNA damage are determinants of mammalian cellular senescence Asako J Nakamura* 1 , Y Jeffrey Chiang 2 , Karen S Hathcock 2 , Izumi Horikawa 3 , Olga A Sedelnikova 1 , Richard J Hodes 2,4 and William M Bonner* 1 Address: 1 Laboratory of Molecular Pharmacology, National Cancer Institute, National Institutes of Health, Rockville Pike, Bethesda, MD 20892, USA, 2 Experimental Immunology Branch, National Cancer Institute, National Institutes of Health, Rockville Pike, Bethesda, MD 20892, USA, 3 Laboratory of Biosystems and Cancer, National Cancer Institute, National Institutes of Health, Rockville Pike, Bethesda, MD 20892, USA and 4 National Institute on Aging, National Institutes of Health, Rockville Pike, Bethesda MD 20892, USA Email: Asako J Nakamura* - nakamuraa@mail.nih.gov; Y Jeffrey Chiang - chiangj@mail.nih.gov; Karen S Hathcock - hathcock@exchange.nih.gov; Izumi Horikawa - horikawi@mail.nih.gov; Olga A Sedelnikova - SedelniO@mail.nih.gov; Richard J Hodes - hodesr@31.nia.nih.gov; William M Bonner* - bonnerw@mail.nih.gov * Corresponding authors Abstract Background: Cellular senescence is a state reached by normal mammalian cells after a finite number of cell divisions and is characterized by morphological and physiological changes including terminal cell-cycle arrest. The limits on cell division imposed by senescence may play an important role in both organismal aging and in preventing tumorigenesis. Cellular senescence and organismal aging are both accompanied by increased DNA damage, seen as the formation of γ-H2AX foci (γ-foci), which may be found on uncapped telomeres or at non-telomeric sites of DNA damage. However, the relative importance of telomere- and non-telomere-associated DNA damage to inducing senescence has never been demonstrated. Here we present a new approach to determine accurately the chromosomal location of γ-foci and quantify the number of telomeric versus non-telomeric γ-foci associated with senescence in both human and mouse cells. This approach enables researchers to obtain accurate values and to avoid various possible misestimates inherent in earlier methods. Results: Using combined immunofluorescence and telomere fluorescence in situ hybridization on metaphase chromosomes, we show that human cellular senescence is not solely determined by telomeric DNA damage. In addition, mouse cellular senescence is not solely determined by non-telomeric DNA damage. By comparing cells from different generations of telomerase-null mice with human cells, we show that cells from late generation telomerase-null mice, which have substantially short telomeres, contain mostly telomeric γ-foci. Most notably, we report that, as human and mouse cells approach senescence, all cells exhibit similar numbers of total γ-foci per cell, irrespective of chromosomal locations. Conclusion: Our results suggest that the chromosome location of senescence-related γ-foci is determined by the telomere length rather than species differences per se. In addition, our data indicate that both telomeric and non-telomeric DNA damage responses play equivalent roles in signaling the initiation of cellular senescence and organismal aging. These data have important implications in the study of mechanisms to induce or delay cellular senescence in different species. Published: 3 November 2008 Epigenetics & Chromatin 2008, 1:6 doi:10.1186/1756-8935-1-6 Received: 25 June 2008 Accepted: 3 November 2008 This article is available from: http://www.epigeneticsandchromatin.com/content/1/1/6 © 2008 Nakamura et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0 ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.