Chapter 2 Ionizing Radiation-Induced DNA Strand Breaks and γ-H2AX Foci in Cells Exposed to Nitric Oxide Kai Rothkamm and Susanne Burdak-Rothkamm Abstract A number of studies have demonstrated that nitric oxide enhances radiosensitivity of anoxic and hypoxic cells in vitro and in vivo, and some evidence points to a role for DNA damage and repair in this phe- nomenon. We have recently observed that nitric oxide enhances the formation of DNA single- and double-strand breaks following ionising irradiation, measured by the alkaline comet assay and immunoflu- orescence microscopy for γ-H2AX. Key words: Nitric oxide, DNA strand break, γ-H2AX, single cell gel electrophoresis, ionising radiation. 1. Introduction Whilst numerous studies have clearly demonstrated that nitric oxide (NO) enhances radiosensitivity of anoxic and hypoxic cells in vitro and in vivo (1, 2), the exact mechanisms underlying the observed radiosensitisation remain elusive. NO has been reported to upregulate p53, PARP and the cat- alytic subunit of DNA-dependent protein kinase (DNA-PKcs), an enzyme involved in repairing DNA double-strand breaks via the non-homologous end-joining pathway (3–5). On the other hand, it has been reported to inhibit nucleotide excision repair (6). However, it is possible that these data were confounded by the presence of nitrogen dioxide (NO 2 ). Furthermore, there is some evidence for the involvement of NO in ‘bystander’ responses (7) which may occur via the radiation-induced activation of nitric oxide synthase (NOS) (8). H.O. McCarthy, J.A. Coulter (eds.), Nitric Oxide, Methods in Molecular Biology 704, DOI 10.1007/978-1-61737-964-2_2, © Springer Science+Business Media, LLC 2011 17