An early molecular response to
DNA double-strand breaks (DSBs) is phosphorylation of the Ser-139 residue within the terminal SQEY motif of the
histone H2AX. This phosphorylation of H2AX is mediated by the phosphatidyl-inosito 3-kinase (PI3K) family of
proteins,
ataxia telangiectasia mutated (ATM),
DNA-
protein kinase catalytic subunit and ATM and RAD3-related (ATR). The phosphorylated form of H2AX, referred to as gammaH2AX, spreads to adjacent regions of
chromatin from the site of the
DSB, forming discrete foci, which are easily visualized by immunofluorescence microscopy. Analysis and quantitation of gammaH2AX foci has been widely used to evaluate
DSB formation and repair, particularly in response to ionizing radiation and for evaluating the efficacy of various radiation modifying compounds and cytotoxic compounds. Given the exquisite specificity and sensitivity of this de novo marker of DSBs, it has provided new insights into the processes of DNA damage and repair in the context of
chromatin. For example, in radiation biology the central paradigm is that the nuclear
DNA is the critical target with respect to radiation sensitivity. Indeed, the general consensus in the field has largely been to view
chromatin as a homogeneous template for DNA damage and repair. However, with the use of gammaH2AX as molecular marker of DSBs, a disparity in gamma-irradiation-induced gammaH2AX foci formation in
euchromatin and
heterochromatin has been observed. Recently, we used a panel of
antibodies to either mono-, di- or tri- methylated
histone H3 at
lysine 9 (H3K9me1, H3K9me2, H3K9me3) which are epigenetic imprints of constitutive
heterochromatin and transcriptional silencing and
lysine 4 (H3K4me1, H3K4me2,
H3K4me3), which are tightly correlated actively transcribing euchromatic regions, to investigate the spatial distribution of gammaH2AX following ionizing radiation. In accordance with the prevailing ideas regarding
chromatin biology, our findings indicated a close correlation between gammaH2AX formation and active transcription. Here we demonstrate our immunofluorescence method for detection and quantitation of gammaH2AX foci in non-adherent cells, with a particular focus on co-localization with other epigenetic markers, image analysis and 3D-modeling.