Abstract
Ionizing radiation causes DNA double strand breaks, which initiate ATM-dependent DNA damage signalling. Activated ATM phosphorylates histone H2AX at serine 139, whose phosphorylation is essential for recruitment of MDC1. MDC1 is another target for ATM, and its phosphorylation recruits RNF8, a ubiquitin ligase, which mediates ubiquitination of histone H2AX. Then, 53BP1 is recruited to the modified chromatin region, and all of the factors form DNA damage checkpoint foci. Although phosphorylation and ubiquitination are important modification for DNA damage foci, there are other major histone modifications, such as acetylation and methylation. Here, we examined a role of these modifications in DNA damage foci formation. Normal human diploid cells were incubated in a special culture condition, in which acetylation or methylation is suppressed. Then, cells were exposed to gamma-rays to investigate the foci of phosphorylated histone H2AX and 53BP1. We found that decreased acetylation of histone H3 and di-methylation of histone H3 at lysine 9 did not affect initial foci formation but they ablated persistent foci formation. These results indicate that chromatin modification is essential for proper formation of DNA damage checkpoint foci in normal human cells exposed to ionizing radiation.
