Histone modifications and DNA double-strand break repair after exposure to ionizing radiations

Chromatin structure and radiation-induced DNA damage: from structural biology to radiobiology

Genomic DNA in eukaryotic cells is basically divided into chromosomes, each consisting of a single huge nucleosomal fiber. It is now clear that chromatin structure and dynamics play a critical role in all processes involved in DNA metabolism, e.g. replication, transcription, repair and recombination. Radiation is a useful tool to study the biological effects of chromatin alterations. Conversely, radiotherapy and radiodiagnosis raise questions about the influence of chromatin integrity on clinical features and secondary effects. This review focuses on the link between DNA damage and chromatin structure at different scales, showing how a comprehensive multiscale vision is required to understand better the effect of radiations on DNA. Clinical aspects related to high- and low-dose of radiation and chromosomal instability will be discussed. At the same time, we will show that the analysis of the radiation-induced DNA damage distribution provides good insight on chromatin structure. Hence, we argue that chromatin "structuralists" and radiobiological "clinicians" would each benefit from more collaboration with the other. We hope that this focused review will help in this regard.

Histone H4 lysine 16 acetylated isoform synthesis opens new route to biophysical studies

Histone H4 lysine acetylation regulated by MOF (males absent on the first) was initially discovered as a dosage compensation epigenetic mark. Recent studies have revealed, however, that the epigenetic mark has a critical role in cellular function both during oogenesis as well as oncogenesis. Detailed molecular analysis of H4K16 isoforms and other posttranslational modified histones has been limited by the lack of means to prepare sufficient material for in vitro study. This paper describes an improved method to prepare acetylated H4K16 as well as other covalently modified histone H4 isoform for biophysical studies.

Role of 53BP1 in the regulation of DNA double-strand break repair pathway choice

The p53-binding protein 1 (53BP1) is a well-known DNA damage response (DDR) factor, which is recruited to nuclear structures at the site of DNA damage and forms readily visualized ionizing radiation (IR) induced foci. Depletion of 53BP1 results in cell cycle arrest in G2/M phase as well as genomic instability in human as well as mouse cells. Within the DNA damage response mechanism, 53BP1 is classified as an adaptor/mediator, required for processing of the DNA damage response signal and as a platform for recruitment of other repair factors. More recently, specific 53BP1 contributions to DSB repair pathway choice have been recognized and are being characterized. In this review, we have summarized recent advances in understanding the role of 53BP1 in regulating DNA DSBs repair pathway choice, variable diversity joining [V(D)J] recombination and class-switch recombination (CSR).

Characterization of two different Asf1 histone chaperones with distinct cellular localizations and functions in Trypanosoma brucei

The anti-silencing function protein 1 (Asf1) is a chaperone that forms a complex with histones H3 and H4 facilitating dimer deposition and removal from chromatin. Most eukaryotes possess two different Asf1 chaperones but their specific functions are still unknown. Trypanosomes, a group of early-diverged eukaryotes, also have two, but more divergent Asf1 paralogs than Asf1 of higher eukaryotes. To unravel possible different functions, we characterized the two Asf1 proteins in Trypanosoma brucei. Asf1A is mainly localized in the cytosol but translocates to the nucleus in S phase. In contrast, Asf1B is predominantly localized in the nucleus, as described for other organisms. Cytosolic Asf1 knockdown results in accumulation of cells in early S phase of the cell cycle, whereas nuclear Asf1 knockdown arrests cells in S/G2 phase. Overexpression of cytosolic Asf1 increases the levels of histone H3 and H4 acetylation. In contrast to cytosolic Asf1, overexpression of nuclear Asf1 causes less pronounced growth defects in parasites exposed to genotoxic agents, prompting a function in chromatin remodeling in response to DNA damage. Only the cytosolic Asf1 interacts with recombinant H3/H4 dimers in vitro. These findings denote the early appearance in evolution of distinguishable functions for the two Asf1 chaperons in trypanosomes.

Significant associations between X-ray repair cross-complementing group 3 genetic polymorphisms and thyroid cancer risk

Polymorphisms in X-ray cross-complementing group 3 (XRCC3) are proposed to be associated with cancer susceptibility, but previous studies on the associations between XRCC3 polymorphisms and thyroid cancer are controversial. We performed a systemic review and meta-analysis to investigate the associations of XRCC3 polymorphisms with thyroid cancer risk. We used odds ratio (OR) with 95 % confidence interval (95%CI) to assess the associations. For XRCC3 C241T polymorphism, meta-analysis of total eligible studies showed that there was no association between XRCC3 C241T polymorphism and thyroid cancer risk, but subgroup analysis in Caucasians showed that there was a significant association between XRCC3 C241T polymorphism and thyroid cancer risk (T versus C: OR = 1.30, 95%CI 1.05-1.62, P = 0.01; TT versus CC: OR = 1.74, 95%CI 1.13-2.70, P = 0.01; TT versus

CC/CT

OR = 1.74, 95%CI 1.16-2.60, P = 0.007). For XRCC3 A17893G polymorphism, meta-analysis of total eligible studies showed that there was an obvious association between XRCC3 A17893G polymorphism and thyroid cancer risk (GG versus AA/AG: OR = 0.57, 95%CI 0.35-0.93, P = 0.02), but subgroup analysis by ethnicity only identify the significant association in Asians. In summary, the meta-analysis suggests that there are significant associations of XRCC3 polymorphisms with thyroid cancer risk. Besides, more studies with large sample sizes are needed to further assess the associations above.

γH2AX foci formation in the absence of DNA damage: mitotic H2AX phosphorylation is mediated by the DNA-PKcs/CHK2 pathway

Phosphorylated H2AX is considered to be a biomarker for DNA double-strand breaks (DSB), but recent evidence suggests that γH2AX does not always indicate the presence of DSB. Here we demonstrate the bimodal dynamic of H2AX phosphorylation induced by ionizing radiation, with the second peak appearing when G2/M arrest is induced. An increased level of γH2AX occurred in mitotic cells, and this increase was attenuated by DNA-PKcs inactivation or Chk2 depletion, but not by ATM inhibition. The phosphorylation-mimic CHK2-T68D abrogated the attenuation of mitotic γH2AX induced by DNA-PKcs inactivation. Thus, the DNA-PKcs/CHK2 pathway mediates the mitotic phosphorylation of H2AX in the absence of DNA damage.

The role of inflammatory pathways in cancer-associated cachexia and radiation resistance

Dysregulated inflammatory responses are key contributors to a multitude of chronic ailments, including cancer. Evidence indicates that disease progression in cancer is dependent on the complex interaction between the tumor and the host microenvironment. Most recently, the inflammatory response has been suggested to be critical, as both the tumor and microenvironment compartments produce cytokines that act on numerous target sites, where they foster a complex cascade of biologic outcomes. Patients with cancer-associated cachexia (CAC) suffer from a dramatic loss of skeletal muscle and adipose tissue, ultimately precluding them from many forms of therapeutic intervention, including radiotherapy. The cytokines that have been linked to the promotion of the cachectic response may also participate in radiation resistance. The major changes at the cytokine level are, in part, due to transcriptional regulatory alterations possibly due to epigenetic modifications. Herein we discuss the role of inflammatory pathways in CAC and examine the potential link between cachexia induction and radiation resistance.