The maintenance of genome stability is essential to prevent loss of genetic information and the development of diseases such as cancer. One of the most common forms of damage to the genetic code is the oxidation of DNA by reactive oxygen species (ROS), of which 8-oxo-7,8-dihydro-guanine (8-oxoG) is the most frequent modification. Previous studies have established that human single-stranded DNA-binding protein 1 (hSSB1) is essential for the repair of double-stranded DNA breaks by the process of homologous recombination. Here we show that hSSB1 is also required following oxidative damage. Cells lacking hSSB1 are sensitive to oxidizing agents, have deficient ATM and p53 activation and cannot effectively repair 8-oxoGs. Furthermore, we demonstrate that hSSB1 forms a complex with the human oxo-guanine glycosylase 1 (hOGG1) and is important for hOGG1 localization to the damaged chromatin. In vitro, hSSB1 binds directly to DNA containing 8-oxoguanines and enhances hOGG1 activity. These results underpin the crucial role hSSB1 plays as a guardian of the genome.

The maintenance of genome stability is an essential cellular process to prevent the development of diseases including cancer. hSSB1 (NABP2/ OBFC2A) is a critical component of the DNA damage response where it participates in the repair of double-strand DNA breaks and in base excision repair of oxidized guanine residues (8-oxoguanine) by aiding the localization of the human 8-oxoguanine glycosylase (hOGG1) to damaged DNA. Here we demonstrate that following oxidative stress, hSSB1 is stabilized as an oligomer which is required for hSSB1 to function in the removal of 8-oxoguanine. Monomeric hSSB1 shows a decreased affinity for oxidized DNA resulting in a cellular 8-oxoguanine-repair defect and in the absence of ATM signaling initiation. While hSSB1 oligomerization is important for the removal of 8-oxoguanine from the genome, it is not required for the repair of double-strand DNA-breaks by homologous recombination. These findings demonstrate a novel hSSB1 regulatory mechanism for the repair of damaged DNA.

Abstract. The Akt pathway is one of the most common molecular alterations in various human malignancies. However, its involvement in nasopharyngeal carcinoma (NPC) tumorigenesis has not been well established. In this study, the status of Akt activation and expression of its upstream and downstream molecules was investigated in 64 NPC and 38 non-malignant nasopharyngeal tissues by immunohistochemistry. The hotspot mutations of PIK3CA, encoding the p110α catalytic subunit of phosphatidylinositol 3-kinase (PI3K), were also determined in 25 of these NPC tissues. No hotspot mutations were found in any of the samples tested. Akt was activated in 27 (42.2%) and 23 (35.9%) NPCs, as indicated by p-Akt (Thr308) and p-Akt (Ser473) immunoreactivity, respectively. PTEN loss did not correlate statistically with activated Akt. However, a positive correlation was observed between activated Akt and phospho-epidermal growth factor receptor (p-EGFR), suggesting that the EGFR signaling might be one of the upstream regulators of the Akt pathway. The phosphorylation of forkhead (FKHR) and Bcl-2 associated death domain (BAD), but not mamalian target of rapamycin and glycogen synthase kinase-3ß, was significantly correlated with Akt activation. This implies that Akt promotes cell proliferation (as estimated by and survival, at least, through the inactivation of FKHR and BAD in NPC. Our data revealed that the EGFR/PI3K/Akt signaling pathway is important in NPC pathogenesis and that PIK3CA hotspot mutations are rare in NPC.