Upon replication fork stalling, the RPA-coated single-stranded DNA (ssDNA) formed behind the fork activates the ataxia telangiectasia-mutated and Rad3-related (ATR) kinase, concomitantly initiating Rad18-dependent monoubiquitination of PCNA. However, whether crosstalk exists between these two events and the underlying physiological implications of this interplay remain elusive. In this study, we demonstrate that during replication stress, ATR phosphorylates human Rad18 at Ser403, an adjacent residue to a previously unidentified PIP motif (PCNA-interacting peptide) within Rad18. This phosphorylation event disrupts the interaction between Rad18 and PCNA, thereby restricting the extent of Rad18-mediated PCNA monoubiquitination. Consequently, excessive accumulation of the tumor suppressor protein SLX4, now characterized as a novel reader of ubiquitinated PCNA, at stalled forks is prevented, contributing to the prevention of stalled fork collapse. We further establish that ATR preserves telomere stability in alternative lengthening of telomere (ALT) cells by restricting Rad18-mediated PCNA monoubiquitination and excessive SLX4 accumulation at telomeres. These findings shed light on the complex interplay between ATR activation, Rad18-dependent PCNA monoubiquitination, and SLX4-associated stalled fork processing, emphasizing the critical role of ATR in preserving replication fork stability and facilitating telomerase-independent telomere maintenance.

Although located at the chromosome end, telomeres are an essential chromosome component that helps maintain genome integrity and chromosome stability from protozoa to mammals. The role of telomere proteins in chromosome end protection is conserved, where they suppress various DNA damage response machineries and block nucleolytic degradation of the natural chromosome ends, although the detailed underlying mechanisms are not identical. In addition, the specialized telomere structure exerts a repressive epigenetic effect on expression of genes located at subtelomeres in a number of eukaryotic organisms. This so-called telomeric silencing also affects virulence of a number of microbial pathogens that undergo antigenic variation/phenotypic switching. Telomere proteins, particularly the RAP1 homologs, have been shown to be a key player for telomeric silencing. RAP1 homologs also suppress the expression of Telomere Repeat-containing RNA (TERRA), which is linked to their roles in telomere stability maintenance. The functions of RAP1s in suppressing telomere recombination are largely conserved from kinetoplastids to mammals. However, the underlying mechanisms of RAP1-mediated telomeric silencing have many species-specific features. In this review, I will focus on Trypanosoma brucei RAP1\'s functions in suppressing telomeric/subtelomeric DNA recombination and in the regulation of monoallelic expression of subtelomere-located major surface antigen genes. Common and unique mechanisms will be compared among RAP1 homologs, and their implications will be discussed.

As the limiting component of the budding yeast telomerase, the Tlc1 RNA must undergo multiple consecutive modifications and rigorous quality checks throughout its lifecycle. These steps will ensure that only correctly processed and matured molecules are assembled into telomerase complexes that subsequently act at telomeres. The complex pathway of Tlc1 RNA maturation, involving 5\'- and 3\'-end processing, stabilisation and assembly with the protein subunits, requires at least one nucleo-cytoplasmic passage. Furthermore, it appears that the pathway is tightly coordinated with the association of various and changing proteins, including the export factor Xpo1, the Mex67/Mtr2 complex, the Kap122 importin, the Sm7 ring and possibly the CBC and TREX-1 complexes. Although many of these maturation processes also affect other RNA species, the Tlc1 RNA exploits them in a new combination and, therefore, ultimately follows its own and unique pathway. In this review, we highlight recent new insights in maturation and subcellular shuttling of the budding yeast telomerase RNA and discuss how these events may be fine-tuned by the biochemical characteristics of the varying processing and transport factors as well as the final telomerase components. Finally, we indicate outstanding questions that we feel are important to be addressed for a complete understanding of the telomerase RNA lifecycle and that could have implications for the human telomerase as well.

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide. It accounts for 2.5% of all new cancer cases and 1.9% of all cancer deaths annually. More than 90% of oral cancers (occurring in the mouth, lip, and tongue) are oral squamous cell carcinoma. The incidence rate of oral cancer varies widely throughout the world, with an evident prevalence in South Asian countries. This high incidence occurs in correlation with oral cancer-associated behaviors such as alcohol, tobacco use. Researchers have reported that these behaviors lead to genetic variations in tumor suppressor genes (APC, p53), proto-oncogenes (Myc), oncogene (Ras) and genes controlling normal cellular processes (EIF3E, GSTM1). Processes such as segregation of chromosomes, genomic copy number, loss of heterozygosity, telomere stabilities, regulations of cell-cycle checkpoints, DNA damage repairs and defects in notch signaling pathways are involved in causing oral cancer. In order to develop preventive and therapeutic options, it is necessary to comprehend the basic molecular mechanisms forcing oral tumorigenesis. This review examines, in detail, the mechanisms of genetic alteration which are considered to be responsible for the initiation of oral cancer.

Radiotherapy is widely applied for treatment of esophageal squamous cell carcinoma (ESCC). The Rad51-related protein XRCC3 plays roles in the recombinational repair of DNA double-strand breaks to maintain chromosome stability and repair DNA damage. The present study aimed to investigate the effect of XRCC3 on the radiotherapy response of ESCC and the underlying mechanisms of the roles of XRCC3 in ESCC radiosensitivity. XRCC3 expression in ESCC cells and tissues was higher than that in normal esophageal epithelial cells and corresponding adjacent noncancerous esophageal tissue. High XRCC3 expression was positively correlated with resistance to chemoradiotherapy in ESCC and an independent predictor for short disease-specific survival of ESCC patients. Furthermore, the therapeutic efficacy of radiotherapy in vitro and in vivo was substantially increased by knockdown of XRCC3 in ESCC cells. Ectopic overexpression of XRCC3 in both XRCC3-silenced ESCC cells dramatically enhanced ESCC cells\' resistance to radiotherapy. Moreover, radiation resistance conferred by XRCC3 was attributed to enhancement of homologous recombination, maintenance of telomere stability, and a reduction of ESCC cell death by radiation-induced apoptosis and mitotic catastrophe. Our data suggest that XRCC3 protects ESCC cells from ionizing radiation-induced death by promoting DNA damage repair and/or enhancing telomere stability. XRCC3 may be a novel radiosensitivity predictor and promising therapeutic target for ESCC.