In some types of cancer, telomere length is maintained by the alternative lengthening of telomeres (ALT) mechanism. In many ALT cancers, the α‐thalassemia/mental retardation syndrome X‐linked (ATRX) gene is mutated leading to the conclusion that the ATRX complex represses ALT. Here, we report that most high‐grade pediatric osteosarcomas maintain their telomeres by ALT, and that the majority of these ALT tumors are ATRX wild‐type (wt) and instead carry an amplified 17p11.2 chromosomal region containing TOP3A. We found that TOP3A was overexpressed in the ALT‐positive ATRX‐wt tumors consistent with its amplification. We demonstrated the functional significance of these results by showing that TOP3A overexpression in ALT cancer cells countered ATRX‐mediated ALT inhibition and that TOP3A knockdown disrupted the ALT phenotype in ATRX‐wt cells. Moreover, we report that TOP3A is required for proper BLM localization and promotes ALT DNA synthesis in ALT cell lines. Collectively, our results identify TOP3A as a major ALT player and potential therapeutic target.

https://hal.science/hal-03877721/document

TOP3A amplification and ATRX inactivation are mutually exclusive events in pediatric osteosarcomas using ALT

Cells must replicate and segregate their DNA to daughter cells accurately to maintain genome stability and prevent cancer. DNA replication is usually fast and accurate, with intrinsic (proofreading) and extrinsic (mismatch repair) error-correction systems. However, replication forks slow or stop when they encounter DNA lesions, natural pause sites, and difficult-to-replicate sequences, or when cells are treated with DNA polymerase inhibitors or hydroxyurea, which depletes nucleotide pools. These challenges are termed replication stress, to which cells respond by activating DNA damage response signaling pathways that delay cell cycle progression, stimulate repair and replication fork restart, or induce apoptosis. Stressed forks are managed by rescue from adjacent forks, repriming, translesion synthesis, template switching, and fork reversal which produces a single-ended double-strand break (seDSB). Stressed forks also collapse to seDSBs when they encounter single-strand nicks or are cleaved by structure-specific nucleases. Reversed and cleaved forks can be restarted by homologous recombination (HR), but seDSBs pose risks of mis-rejoining by non-homologous end-joining (NHEJ) to other DSBs, causing genome rearrangements. HR requires resection of broken ends to create 3' single-stranded DNA for RAD51 recombinase loading, and resected ends are refractory to repair by NHEJ. This Mini Review highlights mechanisms that help maintain genome stability by promoting resection of seDSBs and accurate fork restart by HR.

/pdf/the-safe-path-at-the-fork-ensuring-replication-associated-5dqrscr37v.pdf

The Safe Path at the Fork: Ensuring Replication-Associated DNA Double-Strand Breaks are Repaired by Homologous Recombination.

The number of (TTAGGG)n repeats at the ends of chromosomes is highly variable between individual chromosomes, between different cells and between species. Progressive loss of telomere repeats limits the proliferation of pre-malignant human cells but also contributes to aging by inducing apoptosis and senescence in normal cells. Despite enormous progress in understanding distinct pathways that result in loss and gain of telomeric DNA in different cell types, many questions remain. Further studies are needed to delineate the role of damage to telomeric DNA, replication errors, chromatin structure, liquid-liquid phase transition, telomeric transcripts (TERRA) and secondary DNA structures such as guanine quadruplex structures, R-loops and T-loops in inducing gains and losses of telomere repeats in different cell types. Limitations of current telomere length measurements techniques and differences in telomere biology between species and different cell types complicate generalizations about the role of telomeres in aging and cancer. Here some of the factors regulating the telomere length in embryonic and adult cells in mammals are discussed from a mechanistic and evolutionary perspective.

/pdf/telomere-length-regulation-1ng7zt5m.pdf

Telomere Length Regulation

Break-induced replication: unraveling each step.

Background Hepatocellular carcinoma (HCC) is widely acknowledged as a malignant tumor with rapid progression, high recurrence rate, and poor prognosis. At present, there is a paucity of reliable biomarkers at the clinical level to guide the management of HCC and improve patient outcomes. Our research is aimed at assessing the prognostic value of MAD2L1 in HCC. Methods Four datasets, GSE121248, GSE101685, GSE85598, and GSE62232, were selected from the GEO database to analyze differentially expressed genes (DEGs) between HCC and normal liver tissues. After functional analysis, we constructed a protein-protein interaction network (PPI) for DEGs and identified core genes in this network with high connectivity with other genes. We assessed the relationship between core genes and the pathogenesis and prognosis of HCC. Finally, we explored the gene regulatory signaling mechanisms involved in HCC pathogenesis. Results 145 DEGs were screened from the intersection of the four GEO datasets. MAD2L1 was associated with most genes according to the PPI network and was selected as a candidate gene for further study. Survival analysis suggested that high MAD2L1 expression in HCC correlated with a worse prognosis. In addition, real-time quantitative PCR (RT-qPCR), western blot (WB), and immunohistochemistry (IHC) findings suggested that the expression of MAD2L1 was abnormally increased in HCC tissues and cells compared to paraneoplastic tissues and normal hepatocytes. Conclusion We found that high MAD2L1 expression in HCC was significantly associated with overall patient survival and clinical features. We also explored the potential biological properties of this gene.

/pdf/identification-of-mad2l1-as-a-potential-biomarker-in-2girkjja.pdf

Esther A. Epum

Papers

DNA replication: the recombination connection.