XPF activates break-induced telomere synthesis

Multiple functions of the ALT favorite helicase, BLM

Eukaryotic somatic cells undergo continuous telomere shortening because of end-replication problems. Approximately 10%~15% of human cancers rely on alternative lengthening of telomeres (ALT) to overcome telomere shortening. ALT cells are characterized by persistent telomere DNA replication stress and rely on recombination-based DNA repair pathways for telomere elongation. The Bloom syndrome (BLM) helicase is a member of the RecQ family, which has been implicated as a key regulator of the ALT mechanism as it is required for either telomere length maintenance or telomere clustering in ALT-associated promyelocytic leukemia bodies (APBs). Here, we summarize recent evidence detailing the role of BLM in the activation and maintenance of ALT. We propose that the role of BLM-dependent recombination and its interacting proteins remains a crucial question for future research in dissecting the molecular mechanisms of ALT.

Biomolecular Condensates in Telomere Maintenance of ALT Cancer Cells

Alternative Lengthening of Telomeres (ALT) pathway is a telomerase-independent mechanism that utilizes homology-directed repair (HDR) to sustain telomere length in specific cancers. Biomolecular condensates, such as ALT-associated promyelocytic leukemia nuclear bodies (APBs), have emerged as critical players in the ALT pathway, supporting telomere maintenance in ALT-positive cells. These condensates bring together DNA repair proteins, telomeric repeats, and other regulatory elements. By regulating replication stress and promoting DNA synthesis, ALT condensates create an environment conducive to HDR-based telomere extension. This review explores recent advancements in ALT, focusing on understanding the role of biomolecular condensates in ALT and how they impact telomere dynamics and stability.

Mechanisms of Alternative Lengthening of Telomeres

In recent years, significant advances have been made in understanding the intricate details of the mechanisms underlying alternative lengthening of telomeres (ALT). Studies of a specialized DNA strand break repair mechanism, known as break-induced replication, and the advent of telomere-specific DNA damaging strategies and proteomic methodologies to profile the ribonucleoprotein composition of telomeres enabled the discovery of networks of proteins that coordinate the stepwise homology-directed DNA repair and DNA synthesis processes of ALT. These networks couple mediators of homologous recombination, DNA template-switching, long-range template-directed DNA synthesis, and DNA strand resolution with SUMO-dependent liquid condensate formation to create discrete nuclear bodies where telomere extension occurs. This review will discuss the recent findings of how these networks may cooperate to mediate telomere extension by the ALT mechanism and their impact on telomere function and integrity in ALT cancer cells.

TERRA and the alternative lengthening of telomeres: a dangerous affair

Eukaryotic telomeres are transcribed into the long noncoding RNA TERRA. A fraction of TERRA remains associated with telomeres by forming RNA:DNA hybrids dubbed telR-loops. TERRA and telR-loops are essential to promote telomere elongation in human cancer cells that maintain telomeres through a homology-directed repair pathway known as alternative lengthening of telomeres or ALT. However, TERRA and telR-loops compromise telomere integrity and cell viability if their levels are not finely tuned. The study of telomere transcription in ALT cells will enormously expand our understanding of the ALT mechanism and of how genome integrity is maintained. Moreover, telomere transcription, TERRA and telR-loops are likely to become exceptionally suited targets for the development of novel anti-cancer therapies.

Telomere-related DNA damage response pathways in cancer therapy: prospective targets

Maintaining the structural integrity of genomic chromosomal DNA is an essential role of cellular life and requires two important biological mechanisms: the DNA damage response (DDR) mechanism and telomere protection mechanism at chromosome ends. Because abnormalities in telomeres and cellular DDR regulation are strongly associated with human aging and cancer, there is a reciprocal regulation of telomeres and cellular DDR. Moreover, several drug treatments for DDR are currently available. This paper reviews the progress in research on the interaction between telomeres and cellular DNA damage repair pathways. The research on the crosstalk between telomere damage and DDR is important for improving the efficacy of tumor treatment. However, further studies are required to confirm this hypothesis.

ERCC4: a potential regulatory factor in inflammatory bowel disease and inflammation-associated colorectal cancer

The pathogenesis of inflammatory bowel disease (IBD) remains unclear and is associated with an increased risk of developing colitis-associated cancer (CAC). Under sustained inflammatory stimulation in the intestines, loss of early DNA damage response genes can lead to tumor formation. Many proteins are involved in the pathways of DNA damage response and play critical roles in protecting genes from various potential damages that DNA may undergo. ERCC4 is a structure-specific endonuclease that participates in the nucleotide excision repair (NER) pathway. The catalytic site of ERCC4 determines the activity of NER and is an indispensable gene in the NER pathway. ERCC4 may be involved in the imbalanced process of DNA damage and repair in IBD-related inflammation and CAC. This article primarily reviews the function of ERCC4 in the DNA repair pathway and discusses its potential role in the processes of IBD-related inflammation and carcinogenesis. Finally, we explore how this knowledge may open novel avenues for the treatment of IBD and IBD-related cancer.

Effects of HSV-G47Δ Oncolytic Virus on Telomerase and Telomere Length Alterations in Glioblastoma Multiforme Cancer Stem Cells Under Hypoxia and Normoxia Conditions

BACKGROUND

Due to the existence of tumor stem cells with tumorigenicity properties and resistance patterns, treatment of glioblastoma is not easy. Hypoxia is a major concern in glioblastoma therapy. Telomerase activity and telomere length alterations have been known to play a critical role in glioblastoma progression and invasion.

OBJECTIVE

This study aimed to investigate the effects of HSV-G47Δ oncolytic virus on telomerase and telomere length alterations in U251GBMCSCs (U251-Glioblastoma cancer stem cells) under hypoxia and normoxia conditions.

METHODS

U251-CSCs were exposed to the HSV-G47Δ virus in optimized MOI (Multiplicity of infection= 1/14 hours). An absolute telomere length and gene expression of telomerase subunits were determined using an absolute human telomere length quantification PCR assay. Furthermore, a bioinformatics pathway analysis was carried out to evaluate physical and genetic interactions between dysregulated genes with other potential genes and pathways.

RESULTS

Data revealed that U251CSCs had longer telomeres when exposed to HSV-G47Δ in normoxic conditions but had significantly shorter telomeres in hypoxic conditions. Furthermore, hTERC, DKC1, and TEP1 genes were significantly dysregulated in hypoxic and normoxic microenvironments. The analysis revealed that the expression of TERF2 was significantly reduced in both microenvironments, and two critical genes from the MRN complex, MER11 and RAD50, were significantly upregulated in normoxic conditions. RAD50 showed a significant downregulation pattern in the hypoxic niche. Our results suggested that repair complex in the telomeric structure could be targeted by HSV-G47Δ in both microenvironments.

CONCLUSION

In the glioblastoma treatment strategy, telomerase and telomere complex could be potential targets for HSV-G47Δ in both microenvironments.

R-Loops in Genome Instability and Cancer

R-loops are unique, three-stranded nucleic acid structures that primarily form when an RNA molecule displaces one DNA strand and anneals to the complementary DNA strand in a double-stranded DNA molecule. R-loop formation can occur during natural processes, such as transcription, in which the nascent RNA molecule remains hybridized with the template DNA strand, while the non-template DNA strand is displaced. However, R-loops can also arise due to many non-natural processes, including DNA damage, dysregulation of RNA degradation pathways, and defects in RNA processing. Despite their prevalence throughout the whole genome, R-loops are predominantly found in actively transcribed gene regions, enabling R-loops to serve seemingly controversial roles. On one hand, the pathological accumulation of R-loops contributes to genome instability, a hallmark of cancer development that plays a role in tumorigenesis, cancer progression, and therapeutic resistance. On the other hand, R-loops play critical roles in regulating essential processes, such as gene expression, chromatin organization, class-switch recombination, mitochondrial DNA replication, and DNA repair. In this review, we summarize discoveries related to the formation, suppression, and removal of R-loops and their influence on genome instability, DNA repair, and oncogenic events. We have also discussed therapeutical opportunities by targeting pathological R-loops.

The multistep path to replicative senescence onset: zooming on triggering and inhibitory events at telomeric DNA

Replicative senescence is an essential cellular process playing important physiological functions, but it is better known for its implications in aging, cancer, and other pathologies. One of the main triggers of replicative senescence is telomere shortening and/or its dysfunction and, therefore, a deep understanding of the molecular determinants is crucial. However, replicative senescence is a heterogeneous and hard to study process, especially in mammalian cells, and some important questions still need an answer. These questions concern i) the exact molecular causes triggering replicative senescence, ii) the role of DNA repair mechanisms and iii) the importance of R-loops at telomeres in regulating senescence onset, and iv) the mechanisms underlying the bypass of replicative senescence. In this review, we will report and discuss recent findings about these mechanisms both in mammalian cells and in the model organism Saccharomyces cerevisiae.

TERRA stability is regulated by RALY and polyadenylation in a telomere-specific manner

Telomeric repeat-containing RNA (TERRA) is a long non-coding RNA transcribed from telomeres that plays key roles in telomere maintenance. A fraction of TERRA is polyadenylated, and the presence of the poly(A) tail influences TERRA localization and stability. However, the mechanisms of TERRA biogenesis remain mostly elusive. Here, we show that the stability of TERRA transcripts is regulated by the RNA-binding protein associated with lethal yellow mutation (RALY). RALY depletion results in lower TERRA levels, impaired localization of TERRA at telomeres, and ultimately telomere damage. Importantly, we show that TERRA polyadenylation is telomere specific and that RALY preferentially stabilizes non-polyadenylated TERRA transcripts. Finally, we report that TERRA interacts with the poly(A)-binding protein nuclear 1 (PABPN1). Altogether, our results indicate that TERRA stability is regulated by the interplay between RALY and PABPN1, defined by the TERRA polyadenylation state. Our findings also suggest that different telomeres may trigger distinct TERRA-mediated responses.

The Molecular Mechanisms and Therapeutic Prospects of Alternative Lengthening of Telomeres (ALT)

As detailed by the end replication problem, the linear ends of a cell's chromosomes, known as telomeres, shorten with each successive round of replication until a cell enters into a state of growth arrest referred to as senescence. To maintain their immortal proliferation capacity, cancer cells must employ a telomere maintenance mechanism, such as telomerase activation or the Alternative Lengthening of Telomeres pathway (ALT). With only 10-15% of cancers utilizing the ALT mechanism, progress towards understanding its molecular components and associated hallmarks has only recently been made. This review analyzes the advances towards understanding the ALT pathway by: (1) detailing the mechanisms associated with engaging the ALT pathway as well as (2) identifying potential therapeutic targets of ALT that may lead to novel cancer therapeutic treatments. Collectively, these studies indicate that the ALT molecular mechanisms involve at least two distinct pathways induced by replication stress and damage at telomeres. We suggest exploiting tumor dependency on ALT is a promising field of study because it suggests new approaches to ALT-specific therapies for cancers with poorer prognosis. While substantial progress has been made in the ALT research field, additional progress will be required to realize these advances into clinical practices to treat ALT cancers and improve patient prognoses.

Telomere Length and Telomerase Activity of Granulosa Cells and Follicular Fluid in Women Undergoing In Vitro Fertilization

This study aimed to evaluate the interrelationship between telomere length, telomerase activity and oxidative DNA damage in patients undergoing in vitro fertilization (IVF). This single-center, observational clinical study comprised 102 unselected, consecutive patients with various infertility diagnoses. Granulosa cells (GCs) and follicular fluid (FF) were analyzed simultaneously for telomere functions and for the marker of oxidative DNA damage, 8-hydroxy-2-deoxyguanosine (8-OHdG). An Absolute Human Telomere Lengths Quantification qPCR Assay kit and Telomerase Activity Quantification qPCR Assay kit (Nucleotestbio, Budapest, Hungary), as well as an 8-OHdG ELISA kit (Abbexa Ltd., Cambridge, United Kingdom) were used for analyses. Similar telomere lengths were found in GCs and FF, however telomerase activity was markedly depressed, while 8-OHdG levels were markedly elevated in FF compared with those in GCs (p < 0.01). Telomere lengths were independent of telomerase activity both in GCs and FF. However, GC 8-OHdG was inversely related to telomerase activity in GCs and FF (p < 0.05). Importantly, 8-OHdG levels both in GCs and FF had significant negative impact on the number of the retrieved and MII oocytes (p < 0.01), whereas FF 8-OHdG was negatively related further to the number of fertilized oocytes and blastocysts (p < 0.01). In conclusion, we could not confirm the direct association of telomere function and reproductive potential. However, oxidative DNA damage, as mainly reflected by 8-OHdG, adversely affected early markers of IVF outcome and clinical pregnancies.

Telomere Fragility and MiDAS: Managing the Gaps at the End of the Road

Telomeres present inherent difficulties to the DNA replication machinery due to their repetitive sequence content, formation of non-B DNA secondary structures, and the presence of the nucleo-protein t-loop. Especially in cancer cells, telomeres are hot spots for replication stress, which can result in a visible phenotype in metaphase cells termed "telomere fragility". A mechanism cells employ to mitigate replication stress, including at telomeres, is DNA synthesis in mitosis (MiDAS). While these phenomena are both observed in mitotic cells, the relationship between them is poorly understood; however, a common link is DNA replication stress. In this review, we will summarize what is known to regulate telomere fragility and telomere MiDAS, paying special attention to the proteins which play a role in these telomere phenotypes.