Alternative Lengthening of Telomeres in Cancer Confers a Vulnerability to Reactivation of p53 Function

Metabolic pathways of Alternative Lengthening of Telomeres in pan-carcinoma

Alternative Lengthening of Telomeres (ALT) is a telomerase-independent mechanism deployed by several aggressive cancers to maintain telomere length. This contributes to their malignancy and resistance to conventional therapies. In prior studies, we have identified key proteins linked to the ALT process using multi-omic data integration strategies. In this work, we combined metabolomic datasets with our earlier results to identify targetable metabolic pathways for ALT-positive tumors. 39 ALT-related proteins were found to interact with 42 different metabolites in our analysis. Additional networking analysis revealed a complex interaction between metabolites and ALT-related proteins, suggesting that pan-cancer oncogenes may have an impact on these pathways. Three metabolic pathways have been primarily related with the ALT mechanism: purine metabolism, cysteine and methionine metabolism, and nicotinate and nicotinamide metabolism. Lastly, we prioritized FDA-approved drugs (azathioprine, thioguanine, and mercaptopurine) that could target ALT-positive tumors through purine metabolism. This work provides a wide perspective of the metabolomic pathways associated with ALT and reveals potential therapeutic targets that require further experimental validation.

MSH2, MSH6, MLH1, and PMS2 immunohistochemistry as highly sensitive screening method for DNA mismatch repair deficiency syndromes in pediatric high-grade glioma

Pediatric high-grade glioma (pedHGG) can occur as first manifestation of cancer predisposition syndromes resulting from pathogenic germline variants in the DNA mismatch repair (MMR) genes MSH2, MSH6, MLH1, and PMS2. The aim of this study was to establish a generalized screening for Lynch syndrome and constitutional MMR deficiency (CMMRD) in pedHGG patients, as the detection of MMR deficiencies (MMRD) may enable the upfront therapeutic use of checkpoint inhibitors and identification of variant carriers in the patients' families. We prospectively enrolled 155 centrally reviewed primary pedHGG patients for MMR-immunohistochemistry (IHC) as part of the HIT-HGG-2013 trial protocol. MMR-IHC results were subsequently compared to independently collected germline sequencing data (whole exome sequencing or pan-cancer DNA panel next-generation sequencing) available in the HIT-HGG-2013, INFORM, and MNP2.0 trials. MMR-IHC could be successfully performed in 127/155 tumor tissues. The screening identified all present cases with Lynch syndrome or CMMRD (5.5%). In addition, MMR-IHC also detected cases with exclusive somatic MMR gene alterations (2.3%), including MSH2 hypermethylation as an alternative epigenetic silencing mechanism. Most of the identified pedHGG MMRD patients had no family history of MMRD, and thus, they represented index patients in their families. Cases with regular protein expression in MMR-IHC never showed evidence for MMRD in DNA sequencing. In conclusion, MMR-IHC presents a cost-effective, relatively widely available, and fast screening method for germline MMRD in pedHGG with high sensitivity (100%) and specificity (96%). Given the relatively high prevalence of previously undetected MMRD cases among pedHGG patients, we strongly recommend incorporating MMR-IHC into routine diagnostics.

TP53 gene status can promote sensitivity and resistance to chemotherapeutic drugs and small molecule signal transduction inhibitors

TP53 is normally a tumor suppressor. However, it is mutated in at least 50% of human cancers. Usually, we assume that mutation of the TP53 is associated with loss of sensitivity to various drugs as in most cases wild type (WT) TP53 activity is lost. This type of mutations is often dominant-negative (DN) mutations as they can interfere with the normal functions of WT-TP53 which acts as a tetramer. These mutations can result in altered gene expression patterns. There are some TP53 mutations which may lack some of the normal functions of TP53 but have additional functions; these types of mutations are called gain of function (GOF) mutations. There is another class of TP53 mutations, they are TP53 null mutations as the cells have deleted the TP53 gene (TP53-null). Although TP53 mutations were initially considered undruggable, other approaches have been developed to increase TP53 activity. One approach was to develop mouse double minute 2 homolog (MDM2) inhibitors as MDM2 suppresses TP53 activity. In addition, there have been mutant TP53 reactivators created, which will at least partially restore some of the critical growth suppressing effects of TP53. Some of these mutant TP53 reactivators have shown promise in clinical trial in certain types of cancer patients, especially myelodysplastic syndrome (MDS). In this review, we summarize the development of novel TP53 reactivators and MDM2 inhibitors. Both approaches are aimed at increasing or restoring TP53 activity. Attempts to increase TP53 activity in various TP53 mutant tumors could increase therapy of multiple deadly diseases.

Beginning at the ends: telomere and telomere-based cancer therapeutics

Telomeres, which are situated at the terminal ends of chromosomes, undergo a reduction in length with each cellular division, ultimately reaching a critical threshold that triggers cellular senescence. Cancer cells circumvent this senescence by utilizing telomere maintenance mechanisms (TMMs) that grant them a form of immortality. These mechanisms can be categorized into two primary processes: the reactivation of telomerase reverse transcriptase and the alternative lengthening of telomeres (ALT) pathway, which is dependent on homologous recombination (HR). Various strategies have been developed to inhibit telomerase activation in 85-95% of cancers, including the use of antisense oligonucleotides such as small interfering RNAs and endogenous microRNAs, agents that simulate telomere uncapping, expression modulators, immunotherapeutic vaccines targeting telomerase, reverse transcriptase inhibitors, stabilization of G-quadruplex structures, and gene therapy approaches. Conversely, in the remaining 5-15% of human cancers that rely on ALT, mechanisms involve modifications in the chromatin environment surrounding telomeres, upregulation of TERRA long non-coding RNA, enhanced activation of the ataxia telangiectasia and Rad-3-related protein kinase signaling pathway, increased interactions with nuclear receptors, telomere repositioning driven by HR, and recombination events between non-sister chromatids, all of which present potential targets for therapeutic intervention. Additionally, combinatorial therapy has emerged as a strategy that employs selective agents to simultaneously target both telomerase and ALT, aiming for optimal clinical outcomes. Given the critical role of anti-TMM strategies in cancer treatment, this review provides an overview of the latest insights into the structure and function of telomeres, their involvement in tumorigenesis, and the advancements in TMM-based cancer therapies.

Colorectal Leiomyosarcoma: Demographics Patterns, Treatment Characteristics, and Survival Analysis in the U.S. Population

BACKGROUND

Colorectal leiomyosarcoma (CR-LMS) is a rare neoplasm arising from smooth muscle cells. It accounts for less than 0.1% of all colorectal malignancies. In this population-based study, we aim to understand the demographics, treatment characteristics, and pathologic factors associated with survival in CR-LMS.

METHODS

Data from the SEER Program (2000-2018) were analyzed using SEER*Stat and SPSS. Statistical methods included descriptive analysis, Kaplan-Meier survival curves, log-rank tests, and Cox proportional hazards regression to assess the impact of various factors on disease-specific and overall survival.

RESULTS

A total of 191 cases of CR-LMS were identified. Most patients were 60-69 years of age (median: 64 years) and Caucasian (78%). There was nearly the same distribution in sex (M:F ratio; 1:1.2). The overall 5-year observed survival was 50.3% (95% C.I., 46.3-54.2). The 5-year disease-specific survival (DSS) was 66.1% (95% C.I., 62.0-70.1). The 5-year overall survival after resection was 60.8% (95% C.I., 56.3-65.3). Multivariable analysis identified grades III and IV (p = 0.028) as negative predictors of overall survival. Regional spread and distant stage are negative predictors of overall survival (p < 0.01).

CONCLUSION

Our data reveals that colorectal leiomyosarcoma (CR-LMS) often presents in patients around 64 years old with advanced stages and poor differentiation. Key adverse prognostic factors include older age, high tumor grade, large tumor size, and distant metastases, with surgical resection showing the best survival outcomes. To improve outcomes, further research and consolidation of data are essential for developing targeted therapies and comprehensive guidelines.

SNF2L maintains glutathione homeostasis by initiating SLC7A11 transcription through chromatin remodeling

SNF2L encodes an ISWI chromatin remodeling factor that promotes gene transcription and is consistently elevated in cancers. Previous studies have shown that inhibiting SNF2L expression in cancer cells leads to significant growth suppression, DNA damage, and cell death. However, the underlying mechanisms remain poorly understood. In this study, we demonstrated that cancer cells lacking SNF2L show significantly decreased glutathione (GSH) levels, leading to elevated reactive oxygen species (ROS) and increased oxidative stress. SNF2L deficiency also heightened the sensitivity of cancer cells to APR-246, a drug that depletes GSH and induces oxidative stress, consequently decreasing cell viability and increasing ROS levels, regardless of p53 status. Mechanistically, we found that NRF2 recruits SNF2L to the SLC7A11 promoter, leading to increased chromatin accessibility and facilitating SLC7A11 transcription. This results in decreased cystine uptake and impaired GSH biosynthesis. These findings suggest that targeting the SNF2L/SLC7A11 axis could enhance the effectiveness of APR-246 by depleting GSH and increasing ROS level in cancer cells, highlighting SNF2L as a promising therapeutic target.

Prevalence of alternative lengthening of telomeres in pediatric sarcomas determined by the telomeric DNA C-circle assay

Introduction

Alternative lengthening of telomeres (ALT) occurs in sarcomas and ALT cancers share common mechanisms of therapy resistance or sensitivity. Telomeric DNA C-circles are self-primed circular telomeric repeats detected with a PCR assay that provide a sensitive and specific biomarker exclusive to ALT cancers. We have previously shown that 23% of high-risk neuroblastomas are of the ALT phenotype. Here, we investigate the frequency of ALT in Ewing's family sarcoma (EFS), rhabdomyosarcoma (RMS), and osteosarcoma (OS) by analyzing DNA from fresh frozen primary tumor samples utilizing the real-time PCR C-circle Assay (CCA).

Methods

We reviewed prior publications on ALT detection in pediatric sarcomas. DNA was extracted from fresh frozen primary tumors, fluorometrically quantified, C-circles were selectively enriched by isothermal rolling cycle amplification and detected by real-time PCR.

Results

The sample cohort consisted of DNA from 95 EFS, 191 RMS, and 87 OS primary tumors. One EFS and 4 RMS samples were inevaluable. Using C-circle positive (CC+) cutoffs previously defined for high-risk neuroblastoma, we observed 0 of 94 EFS, 5 of 187 RMS, and 62 of 87 OS CC+ tumors.

Conclusions

Utilizing the ALT-specific CCA we observed ALT in 0% of EFS, 2.7% of RMS, and 71% of OS. These data are comparable to prior studies in EFS and OS using less specific ALT markers. The CCA can provide a robust and sensitive means of identifying ALT in sarcomas and has potential as a companion diagnostic for ALT targeted therapeutics.

Telomeres and telomerase in Sarcoma disease and therapy

Sarcoma is a rare tumor derived from the mesenchymal tissue and mainly found in children and adolescents. The outcome for patients with sarcoma is relatively poor compared with that for many other solid malignant tumors. Sarcomas have a highly heterogeneous pathogenesis, histopathology and biological behavior. Dysregulated signaling pathways and various gene mutations are frequently observed in sarcomas. The telomere maintenance mechanism (TMM) has recently been considered as a prognostic factor for patients with sarcomas, and alternative lengthening of telomeres (ALT) positivity has been correlated with poor outcomes in patients with several types of sarcomas. Therefore, telomeres and telomerases may be useful targets for treating sarcomas. This review aims to provide an overview of telomere and telomerase biology in sarcomas.

Long-term follow-up of patients with intermediate-risk neuroblastoma treated with response- and biology-based therapy: A report from the Children's Oncology Group study ANBL0531

BACKGROUND

We previously reported excellent three-year overall survival (OS) for patients with newly diagnosed intermediate-risk neuroblastoma treated with a biology- and response-based algorithm on the Children's Oncology Group study ANBL0531. We now present the long-term follow-up results.

METHODS

All patients who met the age, stage, and tumor biology criteria for intermediate-risk neuroblastoma were eligible. Treatment was based on prognostic biomarkers and overall response. Event-free survival (EFS) and OS were estimated by the Kaplan-Meier method.

RESULTS

The 10-year EFS and OS for the entire study cohort (n = 404) were 82.0% (95% confidence interval (CI), 77.2%-86.9%) and 94.7% (95% CI, 91.8%-97.5%), respectively. International Neuroblastoma Staging System stage 4 patients (n = 133) had inferior OS compared with non-stage 4 patients (n = 271; 10-year OS: 90.8% [95% CI, 84.5%-97.0%] vs 96.6% [95% CI, 93.9%-99.4%], p = .02). Infants with stage 4 tumors with ≥1 unfavorable biological feature (n = 47) had inferior EFS compared with those with favorable biology (n = 61; 10-year EFS: 66.8% [95% CI, 50.4%-83.3%] vs 86.9% [95% CI, 76.0%-97.8%], p = .02); OS did not differ (10-year OS: 84.4% [95% CI, 71.8%-97.0%] vs 95.0% [95% CI, 87.7%-100.0%], p = .08). Inferior EFS but not OS was observed among patients with tumors with (n = 26) versus without (n = 314) 11q loss of heterozygosity (10-year EFS: 68.4% [95% CI, 44.5%-92.2%] vs 83.9% [95% CI, 78.7%-89.2%], p = .03; 10-year OS: 88.0% [95% CI, 72.0%-100.0%] vs 95.7% [95% CI, 92.8%-98.6%], p = .09).

CONCLUSIONS

The ANBL0531 trial treatment algorithm resulted in excellent long-term survival. More effective treatments are needed for subsets of patients with unfavorable biology tumors.

Primary cardiac sarcoma: demographics, genomic study correlation, and survival benefits of surgery with adjuvant therapy in U.S. population

BACKGROUND

Cardiac sarcomas are rare and aggressive tumors with little known about the demographics, genetics, or treatment outcomes.

OBJECTIVES

The objectives of this study were to characterize the demographics, treatment modality, and survival associated with cardiac sarcomas and evaluate the potential for mutation-directed therapies.

METHODS

All cases from 2000 to 2018 of cardiac sarcoma were extracted from the SEER database. Genomic comparison utilized The Cancer Genome Atlas (TCGA) database, as well as reviews and re-analysis of past applicable genomic studies.

RESULTS

Cardiac sarcomas occurred most often in White patients, compared with national census data cardiac sarcomas occurred at a significantly higher rate in Asians. The majority of cases were undifferentiated (61.7%) and without distant metastases (71%). Surgery was the most common primary treatment modality and offered survival benefit (HR 0.391 (p = 0.001) that was most pronounced and sustained as compared to patients who received chemotherapy (HR 0.423 (p = 0.001) or radiation (HR 0.826 (p = 0.241) monotherapy. There was no difference in survival when stratified by race or sex; however, younger patients (< 50) had better survival. Genomics data on histologically undifferentiated cardiac sarcomas revealed a significant number were likely poorly differentiated pulmonary intimal sarcomas and angiosarcomas.

CONCLUSIONS

Cardiac sarcoma is a rare disease with surgery continuing to be a cornerstone of therapy followed by traditional chemotherapy. Case studies have indicated the potential for therapies directed to specific genetic aberrations to improve survival for these patients and utilization of next-generation sequencing (NGS) will help improve both classification and these therapies for cardiac sarcoma patients.

METTL3 drives telomere targeting of TERRA lncRNA through m6A-dependent R-loop formation: a therapeutic target for ALT-positive neuroblastoma

Telomerase-negative tumors maintain telomere length by alternative lengthening of telomeres (ALT), but the underlying mechanism behind ALT remains poorly understood. A proportion of aggressive neuroblastoma (NB), particularly relapsed tumors, are positive for ALT (ALT+), suggesting that a better dissection of the ALT mechanism could lead to novel therapeutic opportunities. TERRA, a long non-coding RNA (lncRNA) derived from telomere ends, localizes to telomeres in a R-loop-dependent manner and plays a crucial role in telomere maintenance. Here we present evidence that RNA modification at the N6 position of internal adenosine (m6A) in TERRA by the methyltransferase METTL3 is essential for telomere maintenance in ALT+ cells, and the loss of TERRA m6A/METTL3 results in telomere damage. We observed that m6A modification is abundant in R-loop enriched TERRA, and the m6A-mediated recruitment of hnRNPA2B1 to TERRA is critical for R-loop formation. Our findings suggest that m6A drives telomere targeting of TERRA via R-loops, and this m6A-mediated R-loop formation could be a widespread mechanism employed by other chromatin-interacting lncRNAs. Furthermore, treatment of ALT+ NB cells with a METTL3 inhibitor resulted in compromised telomere targeting of TERRA and accumulation of DNA damage at telomeres, indicating that METTL3 inhibition may represent a therapeutic approach for ALT+ NB.

Alternative lengthening of telomeres (ALT) in pediatric high-grade gliomas can occur without ATRX mutation and is enriched in patients with pathogenic germline mismatch repair (MMR) variants

BACKGROUND

To achieve replicative immortality, most cancers develop a telomere maintenance mechanism, such as reactivation of telomerase or alternative lengthening of telomeres (ALT). There are limited data on the prevalence and clinical significance of ALT in pediatric brain tumors, and ALT-directed therapy is not available.

METHODS

We performed C-circle analysis (CCA) on 579 pediatric brain tumors that had corresponding tumor/normal whole genome sequencing through the Open Pediatric Brain Tumor Atlas (OpenPBTA). We detected ALT in 6.9% (n = 40/579) of these tumors and completed additional validation by ultrabright telomeric foci in situ on a subset of these tumors. We used CCA to validate TelomereHunter for computational prediction of ALT status and focus subsequent analyses on pediatric high-grade gliomas (pHGGs) Finally, we examined whether ALT is associated with recurrent somatic or germline alterations.

RESULTS

ALT is common in pHGGs (n = 24/63, 38.1%), but occurs infrequently in other pediatric brain tumors (<3%). Somatic ATRX mutations occur in 50% of ALT+ pHGGs and in 30% of ALT- pHGGs. Rare pathogenic germline variants in mismatch repair (MMR) genes are significantly associated with an increased occurrence of ALT.

CONCLUSIONS

We demonstrate that ATRX is mutated in only a subset of ALT+ pHGGs, suggesting other mechanisms of ATRX loss of function or alterations in other genes may be associated with the development of ALT in these patients. We show that germline variants in MMR are associated with the development of ALT in patients with pHGG.

Highlighting vulnerabilities in the alternative lengthening of telomeres pathway

The alternative lengthening of telomeres (ALT) pathway is a telomere elongation mechanism found in a small but often aggressive subset of cancers. Dependent on break-induced replication, telomere extension in ALT-positive cells relies on a baseline level of DNA replication stress to initiate elongation events. This results in an elevated level of DNA damage and presents a possible vulnerability to be exploited in the development of ALT-targeted cancer therapies. Currently, there are no treatment options that target the ALT mechanism or that are specific for ALT-positive tumors. Here, we review recent developments and promising directions in the development of ALT-targeted therapeutics, many of which involve tipping the balance towards inhibition or exacerbation of ALT activity to selectively target these cells.

Targeting the Molecular and Immunologic Features of Leiomyosarcoma

Leiomyosarcoma (LMS) is a rare, aggressive mesenchymal tumor with smooth muscle differentiation. LMS is one of the most common histologic subtypes of soft tissue sarcoma; it most frequently occurs in the extremities, retroperitoneum, or uterus. LMS often demonstrates aggressive tumor biology, with a higher risk of developing distant metastatic disease than most sarcoma histologic types. The prognosis is poor, particularly in patients with uterine disease, and there is a need for the development of more effective therapies. Genetically, LMS is karyotypically complex and characterized by a low tumor mutational burden, with frequent alterations in TP53, RB1, PTEN, and DNA damage response pathways that may contribute to resistance against immune-checkpoint blockade monotherapy. The LMS immune microenvironment is highly infiltrated with tumor-associated macrophages and tumor-infiltrating lymphocytes, which may represent promising biomarkers. This review provides an overview of the clinical and pathologic behavior of both soft tissue and uterine LMS and summarizes the genomic and immune characteristics of these tumors and how they may provide opportunities for the development of biomarker-based immune therapies.

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.