|Year : 2021 | Volume
| Issue : 4 | Page : 85-91
Pediatric posterior fossa ependymoma and metabolism: A narrative review
Katharine E Halligan1, Antony Michealraj Kulandaimanuvel2, Andrea Cruz2, James T Felker3, Craig Daniels4, Michael D Taylor5, Sameer Agnihotri6
1 Department of Neurological Surgery, University of Pittsburgh School of Medicine; John G. Rangos Sr. Research Center, Children's Hospital of Pittsburgh; Division of Pediatrics, Department of Hematology Oncology, UPMC Children's Hospital of Pittsburgh, Pittsburgh, PA; Division of Pediatrics, Department of Hematology Oncology, Albany Medical Center, Albany, NY, USA
2 Department of Neurological Surgery, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA
3 Division of Pediatrics, Department of Hematology Oncology, UPMC Children's Hospital of Pittsburgh, Pittsburgh, PA, USA
4 The Arthur and Sonia Labatt Brain Tumor Research Center, The Hospital for Sick Children; Department of Developmental and Stem Cell Biology Program, The Hospital for Sick Children, Toronto, ON, Canada
5 The Arthur and Sonia Labatt Brain Tumor Research Center, The Hospital for Sick Children; Department of Developmental and Stem Cell Biology Program, The Hospital for Sick Children; Division of Neurosurgery, The Hospital for Sick Children, Toronto, ON, Canada
6 Department of Neurological Surgery, University of Pittsburgh School of Medicine; John G. Rangos Sr. Research Center, Children's Hospital of Pittsburgh; Division of Pediatrics, Department of Hematology Oncology, UPMC Children's Hospital of Pittsburgh, Pittsburgh, PA, USA
|Date of Submission||13-Oct-2021|
|Date of Decision||17-Nov-2021|
|Date of Acceptance||25-Nov-2021|
|Date of Web Publication||13-Jan-2022|
Dr. Sameer Agnihotri
John G. Rangos Sr. Research Center, UPMC Children's Hospital and University of Pittsburgh, 4401 Penn Avenue, PA 15224
Source of Support: None, Conflict of Interest: None
Ependymomas are a lethal central nervous system (CNS) tumor found in both adults and children. Recent efforts have focused on risk stratification by classifying the molecular variants of CNS ependymoma. Despite this increased knowledge of molecular drivers, much less is known about the metabolism of these subgroups. Disruption of cellular metabolism can drive the transition of normal neuronal cells to tumor cells. A shift from anaerobic to aerobic metabolism as the primary energy source is a hallmark of cancer, promoting cancer cell proliferation, and avoidance of cellular apoptotic cues. This review aims to discuss the current knowledge regarding metabolism in ependymoma cells compared to normal brain cells and the implications of metabolic changes with regard to tumorigenesis, the tumor microenvironment, and possible targets for treatment.
Keywords: Ependymoma, H3K27M, hypoxia, metabolism, posterior fossa ependymoma, review
|How to cite this article:|
Halligan KE, Kulandaimanuvel AM, Cruz A, Felker JT, Daniels C, Taylor MD, Agnihotri S. Pediatric posterior fossa ependymoma and metabolism: A narrative review. Glioma 2021;4:85-91
|How to cite this URL:|
Halligan KE, Kulandaimanuvel AM, Cruz A, Felker JT, Daniels C, Taylor MD, Agnihotri S. Pediatric posterior fossa ependymoma and metabolism: A narrative review. Glioma [serial online] 2021 [cited 2022 Jan 27];4:85-91. Available from: http://www.jglioma.com/text.asp?2021/4/4/85/335758
| Introduction|| |
Ependymomas are a lethal central nervous system (CNS) tumor found in both adults and children. Histologically classified as World Health Organization (WHO) I/II/III Grade tumors arising from glial cells, ependymoma is the third most common pediatric CNS tumor., While these tumors can be found in all three major anatomic compartments of the brain, they are most typically located intracranially (supratentorial region or posterior fossa) in children and as spinal lesions in adults. The current standard of care for ependymoma patients remains surgical resection followed by intense radiotherapy; this treatment confers a 7-year-event-free survival rate of approximately 65% in the pediatric population.,, Unfortunately, due to catastrophic effects on the developing brain, there are limitations regarding the use of radiation treatment for ependymoma in infants and young children. In addition, cytotoxic chemotherapy has failed to show survival benefits.,, However, a role for less intense maintenance chemotherapy is still being investigated.
Recent efforts have focused on risk stratification by classifying the molecular variants of CNS ependymoma. More recently, ten subtypes based on location within the CNS, histopathologic and molecular characteristics reflecting genetic and epigenetic alterations have been classified by the 2021 WHO Classification of Tumors., The most prevalent subtype in pediatric patients is posterior fossa ependymoma located in the hindbrain of infants and children. Posterior fossa ependymomas can be further subtyped into three distinct molecular entities, with clear clinical heterogeneity among the subtypes. Subependymoma is benign and classified as WHO Grade I tumors, while recently Myxopapillary Epenymoma is classified as Grade II. More aggressive and higher-grade posterior fossa ependymomas are classified as posterior fossa A ependymoma (PF-EPN-A) or posterior fossa B ependymoma (PF-EPN-B) [Figure 1]. Compared to PF-EPN-B ependymoma, PF-EPN-A ependymoma occurs more frequently in infants and younger children, has a poor prognosis, and exhibits alteration in polycomb repressor complex 2, a histone methyltransferase that is an important epigenetic regulator of histone H3. There are currently nine PF-EPN-A subgroups and five PFB subgroups. Multiple research groups have shown that gain of chromosome 1q increases the risk of poor outcomes in PF-EPN-A ependymoma,,,, but does not appear to alter prognosis in PF-EPN-B ependymoma. More recently, an ultra-high-risk biologically distinct and extremely aggressive subgroup of PF-EPN-A ependymoma that is characterized by loss of chromosome 6q (independent of 1q status) has been identified.
|Figure 1: Summary of posterior fossa ependymoma subgroups. Common clinical and molecular features of posterior fossa ependymoma by DNA methylation subgroup: type A (PF-EPN-A or PFA ependymoma), type B (PF-EPN-B or PFB ependymoma), and subependymoma. PF-EPN-A: Posterior fossa A ependymoma, PF-EPN-B: Posterior fossa B ependymoma|
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Disruption of cellular metabolism can drive the transition of normal neuronal cells to tumor cells. A shift from anaerobic to aerobic metabolism as the primary energy source is a hallmark of cancer, promoting cancer cell proliferation and avoidance of cellular apoptotic cues. This review aims to discuss the current knowledge regarding metabolism in ependymoma cells compared to normal brain cells and the implications of metabolic changes with regard to tumorigenesis, the tumor microenvironment, and PF-EPN-A treatment. Given the lack of advances in clinical PF-EPN-A treatment, understanding the role of metabolic dysregulation in these aggressive tumors is important and may yield new therapeutic strategies in future.
| Database Search Strategy|| |
Literature review was electronically performed using PubMed database. The following combinations of key words were used to initially select the articles to be evaluated: Ependymoma and metabolism; ependymoma and glycolysis; ependymoma and tricarboxylic acid (TCA); ependymoma and oxidative phosphorylation; ependymoma and amino acid metabolism; ependymoma and epigenetics; brain tumors and epigenetics; pediatric brain tumors and epigenetics; ependymoma management; pediatric ependymoma management; brain tumors and metabolism; and pediatric brain tumors and metabolism. Most of the elected studies (80% of all references) were published from 2009 to 2020.
| Normal Brain Metabolism|| |
Although the human brain only accounts for 2% of the body's total mass, it is responsible for 20% of the body's daily oxygen consumption and over half of the daily glucose intake. Normal cells utilize glucose by generating pyruvate through glycolysis followed by oxidative metabolism to ultimately yield 36 adenosine triphosphates (ATPs), the main energy currency of the cell. In the majority of other human tissues, glucose is stored as glycogen, broken down to glucose, and released into the bloodstream to meet metabolic demands. Neural cells in the brain are unable to store glucose and thus rely on integrated cooperation with neighboring astrocytes that contain glycogen stores. There is also evidence indicating that choroid plexus and ependymal cells are sites of glycogen synthesis and storage. Astrocytes can take up glucose from the circulation via glucose transporter 1 (GLUT1) located on end feet that directly interact with capillary surfaces. Glucose is ultimately converted to glycogen or metabolized to lactate, which is transported out of the astrocytes through monocarboxylate transporters (MCTs) and then taken up by neurons to be utilized. MCT1 is the predominant MCT isoform found in intracranial epithelial cells, astrocytes, and neuronal cells. Lactate can then be oxidized to pyruvate through lactose dehydrogenase 1 and shuttled to the mitochondria for ATP generation.
Astrocytes have high rates of glycolysis, utilizing glucose to generate ATP and lactate. They have high levels of glycolysis enzymes such as phosphofructokinase B3, an enzyme involved in the glucose input into the glycolytic pathway, and high levels of fructose 2,6-bisphosphate, an activator of the key glycolysis enzyme phosphofructokinase. In contrast, neurons are thought to mainly exhibit oxidative metabolism of glucose, involving the mitochondrial TCA cycle and the electron transport chain. More recently, it has been suggested that neurons can utilize lactate efficiently and will preferentially use lactate over glucose in the presence of both. Compared to astrocytes, neurons have low production of fructose 2,6-bisphosphate and PFK3B is essentially absent due to posttranscriptional downregulation (involving proteasomal degradation). High rates of glycolysis in neurons have been observed to increase oxidative stress and apoptosis, further indicating that neurons cannot maintain high glycolytic rates. High glucose levels in neurons have also been shown to activate the hexose monophosphate pathway, resulting in nicotinamide adenine dinucleotide phosphate production. This indicates that neurons utilize glucose for maintaining antioxidant status rather than for bioenergetic purposes., Conversely, others have shown that neurons express all the enzymes needed to metabolize glucose through glycolysis, and while PFK3B is posttranscriptionally downregulated in neurons, this is just one mechanism by which neurons can accomplish glycolysis for glucose metabolism., Yellen reviewed several mechanisms by which neurons can increase their glycolytic capacity under stress, and they suggested the uncoupling of glycolysis and oxidative metabolism may be transient and not the preferred metabolic mechanism of neurons under stress.
Both glycogen storage and energy metabolism are compartmentalized throughout the brain. They appear to be highest in areas with the greatest synaptic density, with the glycogen concentration being twice as high in grey matter compared to white matter. Glycogen levels in these areas are strictly regulated by hormones (adrenaline and noradrenaline) and insulin signaling involving insulin-like growth factor. There is also evidence for metabolic compartmentalization within both neurons and astrocytes, as evidenced by cellular heterogeneity in terms of the distribution of metabolites, proteins, ions, and organelles. Waagepetersen et al. have described distinct mitochondria populations with different metabolic potential (based on α-ketoglutarate dehydrogenase activity) within astrocytes.
| Normal Glycolysis versus Posterior Fossa an Ependymoma Glycolysis|| |
In addition to ATP generation to meet metabolic demands, cells also require biomolecules for the synthesis of lipids, nucleotides, and protein, as well as reducing agents for antioxidant protection.,, Perhaps the most well-described difference between normal neuron metabolism and cancer cell metabolism is the shift from anaerobic to aerobic glycolysis known as the Warburg effect. Tumor cells and other rapidly dividing cells can ferment glucose to lactate in the presence of oxygen. As reviewed by Agnihotri and Zadeh, the shift from oxidative metabolism to aerobic glycolysis in tumor cells is less efficient in energy generation due to the decreased potential yield of ATP through glycolysis but allows for the glycolysis and Kreb cycle intermediates to be shuttled into anabolic pathways to generate biomass.,, Metabolic profiling suggests that glucose is abundantly enriched in PF-EPN-A ependymoma tissues compared to other pediatric brain tumors. Indeed, PF-EPN-A ependymoma cells growing in hypoxic conditions had superior glucose uptake than normoxia-treated cells, and blockade of GLUT1 decreased the proliferation of PF-EPN-A ependymoma cells. Upon glucose uptake, PF-EPN-A ependymoma cells use the glucose molecules for macromolecular synthesis by activating both the pentose phosphate pathway and serine/glycine metabolism by upregulating the expression of transketolase like 1, phosphoglycerate dehydrogenase, and phosphoglycerate dehydrogenase. Given that tumors such as ependymoma grow in hypoxic and nutrient-restricted conditions, the metabolic shift to aerobic glycolysis confers an advantage to the tumor cells as it allows adequate generation of both ATP and essential macromolecules for nucleotide, protein, and lipid synthesis required for cellular proliferation through diversion of carbon from glucose into biosynthetic pathways. Ultimately, this allows PF-EPN-A cells to grow in harsh microenvironment cells that normal cells cannot grow nor survive in.
| Normal Tricarboxylic Acid Cycle versus Posterior Fossa an Ependymoma Tricarboxylic Acid Cycle|| |
In the first comprehensive metabolic landscape study of PF-EPN-A ependymoma, several key findings regarding the TCA cycle (which is also known as the Krebs cycle or the citric acid cycle) in PF-EPN-A ependymoma emerged. The TCA cycle is a critical metabolic pathway that regulates bioenergetics, biosynthetic pathways, and redox balance mechanisms. It is an amphibolic pathway involving anaplerosis and cataplerosis mechanisms that provide the nutrients to run the TCA cycle and remove the TCA cycle intermediates, respectively. In normal or neoplastic cells, glucose is converted into pyruvate, which is then imported into mitochondria to initiate the TCA cycle. In the TCA cycle, a series of enzymatic reactions generate reduced NADH and FADH2 (for use in the electron transport chain) and intermediates. The intermediates are used for fatty acid synthesis, gluconeogenesis, and amino acid biosynthesis. Additionally, they are used as essential cofactors for chromatin modifiers that regulate the physiological and disease epigenetic landscapes. Cancer cells exhibit robust metabolic plasticity, with both glycolysis and the TCA cycle/oxidative phosphorylation in mitochondria being functionally active and providing molecules for bioenergetic and biosynthetic processes. Certain mutations of TCA cycle-related genes can directly lead to enrichment of oncometabolites (2-hydroxyglutarate, succinate, and fumarate) and thereby inhibit the catalytic activity of histone demethylases and DNA demethylases. This leads to aberrant epigenetic patterns and may promote tumorigenesis by blocking differentiation. The tumor microenvironment (including hypoxia) and oncogene/tumor suppressor-dependent cell signaling events can selectively enrich certain intermediates of the TCA cycle to essentially regulate cellular and molecular processes involved in tumor formation and progression. Due to hypoxia and other factors, the TCA cycle in PF-EPN-A ependymoma is somewhat dysregulated but still functional. In actively proliferating PF-EPN-A ependymoma cells, due to dysregulation of the TCA cycle, the levels of TCA cycle intermediates such as succinate, fumarate, malate, and oxaloacetate are reduced, while citrate, isocitrate, and α-ketoglutarate levels are increased. As a result, the cellular ratio of α-ketoglutarate to succinate increases in PF-EPN-A ependymoma, promoting increased demethylation potential. The histone 3 lysine 27 (H3K27) demethylases KDM6A/B use α-ketoglutarate as a cofactor to demethylate histones and promote transcriptional activation of genes that are important for PF-EPN-A ependymoma cell survival. In the TCA cycle, α-ketoglutarate is converted into citrate by reductive enzymatic reactions and exported into the cytosol via the citrate shuttle and then converted to acetyl-coenzyme A (CoA). Once in the cytosol, it serves as a precursor for lipid metabolism and as a cofactor for histone acetyltransferases (HATs). Thus, the TCA cycle plays a vital role in the dynamic regulation of the epigenetic landscape of lethal infantile ependymomas.
| Amino Acid Metabolism|| |
Besides serving as precursors for protein synthesis, amino acids (and their metabolism) are very important for the sustained proliferation of cancer cells. This is because the related homeostasis mechanism controls gluconeogenesis, fatty acid synthesis, the urea cycle, redox balance, nucleotide synthesis, the TCA cycle, energy generation, and more importantly, epigenetic patterns in the cells. Cancers can be auxotrophic regarding amino acids (depending on genetic makeup), and both gluconeogenic and ketogenic amino acids can be either synthesized (nonessential amino acids) or supplied throuh the tumor microenvironment and essential amino acids through oncogenic reprogramming mechanisms. Amino acid metabolism strongly contributes to the regulation of PF-EPN-A ependymoma nucleotide synthesis, redox homeostasis, protein regulation, and epigenetic regulation, selectively enriching specific metabolites/intermediates at the subcellular level. Glutamine and glutamate, which undergo catabolic processing to supply more α-ketoglutarate and acetyl-CoA, play pivotal roles in lipogenesis and histone acetylation in PF-EPN-A ependymoma. Metabolic plasticity in PF-EPN-A ependymoma cells can allow the glutamate pool to be increased using other amino acids such as proline, serine, glycine, and ornithine. On the other hand, amino acid homeostasis of methionine, glycine, and serine control the folate cycle, redox potential, and epigenetic regulation, including maintaining the basal level of histone 3 lysine 27 trimethylation (H3K27 me3), which is indispensable for PF-EPN-A ependymoma cell survival. PF-EPN-A ependymomas are lethal infantile malignancies that may originate during early embryonic development. Amino acid availability and metabolism in the developing embryonic brain can regulate physiological processes such as epigenetic and protein regulation involved in determining the fate of stem cells. However, this is poorly understood. Moving forward, to understand the metabolic and epigenetic plasticity of neoplasms, we need extensive investigations into the role of transcriptional and translational control of amino acid metabolism.
| How Metabolism Influences Epigenetics|| |
The epigenetic landscape of any given cell, whether it is nonneoplastic, neoplastic, or differentiated, is dynamically linked to the availability of metabolites and the activity of metabolic pathways. Recent studies have highlighted that the localization and activity of metabolic enzymes can indeed regulate loci-specific gene expression and epigenomic profiles., Genomic aberrations, dysregulated cell signaling events, the tumor microenvironment, and dysfunctional/compromised immune landscapes can all enhance the activity of metabolic enzymes and thus control the availability of metabolites in tumor cells and tumor-associated cells. The following cofactors are essential for catalytic activity: S-adenosylmethionine (SAM; methyl-donor cofactor) for histone methyltransferases and DNA methyltransferases; Fe (II) and α-ketoglutarate for Tet methylcytosine dioxygenase and histone lysine demethylases (KDMs); acetyl-CoA (acetyl-donating cofactor) for HATs; and nicotinamide adenine dinucleotide (NAD+) for Class III histone deacetylases/sirtuins.
This interdependence involving metabolism and epigenetics is outlined in [Figure 2]. As mentioned above, various cancers essentially reprogram metabolic pathways and alter metabolite availability by potentiating oncogenic mechanisms. However, pediatric brain tumors that have a low mutational burden selectively hijack the cell types (cell of origin) that have potentiating oncogenic mechanisms or inhibiting tumor suppressor mechanisms to support rapid metabolic growth. SAM is essential for histone methyltransferase activity, and the methionine cycle, which is a major source of SAM in the cell, is regulated by several critical metabolic pathways such as the folate cycle, trans-sulfuration pathway, and polyamine metabolism. H3K27 hypomethylation occurs in PF-EPN-A ependymoma through two distinct routes: expression of EZH2 inhibitory protein (EZHIP/CXorf67) and elongin BC and polycomb repressive complex 2 associated protein (EPOP) and restricted availability of SAM. PF-EPN-A ependymoma has elevated expression of nicotinamide N-methyltransferase, which robustly consumes SAM as a cofactor and converts nicotinamide to 1-methyl nicotinamide. By this process, nicotinamide N-methyltransferase outcompetes EZH2 methyltransferases for SAM and generates S-adenosylhomocysteine (SAH) in the cytosol, effectively blocking histone methyltransferases and DNA methyltransferases through direct competition for SAM. An unbalanced SAM/SAH ratio indicates aberrant cellular methylation potential. Polyamine metabolite (putrescine, spermidine, and spermine) pathways, the other major SAM-consuming metabolic pathways, are upregulated in PF-EPN-A ependymoma. This further restricts the availability of SAM, leading to H3K27 hypomethylation and a less repressive chromatin signature.
|Figure 2: Metabolic regulation of the ependymoma epigenome. A hypoxic microenvironment is essential for propagation and growth of posterior fossa A ependymoma (PF-EPN-A) ependymoma. Hypoxia is a key regulator of several metabolic intermediates that maintain the H3K27 hypomethylated state, which is a key feature of PF-EPN-A ependymoma. These metabolic alterations result in S-adenosylmethionine sequestration (reducing H3K27 methylation), increased glycolysis and increased reductive carboxylation (increasing H3K27 acetylation), and increased glutaminolysis (to support active DNA and histone demethylases). EPOP: Elongin BC and polycomb repressive complex 2 associated protein, EZHIP: EZH2 inhibitory protein, SAH: S-adenosylhomocysteine|
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PF-EPN-A ependymoma exhibits hypomethylation and hyperacetylation at H3K27, indicating open chromatin and enhanced gene expression at specific loci., For hyperacetylation to occur at H3K27, PF-EPN-A ependymoma heavily depends on hypoxic conditions. Hypoxia increases the supply of the cofactor α-ketoglutarate to demethylases and thereby removes methyl groups at H3K27, and hypoxia also leads to acetyl-CoA production through reductive TCA cycle-mediated cataplerosis. Acetyl-CoA synthesized by ATP citrate lyase is imported into the nucleus and used as an acetyl-donating cofactor by HATs for hyperacetylation. When nutrient deprivation occurs in the tumor microenvironment, hypoxia promotes preferential uptake of acetate from outside the cell as a direct source of acetyl-CoA. Acetyl-CoA synthetase short-chain family members 2 and 3 (ACSS2/3) are involved in the metabolization of acetate into acetyl-CoA. When there is limited acetyl-CoA generated from the citrate pool, this mechanism serves to generate acetyl-CoA for lipid synthesis and histone acetylation. In summary, while PF-EPN-A ependymoma does not have recurrent H3K27M, mutations to suppress histone hypermethylation, PF-EPN-A cells achieve hypomethylation by metabolic reprogramming to ensure that the essential cofactors and nutrients are supplied to inhibit the activity of histone methyltransferases. Demethylated histone is then hyperacetylated, using the enormous supply of acetyl-CoA through HAT activity in the nuclear compartment. H3K27 acetylation (H3K27Ac) in PF-EPN-A ependymoma leads to promoter activation, recruitment of enhancers and other transcriptional factors, and finally upregulation of gene expression for sustained cell survival, uncontrolled proliferation, and oncogenic cell signaling events. It will be of great interest how future studies incorporate metabolic therapies with the current standard of care treatments for PF-EPN-A such as radiation.
A major obstacle in the development of treatment for posterior fossa ependymoma is the paucity of known molecular features for targeting. Targeting tumor metabolism and the related regulation of epigenetics presents a very encouraging new treatment paradigm to pursue.
This review is a summary of previously published articles related to metabolism and PF-EPN-A ependymoma. Some limitations include bias by authors, which could have led to incomplete or inadvertent interpretation of the existing literature. Other limitations of this review are accidental acts of omission by incomplete searches of the literature, which do not include all of the pertinent articles in this field, which regret to have included.
| Conclusion|| |
Ependymomas are a lethal CNS tumor, and despite recent efforts focused on risk stratification by classifying the molecular variants, no new targeted treatments exist. Increased knowledge of the cellular metabolism in ependymoma cells compared to normal brain cells has enormous implications with regard to tumorigenesis, the tumor microenvironment, and possible targets for treatment. Targeting tumor metabolism and the related regulation of epigenetics presents a very encouraging new treatment paradigm to pursue.
Financial support and sponsorship
KEH was funded by an NIH T32 training grant from the Department of Paediatrics, University of Pittsburgh.
Conflicts of interest
There are no conflicts of interest.
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