ABCG2 Transporter Expression Impacts Group 3 Medulloblastoma Response to Chemotherapy

Tumor-acquired somatic mutation affects conformation to abolish ABCG2-mediated drug resistance

ABCG2 is an important ATP-binding cassette transporter impacting the absorption and distribution of over 200 chemical toxins and drugs. ABCG2 also reduces the cellular accumulation of diverse chemotherapeutic agents. Acquired somatic mutations in the phylogenetically conserved amino acids of ABCG2 might provide unique insights into its molecular mechanisms of transport. Here, we identify a tumor-derived somatic mutation (Q393K) that occurs in a highly conserved amino acid across mammalian species. This ABCG2 mutant seems incapable of providing ABCG2-mediated drug resistance. This was perplexing because it is localized properly and retained interaction with substrates and nucleotides. Using a conformationally sensitive antibody, we show that this mutant appears "locked" in a non-functional conformation. Structural modeling and molecular dynamics simulations based on ABCG2 cryo-EM structures suggested that the Q393K interacts with the E446 to create a strong salt bridge. The salt bridge is proposed to stabilize the inward-facing conformation, resulting in an impaired transporter that lacks the flexibility to readily change conformation, thereby disrupting the necessary communication between substrate binding and transport.

Development and Validation of a Sensitive and Specific LC-MS/MS Method for IWR-1-Endo, a Wnt Signaling Inhibitor: Application to a Cerebral Microdialysis Study

IWR-1-endo, a small molecule that potently inhibits the Wnt/β-catenin signaling pathway by stabilizing the AXIN2 destruction complex, can inhibit drug efflux at the blood−brain barrier. To conduct murine cerebral microdialysis research, validated, sensitive, and reliable liquid chromatography−tandem mass spectrometry (LC-MS/MS) methods were used to determine IWR-1-endo concentration in the murine plasma and brain microdialysate. IWR-1-endo and the internal standard (ISTD) dabrafenib were extracted from murine plasma and microdialysate samples by a simple solid-phase extraction protocol performed on an Oasis HLB µElution plate. Chromatographic separation was executed on a Kinetex C18 (100A, 50 × 2.1 mm, 4 µm particle size) column with a binary gradient of water and acetonitrile, each having 0.1% formic acid, pumped at a flow rate of 0.6 mL/min. Detection by mass spectrometry was conducted in the positive selected reaction monitoring ion mode by monitoring mass transitions 410.40 > 344.10 (IWR-1-endo) and 520.40 > 307.20 (ISTD). The validated curve range of IWR-1-endo was 5−1000 ng/mL for the murine plasma method (r2 ≥ 0.99) and 0.5−500 ng/mL for the microdialysate method (r2 ≥ 0.99). The lower limit of quantification (LLOQ) was 5 ng/mL and 0.5 ng/mL for the murine plasma and microdialysate sample analysis method, respectively. Negligible matrix effects were observed in murine plasma and microdialysate samples. IWR-1-endo was extremely unstable in murine plasma. To improve the stability of IWR-1-endo, pH adjustments of 1.5 were introduced to murine plasma and microdialysate samples before sample storage and processing. With pH adjustment of 1.5 to the murine plasma and microdialysate samples, IWR-1-endo was stable across several tested conditions such as benchtop, autosampler, freeze−thaw, and long term at −80 °C. The LC-MS/MS methods were successfully applied to a murine pharmacokinetic and cerebral microdialysis study to characterize the unbound IWR-1-endo exposure in brain extracellular fluid and plasma.

Quercetin attenuates cyclophosphamide induced-immunosuppressive indoleamine 2,3-dioxygenase in the hippocampus and cerebral cortex of male Wister rats

This study investigated the protective effect of quercetin against cyclophosphamide-induced immunosuppressive indoleamine 2,3-dioxygenase (IDO) via the mechanism of oxidative-inflammatory stress and behavioral indices. Cyclophosphamide (CYP) was administered to male Wister rats at a dose of 100 mg/kg with or without quercetin 50 mg/kg every other day for 7 days. Experimental techniques including western blotting, immunohistochemistry analysis, and inflammatory and oxidative stress marker assays were carried out. We also conducted behavioral analyses such as open field, tail suspension, and Y-maze tests for cognitive assessment. The results indicated that quercetin attenuated oxidative-inflammatory stress induced by CYP in the hippocampus and cerebral cortex of male Wister rats by augmenting the activities of antioxidant enzymes and suppressing lipid peroxidation as well as inflammatory mediators such as interleukin-6 and interferon-γ. Concomitantly, quercetin partially prevented the alteration in brain tissue histological architecture and mitigated the activities of IDO/tryptophan 2,3-dioxygenase (TDO) and protein expression of IDO1. This was corroborated by the IDO-quercetin model obtained in silico, revealing a favorable inhibitory interaction between quercetin and the enzyme. Finally, the results of behavioral tests suggested that quercetin significantly prevented the depressive-like posture of the CYP-treated rats. Our study for the first time revealed that quercetin ameliorates the effect of CYP-instigated IDO/TDO activities in the cerebral cortex and hippocampus via restoration of antioxidant enzymes and preventing oxidative-inflammatory stress.

New Developments in the Pathogenesis, Therapeutic Targeting, and Treatment of Pediatric Medulloblastoma

Pediatric medulloblastoma (MB) is the most common pediatric brain tumor with varying prognoses depending on the distinct molecular subtype. The four consensus subgroups are WNT, Sonic hedgehog (SHH), Group 3, and Group 4, which underpin the current 2021 WHO classification of MB. While the field of knowledge for treating this disease has significantly advanced over the past decade, a deeper understanding is still required to improve the clinical outcomes for pediatric patients, who are often vulnerable in ways that adult patients are not. Here, we discuss how recent insights into the pathogenesis of pediatric medulloblastoma have directed current and future research. This review highlights new developments in understanding the four molecular subtypes’ pathophysiology, epigenetics, and therapeutic targeting. In addition, we provide a focused discussion of recent developments in imaging, and in the surgery, chemotherapy, and radiotherapy of pediatric medulloblastoma. The article includes a brief explanation of healthcare costs associated with medulloblastoma treatment.

Structures of ABCG2 under turnover conditions reveal a key step in the drug transport mechanism

ABCG2 is a multidrug transporter that affects drug pharmacokinetics and contributes to multidrug resistance of cancer cells. In previously reported structures, the reaction cycle was halted by the absence of substrates or ATP, mutation of catalytic residues, or the presence of small-molecule inhibitors or inhibitory antibodies. Here we present cryo-EM structures of ABCG2 under turnover conditions containing either the endogenous substrate estrone-3-sulfate or the exogenous substrate topotecan. We find two distinct conformational states in which both the transport substrates and ATP are bound. Whereas the state turnover-1 features more widely separated NBDs and an accessible substrate cavity between the TMDs, turnover-2 features semi-closed NBDs and an almost fully occluded substrate cavity. Substrate size appears to control which turnover state is mainly populated. The conformational changes between turnover-1 and turnover-2 states reveal how ATP binding is linked to the closing of the cytoplasmic side of the TMDs. The transition from turnover-1 to turnover-2 is the likely bottleneck or rate-limiting step of the reaction cycle, where the discrimination of substrates and inhibitors occurs.

ABCG2 is a transporter contributing to multidrug resistance of cancer cells. Here, structures of human ABCG2 under turnover conditions reveal distinct conformational states, provide insight into the transport cycle and suggest a mechanism of discrimination between substrates and inhibitors.

Role of ATP-Binding Cassette Transporter Proteins in CNS Tumors: Resistance- Based Perspectives and Clinical Updates

Despite gigantic advances in medical research and development, chemotherapeutic resistance remains a major challenge in complete remission of CNS tumors. The failure of complete eradication of CNS tumors has been correlated with the existence of several factors including overexpression of transporter proteins. To date, 49 ABC-transporter proteins (ABC-TPs) have been reported in humans, and the evidence of their strong association with chemotherapeutics' influx, dissemination, and efflux in CNS tumors, is growing. Research studies on CNS tumors are implicating ABC-TPs as diagnostic, prognostic and therapeutic biomarkers that may be utilised in preclinical and clinical studies. With the current advancements in cell biology, molecular analysis of genomic and transcriptomic interplay, and protein homology-based drug-transporters interaction, our research approaches are streamlining the roles of ABC-TPs in cancer and multidrug resistance. Potential inhibitors of ABC-TP for better clinical outcomes in CNS tumors have emerged. Elacridar has shown to enhance the chemo-sensitivity of Dasatanib and Imatinib in various glioma models. Tariquidar has improved the effectiveness of Temozolomide's in CNS tumors. Although these inhibitors have been effective in preclinical settings, their clinical outcomes have not been as significant in clinical trials. Thus, to have a better understanding of the molecular evaluations of ABC-TPs, as well as drug-interactions, further research is being pursued in research labs. Our lab aims to better comprehend the biological mechanisms involved in drug resistance and to explore novel strategies to increase the clinical effectiveness of anticancer chemotherapeutics, which will ultimately improve clinical outcomes.

CNS Penetration of Cyclophosphamide and Metabolites in Mice Bearing Group 3 Medulloblastoma and Non-Tumor Bearing Mice

PURPOSE

Cyclophosphamide is widely used to treat children with medulloblastoma; however, little is known about its brain penetration. We performed cerebral microdialysis to characterize the brain penetration of cyclophosphamide (130 mg/kg, IP) and its metabolites [4-hydroxy-cyclophosphamide (4OH-CTX) and carboxyethylphosphoramide mustard (CEPM)] in non-tumor bearing mice and mice bearing orthotopic Group 3 medulloblastoma.

METHODS

A plasma pharmacokinetic study was performed in non-tumor-bearing CD1- nude mice, and four cerebral microdialysis studies were performed in non-tumor-bearing (M1 and M3) and tumor- bearing mice (M2 and M4). Plasma samples were collected up to 6-hours post-dose, and extracellular fluid (ECF) samples were collected over 60-minute intervals for 24-hours post-dose. To stabilize and quantify 4OH-CTX, a derivatizing solution was added in blood after collection, and either directly in the microdialysis perfusate (M1 and M2) or in ECF collection tubes (M3 and M4). Plasma/ECF cyclophosphamide and CEPM, and 4OH-CTX concentrations were separately measured using different LC-MS/MS methods.

RESULTS

All plasma/ECF concentrations were described using a population-based pharmacokinetic model. Plasma exposures of cyclophosphamide, 4OH-CTX, and CEPM were similar across studies (mean AUC=112.6, 45.6, and 80.8 µmol∙hr/L). Hemorrhage was observed in brain tissue when the derivatizing solution was in perfusate compared with none when in collection tubes, which suggested potential sample contamination in studies M1 and M2. Model-derived unbound ECF to plasma partition coefficients (Kp,uu) were calculated to reflect CNS penetration of the compounds. Lower cyclophosphamide Kp,uu was obtained in tumor-bearing mice versus non-tumor bearing mice (mean 0.15 versus 0.22, p=0.019). No differences in Kp,uu were observed between these groups for 4OH- CTX and CEPM (overall mean 0.10 and 0.07).

CONCLUSIONS

Future studies will explore potential mechanisms at the brain-tumor barrier to explain lower cyclophosphamide brain penetration in tumor-bearing mice. These results will be used to further investigate exposure-response relationships in medulloblastoma xenograft models.

Using Pharmacology to Squeeze the Life Out of Childhood Leukemia, and Potential Strategies to Achieve Breakthroughs in Medulloblastoma Treatment

Eliminating cancer was once thought of as a war. This analogy is still apt today; however, we now realize that cancer is a much more formidable enemy than scientists originally perceived, and in some cases, it harbors a profound ability to thwart our best efforts to defeat it. However, before we were aware of the complexity of cancer, chemotherapy against childhood acute lymphoblastic leukemia (ALL) was successful because it applied the principles of pharmacology. Herein, we provide a historic perspective of the experience at St. Jude Children's Research Hospital. In 1962, when the hospital opened, fewer than 3% of patients experienced durable cure. Through judicious application of pharmacologic principles (e.g., combination therapy with agents using different mechanisms of action) plus appropriate drug scheduling, dosing, and pharmacodynamics, the survival of patients with ALL now exceeds 90%. We contrast this approach to treating ALL with the contemporary approach to treating medulloblastoma, in which genetics and molecular signatures are being used to guide the development of more-efficacious treatment strategies with minimal toxicity. Finally, we highlight the emerging technologies that can sustain and propel the collaborative efforts to squeeze the life out of these cancers. SIGNIFICANCE STATEMENT: Up until the early 1960s, chemotherapy for childhood acute lymphoblastic leukemia was mostly ineffective. This changed with the knowledge and implementation of rational approaches to combination therapy. Although the therapeutics of brain cancers such as medulloblastoma are not as refined (in part because of the blood-brain barrier obstacle), recent extraordinary advances in knowledge of medulloblastoma pathobiology has led to innovations in disease classification accompanied with strategies to improve therapeutic outcomes. Undoubtedly, additional novel approaches, such as immunological therapeutics, will open new avenues to further the goal of taming cancer.

Construction and Validation of a 13-Gene Signature for Prognosis Prediction in Medulloblastoma

Background: Recent studies have identified several molecular subgroups of medulloblastoma associated with distinct clinical outcomes; however, no robust gene signature has been established for prognosis prediction. Our objective was to construct a robust gene signature-based model to predict the prognosis of patients with medulloblastoma.

Methods: Expression data of medulloblastomas were acquired from the Gene Expression Omnibus (GSE85217, n = 763; GSE37418, n = 76). To identify genes associated with overall survival (OS), we performed univariate survival analysis and least absolute shrinkage and selection operator (LASSO) Cox regression. A risk score model was constructed based on selected genes and was validated using multiple datasets. Differentially expressed genes (DEGs) between the risk groups were identified. Kyoto Encyclopedia of Genes and Genomes (KEGG), Gene Ontology (GO), and protein–protein interaction (PPI) analyses were performed. Network modules and hub genes were identified using Cytoscape. Furthermore, tumor microenvironment (TME) was evaluated using ESTIMATE algorithm. Tumor-infiltrating immune cells (TIICs) were inferred using CIBERSORTx.

Results: A 13-gene model was constructed and validated. Patients classified as high-risk group had significantly worse OS than those as low-risk group (Training set: p < 0.0001; Validation set 1: p < 0.0001; Validation set 2: p = 0.00052). The area under the curve (AUC) of the receiver operating characteristic (ROC) analysis indicated a good performance in predicting 1-, 3-, and 5-year OS in all datasets. Multivariate analysis integrating clinical factors demonstrated that the risk score was an independent predictor for the OS (validation set 1: p = 0.001, validation set 2: p = 0.004). We then identified 265 DEGs between risk groups and PPI analysis predicted modules that were highly related to central nervous system and embryonic development. The risk score was significantly correlated with programmed death-ligand 1 (PD-L1) expression (p < 0.001), as well as immune score (p = 0.035), stromal score (p = 0.010), and tumor purity (p = 0.010) in Group 4 medulloblastomas. Correlations between the 13-gene signature and the TIICs in Sonic hedgehog and Group 4 medulloblastomas were revealed.

Conclusion: Our study constructed and validated a robust 13-gene signature model estimating the prognosis of medulloblastoma patients. We also revealed genes and pathways that may be related to the development and prognosis of medulloblastoma, which might provide candidate targets for future investigation.

Methylation Profiling of Medulloblastoma in a Clinical Setting Permits Sub-classification and Reveals New Outcome Predictions

Medulloblastoma (MB) is the most common childhood malignant brain tumor and is a leading cause of cancer-related death in children. DNA methylation profiling has rapidly advanced our understanding of MB pathogenesis at the molecular level, but assessments in Saudi Arabian (SA)-MB cases are sparse. MBs can be sub-grouped according to methylation patterns from FPPE samples into Wingless (WNT-MB), Sonic Hedgehog (SHH-MB), Group 3 (G3), and Group 4 (G4) tumors. The WNT-MB and SHH-MB subgroups are characterized by gain-of function mutations that activate oncogenic cell signaling, whilst G3/G4 tumors show recurrent chromosomal alterations. Given that each subgroup has distinct clinical outcomes, the ability to subgroup SA-FPPE samples holds significant prognostic and therapeutic value. Here, we performed the first assessment of MB-DNA methylation patterns in an SA cohort using archival biopsy material (FPPE n = 49). Of the 41 materials available for methylation assessments, 39 could be classified into the major DNA methylation subgroups (SHH, WNT, G3, and G4). Furthermore, methylation analysis was able to reclassify tumors that could not be sub-grouped through next-generation sequencing, highlighting its superior accuracy for MB molecular classifications. Independent assessments demonstrated known clinical relationships of the subgroups, exemplified by the high survival rates observed for WNT tumors. Surprisingly, the G4 subgroup did not conform to previously identified phenotypes, with a high prevalence in females, high metastatic rates, and a large number of tumor-associated deaths. Taking our results together, we demonstrate that DNA methylation profiling enables the robust sub-classification of four disease sub-groups in archival FFPE biopsy material from SA-MB patients. Moreover, we show that the incorporation of DNA methylation biomarkers can significantly improve current disease-risk stratification schemes, particularly concerning the identification of aggressive G4 tumors. These findings have important implications for future clinical disease management in MB cases across the Arab world.

ABCG2 requires a single aromatic amino acid to "clamp" substrates and inhibitors into the binding pocket

ATP-binding cassette sub-family G member 2 (ABCG2) is a homodimeric ATP-binding cassette (ABC) transporter that not only has a key role in helping cancer cells to evade the cytotoxic effects of chemotherapy, but also in protecting organisms from multiple xeno- and endobiotics. Structural studies indicate that substrate and inhibitor (ligands) binding to ABCG2 can be differentiated quantitatively by the number of amino acid contacts, with inhibitors displaying more contacts. Although binding is the obligate initial step in the transport cycle, there is no empirical evidence for one amino acid being primarily responsible for ligand binding. By mutagenesis and biochemical studies, we demonstrated that the phylogenetically conserved amino acid residue, F439, was critical for both transport and the binding of multiple substrates and inhibitors. Structural modeling implied that the π-π interactions from each F439 monomer mediated the binding of a surprisingly diverse array of structurally unrelated substrates and inhibitors and that this symmetrical π-π interaction "clamps" the ligand into the binding pocket. Key molecular features of diverse ABCG2 ligands using the π-π clamp along with structural studies created a pharmacophore model. These novel findings have important therapeutic implications because key properties of ligands interacting with ABCG2 have been disovered. Furthermore, mechanistic insights have been revealed by demonstrating that for ABCG2 a single amino acid is essential for engaging and initiating transport of multiple drugs and xenobiotics.

An ABC Transporter Drives Medulloblastoma Pathogenesis by Regulating Sonic Hedgehog Signaling

Mutations in Sonic hedgehog (SHH) signaling promote aberrant proliferation and tumor growth. SHH-medulloblastoma (MB) is among the most frequent brain tumors in children less than 3 years of age. Although key components of the SHH pathway are well-known, we hypothesized that new disease-modifying targets of SHH-MB might be identified from large-scale bioinformatics and systems biology analyses. Using a data-driven systems biology approach, we built a MB-specific interactome. The ATP-binding cassette transporter ABCC4 was identified as a modulator of SHH-MB. Accordingly, increased ABCC4 expression correlated with poor overall survival in patients with SHH-MB. Knockdown of ABCC4 expression markedly blunted the constitutive activation of the SHH pathway secondary to Ptch1 or Sufu insufficiency. In human tumor cell lines, ABCC4 knockdown and inhibition reduced full-length GLI3 levels. In a clinically relevant murine SHH-MB model, targeted ablation of Abcc4 in primary tumors significantly reduced tumor burden and extended the lifespan of tumor-bearing mice. These studies reveal ABCC4 as a potent SHH pathway regulator and a new candidate to target with the potential to improve SHH-MB therapy. SIGNIFICANCE: These findings identify ABCC4 transporter as a new target in SHH-MB, prompting the development of inhibitors or the repurporsing of existing drugs to target ABCC4.

Overview of Current Drug Delivery Methods Across the Blood-Brain Barrier for the Treatment of Primary Brain Tumors

Existing drug delivery methods have not led to a significant increase in survival for patients with malignant primary brain tumors. While the combination of conventional therapies consisting of surgery, radiotherapy, and chemotherapy has improved survival for some types of brain tumors (e.g., WNT medulloblastoma), other types of brain tumors (e.g., glioblastoma and diffuse midline glioma) still have a poor prognosis. The reason for the differences in response can be largely attributed to the blood-brain barrier (BBB), a specialized structure at the microvasculature level that regulates the transport of molecules across the blood vessels into the brain parenchyma. This structure hampers the delivery of most chemotherapeutic agents for the treatment of primary brain tumors. Several drug delivery methods such as nanoparticles, convection enhanced delivery, focused ultrasound, intranasal delivery, and intra-arterial delivery have been developed to overcome the BBB in primary brain tumors. However, prognosis of most primary brain tumors still remains poor. The heterogeneity of the BBB in primary brain tumors and the distinct vasculature of tumors make it difficult to design a drug delivery method that targets the entire tumor. Drug delivery methods that combine strategies such as focused ultrasound and nanoparticles might be a more successful approach. However, more research is needed to optimize and develop new drug delivery techniques to improve survival of patients with primary brain tumors.

Human Sialic acid O-acetyl esterase (SIAE) - mediated changes in sensitivity to etoposide in a medulloblastoma cell line

Medulloblastoma (MB), the most common malignant paediatric brain tumour occurs in the cerebellum. Advances in molecular genomics have led to the identification of defined subgroups which are associated with distinct clinical prognoses. Despite this classification, standard therapies for all subgroups often leave children with life-long neurological deficits. New therapeutic approaches are therefore urgently needed to reduce current treatment toxicity and increase survival for patients. GD3 is a well-studied ganglioside which is known to have roles in the development of the cerebellum. Post-partum GD3 is not highly expressed in the brain. In some cancers however GD3 is highly expressed. In MB cells GD3 is largely acetylated to GD3^A. GD3 is pro-apoptotic but GD3^A can protect cells from apoptosis. Presence of these gangliosides has previously been shown to correlate with resistance to chemotherapy. Here we show that the GD3 acetylation pathway is dysregulated in MB and as a proof-of-principle we show that increased GD3 expression sensitises an MB cell line to etoposide.

Accumulation of protoporphyrin IX in medulloblastoma cell lines and sensitivity to subsequent photodynamic treatment

BACKGROUND

Medulloblastoma (MB) is the most common malignant primary brain tumor of childhood. High risk patients still have a poor outcome, and especially young patients suffer from standard therapy induced sequelae. Therefore, other therapeutic options need to be explored. In glioblastoma (GBM), application of 5-aminolaevulinic acid (5-ALA) results in selective accumulation of protoporphyrin IX (PPIX) in the tumor cells, which can be exploited during fluorescence-guided surgery to increase the extent of resection or for photodynamic therapy (PDT) induced phototoxicity. It is not entirely clear, whether MB cells accumulate PPIX and are sensitive to PDT.

METHODS

Human MYC-amplified (Med8A and D283) and non-amplified (UW228-2 and ONS76) MB cell lines were incubated for 2, 4 or 6 h with increasing doses (0-100 μg/ml) of 5-ALA, and PPIX accumulation was determined by flow cytometry. To assess sensitivity to 5-ALA/PDT, cells were incubated with 5-ALA and subsequently exposed to laser light of 635 nm wavelength (18.75 J/cm²). After an additional 24 h culture period, viability of cells was quantified using the WST-1 assay. Expression of ferrochelatase was detected by reverse transcription and quantitative polymerase chain reaction. Ferrochelatase activity was quantified by measuring the enzymatic conversion of PPIX to zinc-protoporphyrin. Expression of the ABCG2 transporter protein CD338 was detected by flow cytometry.

RESULTS

All MB cell lines showed a time- and dose-dependent accumulation of PPIX after exposure to exogenous 5-ALA and became sensitive to 5-ALA/PDT-induced phototoxicity. PPIX accumulation was reduced compared to U373 GBM cells at shorter incubation periods and limiting 5-ALA doses. Moreover, not all MB cells became PPIX positive and overall phototoxicity was lower in the MB cell lines. Notably, the MYC-amplified MB cells demonstrated a more pronounced photosensitivity compared to their non-amplified counterparts. There was no difference in expression of ferrochelatase, but enzymatic activity appeared to be reduced in the MB cells compared to U373 GBM cells, whereas CD338 was expressed on the MB cells only.

CONCLUSION

Medulloblastoma cell lines accumulate PPIX after application of 5-ALA and become sensitive to PDT, associated with low ferrochelatase expression and activity. Photosensitivity is more pronounced in MYC-amplified cell lines. In contrast to GBM cells, however, PPIX accumulation appears to be reduced, restricted to a subset of cells and associated with lower photosensitivity of the MB cell lines, possibly due to expression of the ABCG2 transporter protein CD338 on MB cells.

Ependymomas overexpress chemoresistance and DNA repair-related proteins

BACKGROUND

After surgery and radiation, treatment options for ependymoma are few making recurrence a challenging issue. Specifically, the efficacy of chemotherapy at recurrence is limited. We performed molecular profiling on a cohort of ependymoma cases in order to uncover therapeutic targets and to elucidate the molecular mechanisms contributing to treatment resistance.

RESULTS

This ependymoma cohort showed minimal alterations in gene amplifications and mutations but had high expression rates of DNA synthesis and repair enzymes such as RRM1 (47%), ERCC1 (48%), TOPO1 (62%) and class III β-tublin (TUBB3) (57%), which are also all associated with chemoresistance. This cohort also had high expression rates of transporter proteins that mediate multi-drug resistance including BCRP (71%) and MRP1 (43%). Subgroup analyses showed that cranial ependymomas expressed the DNA synthesis enzyme TS significantly more frequently than spinal lesions did (57% versus 15%; p = 0.0328) and that increased TS expression was correlated with increased tumor grade (p = 0.0009). High-grade lesions were also significantly associated with elevated expression of TOP2A (p = 0.0092) and TUBB3 (p = 0.0157).

MATERIALS AND METHODS

We reviewed the characteristics of 41 ependymomas (21 cranial, 20 spinal; 8 grade I, 11 grade II, 22 grade III) that underwent multiplatform profiling with immunohistochemistry, next-generation sequencing, and in situ hybridization.

CONCLUSIONS

Ependymomas are enriched with proteins involved in chemoresistance and in DNA synthesis and repair, which is consistent with the meager clinical effectiveness of conventional systemic therapy in ependymoma. Adjuvant therapies that combine conventional chemotherapy with the inhibition of chemoresistance-related proteins may represent a novel treatment paradigm for this difficult disease.

Obstacles to Brain Tumor Therapy: Key ABC Transporters

The delivery of cancer chemotherapy to treat brain tumors remains a challenge, in part, because of the inherent biological barrier, the blood-brain barrier. While its presence and role as a protector of the normal brain parenchyma has been acknowledged for decades, it is only recently that the important transporter components, expressed in the tightly knit capillary endothelial cells, have been deciphered. These transporters are ATP-binding cassette (ABC) transporters and, so far, the major clinically important ones that functionally contribute to the blood-brain barrier are ABCG2 and ABCB1. A further limitation to cancer therapy of brain tumors or brain metastases is the blood-tumor barrier, where tumors erect a barrier of transporters that further impede drug entry. The expression and regulation of these two transporters at these barriers, as well as tumor derived alteration in expression and/or mutation, are likely obstacles to effective therapy.

BCRP expression in schwannoma, plexiform neurofibroma and MPNST

Background

peripheral nerve sheath tumors comprise a broad spectrum of neoplasms. Vestibular schwannomas and plexiform neurofibromas are symptomatic albeit benign, but a subset of the latter pre-malignant lesions will transform to malignant peripheral nerve sheath tumors (MPNST). Surgery and radiotherapy are the primary strategies to treat these tumors. Intrinsic resistance to drug therapy characterizes all three tumor subtypes. The breast cancer resistance protein BCRP is a transmembrane efflux transporter considered to play a key role in various biological barriers such as the blood brain barrier. At the same time it is associated with drug resistance in various tumors. Its potential role in drug resistant tumors of the peripheral nervous system is largely unknown.

Objective

to assess if BCRP is expressed in vestibular schwannomas, plexiform neurofibromas and MPNST.

Material and methods

immunohistochemical staining for BCRP was performed on a tissue microarray composed out of 22 vestibular schwannomas, 10 plexiform neurofibromas and 18 MPNSTs.

Results

sixteen out of twenty-two vestibular schwannomas (73%), nine out of ten plexiform neurofibromas (90%) and six out of eighteen MPNST (33%) expressed BCRP in the vasculature. Tumor cells were negative.

Conclusion

BCRP is present in the vasculature of vestibular schwannomas, plexiform neurofibromas and MPSNT. Therefore, it may reduce the drug exposure of underlying tumor tissues and potentially cause failure of drug therapy.

Upregulated heme biosynthesis, an exploitable vulnerability in MYCN-driven leukemogenesis

The increased heme biosynthesis long observed in leukemia was previously of unknown significance. Heme, synthesized from porphyrin precursors, plays a central role in oxygen metabolism and mitochondrial function, yet little is known about its role in leukemogenesis. Here, we show increased expression of heme biosynthetic genes, including UROD, only in pediatric AML samples that have high MYCN expression. High expression of both UROD and MYCN predicts poor overall survival and unfavorable outcomes in adult AML. Murine leukemic progenitors derived from hematopoietic progenitor cells (HPCs) overexpressing a MYCN cDNA (MYCN-HPCs) require heme/porphyrin biosynthesis, accompanied by increased oxygen consumption, to fully engage in self-renewal and oncogenic transformation. Blocking heme biosynthesis reduced mitochondrial oxygen consumption and markedly suppressed self-renewal. Leukemic progenitors rely on balanced production of heme and heme intermediates, the porphyrins. Porphyrin homeostasis is required because absence of the porphyrin exporter, ABCG2, increased death of leukemic progenitors in vitro and prolonged the survival of mice transplanted with Abcg2-KO MYCN-HPCs. Pediatric AML patients with elevated MYCN mRNA display strong activation of TP53 target genes. Abcg2-KO MYCN-HPCs were rescued from porphyrin toxicity by p53 loss. This vulnerability was exploited to show that treatment with a porphyrin precursor, coupled with the absence of ABCG2, blocked MYCN-driven leukemogenesis in vivo, thereby demonstrating that porphyrin homeostasis is a pathway crucial to MYCN leukemogenesis.

Expression and function of ABCG2 and XIAP in glioblastomas

Despite multimodal treatment that includes surgery, radiation and chemotherapy, virtually all glioblastomas (GBM) recur, indicating that these interventions are insufficient to eradicate all malignant cells. To identify potential new therapeutic targets in GBMs, we examined the expression and function of proteins that are associated with therapy resistance and cancer cell survival. We measured the expression of eight such proteins in 50 GBM samples by immunohistochemistry and analyzed patient survival. We report that GBM patients with high expression of ABCG2 (also called BCRP) or XIAP at the protein level had worse survival than those with low expression. The adjusted hazard ratio for ABCG2 was 2.35 and for XIAP was 2.65. Since glioma stem cells (GSCs) have been shown to be more resistant than bulk tumor cells to anti-cancer therapies and to express high levels of these proteins, we also sought to determine if ABCG2 and XIAP have functional roles in GSCs. We used small molecule inhibitors to treat patient-derived GBM tumorspheres in vitro and observed that inhibitors of ABCG2, Ko143 and fumitremorgin, significantly reduced self-renewal. These results suggest that ABCG2 and XIAP proteins may be useful indicators of patient survival and that inhibition of ABCG2 may be a promising therapeutic strategy in GBMs.

Preclinical studies of 5-fluoro-2'-deoxycytidine and tetrahydrouridine in pediatric brain tumors

Chemotherapies active in preclinical studies frequently fail in the clinic due to lack of efficacy, which limits progress for rare cancers since only small numbers of patients are available for clinical trials. Thus, a preclinical drug development pipeline was developed to prioritize potentially active regimens for pediatric brain tumors spanning from in vitro drug screening, through intracranial and intra-tumoral pharmacokinetics to in vivo efficacy studies. Here, as an example of the pipeline, data are presented for the combination of 5-fluoro-2'-deoxycytidine and tetrahydrouridine in three pediatric brain tumor models. The in vitro activity of nine novel therapies was tested against tumor spheres derived from faithful mouse models of Group 3 medulloblastoma, ependymoma, and choroid plexus carcinoma. Agents with the greatest in vitro potency were then subjected to a comprehensive series of in vivo pharmacokinetic (PK) and pharmacodynamic (PD) studies culminating in preclinical efficacy trials in mice harboring brain tumors. The nucleoside analog 5-fluoro-2'-deoxycytidine (FdCyd) markedly reduced the proliferation in vitro of all three brain tumor cell types at nanomolar concentrations. Detailed intracranial PK studies confirmed that systemically administered FdCyd exceeded concentrations in brain tumors necessary to inhibit tumor cell proliferation, but no tumor displayed a significant in vivo therapeutic response. Despite promising in vitro activity and in vivo PK properties, FdCyd is unlikely to be an effective treatment of pediatric brain tumors, and therefore was deprioritized for the clinic. Our comprehensive and integrated preclinical drug development pipeline should reduce the attrition of drugs in clinical trials.