Somatostatin receptor subtype 2 (sst₂) is a potential prognostic marker and a therapeutic target in medulloblastoma

Immunohistochemical assessment and clinical, histopathologic, and molecular correlates of membranous somatostatin type-2A receptor expression in high-risk pediatric central nervous system tumors

INTRODUCTION

177Lu-DOTATATE, a radionuclide therapy that binds somatostatin type-2A receptors (SST2A), has demonstrated efficacy in neuroendocrine tumors and evidence of central nervous system (CNS) penetration, supporting potential expansion within pediatric neuro-oncology. Understanding the prevalence of SST2A expression across pediatric CNS tumors is essential to identify patients who may benefit from somatostatin receptor-targeted therapy and to further elucidate the oncogenic role of SST2A.

METHODS

SST2A immunohistochemistry (IHC) was performed on tumor specimens and interpreted by an experienced pathologist (blinded), utilizing semi-quantitative scoring of membranous expression within viable tumor. Immunoreactive cell percentage was visually scored as 0 (none), 1 (<10%), 2 (10-50%), 3 (51-80%), or 4 (>80%). Staining intensity was scored as 0 (none), 1 (weak), 2 (moderate), or 3 (strong). Combined scores for each specimen were calculated by multiplying percent immunoreactivity and staining intensity values (Range: 0-12).

RESULTS

A total of 120 tumor samples from 114 patients were analyzed. Significant differences in SST2A IHC scores were observed across histopathologic diagnoses, with consistently high scores in medulloblastoma (mean ± SD: 7.5 ± 3.6 [n=38]) and meningioma (5.7 ± 3.4 [n=15]), compared to minimal or absent expression in ATRT (0.3 ± 0.6 [n=3]), ETMR (1.0 ± 0 [n=3]), ependymoma (grades I-III; 0.2 ± 0.7 [n=27]), and high-grade glioma (grades III-IV; 0.4 ± 0.7 [n=23]). Pineoblastoma (3.8 ± 1.5 [n=4]) and other embryonal tumors (2.0 ± 4.0 [n=7]) exhibited intermediate, variable expression. Among medulloblastomas, SST2A IHC scores were higher in non-SHH (8.5 ± 3.1) than SHH (5.0 ± 3.3) molecular subgroups (p=0.033). In a subset of paired primary and recurrent specimens from four patients, SST2A IHC scores remained largely unchanged.

DISCUSSION

High membranous SST2A expression was demonstrated in medulloblastoma, meningioma, and some rarer embryonal tumors with potential diagnostic, biologic, and therapeutic implications. Somatostatin receptor-targeted therapy such as 177Lu-DOTATATE deserves further investigation in these highly SST2A-expressing pediatric CNS tumors.

Medulloblastoma in the Modern Era: Review of Contemporary Trials, Molecular Advances, and Updates in Management

Critical discoveries over the past two decades have transformed our understanding of medulloblastoma from a single entity into a clinically and biologically heterogeneous disease composed of at least four molecularly distinct subgroups with prognostically and therapeutically relevant genomic signatures. Contemporary clinical trials also have provided valuable insight guiding appropriate treatment strategies. Despite therapeutic and biological advances, medulloblastoma patients across the age spectrum experience tumor- and treatment-related morbidity and mortality. Using an updated risk stratification approach integrating both clinical and molecular features, ongoing research seeks to (1) cautiously reduce therapy and mitigate toxicity in low-average risk patients, and (2) thoughtfully intensify treatment with incorporation of novel, biologically guided agents for patients with high-risk disease. Herein, we review important historical and contemporary studies, discuss management updates, and summarize current knowledge of the biological landscape across unique pediatric, infant, young adult, and relapsed medulloblastoma populations.

Peptide receptor radionuclide therapy for treatment of metastatic neuroendocrine tumors in children

Neuroendocrine tumors (NETs) of the pancreas and midgut are extremely rare in children, and patients presenting with metastatic disease have poor survival. Given this rarity, treatments are extrapolated from guidelines for adults with NET. Recent clinical trials in adults with NETs have shown that the addition of peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷ Lu-DOTATATE resulted in a disease control rate of nearly 80%, with minimal side effects. We report our experience using ¹⁷⁷ Lu-DOTATATE to treat two pediatric patients with metastatic NET.

Understanding the influence of lipid bilayers and ligand molecules in determining the conformational dynamics of somatostatin receptor 2

Somatostatin receptor 2 (SSTR2) is a G-protein coupled receptor (GPCR) that controls numerous cellular processes including cell-to-cell signaling. In this study, we report how the lipid and ligand molecules influence the conformational dynamics of the membrane-bound SSTR2. Molecular simulations of different holo and apoenzyme complexes of SSTR2 in the presence and absence of a lipid bilayer were performed, observed, and correlated with previously reported studies. We identified the important SSTR2 residues that take part in the formation of the SSTR2-ligand complex. On analyzing the molecular simulation trajectories, we identified that the residue D3.32 is crucial in determining the bioactive conformation of SSTR2 ligands in the binding site. Based on the results, we suggest that designing a novel SSTR2 ligand with an H-bond donor group at the R1 position, and hydrophobic groups at R2 and R3 might have higher activity and SSTR2-selectivity. We analyzed the simulated systems to identify other important structural features involved in SSTR2-ligand binding and to observe the different conformational changes that occur in the protein after the ligand binding. Additionally, we studied the conformational dynamics of N- and C-terminal regions of SSTR2 in the presence and absence of the lipid bilayer. Both the systems were compared to understand the influence of lipid molecules in the formation of secondary structural domains by these extracellular regions. The comparative study revealed that the secondary structural elements formed by C-terminal residues in presence of lipid molecules is crucial for the functioning of SSTR2. Our study results highlight the structural complexities involved in the functioning of SSTR upon binding with the ligands in the presence and absence of lipid bilayer, which is essential for designing novel drug targets.

Understanding the influence of lipid bilayers and ligand molecules in determining the conformational dynamics of somatostatin receptor 2

Somatostatin receptor 2 (SSTR2) is a G-protein coupled receptor (GPCR) that controls numerous cellular processes including cell-to-cell signaling. In this study, we report how the lipid and ligand molecules influence the conformational dynamics of the membrane-bound SSTR2. Molecular simulations of different holo and apoenzyme complexes of SSTR2 in the presence and absence of a lipid bilayer were performed, observed, and correlated with previously reported studies. We identified the important SSTR2 residues that take part in the formation of the SSTR2-ligand complex. On analyzing the molecular simulation trajectories, we identified that the residue D3.32 is crucial in determining the bioactive conformation of SSTR2 ligands in the binding site. Based on the results, we suggest that designing a novel SSTR2 ligand with an H-bond donor group at the R1 position, and hydrophobic groups at R2 and R3 might have higher activity and SSTR2-selectivity. We analyzed the simulated systems to identify other important structural features involved in SSTR2-ligand binding and to observe the different conformational changes that occur in the protein after the ligand binding. Additionally, we studied the conformational dynamics of N- and C-terminal regions of SSTR2 in the presence and absence of the lipid bilayer. Both the systems were compared to understand the influence of lipid molecules in the formation of secondary structural domains by these extracellular regions. The comparative study revealed that the secondary structural elements formed by C-terminal residues in presence of lipid molecules is crucial for the functioning of SSTR2. Our study results highlight the structural complexities involved in the functioning of SSTR upon binding with the ligands in the presence and absence of lipid bilayer, which is essential for designing novel drug targets.

Posterior fossa extra-axial variations of medulloblastoma: a pictorial review as a primer for radiologists

Manifestations of an atypical variant of medulloblastoma of the posterior fossa in extra-axial locations have been reported, and key questions concerning its interpretation have been raised previously. This review illustrated the clinico-radiological and histopathological features of the posterior fossa extra-axial medulloblastoma and described possible management strategies. We thoroughly reviewed all atypical anatomical locations of medulloblastoma reported within the posterior fossa and extra-axial spaces. The main characteristics of diagnostic imaging and histopathological results, primarily the distinctive radiopathological characteristics, were summarized to distinguish between intra- and extra-axial medulloblastoma, or pathologies mimicking this tumor. Most cases of posterior fossa extra-axial medulloblastoma have been reported in the cerebellopontine angle, followed by the tentorial and lateral cerebellar locations. The dural tail sign, which is commonly observed in meningioma, is rarely seen in intra- or extra-axial medulloblastoma and might be associated with other benign or malignant lesions. In addition to magnetic resonance imaging, the proposed new imaging techniques, including advances in modern neuroimaging modalities, were discussed, as potentially efficient modalities for characterizing extra-axial medulloblastoma. Radionuclide imaging and magnetic resonance perfusion imaging are practical alternatives to limit the number of differential diagnoses. We believe that medulloblastoma cases are likely under-reported because of publication bias and frequent tumors in unusual locations. Addressing these issues would help establish a more accurate understanding of this entity.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

Somatostatin, also known as somatotropin-release inhibitory factor, is a cyclopeptide that exerts potent inhibitory actions on hormone secretion and neuronal excitability. Its physiologic functions are mediated by five G protein-coupled receptors (GPCRs) called somatostatin receptor (SST)1-5. These five receptors share common structural features and signaling mechanisms but differ in their cellular and subcellular localization and mode of regulation. SST₂ and SST₅ receptors have evolved as primary targets for pharmacological treatment of pituitary adenomas and neuroendocrine tumors. In addition, SST₂ is a prototypical GPCR for the development of peptide-based radiopharmaceuticals for diagnostic and therapeutic interventions. This review article summarizes findings published in the last 25 years on the physiology, pharmacology, and clinical applications related to SSTs. We also discuss potential future developments and propose a new nomenclature.

Toward a better understanding of the interaction between somatostatin receptor 2 and its ligands: a structural characterization study using molecular dynamics and conceptual density functional theory

This study is a part of the extensive research intending to provide the structural insights on somatostatin and its receptor. Herein, we have studied the structural complexity involved in the binding of somatostatin receptor 2 (SSTR2) with its agonists and antagonist. A 3D QSAR study based on comparative molecular field analysis and comparative molecular similarity analysis (CoMSIA) discerned that a SSTR2 ligand with electronegative, less-bulkier, and hydrogen atom donating/accepting substitutions is important for their biological activity. A conceptual density functional theory (DFT) study was followed to study the chemical behavior of the ligands based on the molecular descriptors derived using the Fukui's molecular orbital theory. We have performed molecular dynamics simulations of receptor-ligand complexes for 100 ns to analyze the dynamic stability of the backbone Cα atoms of the receptor and strength and approachability of the receptor-ligand complex. The findings of this study could be efficacious in the further studies understanding intricate structural features of the somatostatin receptors and in discovering novel subtype-specific ligands with higher affinity. Communicated by Ramaswamy H. Sarma.

G-protein coupled receptor expression patterns delineate medulloblastoma subgroups

Background

Medulloblastoma is the most common malignant brain tumor in children. Genetic profiling has identified four principle tumor subgroups; each subgroup is characterized by different initiating mutations, genetic and clinical profiles, and prognoses. The two most well-defined subgroups are caused by overactive signaling in the WNT and SHH mitogenic pathways; less is understood about Groups 3 and 4 medulloblastoma. Identification of tumor subgroup using molecular classification is set to become an important component of medulloblastoma diagnosis and staging, and will likely guide therapeutic options. However, thus far, few druggable targets have emerged. G-protein coupled receptors (GPCRs) possess characteristics that make them ideal targets for molecular imaging and therapeutics; drugs targeting GPCRs account for 30-40% of all current pharmaceuticals. While expression patterns of many proteins in human medulloblastoma subgroups have been discerned, the expression pattern of GPCRs in medulloblastoma has not been investigated. We hypothesized that analysis of GPCR expression would identify clear subsets of medulloblastoma and suggest distinct GPCRs that might serve as molecular targets for both imaging and therapy.

Results

Our study found that medulloblastoma tumors fall into distinct clusters based solely on GPCR expression patterns. Normal cerebellum clustered separately from the tumor samples. Further, two of the tumor clusters correspond with high fidelity to the WNT and SHH subgroups of medulloblastoma. Distinct over-expressed GPCRs emerge; for example, LGR5 and GPR64 are significantly and uniquely over-expressed in the WNT subgroup of tumors, while PTGER4 is over-expressed in the SHH subgroup. Uniquely under-expressed GPCRs were also observed. Our key findings were independently validated using a large international dataset.

Conclusions

Our results identify GPCRs with potential to act as imaging and therapeutic targets. Elucidating tumorigenic pathways is a secondary benefit to identifying differential GPCR expression patterns in medulloblastoma tumors.