The glioma-associated gangliosides 3'-isoLM1, GD3 and GM2 show selective area expression in human glioblastoma xenografts in nude rat brains

GD3 ganglioside is a promising therapeutic target for glioma patients

Glioblastoma is the most frequent and aggressive primary brain tumor in adults. Currently, no curative treatment is available. Despite first-line treatment composed by the association of surgery, radiotherapy, and chemotherapy, relapse remains inevitable in a median delay of 6 to 10 months. Improving patient management and developing new therapeutic strategies are therefore a critical medical need in neuro-oncology. Gangliosides are sialic acid-containing glycosphingolipids, the most abundant in the nervous system, representing attractive therapeutic targets. The ganglioside GD3 is highly expressed in neuroectoderm-derived tumors such as melanoma and neuroblastoma, but also in gliomas. Moreover, interesting results, including our own, have reported the involvement of GD3 in the stemness of glioblastoma cells. In this review, we will first describe the characteristics of the ganglioside GD3 and its enzyme, the GD3 synthase (GD3S), including their biosynthesis and metabolism. Then, we will detail their expression and role in gliomas. Finally, we will summarize the current knowledge regarding the therapeutic development opportunities against GD3 and GD3S.

The cancer glycocode as a family of diagnostic biomarkers, exemplified by tumor-associated gangliosides

One unexploited family of cancer biomarkers comprise glycoproteins, carbohydrates, and glycolipids (the Tumor Glycocode).A class of glycolipid cancer biomarkers, the tumor-marker gangliosides (TMGs) are presented here as potential diagnostics for detecting cancer, especially at early stages, as the biological function of TMGs makes them etiological. We propose that a quantitative matrix of the Cancer Biomarker Glycocode and artificial intelligence-driven algorithms will expand the menu of validated cancer biomarkers as a step to resolve some of the challenges in cancer diagnosis, and yield a combination that can identify a specific cancer, in a tissue-agnostic manner especially at early stages, to enable early intervention. Diagnosis is critical to reducing cancer mortality but many cancers lack efficient and effective diagnostic tests, especially for early stage disease. Ideal diagnostic biomarkers are etiological, samples are preferably obtained via non-invasive methods (e.g. liquid biopsy of blood or urine), and are quantitated using assays that yield high diagnostic sensitivity and specificity for efficient diagnosis, prognosis, or predicting response to therapy. Validated biomarkers with these features are rare. While the advent of proteomics and genomics has led to the identification of a multitude of proteins and nucleic acid sequences as cancer biomarkers, relatively few have been approved for clinical use. The use of multiplex arrays and artificial intelligence-driven algorithms offer the option of combining data of known biomarkers; however, for most, the sensitivity and the specificity are below acceptable criteria, and clinical validation has proven difficult. One strategic solution to this problem is to expand the biomarker families beyond those currently exploited. One unexploited family of cancer biomarkers comprise glycoproteins, carbohydrates, and glycolipids (the Tumor Glycocode). Here, we focus on a family of glycolipid cancer biomarkers, the tumor-marker gangliosides (TMGs). We discuss the diagnostic potential of TMGs for detecting cancer, especially at early stages. We include prior studies from the literature to summarize findings for ganglioside quantification, expression, detection, and biological function and its role in various cancers. We highlight the examples of TMGs exhibiting ideal properties of cancer diagnostic biomarkers, and the application of GD2 and GD3 for diagnosis of early stage cancers with high sensitivity and specificity. We propose that a quantitative matrix of the Cancer Biomarker Glycocode and artificial intelligence-driven algorithms will expand the menu of validated cancer biomarkers as a step to resolve some of the challenges in cancer diagnosis, and yield a combination that can identify a specific cancer, in a tissue-agnostic manner especially at early stages, to enable early intervention.

Ganglioside Composition Distinguishes Anaplastic Ganglioglioma Tumor Tissue from Peritumoral Brain Tissue: Complementary Mass Spectrometry and Thin-Layer Chromatography Evidence

Gangliosides serve as antitumor therapy targets and aberrations in their composition strongly correlate with tumor growth and invasiveness. Anaplastic ganglioglioma is a rare, poorly characterized, malignant neuronal–glial tumor type. We present the first comparative characterization of ganglioside composition in anaplastic ganglioglioma vs. peritumoral and healthy brain tissues by combining mass spectrometry and thin-layer chromatography. Anaplastic ganglioglioma ganglioside composition was highly distinguishable from both peritumoral and healthy tissue despite having five to six times lower total content. Ten out of twelve MS-identified ganglioside classes, defined by unique glycan residues, were represented by a large number and considerable abundance of individual species with different fatty acid residues (C16–C24) in ceramide portions. The major structurally identified class was tumor-associated GD3 (>50%) with 11 species; GD3 (d18:1/24:0) being the most abundant. The dominant sphingoid base residue in ganglioside ceramides was sphingosine (d18:1), followed by eicosasphingosine (d20:1). The peritumoral tissue ganglioside composition was estimated as normal. Specific ganglioside composition and large variability of ganglioside ceramide structures determined in anaplastic ganglioglioma demonstrate realistic ganglioside expression patterns and correspond to the profile of high-grade malignancy brain tumors.

Aiming for the Sweet Spot: Glyco-Immune Checkpoints and γδ T Cells in Targeted Immunotherapy

Though a healthy immune system is capable of recognizing and eliminating emergent cancerous cells, an established tumor is adept at escaping immune surveillance. Altered and tumor-specific expression of immunosuppressive cell surface carbohydrates, also termed the “tumor glycocode,” is a prominent mechanism by which tumors can escape anti-tumor immunity. Given their persistent and homogeneous expression, tumor-associated glycans are promising targets to be exploited as biomarkers and therapeutic targets. However, the exploitation of these glycans has been a challenge due to their low immunogenicity, immunosuppressive properties, and the inefficient presentation of glycolipids in a conventional major histocompatibility complex (MHC)-restricted manner. Despite this, a subset of T-cells expressing the gamma and delta chains of the T-cell receptor (γδ T cells) exist with a capacity for MHC-unrestricted antigen recognition and potent inherent anti-tumor properties. In this review, we discuss the role of tumor-associated glycans in anti-tumor immunity, with an emphasis on the potential of γδ T cells to target the tumor glycocode. Understanding the many facets of this interaction holds the potential to unlock new ways to use both tumor-associated glycans and γδ T cells in novel therapeutic interventions.

Affinity-matured recombinant immunotoxin targeting gangliosides 3'-isoLM1 and 3',6'-isoLD1 on malignant gliomas

About 60 percent of glioblastomas highly express the gangliosides 3'-isoLM1 and 3',6'-isoLD1 on the cell surface, providing ideal targets for brain tumor immunotherapy. A novel recombinant immunotoxin, DmAb14m-(scFv)-PE38KDEL (DmAb14m-IT), specific for the gangliosides 3'-isoLM1 and 3',6'-isoLD1, was constructed with improved affinity and increased cytotoxicity for immunotherapeutic targeting of glioblastoma. We isolated an scFv parental clone from a previously established murine hybridoma, DmAb14, that is specific to both 3'-isoLM1 and 3',6'-isoLD1. We then performed in vitro affinity maturation by CDR hotspot random mutagenesis. The binding affinity and specificity of affinity-matured DmAb14m-IT were measured by surface-plasmon resonance, flow cytometry, and immunohistochemical analysis. In vitro cytotoxicity of DmAb14m-IT was measured by protein synthesis inhibition and cell death assays in human cell lines expressing gangliosides 3'-isoLM1 and 3',6'-isoLD1 (D54MG and D336MG) and xenograft-derived cells (D2224MG). As a result, the KD of DmAb14m-IT for gangliosides 3'-isoLM1 and 3',6'-isoLD1 was 2.6 × 10(-9)M. Also, DmAb14m-IT showed a significantly higher internalization rate in cells expressing 3'-isoLM1 and 3',6'-isoLD1. The DmAb14m-IT IC 50 was 80 ng/mL (1194 pM) on the D54MG cell line, 5 ng/ml (75 pM) on the D336MG cell line, and 0.5 ng/ml (7.5 pM) on the D2224MG xenograft-derived cells. There was no cytotoxicity on ganglioside-negative HEK293 cells. Immunohistochemical analysis confirmed the specific apparent affinity of DmAb14m-IT with 3'-isoLM1 and 3',6'-isoLD1. In conclusion, DmAb14m-IT showed specific binding affinity, a significantly high internalization rate, and selective cytotoxicity on glioma cell lines and xenograft-derived cells expressing 3'-isoLM1 and 3',6'-isoLD1, thereby displaying robust therapeutic potential for testing the antitumor efficacy of DmAb14m-IT at the preclinical level and eventually in the clinical setting.

Glycosphingolipid antigens in neural tumor cell lines and anti-glycosphingolipid antibodies in sera of patients with neural tumors

To characterize biomarkers in neural tumors, we analyzed the acidic lipid fractions of 13 neural tumor cell lines using enzyme-linked immunoabsorbent assay (ELISA) and high-performance thin-layer chromatography (HPTLC) immunostaining. Sulfated glucuronosyl glycosphingolipids (SGGLs) are cell surface molecules that are endowed with the Human Natural Killer-1 (HNK-1) carbohydrate epitope. These glycosphingolipids (GSLs) were expressed in all cell lines with concentrations ranging from 210 to 330 ng per 2 x 10(6) cells. Sulfoglucuronosyl paragloboside (SGPG) was the prominent species with lesser amounts of sulfoglucuronosyl lactosaminyl paragloboside (SGLPG) in these tumor cell lines as assessed by quantitative HPTLC immunostaining. Among the gangliosides surveyed, GD3 and 9-O-acetylated GD3 (OAc-GD3) were expressed in all tumor cell lines. In contrast, fucosyl-GM1 was not found to restrict to small cell lung carcinoma cells. In addition, we have analyzed serum antibody titers against SGPG, GD3, and OAc-GD3 in patients with neural tumors by ELISA and HPTLC immunostaining. All sera had high titers of antibodies of the IgM isotype against SGPG (titers over 1:3,200), especially in tumors such as meningiomas, germinomas, orbital tumors, glioblastomas, medulloblastomas, and subependymomas. Serum in a patient with subependymomas also had a high anti-SGGL antibody titer of the IgG and IgA types (titers over 12,800). The titer of anti-GD3 antibody was also elevated in patients with subependymomas and medulloblastomas; the latter cases also had a high titer of antibody against OAc-GD3. Our data indicate that certain GSL antigens, especially SGGLs, GD3, and OAc-GD3, are expressed in neural tumor cells and may be considered as tumor-associated antigens that represent important biomarkers for neural tumors. Furthermore, antibody titers in sera of patients with these tumors may be of diagnostic value for monitoring the presence of tumor cells and tumor progression.

Approaches to mitochondrially mediated cancer therapy

For some malignant cancers even combined surgical, radiotherapeutic and chemotherapeutic approaches are not curative, indeed, in certain tumour types even a modest survival benefit is difficult to achieve. There are various biological reasons which underlie this profound resistance but the propensity of cancer cells to repair breaks caused by DNA-damaging radiation and cytotoxic drugs is of major significance in this context. Such highly resistant tumours include the malignant gliomas which are intrinsic to and directly affect the brain and spinal cord. In evaluating approaches which do not elicit tumour cell death directly by DNA damage, it is intriguing to consider mitochondrially mediated apoptosis as a potentially effective alternative. Since the mitochondrial membrane potentials in cancer cells are frequently reduced in comparison with those of non-neoplastic cells this allows a window of opportunity for small molecule agents to enter the tumour cell mitochondria and reduce oxygen consumption with subsequent release of cytochrome c and activation of a caspase pathway to apoptosis which is cancer cell specific. In the quest for agents which can selectively destroy neoplastic cells in this manner, whilst leaving normal adjacent cells intact, various tricyclic drugs have come under scrutiny. In a range of laboratory assays we, and others, have established that certain cancers and, in particular, malignant glioma, are intrinsically sensitive to this approach. We have also established the cellular, molecular and biochemical mechanisms underlying this process. While such archival tricyclics as the antidepressants, clomipramine and amitriptyline, have been used in these experiments their commercial development in cancer therapy has not been forthcoming and their clinical use in glioma has been confined to anecdotal cases. In addition, the dose-dependant role of agents such as anticonvulsants and steroids commonly used in glioma patients in modulating efficacy of the tricyclics is a matter for continued investigation. Other ways of targeting the mitochondrion for cancer therapy include exploitation of the 18kDa translocator protein (peripheral-type benzodiazepine receptor) within the mitochondrial permeability transition pore and enzymatic or molecular modification of a species of ganglioside (GD3/GD3(A)) expressed on the surface of neoplastic cells which are determinants of mitochondrially mediated apoptosis. It is hoped that such approaches may lead to clinical programmes which will improve the prognosis for patients suffering from highly resistant neoplasms.

Human gliosarcoma-associated ganglioside composition is complex and distinctive as evidenced by high-performance mass spectrometric determination and structural characterization

Gangliosides (GGs), involved in malignant alteration and tumor progression/invasiveness, are considered as tumor biomarkers or therapeutic targets. Here, we describe the first systematic GG composition characterization in human gliosarcoma versus normal brain tissue using our recently developed mass spectrometry (MS) methods, based on nano-electrospray (nano-ESI), Fourier-transform ion cyclotron resonance (FT-ICR), and chip nano-ESI quadrupole time-of-flight (QTOF), complemented by thin-layer chromatographic (TLC) analysis and quantification. Combined MS enabled detection and structural assignment of 73 distinct GG species: many more than reported so far for investigated gliomas. Apart from the 7.4-times lower total GG content, gliosarcoma contained all major brain-associated species, however, in very altered proportions, exhibiting a highly distinctive pattern: GD3 (48.9%)>GD1a/nLD1>GD2/GT3>GM3>GT1b>GM2>GM1a/GM1b/nLM1>LM1>GD1b>GQ1b. MS also revealed abundant O-Ac-GD3; its sequencing provided structural evidence to postulate a novel O-Ac-GD3 isomer O-acetylated at the inner Neu5Ac-residue, previously not structurally confirmed. The high sensitivity and mass accuracy permitted the assignment of unusual minor species: GM4, Hex-HexNAc-nLM1, Gal-GD1, Fuc-GT1, GalNAc-GT1, O-Ac-GM3, di- O-Ac-GD3O-Ac-GD3, and O-Ac-GT3, not previously reported as glioma-associated. The gliosarcoma-expressed GA2 might represent a marker distinguishing astrocytic from oligodendroglial tumors. This is, to our knowledge, so far the most complete GG composition characterization of certain glioma, which demonstrates that our MS-based approach could provide essential structural information relevant to glycosphingolipid role(s) in brain tumor biology, differential diagnosis/prognosis and novel treatment concepts.

Monoclonal antibodies for brain tumour treatment

Conventional treatment of brain tumours includes surgical, radiotherapeutic and chemotherapeutic modalities. Nonetheless, the outcome of patients with brain tumours, in particular glioblastoma, remains poor. Immunotherapy with armed or unarmed monoclonal antibodies targeting tumour-specific antigens has emerged in the last two decades as a novel potential adjuvant treatment for all types of neoplasia. Many challenges to its implementation as a safe and viable therapy for brain tumours still need to be addressed; nevertheless, results from ongoing Phase I/II clinical trials are encouraging, as disease stabilisation and patient survival prolongation have been observed. Advances in preclinical and clinical research indicate that treatment of brain tumours with monoclonal antibodies can be increasingly adjusted to the characteristics of the targeted tumour and its environment. This aspect relies on the careful selection of the target antigen and corresponding specific monoclonal antibody, and antibody format (size, class, affinity), conjugation to the appropriate toxin or radioactive isotope (half-life, range), and proper compartmental administration.

Gangliosides as therapeutic targets for cancer

Gangliosides, sialic acid-containing glycosphingolipids, have engendered great interest for more than 20 years in the search for target molecules of relevance for tumour growth and formation of metastases and as potential targets for immunotherapy. These molecules show large quantitative and structural variability, which is related to cell type and developmental stage. Their potential role in the formation of tumour metastases was suggested from data supporting that they are involved in cell growth regulation and in cell-cell and cell-matrix adhesion. Moreover, gangliosides are expressed on the cell surface and thereby are accessible for antibodies or other ganglioside-binding molecules to induce cell death, inhibit cell growth and/or inhibit formation of tumour metastasis. All tumours exhibit aberrant ganglioside expression. This includes overexpression of normal ganglioside constituents, which appears to be common among various tumours, and expression of gangliosides not found in normal adult tissue but often found during fetal development. The ganglioside composition of melanoma cells has been found to correlate with their metastatic potential and also to be selectively expressed in cells of a tumour mass and invading tumour cells. Passive immunotherapy using murine or murine/human chimeric monoclonal antiganglioside antibodies in their native form or combined with various effector molecules has been investigated. However, the vaccination strategy using native or structurally modified tumour-associated gangliosides in combination with adjuvants is currently the dominant method in clinical trials. The outcomes reported so far vary between type of tumour and treatment strategies. However, we believe that targeting gangliosides is as promising as any other immune therapeutic strategy, and basic research as well as clinical trials utilising new aspects is encouraged.

The glial precursor proteoglycan, NG2, is expressed on tumour neovasculature by vascular pericytes in human malignant brain tumours

Glial precursor cells express NG2 and GD3 in the developing brain. These antigens are both over-expressed during neoplasia, which suggests they may have specific functions in the malignant progression of human brain tumours. This study describes the expression of NG2 and GD3 in 28 paediatric and adult brain tumours. Glioblastoma biopsy spheroids were also implanted into nude rats to assess the regional distribution of the molecules within the tumour. These xenografts showed extensive infiltration and growth that mimicked the growth patterns of human gliomas in situ. NG2 was identified in 20 out of 28 brain tumours, where the expression was confined to the main mass of the tumour, and was reduced towards the tumour periphery. NG2 was mainly associated with blood vessels on both the pericyte and basement membrane components of the tumour vasculature. Ki67 (MIB-1) labelling indicated that NG2 expression was associated with areas of high cellular proliferation. Conversely, all the tumours expressed GD3, which was present both in the tumour main mass and throughout the periphery. Thus, the expression of NG2 may be indicative of tumour progression and might be an amenable target for future therapeutic interventions.