Sialic acids are important for cell-cell recognition and cell-pathogen interactions. They can be found on gangliosides localized to plasma membranes, where sialylated sugar chains protrude out of cells, serving as specific antigens in immune reactions and contacts for glycan binding proteins to mediate cell-cell interactions.1 N-Acetylneuraminic acid (Neu5Ac) and its hydroxylated form, N-glycolylneuraminic acid (Neu5Gc), are the two major variants of sialic acid found in most mammals.2 Neu5Gc is mainly derived from Neu5Ac through enzymatic hydroxylation of cytidine-5′-monophospho-NeuAc (CMP-Neu5Ac) but can also be synthesized by hydroxylation of free Neu5Ac or glycoconjugate-linked Neu5Ac. CMP-Neu5Ac hydroxylase, a cytosolic NADH-dependent monooxygenase that requires cytochrome b5 and cytochrome b5 reductase for activity, plays a role in regulating Neu5Gc expression in glycoconjugates by controlling the ratio of Neu5Gc to Neu5Ac.
Though Neu5Gc is found in most mammals, its presence in humans is limited due to a partial truncation of the CMP-Neu5Ac hydroxylase gene that renders the biosynthetic enzyme inactive.3 Therefore, human cells are not capable of synthesizing their own Neu5Gc-sialoconjugates.4 The possibility that other Neu5Gc-biosynthesis pathways are present in humans has been ruled out since no other biosynthetic enzymes have been identified. However, Neu5Gc can be metabolically incorporated into human cells through diet, primarily from red meat and milk products.5,6 Neu5Gc is highly immunogenic to humans, and human heterophile antibodies that agglutinate animal erythrocytes, termed Hanganutziu-Deicher (HD) antibodies, detect non-human Neu5Gc.7 A prolonged antibody reaction to this type of xeno-autoantigen can generate chronic inflammation that leads to cancer or the development of inflammatory or autoimmune diseases.
Despite zero to minimal amounts of Neu5Gc being found in healthy human tissues, higher levels of Neu5Gc have been detected in a variety of tumors and are associated with their progression and metastasis.8,9 For example, abnormal expression of N-glycolyl-monosialoganglioside GM3 (N-glycolyl-GM3), a Neu5Gc-containing ganglioside, is found mostly on the surface of malignant tumors.10 In breast cancer, both O-acetylated and Neu5Gc-containing gangliosides have been detected.9 Any alterations to the ceramide portion of these unusual gangliosides could weaken its anchor in the cell membrane, enabling them to be actively shed and taken up by other cells.4 In both pediatric and adult sarcomas, N-glycolyl-GM3 expression was increased by 59.3-100%.10 Another study conducted on 27 cases of neuroectodermal tumors found 85% were reported to have N-glycolyl-GM3.11 These 23 N-glycolyl-GM3-positive tumors were more aggressive than those in the other four cases. The most aggressive urinary bladder tumors and malignant gliomas also contain N-glycolyl-GM3. Reports of reduced survival have been linked to colon adenocarcinomas and non-small cell lung cancer (NSCLC) cells bearing N-glycolyl-GM3.10
Because N-glycolyl-GM3 is easily detected on the surface of malignant cells, and this expression is minimal in healthy cells, the presence of these tumor-associated antigens provides a basis for early (and potentially pre-malignant) immunological localization and diagnosis. These antigens also serve as an excellent target for immunotherapy to suppress tumor growth through macrophage activation or antibody-dependent cytotoxicity. At least two vaccines, racotumomab and NGcGM3/VSSP, that specifically target Neu5Gc tumor antigens are being developed to fight breast tumors, melanoma, and NSCLC.12-14 Both of these vaccines have been found to improve the survival of patients with an immune response to the N-glycolyl-GM3 antigen.
As gangliosides are minor components in human tissues, sensitive methods for their quantitative isolation and analysis are needed. Matreya’s scientists have developed a high-quality standard for N-glycolyl-GM3 detection to advance this research.
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