From Glycocode to Precision Oncology: Therapeutic Strategies Targeting Aberrant O-GalNAc Glycosylation in Cancer

Aberrant O-glycosylation is a prominent feature of cancer and contributes directly to multiple hallmarks of malignant transformation. Cancer-associated O-glycans include both truncated structures that are unique to mucin-type O-glycosylation and more elongated terminal motifs that function as ligands for glycan-binding proteins. Instead of representing a simple binary change from elongated to truncated structures, it is increasingly understood that these glycans decorate the cancer cell surface with a heterogeneous pattern of truncated, elongated, capped, and partially processed glycoforms, which coexist even on individual proteins and within the tumor glycocalyx. Functionally, truncated aberrant O-GalNAc glycans, such as Tn, STn, and T perturb epithelial differentiation, barrier integrity, signaling, and immunosuppression through interactions with glycan binding receptors, such as Siglecs and MGL. In addition, more elongated O-glycans, including Lewis antigens, interact with selectins promoting metastasis and cellular trafficking. Moreover, individual GalNAc-transferase isoenzymes, governing site-specific O-glycosylation, are emerging as context-dependent regulators of oncogenic processes through glycosylation of selected proteins. These observations establish cancer-associated O-glycans as context-dependent molecular drivers of tumor progression and promising therapeutic entry points. In this review, we discuss the expression, regulation, and functions of cancer-associated O-glycans, their roles in glycan-binding protein biology, and emerging therapeutic strategies that may enable a new generation of glycan-directed precision treatments in oncology.