FTC/HHS is a rare autosomal recessive disorder marked by ectopic soft tissue calcifications, especially around large joints, along with bone hyperostosis and ongoing hyperphosphatemia. The condition is closely linked to mutations in GALNT3, which is vital for the glycosylation pathway that regulates the phosphaturic hormone FGF23 (29,30). FGF23 was shown to inhibit renal phosphate reabsorption and modulate vitamin D metabolism, thereby regulating the serum phosphate levels (31). Improper processing of FGF23 can result from mutations in GALNT3, leading to defective secretion and function, which ultimately causes hyperphosphatemia (22,23,32). When the levels are elevated, serum phosphate reacts with calcium to form hydroxyapatite crystals, which deposit in the soft tissues such as those of the shoulders and hips, leading to the development of painful calcified masses and abnormal bone growth. In addition, the phenotypic expression of FTC and HHS differs greatly among individuals with GALNT3 mutations. Some may exhibit extensive calcifications and cortical hyperostosis, while others may show dental anomalies or other clinical features (33). Despite this variability, persistent hyperphosphatemia frequently occurs in the affected individuals. This phenotypic heterogeneity highlights the importance of genetic analysis in diagnosing and comprehending the full spectrum of these disorders.

Diagnosing FTC/HHS involves genetic testing for GALNT3 mutations, as well as imaging techniques such as X-rays or computed tomography (CT) scans that show lobulated calcified masses, and biochemical markers, including hyperphosphatemia with low intact FGF23 levels. The current treatment primarily involves surgical removal of calcified lesions and management of the phosphate levels via dietary adjustments and the use of binders (34). Targeted therapies modulating the GALNT3-FGF23 pathway are still under development.

CAD is a complex condition affected by various genetic and molecular factors. One of the factors is the dysregulation of GALNT3, which has been linked to the development of CAD. Research has identified specific GALNT3 genetic variants that increase susceptibility to diseases. For instance, a study involving a Chinese cohort identified two single-nucleotide polymorphisms, rs13427924 and rs4621175, which were significantly associated with CAD. The risk A allele of rs4621175 was correlated with reduced GALNT3 expression at both the mRNA and protein levels, indicating a genetic predisposition to CAD owing to reduced GALNT3 expression (35). Another study found that low GALNT3 levels in patients with CAD were linked to vascular endothelial damage. This damage occurs by inducing apoptosis and increasing matrix metalloproteinase (MMP) expression via the activation of the p38 mitogen-activated protein kinase (MAPK) signaling pathway (36). This finding underscores the role of genetic regulation in CAD development and highlights the potential of GALNT3 as a novel gene influencing CAD risk.

Furthermore, GALNT3 dysregulation is a part of a complex network of molecular interactions that contribute to CAD. For example, reduced GALNT3 downregulation has been linked to increased expression of MMP-2 and MMP-14, enzymes that degrade ECM components and promote vascular remodeling and plaque instability. This molecular pathway emphasizes the importance of GALNT3 in maintaining vascular health and its possible role in the progression of CAD (35,36).

In summary, the dysregulation of GALNT3 plays a crucial role in the development of CAD by affecting endothelial cell function and influencing its molecular pathogenesis. The genetic variants of GALNT3 further modulate CAD risk, offering valuable insights into potential treatments and genetic markers for early diagnosis and intervention.

During IAV infection, the GALNT3 expression is markedly altered, which can significantly influence viral replication and the host immune response. However, the role of GALNT3 in IAV infection is complex and appears paradoxical, with studies displaying both proviral and antiviral effects.

One study suggested that the induced GALNT3 enhances mucin-type O-glycosylation in infected respiratory epithelial cells. This modification appears to be vital for effective viral replication, as knockdown of GALNT3 or its upstream miRNAs significantly decreases viral titers and genomic RNA levels during the early stages of infection (20). A minireplicon assay revealed that GALNT3 enhances the activity of the viral polymerase complex. In vivo, Galnt3-knockout mice exhibited lower initial viral loads in the lungs, but ultimately developed significantly higher viral titers, more severe lung damage, and increased mortality when compared with the wild-type mice (20). This observation suggested a dual role: GALNT3 promotes intracellular viral replication while also potentially regulating the overall viral spread and lung damage, possibly through its involvement in mucin production and airway defense.

By contrast, Wang et al (37) conducted a study that identified an antiviral role for GALNT3. They found that, following an infection, the levels of GALNT3 protein were notably reduced in the lungs of mice susceptible to IAV (37). Unlike the findings of Nakamura et al (20), their experiments revealed that overexpressing GALNT3 in cell lines significantly reduced IAV replication. Wang et al (37) associated this antiviral effect with the inhibition of the NF-κB-signaling pathway, which plays a central role in inflammation and viral replication. They found that GALNT3 overexpression suppressed the NF-κB-promoter activity and, importantly, prevented the nuclear translocation of phosphorylated p65 (RelA) after IAV infection. By preventing the translocation of this critical transcription factor, GALNT3 dampens the expression of proinflammatory genes and other host factors that the virus may utilize to promote its own replication (37).

The seemingly contradictory roles of GALNT3 in IAV infection, that is, enhancing viral replication according to Nakamura et al (20) but inhibiting it as per Wang et al (37) likely results from different biological contexts and time points studied. Its proviral activity can mainly be observed during the initial infection phases, where GALNT3-driven O-glycosylation may support early viral replication. Conversely, its antiviral effect appears later, via the suppression of the NF-κB-signaling pathway, which reduces inflammation. This dual function suggests that GALNT3 operates at the intersection of viral replication processes and host immune responses, and its overall impact depends on the specific cellular environment and the stage of infection.

The distinct and common roles of GALNT3 in FTC/HHS, CAD, and IAV have been systematically summarized in Table II, whereas Fig. 4 provides a unified schematic of the underlying pathophysiological mechanisms.

In lung cancer, GALNT3 functions as a powerful tumor suppressor via two main mechanisms: Influencing the intrinsic characteristics of tumor cells and the tumor microenvironment (TME) (38). Clinically, reduced GALNT3 expression has been shown to be correlated with advanced tumor stage, poor differentiation, and decreased patient survival (39-41). In vivo experiments demonstrated that overexpressing GALNT3 inhibits tumor growth in xenograft (H2030 and PC9) and syngeneic (LLC) models, whereas its knockdown accelerates tumor development (38). Mechanistically, GALNT3 combats lung cancer via the following two connected pathways: i) Inhibiting cancer stemness by decreasing the catenin mRNA/protein levels, limiting its nuclear translocation, and downregulating Wnt target genes such as homeobox B9 (HOXB9) and jagged canonical notch ligand 2 (JAG2), which reduces tumor-initiating ability; ii) altering the immunosuppressive TME via O-GalNAcylation-mediated suppression of TNFR1-NF-κB- and c-MET-PI3K-AKT-signaling pathways, which lead to decreased C-X-C motif chemokine ligand 1 (CXCL1) secretion. This decrease suppresses the recruitment of proangiogenic polymorphonuclear-myeloid-derived suppressor cells (PMN-MDSCs) (CD11b⁺Gr1⁺Ly6G⁺), ultimately disrupting tumor vascularization, as evidenced by diminished CD31⁺ vasculature. Therapeutically, the GALNT3-regulated gene signature serves as a prognostic biomarker for favorable survival outcomes, while GALNT3 itself appears to be a promising therapeutic target. Restoring GALNT3 offers a multifaceted approach to combating lung cancer by simultaneously reducing cancer stemness and disrupting MDSC-driven immunosuppression and angiogenesis (38). Considering the complexity of these interacting pathways, the multifaceted tumor-suppressive mechanism of GALNT3 is schematically summarized in Fig. 5 to provide a clearer overview.

GALNT3 plays an essential oncogenic role in CRC progression. Evidence from clinical studies indicates that GALNT3 is considerably upregulated in CRC tissues compared with that in the adjacent normal tissues, with higher expression linked to advanced tumor stages, metastasis, and poorer prognosis (15,19). Mechanistically, its dysregulation is regulated by ncRNA networks. For example, circ-RAPGEF5 was shown to act as a sponge for miR-545-5p, reducing its suppression of GALNT3. The circ-RAPGEF5/miR-545-5p/GALNT3 axis was demonstrated to enhance CRC cell proliferation, migration, and invasion, while decreasing apoptosis and promoting tumor growth in vivo. The knockdown of GALNT3 reversed these malignant characteristics, highlighting its critical role in CRC development (15).

GALNT3-mediated O-glycosylation has a direct influence on oncogenic signaling pathways in CRC. A key regulatory axis involves the lncRNA linc01296, which binds miR-26a to suppress the GALNT3 expression. GALNT3 catalyzes aberrant O-glycosylation of mucin 1 (MUC1), stabilizing its active form and activating the PI3K/AKT pathway (19). This sequence of events fosters increased cell survival, metastasis, and resistance to chemotherapy (such as 5-fluorouracil). Animal studies have demonstrated that targeting the linc01296/miR-26a/GALNT3 axis can reduce CRC liver metastasis and tumor growth in xenograft models, positioning GALNT3 as a key link between glycosylation and oncogenic signaling in CRC (19). As detailed above, GALNT3 sits at the center of a complex ceRNA network in CRC. A consolidated overview of these regulatory axes is shown in Table III. The critical downstream mechanism, whereby GALNT3-mediated MUC1 glycosylation drives oncogenic signaling and chemoresistance, can be observed in Fig. 6.

In epithelial ovarian cancer (EOC), GALNT3 is frequently overexpressed in advanced tumors and linked to poor outcomes. Its oncogenic function involves complex regulatory networks, particularly involving the lncRNA PSMA3-AS1, which binds to miR-378a-3p to increase the GALNT3 levels (17). GALNT3 promotes O-glycosylation, which stabilizes oncoproteins such as MUC1(26), leading to the activation of the prosurvival PI3K/Akt pathway and driving tumor growth, invasion, and metabolic changes.

A key feature of GALNT3 in ovarian cancer is its functional redundancy with GALNT6(26). Suppressing GALNT3 alone causes a compensatory upregulation of GALNT6, which maintains the O-glycosylation of shared substrates and partially restores the oncogenic phenotypes. Therefore, the dual suppression of GALNT3 and GALNT6 is necessary to maximize therapeutic effects, resulting in a greater reduction in proliferation, migration, invasion, and intraperitoneal tumor formation when compared to targeting either enzyme alone (26). This redundancy highlights the importance of combination strategies in overcoming adaptive resistance and identifies GALNT3 and GALNT6 as synergistic biomarkers and therapeutic targets in EOC. The therapeutic challenge posed by the functional redundancy between GALNT3 and GALNT6 and the superior efficacy of their co-suppression is illustrated in Fig. 7.

Research on the expression and role of GALNT3 in breast cancer suggests that it influences tumor development. A 2005 immunogenomics study identified GALNT3 as one of five tumor-associated antigens whose expression increased in proportion to the tumor size in a HER2-driven murine mammary carcinoma model (BALB-neuT) (42). Although the GALNT3 levels were lower than those of HER2, it was consistently present in human breast tumors, indicating its potential as a target for immunopreventive therapies (42).

Conversely, a study by Raghu et al (14) showed that GALNT3 is a key suppressor of EMT in breast cancer. According to their results, GALNT3 expression is significantly decreased in aggressive, mesenchymal-like claudin-low breast cancer cells, such as SUM159, when compared with normal human mammary epithelial cells. This reduction in GALNT3 has significant functional consequences, resulting in a widespread decrease in O-GalNAc glycosylation. Notably, GALNT3 directly facilitates the O-GalNAc glycosylation of E-cadherin. Without GALNT3, E-cadherin cannot reach the plasma membrane and instead remains inside the cell within the Golgi apparatus. This mislocalization hinders the formation of adherens junctions, weakens cell-cell adhesion, and promotes the development of mesenchymal features, such as increased motility and invasiveness. While restoring GALNT3 can reverse EMT in some cell models, doing so in established claudin-low breast cancer cells does not restore epithelial traits, hinting that GALNT3 loss may be an early, pivotal event in a wider, possibly irreversible EMT process during breast cancer development (14). Overall, GALNT3 is a crucial post-translational regulator of epithelial integrity, and its decreased expression is a characteristic of the aggressive, mesenchymal breast cancer subtype.

These apparently contradictory findings can be explained by the heterogeneity of breast cancer. Although GALNT3 might be upregulated in certain subtypes as a part of a tumor-associated antigen profile, its loss in aggressive claudin-low cancers could promote mesenchymal transition and invasion. Therefore, GALNT3 may serve as a double-edged sword in breast cancer: Enhancing tumor antigenicity in some situations while maintaining epithelial integrity in others. Additional research is thus necessary to comprehend the regulatory mechanisms and context-specific roles of GALNT3 across various breast cancer subtypes. The context-dependent dual role of GALNT3 in different breast cancer subtypes is summarized in Fig. 8, aiding to understand its seemingly contradictory functions.

The role of GALNT3 in pancreatic ductal adenocarcinoma (PDAC) is complex and context-dependent. Foundational work has established its tumor-promoting function, suggesting that GALNT3 is overexpressed in the majority of PDAC cell lines (6 of 8) while being undetectable in poorly differentiated lines such as PANC-1 and MIA PaCa-2(43). Functionally, GALNT3 suppression attenuated cancer cell growth both in vitro and in vivo by inducing apoptosis, partly through O-glycosylation of its substrate guanine nucleotide-binding protein G(t) subunit α1 (GNAT1), which regulates GNAT1 stability and subcellular distribution (43).

Subsequently, Chugh et al (44) systematically elucidated the context-dependent duality of GALNT3. Their study first revealed that, in poorly differentiated PDAC, GALNT3 expression is significantly lost and that its knockdown enhances proliferation, motility, and tumor-endothelium adhesion. This phenotype was mechanistically linked to aberrant O-glycosylation and hyperactivation of ErbB receptors (namely EGFR and Her2) (44).

Subsequently, the same team identified a contrasting role for GALNT3 in pancreatic cancer stem cells (PCSCs), where it is highly expressed and essential for maintaining stemness. GALNT3 knockdown in PCSCs reduced the core stemness markers (SOX2, OCT3/4) and surface marker epithelial-specific antigen, which led to the impairment of the self-renewal capacity and tumorigenicity (45).

The role of GALNT3 in PDAC is complex and exhibits striking context-dependent duality, functioning as both a tumor promoter and a tumor suppressor. This paradox is resolved when considering the cellular differentiation state, that is, while GALNT3 promotes tumor growth in well-differentiated contexts and is essential for maintaining stemness in PCSCs, its loss in poorly differentiated carcinomas is associated with enhanced aggressiveness. To integrate these seemingly contradictory findings and provide a unified visual model, this context-dependent duality is summarized in Fig. 9. Collectively, these studies illustrate the context-dependent duality of GALNT3 in PDAC. However, it must be acknowledged that the overall research specifically focusing on GALNT3 in this malignancy remains limited. The scarcity of data, particularly concerning its direct mechanistic link to survival pathways such as PI3K/AKT/MAPK in PCSCs, represents a significant knowledge gap that future work must address to fully elucidate its roles.

The composition of the TME, encompassing the ECM, immune cell infiltration, and stromal interactions, varies between tissues and cancer subtypes (48). For example, in breast cancer, the claudin-low subtype has a microenvironment that is different from that of HER2-driven tumors (14,42,49). These variations probably affect the substrates accessible to GALNT3 and influence the downstream effects of its glycosylation, thereby impacting tumor development.

The function of a gene is not executed in isolation, but is critically determined by its specific molecular partners and the broader signaling network in which it operates, a phenomenon aptly described as ‘substrate specificity and signaling pathway crosstalk’ (50-52). The functional role of GALNT3 depends on its key protein substrates within a specific cellular environment. In lung cancer (38), GALNT3-driven O-GalNAcylation was shown to suppress oncogenic signaling by modifying receptors such as TNFR1 and c-MET, leading to decreased activity of the NF-κB and PI3K/AKT pathways. By contrast, in CRC (19), GALNT3 was demonstrated to glycosylate and stabilize oncoproteins such as MUC1, resulting in the activation of the prosurvival PI3K/AKT pathway. The contradictory effects observed in breast cancer can be explained by different substrate choices: In aggressive claudin-low subtypes, GALNT3 has been shown to glycosylate E-cadherin to maintain epithelial integrity and prevent EMT, whereas, in other contexts, it may target different substrates that promote tumorigenicity.

The expression and function of a gene are profoundly influenced by its position within tissue-specific and subtype-specific regulatory networks (53). For instance, specific ceRNA networks involving lncRNAs, miRNAs, and mRNAs have been identified in various cancers, which post-transcriptionally regulate oncogenes or tumor suppressors in a manner that is often dependent on clinical features such as tumor stage, subtype classification, or patient survival (54-56). The expression of GALNT3 is governed by distinct, tissue-specific, and subtype-specific regulatory networks involving ncRNAs (15-19,21). These networks determine GALNT3 expression, effectively positioning it either as a protector against tumor development or as a promoter of malignant progression.

In summary, the dual role of GALNT3 in cancer exemplifies context-dependent biology, influenced by the tissue type, molecular subtype, TME, substrate choice, and signaling crosstalk within the GALNT enzyme family.