Liver Fluke–Associated Cholangiocarcinogenesis in East Asia: Oxidative DNA Damage, Chronic Inflammation, and Epigenetic Transformation

Epidemiologic Context and Carcinogenic Classification

Cholangiocarcinoma (CCA), a lethal malignancy arising from bile duct epithelium, carries a disproportionate burden in East and Southeast Asia owing to the endemic prevalence of two hepatobiliary flukes: Opisthorchis viverrini (OV) and Clonorchis sinensis (CS). Both parasites are classified as definitive Group 1 carcinogens by the International Agency for Research on Cancer (IARC), reflecting robust epidemiologic and mechanistic evidence 1528. Approximately 10 million individuals in the Greater Mekong Subregion are estimated to carry active OV infection, with over 6 million affected in Northeastern Thailand alone—a region where liver and bile duct cancer ranks among the leading causes of mortality 27. For CS, approximately 35 million individuals are currently infected across Korea, China, Vietnam, Taiwan, and the Russian Far East, with nearly 700 million at risk 28. In Korea, the 8th National Survey on Intestinal Parasites (2012) identified CS as the most prevalent parasitic infection nationally (1.86%), with incidence of CCA highest in southern endemic river basins. In China, CS infestation has been reported in 27 of 34 provinces, with national prevalence rising 75% compared to the first national survey 28. The pooled odds ratio for CS-associated CCA ranges from 4.5 to 6.1, and the latency between infection and malignancy typically spans 10–30 years, reflecting a multistep carcinogenic process rather than a direct oncogenic trigger 928.

Oxidative and Nitrative DNA Damage

A central mechanism linking chronic fluke infection to malignancy is the sustained generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) within the biliary microenvironment. In both OV and CS infection, activated macrophages, neutrophils, and biliary epithelial cells produce ROS through enzymatic systems including NADPH oxidase (NOX), xanthine oxidase (XO), lipoxygenase (LO), cyclooxygenase (COX), and inducible nitric oxide synthase (iNOS) 28. ROS and RNS react with DNA to form two hallmark oxidative lesions: 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG) and 8-nitroguanine, both of which are highly mutagenic if not repaired 528.

In OV-infected tissues and urine from patients with CCA, 8-oxo-dG levels are significantly elevated compared to infected individuals without cancer and to healthy controls; these levels decrease following anti-parasitic therapy, establishing 8-oxo-dG as a clinically informative biomarker of active carcinogenic damage 5. Critically, oxidative DNA damage occurs preferentially in cancer stem-like cells expressing CD133 and Oct3/4, and high 8-oxo-dG formation in these cells correlates with poor patient prognosis—suggesting that stem/progenitor-like populations represent particularly vulnerable targets for fluke-driven mutagenesis 5.

Beyond direct DNA lesions, oxidative stress creates a self-sustaining vicious cycle: ROS damage transferrin, releasing free iron ions that catalyze Fenton reactions and generate additional hydroxyl radicals; simultaneously, carbonylation of antioxidant proteins impairs cellular redox defenses 5. CS excretory–secretory products (ESPs) further disrupt redox homeostasis by upregulating tripartite motif protein 22 (TRIM22), which activates the AKT/mTOR signaling pathway while suppressing nuclear factor erythroid 2-related factor 2 (Nrf2)—a master transcriptional regulator of antioxidant and detoxifying enzymes—thereby further impairing the cell's capacity to counteract oxidative injury 16. A novel dimension recently identified in CS infection is ferroptosis, a form of iron-dependent cell death involving lipid peroxidation: infected mice exhibit downregulation of glutathione peroxidase 4 (GPX4), solute carrier SLC7A11, and Nrf2, alongside glutathione depletion and mitochondrial structural damage; pharmacological inhibition of ferroptosis (with Fer-1) reduces liver fibrosis, parasite burden, and pro-inflammatory cytokine production, indicating that ferroptosis links iron dysregulation to hepatic inflammation and fibrotic progression 21.

Chronic Inflammation and Immune-Mediated Carcinogenesis

Fluke-driven cholangiocarcinogenesis is fundamentally an inflammation-driven process. Both OV and CS ESPs activate pattern recognition receptors on biliary epithelial cells (BECs), triggering innate immune cascades that amplify inflammation and fibrosis. Toll-like receptor 2 (TLR2) on BECs is significantly upregulated following CS infection; ESP-mediated TLR2 activation phosphorylates AKT and p38 mitogen-activated protein kinases (MAPKs), driving interleukin-6 (IL-6) secretion 1. IL-6 subsequently engages the transforming growth factor-β1 (TGF-β1)/Smad2/3 pathway, inducing myofibroblast activation (marked by α-smooth muscle actin, α-SMA) and excessive extracellular matrix (ECM) deposition characteristic of periductal fibrosis 1. TLR2-deficient mice exhibit markedly reduced biliary fibrosis, decreased mortality, and attenuated IL-6 and TNF-α production, confirming TLR2's central role in pro-fibrotic signaling 1.

CS extracellular vesicles (CsEVs)—80–120 nm parasite-derived particles carrying nucleic acids and proteins—activate TLR9 on BECs to stimulate ERK, AKT, and NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) phosphorylation, driving further IL-6 and TNF-α secretion 6. Importantly, a contrasting protective pathway exists: CsEV-associated double-stranded RNA activates TLR3, which suppresses IL-6 and TNF-α by inhibiting p38 and ERK phosphorylation; TLR3-deficient mice develop more severe infection, and treatment with the TLR3 agonist poly(I:C) dramatically reduces parasite burden and fibrosis 4. This TLR2/TLR3 dichotomy illustrates how the balance between innate immune receptor signaling determines disease severity.

Macrophage polarization constitutes an additional layer of immune-mediated carcinogenesis. CS granulin (CsGRN), a major ESP component, stimulates BECs to produce monocyte chemoattractant protein-1 (MCP-1), recruiting monocytes and polarizing them toward an M2 (alternatively activated, pro-tumorigenic) phenotype characterized by high CD206 and IL-10 expression 3. M2-derived IL-6 then activates the IL-6/Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) pathway in human intrahepatic biliary epithelial cells, upregulating oncogenic targets c-Myc, trefoil factor 3 (TFF3), and BCL-2, and promoting epithelial-to-mesenchymal transition (EMT)—a process in which epithelial cells acquire mesenchymal, migratory properties—characterized by increased N-cadherin and vimentin with decreased tight junction protein ZO-1 3. Concomitant MEK/ERK pathway activation amplifies STAT3 phosphorylation; inhibition of MEK/ERK with PD98059 significantly represses this malignant transformation, identifying MEK/ERK as a druggable node 3.

CS ESPs also induce biliary epithelium to secrete interleukin-17A (IL-17A), which acts on hepatic stellate cells to upregulate α-SMA and collagen I, driving fibrosis independently of TGF-β pathways 8. Knockdown of IL-17A in ESP-stimulated cholangiocytes attenuates stellate cell activation, establishing a parasite-induced epithelium-stellate cell axis as a new mechanistic contributor 8. In parallel, OV–Helicobacter pylori co-infection—frequent in Lower Mekong Basin populations—modulates neutrophil lifespan and membrane integrity, generating propidium iodide-positive neutrophils with leaky plasma membranes that release pro-inflammatory contents, amplifying persistent biliary inflammation 22.

Parasite-Derived and Microenvironmental Contributors

Beyond direct immune activation, liver flukes reshape the biliary microenvironment through multiple additional mechanisms. Physical activities of migrating worms cause mechanical irritation and bile duct obstruction, promoting metaplasia of mucin-producing cells, progressive periductal fibrosis, and epithelial hyperplasia—changes whose severity correlates with worm burden and infection duration 28. ESP-driven growth signaling through epidermal growth factor receptor (EGFR) and TLR4-associated metalloproteinases significantly increases BEC proliferation and migration; EGFR inhibition abolishes these effects, identifying EGFR as a convergent signaling node 19. CS ESPs further upregulate the transcription factor E2F1, which promotes fatty acid synthase (FASN) expression and de novo lipogenesis in tumor cells, accelerating intrahepatic CCA progression; FASN knockdown abolishes this ESP-driven growth advantage 26.

Microbiome dysbiosis represents an emerging pathogenic interface. Among the three fluke species studied (OV, CS, O. felineus), OV induced the most pronounced biliary microbiota alterations—significantly expanding Enterobacteriaceae while depleting commensal taxa (Parabacteroides, Roseburia)—consistent with its greater carcinogenic potency 2. Dysbiotic communities produce altered metabolites and lipopolysaccharides that further activate pattern recognition receptors on epithelial and immune cells, amplifying local inflammation 212. These microbiome-mediated effects are a frontier area where detailed mechanistic evidence remains limited.

Epigenetic Alterations and Tumor Suppressor Silencing

Epigenetic dysregulation is a critical mechanism by which chronic inflammation and oxidative stress are translated into heritable silencing of tumor suppressor genes. In OV-associated CCA (Ov-CCA) tissues, aberrant promoter hypermethylation—mediated partly by IL-6-induced upregulation of DNA methyltransferase 1 (DNMT1) through a microRNA-dependent mechanism—has been documented for a broad set of tumor suppressors including hMLH1 (mismatch repair), PTEN, SFRP1 and OPCML (Wnt/β-catenin pathway antagonists), RUNX3, TP53I3, RASSF1, CDKN2A (p16INK4A and p14ARF), and HIC1 2728. Network analysis reveals that many of these silenced genes are negative regulators of the Wnt/β-catenin signaling pathway, whose deregulation is a central event in Ov-CCA pathogenesis 27. For CS-associated disease, ESPs alter histone modifications at carcinogenic target genes—including Mcm7, a DNA replication licensing factor—and drive broad transcriptome, proteome, and miRNA profile changes in human CCA cells 28.

MicroRNA (miRNA) dysregulation contributes both oncogenic and tumor-suppressive alterations. During CS-associated cholangiocarcinogenesis, miRNAs regulate cell proliferation, apoptosis, migration, oncogene activation, and DNA methylation pathways 28. A recently validated three-miRNA panel (miR-99a-5p, miR-516a-5p, miR-526b-5p) distinguishes CCA from normal controls (AUC = 0.899) and from hepatocellular carcinoma (AUC = 0.937); incorporating conventional tumor markers CA19-9 and CEA further raises AUC to 0.959, establishing a promising non-invasive diagnostic framework 17. At the genomic level, CREBBP mutations—present in 50% of Thai iCCA patients versus 30.8% of controls—and KRAS codon 13 polymorphisms (exclusive to iCCA, 21.4%) represent population-specific oncogenic signatures enriched in fluke-associated disease, while TP53 mutation status independently predicts metastatic risk 7.

Integrated Mechanisms, Clinical Implications, and Future Directions

The transition from chronic liver fluke infection to cholangiocarcinoma follows a sequential multistep model: (1) mechanical and chemical epithelial injury by migrating flukes and ESPs; (2) acute and chronic TLR-mediated inflammation generating IL-6, TNF-α, and NF-κB activation; (3) oxidative/nitrative DNA damage accumulating as 8-oxo-dG and 8-nitroguanine, preferentially in stem-like cells; (4) fibrotic periductal remodeling via TGF-β1/Smad2/3, IL-17A, and ferroptosis-mediated pathways; (5) M2 macrophage–driven epithelial transformation through IL-6/JAK2/STAT3 and MEK/ERK signaling; (6) epigenetic silencing of tumor suppressors via DNMT1-mediated promoter methylation and miRNA-mediated reprogramming; and (7) accumulation of driver mutations (KRAS, CREBBP, TP53) locking in malignant phenotypes over decades 591027.

The table below summarizes the major mechanisms, molecular mediators, biological effects, and clinical implications identified across these evidence streams.

Conclusion

The carcinogenic pathway linking liver fluke infection to cholangiocarcinoma is not a single-mechanism event but a decades-long convergence of parasite-mediated tissue injury, sustained innate immune activation, oxidative genomic damage, fibrotic stromal remodeling, and progressive epigenetic silencing. O. viverrini and C. sinensis share fundamental mechanisms—including ROS/RNS-driven DNA damage, IL-6/STAT3 and NF-κB inflammatory amplification, and promoter hypermethylation of tumor suppressors—but exhibit species-specific differences in microbiome remodeling potency, ESP composition, and associated genetic mutation spectra that warrant further comparative investigation 279. Clinically, early anti-parasitic therapy with praziquantel remains the cornerstone of prevention, with evidence that treatment reduces biomarkers of oxidative damage. Emerging investigational strategies include non-invasive detection approaches based on serum miRNA panels, urinary 8-oxo-dG, and parasite antigen lateral flow assays, although further clinical validation is required; and pathway-targeted interventions including JAK2/STAT3 inhibitors, MEK/ERK inhibitors, TLR3 agonists, and epigenetic modulators, several of which have demonstrated efficacy in experimental models 431718. Translation of these molecular insights into community-based screening programs and prospective clinical trials—particularly in high-burden regions of Thailand, China, Korea, Laos, and Vietnam—represents the critical next step toward reducing the substantial preventable mortality from fluke-associated cholangiocarcinoma in East Asia 91427.