A Cell-free DNA Methylation Liquid Biopsy for Diagnosis and Management of Biliary Tract Cancers

Biliary tract carcinoma (BTC), encompassing gallbladder cancer, intrahepatic cholangiocarcinoma, and extrahepatic cholangiocarcinoma, is an aggressive malignancy with poor prognosis. Globally, it ranks sixth in incidence among gastrointestinal cancers and tenth in cancer-related mortality. BTC is characterized by the absence of specific early symptoms, high degree of malignancy, and a strong tendency for recurrence and metastasis. The curative resection rate remains around 16.5%, and the overall 5-year survival rate is less than 5%.

Biliary tract inflammation (such as cholangitis, acute cholecystitis, sclerosing cholangitis, and autoimmune cholangitis) can lead to abnormal CA19-9 elevation. In addition, IgG4-related sclerosing cholangitis often affects elderly patients and may mimic hilar cholangiocarcinoma, creating diagnostic challenges. Imaging alone often lacks accuracy in differentiating benign from malignant biliary lesions, resulting in misdiagnosis and inappropriate clinical decision-making. The number of patients with incidentally detected BTC during surgery is rapidly increasing, and reoperations impose substantial trauma and socioeconomic burden.

Circulating tumor DNA (ctDNA) refers to DNA fragments released into the bloodstream from tumor cells through apoptosis, necrosis, or secretion. ctDNA carries genomic alterations (point mutations, indels, CNVs, fusions), epigenetic modifications (DNA methylation [5mC], hydroxymethylation [5hmC]), and structural features (fragment length, fragmentation patterns). Circulating free DNA (cfDNA) represents the total extracellular DNA in plasma or serum, of which ctDNA accounts for less than 1%. Accumulating evidence shows that ctDNA is detectable in the early stages of tumorigenesis, highlighting its clinical potential in early detection, diagnosis, treatment guidance, and post-treatment monitoring.

The choice of biomarker type is key to liquid biopsy applications, and ctDNA methylation offers distinct advantages. Liquid biopsy analytes include circulating tumor cells (CTCs), ctDNA, exosomes, and microRNAs (miRNAs), each with unique detection characteristics. ctDNA, directly derived from tumor cells, provides relatively high sensitivity for early detection and is currently the most widely used target in clinical practice. Research on ctDNA has focused mainly on methylation, somatic mutations, and copy number variations. Among these, ctDNA methylation balances both signal abundance and signal strength, offering clear advantages over other approaches. Methylation analysis methods include restriction enzyme digestion, affinity enrichment, and bisulfite conversion. Among them, bisulfite-based approaches are the most established and widely used.

Therefore, the application of cfDNA methylation liquid biopsy in BTC for early screening, differential diagnosis, prognostic monitoring, and therapeutic guidance holds great significance for improving diagnosis, treatment, and patient outcomes.