Cellular and molecular determinants of lymph node metastasis in papillary thyroid carcinoma: Integrated multi-omics profiling and machine learning models.

The mechanisms underlying lymph node metastasis (LNM) in papillary thyroid carcinoma (PTC) remain elusive. This study integrated single-cell RNA sequencing (scRNA-seq) data, spatial transcriptomics (ST) data, and bulk transcriptomic data. Analyses revealed that N1-stage tumors exhibit a highly active immune-inflammatory microenvironment with an increase in immunosuppressive cells. Successfully disseminated tumor clones achieve immune escape by downregulating antigen presentation mechanisms. Pseudotime analysis uncovered a shared developmental trajectory and significant functional heterogeneity of malignant cells between primary and metastatic sites. An increased proportion of the State 1 subpopulation in metastases suggests its role as a potential metastasis-initiating cluster. A 17-key-gene LNM signature was identified, wherein the hub gene FN1 dynamically regulates migration and colonization via the FN1-SDC4 axis and stromal interactions during metastasis, which was validated by spatial transcriptomics. A random forest model based on this signature was constructed for predicting LNM. Computational analysis suggests that the high-risk group is potentially sensitive to targeted drugs such as ZM447439. Furthermore, in vitro experiments confirmed that FN1 silencing significantly suppresses the migration and proliferation of PTC cells. This study systematically deciphers the functional adaptive reprogramming during PTC LNM, providing novel insights and tools for LNM prediction and targeted therapy.