Integrative Bioinformatics and Experimental Validation Reveal that METTL9 Drives Lung Adenocarcinoma Progression via...

Methyltransferase-like 9 (METTL9) has been implicated in tumor progression, yet its clinical significance, functional roles, and immunoregulatory functions in lung adenocarcinoma (LUAD) remain unclear. This study aimed to integrate multi-omics datasets and experimental evidence to systematically evaluate the expression profile, prognostic value, molecular mechanisms, and tumor microenvironment (TME)-related functions of METTL9 in LUAD.

Transcriptome and clinical data for LUAD and METTL9 protein expression were obtained from public databases. Survival analysis was performed using Kaplan-Meier analysis and Cox proportional hazards models. METTL9-related differentially expressed genes (DEGs) were screened, followed by functional annotation and pathway enrichment analysis. A protein-protein interaction (PPI) network associated with METTL9 was constructed using STRING. METTL9 gene mutations and promoter methylation levels were analyzed via cBioPortal and UALCAN. Immune infiltration was evaluated through gene set variation analysis (GSVA) and the TIMER platform. The role of METTL9 depletion in regulating transforming growth factor-β (TGF-β)/Smad signaling, epithelial-mesenchymal transition (EMT), as well as the behavior of A549 and H1975 cells was investigated.

METTL9 was markedly elevated in LUAD, which was confirmed in external datasets and immunohistochemistry. Increased METTL9 expression showed a strong association with advanced TNM stage, unfavorable clinicopathological parameters, and poor survival outcomes in univariate analysis. Receiver operating characteristic (ROC) analysis indicated that METTL9 showed moderate diagnostic utility for LUAD, with an Area Under the Curve (AUC) of 0.704 (95% Confidence Interval [CI]: 0.650-0.758) in the The Cancer Genome Atlas (TCGA) dataset and 0.815 (95% CI: 0.758-0.872) in the GSE31210 dataset. Moreover, high METTL9 expression was significantly correlated with poor survival outcomes (Hazard Ratio [HR] = 1.349, 95% CI: 1.010-1.802, p = 0.043). METTL9 interacted with multiple oncogenic proteins in the PPI network and showed significant correlations with cuproptosis-related genes. Immune infiltration analysis further demonstrated that METTL9 expression showed significantly associated with immune cell infiltration and the tumor immune microenvironment in LUAD. METTL9 knockdown suppressed LUAD cell proliferation, migration, invasion, and EMT, while promoting apoptosis and reducing the expression of B-cell lymphoma 2 (Bcl-2), phosphorylated Smad2 (p-smad2), and phosphorylated Smad3 (p-smad3).

METTL9 is upregulated in LUAD and correlates with an unfavorable prognosis. Our results indicate that METTL9 may contribute to LUAD progression and EMT, potentially in relation to the TGF-β/Smad signaling pathway.