Pan-Cancer Analysis and Functional Validation Reveal a Role for MAN1A1 in Promoting Metastasis-Associated Phenotypes in Triple-Negative Breast Cancer

Background/Objectives: Triple-negative breast cancer (TNBC) is aggressive and prone to distant metastasis. MAN1A1 encodes a Golgi α-1,2-mannosidase involved in N-glycan processing, but its pan-cancer features and role in TNBC metastasis remain unclear. Methods: We integrated The Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx), Human Protein Atlas (HPA), and Gene Expression Omnibus (GEO) data to characterize MAN1A1 expression, clinical outcomes, Hallmark pathways, and immune features across cancers, with further analyses focused on breast cancer and TNBC. Functional effects were evaluated using MAN1A1 knockdown in LM2 cells, overexpression in MDA-MB-231 cells, Transwell assays, and a tail-vein injection model in nude mice. LM2 RNA sequencing (RNA-seq), quantitative real-time PCR (qPCR) validation, and TCGA-TNBC transcriptomic and immune microenvironment analyses were integrated to investigate MAN1A1-associated molecular features. Results: MAN1A1 exhibited marked cancer-type heterogeneity in expression, prognostic associations, and molecular alterations. It was not specifically overexpressed in TNBC; however, higher MAN1A1 expression within breast cancer and TNBC was associated with poorer overall survival and metastasis-related features. MAN1A1 knockdown suppressed LM2 migration, invasion, and experimental lung metastasis, whereas overexpression enhanced MDA-MB-231 migration and invasion. LM2 RNA-seq revealed alterations in transcriptional programs related to epithelial–mesenchymal transition, cell adhesion, extracellular matrix remodeling, and inflammation. TCGA-TNBC showed concordant transcriptional patterns and distinct immune and stromal microenvironmental features. Conclusions: MAN1A1 promotes TNBC cell migration, invasion, and experimental lung metastasis, accompanied by alterations in transcriptional programs related to cell adhesion, extracellular matrix (ECM) remodeling, migration, and inflammation. These findings support MAN1A1 as a context-dependent metastasis-associated molecule in TNBC and provide a basis for further investigation of its potential molecular mechanisms.