Comprehensive analysis of BTN3A1 in cancers: mining of omics data and validation in patient samples and cellular models.

Butyrophilin 3A1 (BTN3A1), a major histocompatibility complex-associated gene that encodes a membrane protein with two extracellular immunoglobulin domains and an intracellular B30.2 domain, is critical in T-cell activation and adaptive immune response. Here, the expression of BTN3A1 in cancers was analyzed in eight databases comprising 86 733 patients of 33 cancers, and the findings were validated in patient samples and cell models. We showed that BTN3A1 was expressed in most cancers, and its expression level was strongly correlated with clinical outcome of 13 cancers. Mutations of BTN3A1 were detected, and the mutations were distributed throughout the entire gene. Gene set enrichment analysis showed that BTN3A1 co-expression genes and interacting proteins were enriched in immune regulation-related pathways. BTN3A1 was associated with tumor-infiltrating immune cells and was co-expressed with multiple immune checkpoints in patients with breast cancer (BRCA) and non-small cell lung cancer (NSCLC). We reported that BTN3A1 was downregulated in 46 of 65 (70.8%) NSCLCs, and its expression level was inversely associated with clinical outcome of the patients. BTN3A1 in tumor samples was lower than in counterpart normal tissues in 31 of 38 (81.6%) BRCAs. Bioinformatics analyses showed that BTN3A1 could be a target gene of transcription factor Spi-1 proto-oncogene (SPI1), and our 'wet' experiments showed that ectopic expression of SPI1 upregulated, whereas silencing of SPI1 downregulated, BTN3A1 expression in cells. These results suggest that BTN3A1 may function as a tumor suppressor and may serve as a potential prognostic biomarker in NSCLCs and BRCAs.