APOBEC3F expression in triple-negative breast cancer is associated with tumor microenvironment infiltration and activation of cancer immunity and improved survival.

The apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) causes a point mutation from cytidine to uracil in DNA and/or RNA. The role of APOBEC3A and APOBEC3B in breast cancer has been well described, whereas that of APOBEC3F remains unknown. To investigate the clinical relevance of APOBEC3F expression, we analyzed a total of 3000 breast cancer cases from multiple independent large patient cohorts including METABRIC, TCGA, GSE75688, and GSE114725. High expression of was associated with improved disease-specific and overall survival in triple negative breast cancer (TNBC). is not usually a reflection of cancer cell biology in TNBC or luminal breast cancer, except for homologous recombination deficiency in TNBC. In the TNBC homologous recombination deficiency group, expression was not consistently associated with intratumor heterogeneity, mutation rates, or neoantigens. expression did not correlate with response to any of the drugs tested in breast cancer cell lines in vitro. However, high expression was associated with enrichment of several immune-related gene sets and immune activity. High expression also accompanied higher infiltration of anti-cancer immune cell infiltration in TNBC. However, in luminal breast cancer, high tumor significantly enriched not only immune-related gene sets, but also cell proliferation-, metastasis-, and apoptosis-related gene sets. Analysis of single-cell transcriptomes showed exclusively expressed in immune cells and significantly associated with cytolytic activity of the immune cells, immune response, and immune cell proliferation. Expression of immune checkpoint genes was uniformly elevated in -high tumors. We conclude that is exclusively expressed in immune cells and this expression is associated with enhanced anti-cancer immune response as well as improved survival in TNBC.