Activity-Regulated Cytoskeleton-Associated Protein Gene Expression Is Associated With High Infiltration of Stromal Cells and Immune Cells, but With Less Cancer Cell Proliferation and Better Overall Survival in Estrogen Receptor-Positive/Human Epidermal Growth Factor Receptor 2-Negative Breast Cancers.

Background: Peritumoral lidocaine infiltration prior to excision is associated with better survival in breast cancer (BC), which led us to hypothesize that innervation to the tumor affects its biology and patient survival. Activity-regulated cytoskeleton-associated protein (ARC) gene expression is known to be regulated by neuronal activity. Therefore, we studied the clinical relevance of ARC gene expression as a surrogate of neuronal activity in BC.

Methods: Sweden Cancerome Analysis Network - Breast (SCAN-B (GSE96058), n = 3,273) cohort and The Cancer Genome Atlas (TCGA, n = 1,069) were analyzed.

Results: High ARC expression was significantly associated with smaller tumor size, without lymph node metastasis, and less stage IV disease in one cohort, but not validated by the other. Estrogen receptor-positive (ER⁺)/human epidermal growth factor receptor 2-negative (HER2^-) and luminal A expressed significantly higher ARC compared to the other subtypes in both cohorts (P < 0.005). High ARC BC was significantly associated with lower Nottingham histological grade and lower Ki67 gene expression consistently in ER⁺/HER2^- but not triple negative breast cancer (TNBC) in both cohorts (P < 0.001). Cell proliferation-related gene sets in the Hallmark collection (E2F targets, G2M checkpoint, and mitotic spindle) were significantly enriched to low ARC BC in ER⁺/HER2^- but not TNBC in TCGA. The stromal cells (fibroblasts, vascular endothelial cells, and adipocytes) were all significantly infiltrated in high ARC ER⁺/HER2^-, but not in TNBC, except for neurons. Homologous recombination deficiency, intratumor heterogeneity, fraction altered, silent or non-silent mutation rate were all significantly lower in high ARC ER⁺/HER2^- but not TNBC. Although there was no difference in single nucleotide variant or indel neoantigens, tumor infiltrating lymphocytes, and cytolytic activity by ARC expression regardless of subtype, multiple immune cells were significantly infiltrated in high ARC ER⁺/HER2^-, including CD8, CD4 memory cells, helper type II T cells, regulatory T cells, M2 macrophages, and B cells (all P < 0.03 in both cohorts), but not in TNBC. Disease-specific and overall survival were significantly improved in high ARC ER⁺/HER2^- consistently in both cohorts (all P < 0.05), but this was not the case in TNBC.

Conclusion: ARC gene expression was associated with less cancer cell proliferation, high infiltration of stromal cells and immune cells, and better survival in the ER⁺/HER2^- but not TNBC subtype.