Abstract PS3-13-11: Expression of Brutons tyrosine kinase (BTK) in breast cancer associates with tumor immune microenvironment but lacks predictive value for clinical outcome

Abstract Background: Bruton’s tyrosine kinase (BTK), a downstream mediator of the B-cell receptor (BCR) signaling pathway, is known to be essential in differentiation and proliferation of B-cell. Thus, BTK inhibitors are used to treat refractory mantle cell lymphoma and chronic lymphocytic leukemia. Recent discoveries have revealed the expression of BTK in myeloid-derived suppressor cells, known to worsen breast cancer (BC) outcomes. To this end, it was of interest to investigate the clinical relevance of BTK expression in BC. Methods: Total of 9297 BC patients from 23 independent cohorts with tumor transcriptome and clinical data were analyzed. Results: Based on two independent single-cell sequencing cohorts, approximately 15% of B-cells and 30% of myeloid cells within the tumor microenvironment expressed BTK, while no other cells exhibited BTK expression, suggesting that the BTK expression signal from bulk tumor samples primarily originates from these cells. Correlation with BTK expression was highest in macrophage regulation (r=0.94), and leukocyte fraction, lymphocyte infiltration, TCR Shannon and TCR Richness (all r>0.74), but not with BCR Shannon nor BCR Richness. Total macrophage, M1 and M2 macrophage, dendritic cells, total B-cells, memory B-cells, as well as CD8 and CD4 cells were all highly infiltrated in BTK high BCs. Moreover, BTK expression correlated with both PD1 & PD-L1 in all BC subtypes. Biologically, BTK high BC expression was linked to immune-related gene sets, with gene set enrichment analysis (GSEA) showing enrichment in pathways related to immune response such as Complement, Inflammatory response, Allograft rejection, IFN-γ, IFN-α, IL2, IL6, and TNFα signaling. At the same time, BTK high BC enriched tumor-aggravating signaling pathways such as PI3k/AKT/mTOR signaling and mTORC1 signaling. The cytolytic activity score, reflecting the global immune killing, was significantly higher in the BTK high BC. Given that BC with abundant tumor infiltrating lymphocytes respond better to chemotherapy, the relationship between BTK expression and response to neoadjuvant chemotherapy was assessed. Somewhat unexpectedly, BTK expression was associated with better response in only one out of ten neoadjuvant cohorts. Further, no change in BTK expression were observed before and after neoadjuvant chemotherapy in six cohorts. While BTK expression was higher in primary sites compared to metastatic sites, it was not associated with distant recurrence. Across the TCGA, METABRIC, and SCAN-B cohorts, no significant overall survival differences were observed between the BTK high and low expression groups. All findings were consistent regardless of the BC subtypes. Conclusions: Myeloid cells and B-cells were the source of BTK expression, and it was associated with macrophage regulation and infiltration of various types of immune cells and proteins including PD1 & PD-L1 and enriched immune related gene sets. However, BTK expression was not associated with response to neoadjuvant chemotherapy or Impact on survival, regardless of BC subtypes. Citation Format: T. ALRammah. Expression of Brutons tyrosine kinase (BTK) in breast cancer associates with tumor immune microenvironment but lacks predictive value for clinical outcome [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-13-11.