Bioelectric and Epigenetic Landscapes in Lateralized Breast Tumors Reveal Distinct Tumor Microenvironment Signatures.

Cell communication within the tumor microenvironment (TME) plays a pivotal role in cancer progression, yet little is known about whether this communication differs between anatomically paired organs. Here, we report that breast tumors arising in the left (L) versus right (R) mammary glands exhibit significant asymmetry in composition, bioelectric state, and epigenetic regulation suggesting the existence of a stable, lateralized tumor TME shaped by differential cell communication. By mining TCGA data of invasive ductal carcinomas, we found that R-sided tumors display a higher stromal content, particularly enriched in cancer-associated fibroblasts (CAFs). Further subtype analysis revealed a predominance of inflammatory CAFs (iCAFs) in R tumors and dividing CAFs (dCAFs) in L tumors, suggesting distinct TME profiles. These CAF differences were confirmed in paired L-R xenografts, where R tumors showed increased -SMA content. Conditioning cell culture experiments with mammary tissue extracts demonstrated that L-sided environments induce greater membrane depolarization in breast cancer cells. These bioelectric differences were as well replicated in paired L-R xenograft tumors, where L tumors consistently exhibited a depolarized membrane potential relative to their R counterparts. Methylome profiling by Nanopore Sequencing revealed that L tumors were hypermethylated at key ion channel genes involved in electrical cell communication-most notably connexins-correlating with their reduced gene expression. A computational model further showed that bistable membrane potential and methylation states can emerge spontaneously, governed by the strength of Gap Junction-mediated communication and the initial state. Taking together, our results reveal a previously unrecognized L-R asymmetry in tumor biology, in which stromal composition and cell communication mechanisms establish stable, lateralized bioelectric and epigenetic states. These findings have broad implications for understanding tumor heterogeneity, TME conditioning, and the possibility of side-specific therapeutic strategies.