Compartment-Specific CD138 Expression Defines an Aggressive Breast Cancer Phenotype with Distinct Transcriptomic Features

Background/Objectives: CD138 (syndecan-1) is a cell-surface heparan sulfate proteoglycan involved in cell–matrix interactions and growth factor signaling, and it has been implicated in tumor progression. However, the clinical significance of compartment-specific CD138 expression in breast cancer remains unclear. In this study, we investigated the prognostic and transcriptomic features of compartment-specific CD138 expression in invasive breast cancer. Methods: We performed an integrated analysis of immunohistochemistry and RNA sequencing of 111 invasive ductal carcinoma specimens. Tumors were classified into four groups according to CD138 expression in the tumor and stromal compartments. Clinicopathological data and survival outcomes were obtained from medical records, and transcriptomic profiles were analyzed using RNA sequencing. Results: The tumor-positive/stroma-negative phenotype (Group 1) was associated with poorer recurrence-free survival than the other phenotypes. According to multivariable Cox regression analysis, Group 1 remained an independent prognostic factor after adjustment for age, lymph node status, and Ki-67 index (hazard ratio, 5.493; p = 0.0028). Group 1 was also associated with lymph node metastasis and HER2 expression. All brain metastases occurred in Group 1, although the number of events was low. Transcriptomic profiling identified the upregulation of small nucleolar RNAs in Group 1 tumors, with the enrichment of pathways related to ribosome biogenesis, RNA processing, and translational regulation. Conclusions: Compartment-specific CD138 expression identifies an aggressive breast cancer phenotype with distinct transcriptomic features. This phenotype may have prognostic value and warrants validation using larger, independent cohorts.