Lineage Tracing and Single-Cell RNA Sequencing Reveal a Common Transcriptional State in Breast Cancer Tumor-Initiating Cells Characterized by IFN/STAT1 Activity.

A tumor cell subpopulation of tumor-initiating cells (TIC) or "cancer stem cells" is associated with therapeutic resistance, as well as both local and distant recurrences. Signal transducer and activator of transcription (STAT) activity is elevated in TICs in claudin-low models of human triple-negative breast cancer, which enables enrichment of TICs using a STAT-responsive reporter. Lineage tracing of TICs as they undergo cell state changes could enable a better understanding of the molecular phenotypes of TIC and uncover strategies to selectively target TICs. In this study, we developed a STAT-responsive lineage-tracing system and used it in conjunction with the original reporter to enrich for cells with enhanced mammosphere-forming potential. This approach was able to detect TICs in some, but not all, basal-like triple-negative breast cancer xenograft models, indicating that STAT signaling has both TIC-related and TIC-independent functions. Single-cell RNA sequencing (RNA-seq) of reporter-tagged xenografts and clinical samples identified a common IFN/STAT1-associated transcriptional state in TICs that was previously linked to inflammation and macrophage differentiation. Surprisingly, most of the identified genes were not present in previously published TIC signatures derived using bulk RNA-seq. Finally, bone marrow stromal cell antigen-2 was identified as a cell surface marker of this state that functionally regulated TIC frequency. These results suggest that TICs may exploit the IFN/STAT1 signaling axis to promote their activity and that targeting this pathway may help eliminate TICs. Significance: Coupling single-cell transcriptomics with tumor-initiating cell enrichment identified IFN response gene expression not previously reported in bulk RNA-sequencing-derived signatures and proposed IFN/STAT1 signaling as a candidate therapeutic target in breast cancer.