Identification of RFTN1 as a potential regulator for BRCA metastasis in lymph nodes with better prognosis
To investigate the genes that promote metastasis in BC, we performed WGCNA on bulk RNA sequencing data from BC primary tumor tissues and various metastatic sites like lymph node, lung, liver, bone, brain, skin and so on, using datasets GSE110590 and GSE56493 (Fig. 1A). The heatmap revealed gene modules associated with metastatic sites. Given that lymph nodes are often the first site of metastasis for BC cells, we focused on gene modules related to lymph nodes. Our selection criteria for these modules were a gene significance (GS) score greater than 0.2 and a module membership (MM) score greater than 0.8, leading us to identify the gene RFTN1 (Fig. 1B). RFTN1 expression showed a significant difference between BC tumors and normal tissues from TCGA-BRCA (Fig. 1C). To understand RFTN1’s role in BC, we conducted survival analyses across multiple BC subtypes (Fig. 1D-F). The results indicated significant difference in survival between groups when analyzing all BC samples with subtype basal like and HER2-positive using the median cutoff for all samples. However, no clear differences in survival were observed in Luminal A samples and Luminal B patients, suggesting that RFTN1 is significantly related to the prognosis of BC patients with basal like or HER2-positive subtype (extend Fig. 2C-D). To determine the impact of RFTN1 on BC prognosis, it is essential to identify an optimal cutoff to categorize samples into distinct groups. For clarity, we identified differential genes between two groups grouped by the median of RFTN1 expression in TCGA-BRCA (Fig. 1G), resulting in 899 upregulated genes and nine downregulated genes. Subsequent consensus clustering by these differential genes revealed that the optimal number of clusters (optK) is 2 (Fig. 1H), allowing for the division of TCGA-BRCA samples into two groups (Fig. 1I). A significant difference in RFTN1 expression was observed between the two groups (Fig. 1J), with higher RFTN1 expression in the second cluster. Analysis of overall survival (OS) and progression-free interval (PFI) demonstrated the prognostic significance of RFTN1 between these groups (Fig. 1K-L), suggesting that RFTN1 may be associated with a more favorable prognosis in BC patients. The results indicates that RFTN1 potentially acts as a protective factor in BC.
RFTN1 is related to the changes of immuno system in BC
The relationship between RFTN1 expression and the immune system in BC patients was highlighted by the immune score, which showed a clear correlation (Fig. 2A-C). This suggests that RFTN1 may influence the immune system within TME. Analysis using Cibersort for the two clusters identified in BRCA revealed significant differences (Fig. 2D), with anti-tumor immunity notably suppressed in cluster one, which had lower RFTN1 expression. Conversely, the presence of pro-tumor M2 macrophages was higher in cluster one compared to cluster two.
To further understand RFTN1’s functions, we enriched pathways using the differentially expressed genes previously identified (Fig. 2E). Notably, significant alterations in the epithelial-mesenchymal transition (EMT) pathway and allograft rejection pathway were observed, which could be mechanisms through which tumor cells metastasize. GSVA of these clusters revealed pathway differences that aligned with the Cibersort findings (Fig. 2F), indicating that higher RFTN1 expression is associated with increased activation of anti-tumor immunity in BC. These findings suggest that RFTN1 may play a role in modulating the immune response in the TME, potentially affecting the progression and metastasis of BC.
RFTN1 expressed in multi-kinds of cells and may relate to the active anti-tumor immunity
To elucidate RFTN’s role in TME, we analyzed single-cell RNA sequencing data from BC patients' tumor tissues, specifically from studies GSE176078 and GSE161529. We selected 38 samples that exhibited notable RFTN1 expression and divided them into two groups based on the mean RFTN1 expression across all samples. The expression markers for each cell type were visualized in a bubble plot (Fig. 3A-B). Analysis revealed that RFTN1 is expressed in multiple cell types (Fig. 3C).
Our initial focus was at the cellular level, examining the proportions of various cell types. We compared these proportions between the RFTN1 Higher Expression Group (RHEG) and the RFTN1 lower expression group (RLEG) (Fig. 3D-E). In the RHEG, we observed higher proportions of CD4 + T cells, CD8 + T cells, and plasmacytoid dendritic cells (pDCs), alongside fewer malignant cells and conventional dendritic cells (DCs). This suggests a more robust activation of the immune system in the RHEG, which may target malignant cells more effectively.
To support this hypothesis, we employed CellChat to analyze intercellular communication. We found that both the number and strength of inferred interactions were greater in the RHEG compared to the RLEG (Fig. 3F). Furthermore, molecular signals associated with inflammatory and immune activation, such as BAFF, GAS, MIF, and CXCL, were significantly more prevalent in the RHEG than in the RLEG (Fig. 3G). These findings lead to the conclusion that anti-tumor immunity may be more activated in the RHEG compared to the RLEG, potentially due to the influence of RFTN1.
Correlations exist between RFTN1 and immune cells immigration
To delve deeper into the alterations within various cell types, we utilized CellChat for further analysis. When comparing the RFTN1 Lower Expression Group (RLEG) to other group, we noticed that mesenchymal and endothelial cells exhibited a significant reduction in the number of interactions with other cells, while other cell types showed an increase in interaction numbers (Fig. 4A-B). In the RHEG, there was a noticeable decrease in the strength of interactions received by CD4 + T cells, a trend that was also observed in CD8 + T cells.
To understand these changes, we examined the signaling interactions, observing increased signaling in the RLEG and decreased signaling in the RHEG (Fig. 4C-D). In the RLEG, CD4 + T cells and CD8 + T cells received increased signaling from various sources, including MIF signaling from B cells, CD4 + T cells, dendritic cells (DCs), plasmacytoid DCs (pDCs), and MDK from malignant cells, cycling cells, and DCs. This suggests a potential activation of cell-mediated immunity directed towards these cells (Fig. 4C). Conversely, the decrease in signaling to CD4 + T cells and CD8 + T cells in the RLEG from multiple cell types involved molecules like MIF, LGALS9, and CXCL12 (Fig. 4D).
Given the overall trend of decreased CD4 + T cell and CD8 + T cell populations in the RLEG, it appears that the recruitment of these cells is downregulated in this group. Further analysis of chemotactic factors (CFs) across all cells between the two groups revealed higher expression levels of CFs (Fig. 4F-G). This leads to the conclusion that RFTN1 is associated with increased recruitment of CD4 + T cells and CD8 + T cells, potentially enhancing the anti-tumor immune response in the TME.
RFTN1 may brought higher immunity activation
To explore RFTN1's functions at the molecular level within the RHEG and RLEG, we applied hdWGCNA. This analysis identified 15 gene modules, with modules M6, M1, M4, M5, M3, M13, M14, M7, and M12 showing higher correlation with expression in the RLEG, whereas the remaining modules were more associated with the RHEG (Fig. 5A-B). Notably, the M3 module was found to be highly expressed in CD4 + T cells and CD8 + T cells (Fig. 5C).
Focusing on the M3 module, we enriched pathways to understand its role (Fig. 5D). The significantly altered pathways were related to the activation of the immune system. Further analysis, including immune response gene set enrichment analysis (irGSEA) of CD4 + T cells and CD8 + T cells, along with the M3 module, indicated a pronounced suppression in these T cell types (Fig. 5E-F). This suppression could be attributed to the reduced chemotactic factor (CF) reception by CD4 + T cells and CD8 + T cells.
With T cell activation, the programmed apoptosis of tumor cells is expected to follow. Indeed, the proportion of malignant cells in the RHEG was found to be lower than in the RLEG, as previously observed (Fig. 3E). Subsequent irGSEA of tumor cells within these groups revealed that pathways related to cell apoptosis, such as those involving IFN-α, IFN-γ, and apoptosis processes, were significantly enhanced in the RHEG (Fig. 5G).
Further examination of the Gene Ontology (GO) enriched pathways within the M3 module highlighted significant roles in signal and protein transportations, shedding light on RFTN1’s function at the molecular level. In summary, the increased activation of T cells in the presence of higher RFTN1 expression may lead to the programmed apoptosis of breast tumor cells, highlighting RFTN1’s potential role in enhancing the anti-tumor immune response.
RFTN1 may regulate immunity system by controlling transform function in cell
RFTN1 is known to be associated with receptor internalization27, suggesting it may have similar roles in the BC TME. To understand its function, we identified significant gene expression differences between the RHEG and RLEG, finding 610 upregulated and 544 downregulated genes (Fig. 6A). Pathway enrichment analysis of these differentially expressed genes highlighted functions related to protein transport, endocytic vesicle membrane, and organelle inner membrane in the Gene Ontology Cellular Component (GO CC) category, as well as passive transmembrane transporter activity in the Gene Ontology molecular function (GOMF) category (Fig. 6B-C). These findings suggest RFTN1’s involvement in molecular signaling processes.
GSEA analysis of the differentially expressed genes between the RHEG and RLEG (Fig. 6D-F) revealed that upregulated pathways in the RHEG are indicative of immune activation. These pathways include increased recognition of signaling factors and transmembrane signaling, while downregulated pathways are associated with reduced energy metabolism. This pattern supports the hypothesis that RFTN1 may function as a receptor transfer carrier, potentially activating the immune system in the TME by facilitating the transfer of receptors in immune cells.