In this study, by analyzing the clinical case characteristics and survival prognosis of 279 OSCC samples within the TCGA database grouped according to whether or not PNI occurred, we found that PNI was detected more frequently in tumors of patients at T3 and T4 stages or Stage III and Stage IV, indicating that tumor cell invasion of the tumor microenvironment within the tumor microenvironment is more likely to occur when oral cancer tumors develop into intermediate or advanced stages of the neural tissue. With the increased study of neural tissue in cancer microenvironment, new theories of nerve-cancer crosstalk have now been proposed[16]. Whether this phenomenon occurs as an inevitable phenomenon that occurs passively as the tumor grows, invades more normal tissues, and potentially comes into contact with more nerve tissues, or whether it is the result of active invasion of nerve tissues by tumor cells with higher invasive ability, has not been reported in the literature, and further research is underway in this study. In the survival prognostic analysis, we found that the overall survival time was significantly shorter in samples that developed PNI compared with those that did not, a result that is consistent with the conclusion of most studies that PNI is one of the poor prognostic factors for oral squamous carcinoma. This may be due to the fact that nerve infiltration makes it easier for tumor cells to migrate and spread along the nerve bundles, increasing the risk of distant metastasis[17; 18; 19]. Also, nerve infiltration may affect the tumor's response to treatment, making it less effective.
The results of the two groups of samples in terms of immune score, stroma score, and ESTIMATE Score showed significant differences between the two groups in terms of stroma score, whereas the differences in terms of immune score and ESTIMATE score were not significant. This result may imply that nerve infiltration mainly affects the interaction between the tumor and the stroma, while the effect on the immune system may be relatively minor. However, this still needs to be confirmed by further studies.
In addition, we mapped the dataset of OSCC samples to inhibitor-treated melanoma samples using the SubMap algorithm and preliminarily predicted the differences in PNI subgroups on immunotherapy. The results showed that nerve-infiltrating samples were more sensitive to CTLA4 inhibitors, while sensitivity to PD-1 inhibitors did not reach a significant level. And the proportion of T cells follicular helper was significantly higher in the PNI- samples than in the PNI + samples, and only macrophages, especially the M2 tumor-associated macrophages was significantly higher in the PNI + OSCC. CTLA-4 is one of the important targets for tumour immunotherapy. By blocking the CTLA-4 signalling pathway, the inhibitory state of T cells can be lifted and the anti-tumour immune response can be enhanced[20]. For example, Yervoy (Ipilimumab) manufactured by Bristol-Myers Squibb is the world's first approved and marketed CTLA-4 antibody drug, which has been used for the treatment of a variety of cancers[21]. Combining these 2 results, we can speculate that CTLA-4 can up-regulate T cells in PNI + OSCC tumours and thus have an immunotherapeutic effect. This finding provides clues for developing personalized immunotherapy strategies for patients with different subtypes of oral squamous carcinoma. However, it is important to note that these predictive results are only preliminary explorations and further clinical trials are needed to validate their effectiveness.
We extracted the differentially expressed genes and analyzed the functional enrichment of the differentially expressed genes between the PNI + samples and the PNI- samples using the limma package. The results showed that there were significant gene expression differences between the two groups of samples, and that these differential genes were mainly involved in aspects of muscle structure and movement. We selected the ACTA1 gene, which was significantly highly expressed in the PNI-positive group, for validation. Actin is a key structural protein that constitutes the cytoskeleton and plays a role in functions such as cell division, migration and vesicular transport[22]. It consists of six cell type-specific isoforms: ACTA1, ACTA2, ACTB, ACTC1, ACTG1, and ACTG2. Abnormal expression of actin isoforms has been reported in a number of cancers. ACTA1 is a gene encoding actin α1, which is the major isoform found in skeletal muscle and is essential for muscle contraction[23]. High expression of ACTA1 has been reported to be associated with shortened survival in oral squamous cell carcinoma[24]. In addition, in basal-like breast cancer, ACTA1 is a biomarker associated with chemotherapy resistance. There are many possible mechanisms by which ACTC1 protein promotes tumorigenesis. One may be through annexin, which is a Ca2+-dependent phospholipid-binding protein that plays a role in vesicle trafficking, cell proliferation, and apoptosis[25]. Annexin expression has been associated with shortened survival, tumorigenesis, and progression of malignant ovarian cancer[26]. Notably, annexin physically interacts with ACTA1[27]. In pancreatic ductal adenocarcinoma, ACTA1 expression is a feature of cancer-associated fibroblasts[28]. Stromal progenitor cells and fibroblast-like cells show increased ACTA1 expression when they become cancerous[29]. Coincidentally this change in cell morphology also occurs during the epithelial-mesenchymal transition when cells undergo malignant transformation and confers a more migratory phenotype. Immunohistochemical experiments verified that high expression of ACTA1 was observed in tumor tissue samples undergoing PNI, and also the prognosis was worse in the ACTA1high group, which was largely consistent with previous bioinformatics predictions and reports by other scholars. Although there was no statistically significant difference in T stage and LNM between the 2 groups of patients, there was still a tendency for a higher proportion of T stage 3–4, and the reason for the lack of a statistically significant difference may be related to the sample size.
Based on the function of actin α1 protein, we hypothesized that after tumor cells invaded nerve tissues, the invaded nerves could up-regulate the ACTA1 expression of tumor cells, which led to epithelial-mesenchymal transition of the cells, promoting invasion and metastasis of the tumor cells, resulting in a worse prognosis. However, the specific regulatory mechanism may be related to the neurotransmitters secreted by nerves, ACTA1 protein affecting cytoskeletal structure, intercellular junctions, or extracellular matrix degradation, which has not yet been reported in detail and still needs to be further explored.
In summary, this study revealed the clinical features and prognostic significance of nerve infiltration in oral squamous carcinoma by analyzing OSCC samples within the TCGA database, and initially explored its relationship with immunotherapy sensitivity and gene expression. What’s more, PNI + OSCC patients with up-regulated of Actin α1 could benefit from cytotoxic T cell-mediated immunotherapy. However, these results are only preliminary explorations, and further studies are needed to validate and deepen our understanding of nerve infiltration in oral squamous carcinoma.