Previous studies have shown that OS is closely associated with aberrant genetic and epigenetic changes, leading to abnormal expression of oncogenes or methylation of antioncogenes [24]. Meanwhile, a large number of studies have confirmed the reliability of clustering subtype analysis of solid tumors based on methylation sites [25–27]. In this study, 33519 methylation sites were found to be statistically associated with prognosis in OS patients. Subsequently, we categorized OS into three clustering subtypes based on prognosis-related methylation sites, with the C3 subtype possessing the poorest prognosis of the three subtypes, according to the survival analysis. Meanwhile, we observed that the C3 subtype had the lowest CD8 T cell infiltration, but the highest macrophage M1 infiltration. Immune infiltration in the tumor serves a crucial function in tumor progression [28]. Previous studies have linked an increase in the number of CD8 T cells infiltration tumors to a favorable anti-tumor immune response, suggesting that it could be used as a prognostic indicator [29, 30], and that CD8 T lymphocyte infiltration improves the prognosis of patients with osteosarcoma [31]. Macrophages, which are polarized M1 and M2 macrophages, are a prominent component of the tumor microenvironment [32]. The formation of the tumor microenvironment encourages M1 to M2 conversion. Early tumor tissue is dominated by M1 macrophages, which limit angiogenesis and stimulate tumor immunity [33]. Contrary to expectations, we discovered that M1 macrophages of the C3 subtype exhibited the highest infiltration abundance among the three subtypes. In osteosarcoma, Buddingh et al. discovered that macrophages' anticancer efficacy outweighed their probable tumor-supporting role [34]. Tumor-infiltrating lymphocytes (TILs) have been shown to have a profound impact on the prognosis of a variety of cancers [35–37]. Previous studies have utilized the methylation signature of TILs compared to gene expression-based immune markers to measure TILs distribution and to predict survival and tumor immune response [38]. Therefore, we evaluated the differences in MeTIL scores among the three osteosarcoma subtypes. We observed significantly lower scores for the C3 subtype than for the other two subtypes. In general, elevated levels of tumor-infiltrating lymphocytes are thought to be associated with a better prognosis. In particular, the synergistic effect of tumor stromal and TILs has an enhanced prognostic impact [39, 40]. Based on the TIDE website, we evaluated the efficacy of immunotherapy against three OS subtypes, and the results suggested that the C3 subtype responded well to immunotherapy. TIDE has shown potential in published clinical trials for predicting patient response to immunotherapy, but more studies are needed to prove its utility in OS.
Next, we screened for genes that were differentially expressed between the C3 subtype and other subtypes. The pathway enrichment analysis suggested that these DEGs were substantially related with hypoxia pathway activation and oxidative phosphorylation pathway inhibition. Studies have shown that patients with lower oxygen levels are not only more tolerant to radiation than patients with higher oxygen levels, but also have a greater likelihood of recurrence [41]. Hypoxia is also linked to chemotherapeutic resistance[42, 43]. Anti-angiogenic medications cut off the tumor's blood supply, causing or exacerbating a hypoxic microenvironment in the tumor, which is the primary cause of resistance to anti-angiogenic therapy after the first response, resulting in treatment failure [44]. This treatment-induced hypoxia has been proven to be the catalyst for secondary anti-vegf therapeutic resistance [45, 46]. Thus, we speculate that patients with C3 subtype have a worse prognosis, which could be connected to chemotherapy resistance due to increased hypoxia pathway activation.
We obtained 200 hypoxia pathway genes from the MSigDB database, and screened 12 genes by univariate and multivariate COX regression analysis to construct HRPM and calculate the risk scores of the patients. According to the median risk score, patients were separated into high and low risk groups, and we found that patients in the high risk group had a lower overall survival rate. Furthermore, the ROC analysis revealed that the risk score model is highly reliable. We externally validated the model in the GSE21257 dataset and also obtained more satisfactory results. In a multivariate regression analysis, we included age, sex, metastasis, and risk score, and the result indicated that risk score could be an independent prognostic factor. Notably, CD8 T cell infiltration was markedly lower in individuals with high risk scores than in patients with low risk scores. Furthermore, patients in the high-risk group had lower immune score and higher tumor purity than those in the low-risk group.
Overall, in this study, we proposed a novel 12-gene signature for OS prognosis encompassing ANXA2, CASP6, CAVIN1, DCN, FAM162A, HMOX1, MAFF, PDK1, RBPJ, SDC3, STC2 and TES, which can be used to predict high-risk groups of patients with osteosarcoma. The contribution of HRPGs as biomarkers of tumor progression as well as potential therapeutic targets might be confirmed by further studies.