Construction of Weighted Gene Co-expression Modules
First, combining the mRNA data in TCGA-glioma and GTEx database, 10, 550 differentially expressed genes between glioma and normal brain tissue were detected. Among these genes, 260 amino acid metabolism related genes were ensured (Figure S1A). KEGG and GO analysis confirmed that these nods were mainly related to the biological function and pathway of amino acid metabolism, PPI network showed a strong co-expressed correlation among the genes (Fig. S1B-D). Then, samples in the training set were hierarchically clustered in immunity-high (Immunity-H) or immunity-low (Immunity-L) group by ssGSEA (Figure S2A, B). Boxplot of immune cells’ fraction in glioma tissues were significantly different among immunity-H and immunity-L groups (Figure S2C). Consistently, stromal scores, immune scores and ESTIMATE scores of glioma samples in the Immunity-H group remarkably increased compared with those in the Immunity-L group (Figure S2D-F), meanwhile, the tumour purity in Immunity-H was significantly lower than in Immunity-L group (Figure S2G). Besides, patients in Immunity-H group had a significantly poorer prognosis than other groups (Figure S2H).
To find the correlation between amino acid metabolism-related genes and immune infiltration in TCGA-glioma, the gene co-expression networks were constructed from the TCGA glioma datasets with the WGCNA package. 2 modules in the TCGA-glioma were recognized and assigned different colors with each module (Fig. 2A). Then, we created a heatmap of module-immune relationships to evaluate the association between each module and different immune score (high and low). The results of the module-immune relationships showed that the grey module had the highest association with immune-high group (pink module: r = 0.27, p < 0.001) in TCGA-glioma (Fig. 2B). The module membership and gene significance were highly correlated in grey module (Fig. 2C).
Identification of a 12-gene risk signature associated with amino acid metabolism and immune in glioma
To identify the amino acid metabolism-related and immuno-associated risk signature, the univariate Cox regression analysis was used to select 30 genes in the training set, which were related with the prognosis of patients (Fig. 3A). Thereafter, the most relevant biomarkers for prognosis were identified through the LASSO Cox regression model and overfitting was counteracted by ten-fold cross-validation. As a result, the group of 12 genes (PSMC5, GLUD1, DHTKD1, OGDH, PSMF1, PSMD3, PSMB8, PSMB9, PSMD5, PSMD12, PSMC1, PSMD6) were extracted according to LASSO coefficients (Fig. 3B, C). The median of risk score was defined as the cutoff value to divide training set into two subgroups including high-risk and low-risk group, and the significant difference was found in both molecular and clinical characteristics between these subgroups (Fig. 3D).
At the same time, there were significant differences according to the risk signature values of age-stratified and WHO grade-stratified clinical samples in both TCGA and CGGA cohorts (Fig. 4A, B, E, F). The molecular pathological diagnosis of glioma has been put forward in clinical practice, IDH wild type and 1p19q non-codeletion gliomas were all the poor prognostic factors and had an inadequate response to traditional radiotherapy or chemotherapy of glioma patients [24]. Such being the case, the distribution of the 12-gene signature was explored based on IDH status-stratified clinical samples were (Fig. 4C, G) and 1p/19q codeletion status (Fig. 4D, H). Overall, these results indicated that the risk score base on the gene signature was significantly associated with clinical features.
Development of the Risk Score Signature and Assessment of the Predicting Capacity
Based on groups of high-risk and low-risk score, Kaplan-Meier analysis was performed and showed that patients with high-risk score had dramatically reduced overall survival comparing with patients with low-risk score in both TCGA and CGGA dataset (Fig. 5A, B). Besides, as far as 1-year, 3-year and 5-year overall survival, the values of area under the curve (AUC) of ROC curve for TCGA glioma cohort were 0.875, 0.933 and 0.854. Consistently, with regard to 1-year, 3-year and 5-year overall survival, the values of area under the curve (AUC) for CGGA cohort were 0.641, 0.678 and 0.687, respectively (Fig. 5C, D). Furtherly, the plots were listed to show the distributions of gene expression, risk score and survival status basing on the amino acid metabolism related- and immune-related signature in TCGA and CGGA (Fig. 5E, F). To furtherly explore the significance of our model in evaluating prognosis independently, we performed univariate analysis as well as multivariate analysis, which showed that the value of the risk score might defined as an independent factor to evaluate the prognosis of glioma patients in both TCGA and CGGA (Table 1).
Table 1
Univariate and Multivariate Cox Regression Analyses of Clinicopathologic Characteristics Associated with Overall Survival in TCGA dataset and CGGA dataset
|
TCGA Dataset
|
|
CGGA Dataset
|
|
Univariate analysis
|
|
Multivariate analysis
|
|
Univariate analysis
|
|
Multivariate analysis
|
Variables
|
HR
|
95%CI
|
P-value
|
|
HR
|
95%CI
|
P-value
|
|
HR
|
95%CI
|
P-value
|
|
HR
|
95%CI
|
P-value
|
Grade
|
4.987
|
3.873–6.421
|
< 0.001
|
|
2.059
|
1.530–2.772
|
< 0.001
|
|
2.635
|
2.353–2.952
|
< 0.001
|
|
2.079
|
1.820–2.374
|
< 0.001
|
Gender
|
1.011
|
0.747–1.368
|
0.944
|
|
0.974
|
0.710–1.336
|
0.870
|
|
1.022
|
0.870–1.202
|
0.787
|
|
1.020
|
0.866-1.200
|
0.815
|
Age
|
4.863
|
3.391–6.975
|
< 0.001
|
|
2.296
|
1.465–3.596
|
< 0.001
|
|
1.922
|
1.638–2.256
|
< 0.001
|
|
1.227
|
1.033–1.457
|
0.020
|
IDH mutation status
|
0.090
|
0.063–0.129
|
< 0.001
|
|
0.553
|
0.306–1.001
|
< 0.051
|
|
0.367
|
0.315–0.428
|
< 0.001
|
|
0.657
|
0.558–0.773
|
< 0.001
|
1p19q codeletion status
|
0.217
|
0.128–0.370
|
< 0.001
|
|
0.543
|
0.287–1.025
|
0.060
|
|
0.527
|
0.447–0.621
|
< 0.001
|
|
0.781
|
0.661–0.923
|
0.004
|
Risk Score
|
9.425
|
6.355–13.978
|
< 0.001
|
|
2.672
|
1.510–4.729
|
< 0.001
|
|
2.448
|
2.076–2.887
|
< 0.001
|
|
1.364
|
1.131–1.646
|
0.001
|
Abbreviations: HR, hazard ratio; CI, confidence interval. |
Combination of the risk signature and clinicopathological features improves risk stratification and survival prediction
To better enhance risk stratification of prognosis, we constructed a decision tree through patients with different grade of glioma from TCGA. As a result, the difference of overall survival was obviously observed in subgroups with different risk score (Fig. 6A). Developing individualized treatment for individual glioma patients is necessary, consistently, accessing the potential risk and prognosis for individual glioma patients is also important. Consequently, we built a nomogram with risk score as well as clinic-pathological features including IDH mutation and 1p19q (Fig. 6D). Besides, the calibration analysis was performed to elevate the accuracy of our nomogram. And the results showed the prediction line of the nomogram was extremely close to the ideal performance (45-degree dotted line) (Fig. 6B, C).
The Differences in Immunocyte Infiltration Degree and enrichment plots of immune related gene sets from gene set enrichment analysis between High- and Low-Risk TCGA Cohorts
Next, to explore whether our risk score partly accessed the immune status of tumor microenvironment, the relationship of amino acid metabolism and immune related gene signature with the immunocyte infiltration degree was explored in gliomas. Interestingly, our results indicated that M2 (Cor = 0.31; p = 8.8e − 6) and Tregs (Cor = 0.169; p = 0.0093) were obviously positive related to risk score (Fig. 7A, B). Furtherly, NK cells (Cor = -0.39; p = 1.9e − 08) and CD4 + T cells (Cor = -0.24; p = 0.00058) (Fig. 7C, D) showed negative correlation with the risk score.
Immunotherapy is increasingly becoming an important part of tumor therapy and can significantly improve the prognosis of cancer patients in a variety of solid tumors [25]. Hence, we detected the expression of immune checkpoint in subgroups with high- or low-risk score. According to our gene model, the expression level of PD- L1, PD-1 and CTLA-4 was lower in the glioma patients with high-risk score (P < 0.05) (Fig. 7E, F). This result showed that high risk score group may be less sensitivity to immunotherapy. Furtherly, glioma with high-risk score was obviously enriched in downregulation of the effect immunity pathway, for instance, negative regulation of T cell migration and tumor necrosis factor function (Fig. 7G, H), which indicates an immunosuppressive microenvironment.
Identification of Hub Genes from risk signature as biomarkers in glioma
The PPI network among the overlapped genes was established by using the STRING database and performed GO and KEGG (Figure S3). MCC algorithm of CytoHubba plugin was used to select hub genes of PPI network and the hub genes were listed in TableS1. Basing on the MCC scores, we selected the top ten highest-scored genes from hub genes, including ODC1, OAZ2, PSMD2, PSMD12, PSMC1, PSMC5, PSMD3, PSME3, PSMD10, PSMD5. The expression levels of these genes were verified according to TCGA database. Kaplan–Meier plotter and the expression level of the top ten genes were performed as shown in Figure S4 and FigureS5. Then, we performed multivariate Cox analysis to evaluate the prognostic value of these genes in gliomas (Fig. 8A and TableS2). In addition, we performed RT PCR in our own clinical samples which includes 6 normal brain tissues, 24 WHO Grade II and 55 GBM samples. Consistently, we found that the expression level of PSMD3 was positive with the grade of glioma, inversely, the expression level of PSMC5 was negative with the grade of glioma (Fig. 8B, D). Moreover, the results of Kaplan–Meier analysis indicated that PSMD3 was significantly associated with worse overall survival of the glioma patients (P < 0.05) (Fig. 8C), conversely, PSMC5 was significantly associated with better overall survival of the glioma patients (P < 0.05) (Fig. 8E). All the results confirmed that PSMD3 and PSMC5 can be identified as potential biomarkers in glioma patients.