Identifying the prognostic signature in the training set
Following the aforementioned inclusion criteria, a total of 487 and 579 patients with CRC was enrolled as the training (TCGA) and validation (GEO) set, respectively. The clinical characteristics was presented in Table 1. The median age of training set was 68 years. At the time of enrollment, most patients were alive (77.8% in the training and 66.6% in the validation set), the median surveillance time was 699 and 1582 days in the training and validation wing, respectively. Most patients don’t have lymph node involvement (stage I to II). Most patients have tumor infiltration to the subserosa and serosa level (80.5% and 85.6%, respectively).
[insert Table 1 here]
After cross-comparing with ImmPort database, 2112 immune-related gene were selected. Then, repeated, not or inconsistently expressed genes were excluded, leaving 1684 genes as candidates. For each gene, univariate COX regression was performed and 143 suggested significant protective or risk effect (Fig 2a & Table S1). Via RSFVH, nine immune-related genes stood out as independent prognostic predictors (Fig 2b).
Next, to explore the best predictive signature and to preclude over-fitting, we formed a penal of full-size combinations of these nine genes (29-1=511 combinations, Table S2). Using the risk formula previous discussed, 511 possible predictive signatures were calculated. Each signature’s performance was verified via time-ROC curve. The AUC value of each combination was rated. A combination of the following gene CCL22, LIMK1, MAPKAPK3, FLOT1, GPRC5B, IL20RB was screened out with the highest prediction precision (AUC = 0.746, Fig 2c and d). Their hazard ratio (HR) and p value is listed in Table 2. Thus, the designated risk model was: Risk score = (-0.421× expression value of CCL22) + (0.402 × expression value of LIMK1) + (-0.465× expression value of MAPKAPK3) + (0.599× expression value of FLOT1) +(0.613× expression value of GPRC5B) +(0.596× expression value of IL20RB).
[insert Figure 2 here]
[insert Table 2 here]
The validation of performance in predicting overall survival
Using the algorithm, a risk score was calculated for individuals. Kaplan-Meier (KM) test was performed to verify survival difference between high (n=243) and low risk (n=244) population divided by median risk-score-value, the method was consistent with other studies24, 25. As is demonstrated in Fig 3a, significant longevity was observed in low-risk patients in training wing (p<0.001). The median survival time was 8.46 years for low-risk patients versus 4.12 years in high-risk population. To explore this in another independent population, the same methodology was then adopted in the external validation set with relatively larger sample pool (Fig 3b). Again, the model showed significant differentiation capability (p<0.001).
When the training and validation array were sorted by risk score, clusters in gene expression level and survival information could be observed in Figure 3c and d, respectively. Indeed, genes with adverse prognostic effect, namely LIMK1, FLOT1, GPRC5B and IL20RB, showed consistent elevation in expression in high-risk population.
[insert Figure 3 here]
The relationship between the signature and clinical characteristics
We further explored the potential relationship between gene signature and clinical characteristics in TCGA and GEO database (Table 3). Neither patients’ age (stratified by 68 years) nor gender showed co-relation with gene signature via Pearson’s Chi square. Rather, tumors’ TNM staging was significantly advanced in high-risk population (p<0.001). In univariate COX regression, old age (> 68), more advanced tumor stage (stage III and IV) and immune-related gene signature revealed statistical significance and was confirmed as independent adverse predictor via the following multivariate COX regression. In both training and validation pool, the gene signature suggested great predictive potential regarding patients’ clinical outcome. (High- vs. Low-risk, HR training =4.56 95% CI 2.81-7.40, P < 0.001, n=192; HR test = 4.53, 95% CI 2.79–7.34, P < 0.001, n=193, Table 4).
[insert Table 3 here]
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Comparison of predictive performance with other clinical variables
The model’s performance was competed against tumor TNM staging in predicting clinical outcome. To this end, time-ROC curves in the training and validation database were constructed to compare both models (Fig S1). In the training set (Fig S1a), the C-index was 0.746 (95CI: 0.695-0.796). While in the test set (Fig S1b), the C-index was 0.622 (95CI: 0.574-0.670). Tumor staging, which is most used for clinical prognosis expectancy, was borrowed as contrast. As was indicated in Supplement Fig S1, in both wings, the six-gene signature yielded superior accuracy against traditional staging.
Exploring the high-dimensional functions of IRGs
To bring new perspective into the biological importance of the obtained IRG panel, we conducted GO analyses based on A total of 355 relative CEGs which were selected using online library. Via GO analyses (Fig 4a), the CEGs were most significantly involved in metabolic process and immune regulations (GO terms, such as multiple cellular components biogenesis and processing, leukocyte cell-cell adhesion, regulation of T cell proliferation and interleukin). The findings were later mirrored in KEGG analysis, where the CEGs were actively related to “Primary immunodeficiency”. We then explored the CEGs on the level of proteins using PPI analysis (Fig 4b) for potential treatment targets. From the major continent of the network, we could tell that a good number of proteins (like UBA52, NHP2 and FBL) played as junctions (hubs).
[insert Figure 4 here]
Additionally, we conducted leukocyte subsets analysis using CIBERSORT. The patients were sorted by the risk-value and the detailed proportion of twenty-two immune cell in each patient was achieved. The map of subsets is in Supplement Fig S2 and the statistical significance of TIIC in the two groups were given in Supplement table 3. Unfortunately, no significantly variated TIIC subset was discovered.
Development of a predictive gene-clinical nomogram for clinical outcome
To facilitate comprehensive outcome prediction, the six-gene prediction model was combined with clinical independent risk factors, namely tumor stage and age, and transformed into a predictive nomogram (Fig 5a) to provide a straightforward estimation of survival at 1,3,5-year. For instance, an old-aged (> 68 years) advanced-staged (stage III to IV) patient with a gene-signature value of four would have a total risk score of roughly 60, and the odds of survival would be 80, 55 and 35 percent. Via time-ROC (Fig 5b), the AUC value in the training group at 1,3 and 5 years were 0.822 (95CI: 0.761-0.883), 0.835 (95CI: 0.775-0.895) and 0.798 (95CI: 0.715-0.881), respectively. The nomogram was tested for performance in the GRO database, which yielded overall comparable precision: the AUC at 1,3 and 5 years were 0.707 (95CI: 0.622-0.792), 0.692 (95CI: 0.641-0.744) and 0.681 (95CI: 0.628-0.733) (Fig 5c), respectively. It could be judged from the nomogram that the six-gene signature was the most prominent factor affecting patients’ survival and the performance was consistent over various time points.
To access how this nomogram mimics real situation, calibration curves using 1000-time bootstrap test were plotted. As is shown in Fig 5d, in the training data set, the nomogram presents excellent calibration. What’s more, in the external calibration (Fig 5e), the calibration curve showed minor wobble, but still in the near proximity of the 45-degree-dashed line. These suggested that our nomogram close mimics real-life situation, and via calibration in two different large-scale databases, the nomogram showed great utility.
[insert Figure 5 here]