Identification of DEGs and enrichment analysis in training cohort
The Kaplan-Meier survival analysis showed that the OS of carcinoids was significantly superior to that of LCNEC and SCLC (p < 0.001), while there was no statistically distinction between LCNEC and SCLC (p = 0.131) (Supplementary Fig. 1). 314 DEGs out of 20183 genes were screened out in the training cohort by using ‘edgeR’ package in R with P value < 0.05 and FDR < 0.05. For biological process, GO analysis indicated that those DEGs enriched mostly in (Fig. 1A). For cellular component and molecular function, the DEGs enriched mostly in (Fig. 1B) and (Fig. 1C). In addition, KEGG pathway analysis indicated that the DEGs were mainly enriched in (Fig. 1D).
Construction Of Prognostic Risk Model In Training Cohort
The univariate cox regression analysis of 314 DEGs in 40 high-grade LNET patients of training cohort screened out 75 prognosis-related DEGs. Subsequently, we used the least absolute shrinkage and selection operator (LASSO) method to further filter the prognosis-related DEGs, resulting in a model with 8 genes (SLC7A5, MEST, RAD51AP1, PMAIP1, GPER1, OIP5, IGLL3P, CDCA7) (Fig. 2A, 2B). Finally, a prognostic risk model was built as follows: risk score = SLC7A5 * (0.0975810777298566) + MEST * (0.141016923410625) + RAD51AP1 * (0.0738289697839021) + PMAIP1 * (0.00581195941306771) + GPER1 * (-0.24049872761534) + OIP5 * (0.0729507404044556) + IGLL3P * (-0.0979494191126777) + CDCA7 * (0.0117057735690952). The expression of SLC7A5, MEST, RAD51AP1, PMAIP1, OIP5, IGLL3P and CDCA7 were upregulated in tumor tissues, while the expression of GPER1 was downregulated in tumor tissues (Fig. 2C-J). The gene expression profiles in high-risk and low-risk groups were shown in a heatmap (Fig. 2K).
40 LNET patients were divided into high-risk and low-risk groups according to the median value of their risk score calculated from the prognostic risk formula. Kaplan-Meier survival analysis showed that OS in the high-risk group was significantly better than that in the low-risk group (p = 3e − 05) (Fig. 3A). To evaluate the predictive value of this prognostic risk model, time-dependent ROC curve analysis was performed and the AUC values of 1-, 3- and 5-year OS were 0.746, 0.926 and 0.917 (Fig. 3B-D).
Validation Of Prognostic Risk Model In Validation Cohort And Lnets Subgroups
The predictive value of this prognostic risk model was validated in the validation cohort composed of the other 40 high-grade LNETs patients. Similarly, by using the median risk score as the cutoff, 40 LNET patients were classified into the high-risk (n = 20) and low-risk groups (n = 20). K-M survival analysis revealed that patients in high-risk group had significantly higher mortality than patients with low risk group (p = 0.012) (Fig. 3E). Moreover, time-dependent ROC curve analysis revealed that the AUC values of 1-, 3- and 5-year OS were 0.758, 0.781 and 0.771 (Fig. 3F-H).
Since this prognostic risk model derived from the DEGs between high-grade LNETs and low-grade LNETs (carcinoids), we further check the predictive ability of the risk model in several subgroups of LNET patients. Based on the risk classification criteria and K-M survival analysis, a higher mortality was observed in LCNEC patients with high-risk, compared with those in low-risk group (p = 0.0015) (Fig. 4A), and the AUC values of 1-, 3- and 5-year OS were 0.715, 0.813 and 0.804 (Fig. 4B-D). Similar results were seen in 21 SCLC patients, with significant survival distinction (p = 0.013) (Fig. 4E) and AUC values of 0.769, 0.926 and 0.974 corresponding to 1-, 3- and 5-year OS (Fig. 4F-H). Interestingly, when this prognostic risk model was applied to carcinoid patient group, which was initially set as control group, a significant worse OS in high-risk carcinoid patients was observed (p = 0.003) (Fig. 4I), with AUC values of 0.957, 0.957 and 0.857 corresponding to 1-, 3- and 5-year OS (Fig. 4J-L). Moreover, when stratified by gene signature risk model, high-grade LNET patients with early stage (stage I and II) disease in high-risk group had significant worse survival (p = 0.047) (Supplementary Fig. 2A), while the gene signature stratified LNET patients with advanced stage (stage III and IV) disease into subgroups with significant distinction (p = 0.037) (Supplementary Fig. 2B).
Identification Of Prognostic Predictor
Due to the great performance of our gene signature risk model in predicting OS, we explored whether this risk model was an independent prognostic predictor in total 80 high-grade LNET patients. Clinicopathological features, including age, gender, TNM stage classification system and this risk model were enrolled in further prognostic analysis. As a result, univariate cox regression analysis showed that pathological stage (p = 0.002), N stage (p = 0.048), M stage (p = 0.024), as well as the risk model (p = 2.81E-05) were significantly associated with OS, while age (p = 0.24), gender (p = 0.16) and T stage (p = 0.067) were not (Table 1). Further multivariate cox regression analysis revealed that our gene signature risk model was the only prognostic predictor (p = 6.95E-04) (Table 1, Fig. 5).
Table 1
Univariate and multivariate cox regression analysis in high-grade LNET patients based on gene signature.
Variables | Univariate analysis | Multivariate analysis |
HR | 95% CI | P-value | HR | 95% CI | P-value |
age | 1.36 | 0.81–2.28 | 0.238 | | | |
gender | 2.09 | 0.75–5.82 | 0.157 | | | |
Pathological stage | 2.28 | 1.35–3.85 | 0.002 | 2.23 | 0.76–6.59 | 0.146 |
T | 1.63 | 0.97–2.77 | 0.068 | | | |
N | 1.73 | 1.00-2.98 | 0.048 | 0.62 | 0.22–1.74 | 0.365 |
M | 3.94 | 1.19-13.00 | 0.024 | 1.75 | 0.48–6.41 | 0.395 |
riskScore | 5.88 | 2.57–13.49 | 2.81E-05 | 4.66 | 1.92–11.35 | 6.95E-04 |
Gene Set Enrichment Analysis
GSEA was performed based on phenotypes of high risk and low-risk in the high-grade LNET patients. Genes in the high-risk group enriched mostly in the KEGG pathways of spliceosome, cell cycle, RNA polymerase, basal transcription factors, nucleotide excision repair and RNA degradation, responding the active tumor cell proliferation and metabolization (Fig. 6).