Identification of hub module by constructing WGCN
Bioinformatics analysis of publicly available datasets was performed according to the flow chart shown in Fig. 1. First, WGCNA of 471 SKCM samples with corresponding clinical data from TCGA cohort was carried out on 1399 extracted OS genes. As shown in Fig. 2A, six clinical characteristics of patients with SKCM, including overall survival state, overall survival time, age, gender, tumor stage, and metastasis, were included for analysis. To construct a scale-free network, β = 3 (scale-free R2 = 0.89) was selected as the soft threshold (Fig. 2B), and a total of four co‑expressed modules were identified (Fig. 3A–B). Subsequently, each module was assigned a different color to identify the module most associated with overall survival of SKCM. Among all modules, the turquoise module was specifically positively associated with tumor survival (p < 0.05). Interestingly, the 695 genes in the turquoise module also had the closest associations with SKCM metastasis (p < 0.05, Fig. 3C–D). Thus, the turquoise module was identified as the key module of interest in TCGA cohort.
Functional enrichment analysis
GO enrichment analysis suggested that the 695 OS genes were mainly enriched in the BP category, related to response to oxidative stress, oxygen levels, nutrient levels, hypoxia, and neuron death (Fig. 4A). In terms of MF, the key module was notably enriched in protein serine/threonine kinase activity, coenzyme binding, and oxidoreductase activity acting on NAD(P)H. With regard to the CC category, the key module was significantly enriched in mitochondrial matrix, neuronal cell body, and axon part. Gene symbols and their interaction with the enriched functions in GO are shown in Fig. 4B. In addition, the results of KEGG enrichment analysis indicated that the 695 OS genes in the turquoise module were mainly enriched in pathways of Hepatitis B, pancreatic cancer, fluid shear stress and atherosclerosis, chronic myeloid leukemia, and Kaposi sarcoma-associated herpesvirus infection (Fig. 4C). OS genes enriched in these pathways are displayed in Fig. 4D.
Screening of prognosis-related OS genes and construction of a genetic risk score model for patients with SKCM
Twenty-six highly relevant OS genes were screened from the turquoise module (Fig. 5A–B). Subsequently, transcription expression levels of these genes were compared between SKCM samples and normal tissues (Fig. 5C). Ultimately, 23 significantly differentially expressed OS genes were identified as candidate SKCM metastasis- and prognosis-associated genes. These genes further underwent univariate Cox regression analysis to identify nine OS genes with p < 0.05 (Fig. 6A). Thereafter, a multivariate Cox proportional hazards regression model was constructed (Fig. 6B), and four OS genes (AKAP9, VPS13C, ACSL4, and HMOX2) were ultimately selected for calculation of the genetic risk score. Meanwhile, all patients with SKCM in TCGA or GSE65904 cohorts were separated into low- and high-risk groups according to the median risk score (Fig. 6C–D).
Evaluating prognostic value of hub OS genes in patients with SKCM
First, we extracted the expression value of each key gene from TCGA cohort and drew a violin plot and heatmap. As shown in Fig. 7A–B, ACSL4 and HMOX2 were significantly overexpressed in SKCM samples, while expression of AKAP9 and VPS13C was decreased in comparison to that in normal tissues. Similar results were validated by analyzing the protein expression level of the key OS genes in accordance with the immunohistochemistry results from the HPA database (Fig. 7C–F). Subsequently, Kaplan-Meier analysis was also carried out to investigate the prognostic value of the four OS genes in SKCM. As shown in Fig. 8A–D, increased overall survival was significantly associated with elevated expression of AKAP9 (p = 0.004), VPS13C (p = 1.43e-04), and ACSL4 (p = 0.042); however, HMOX2 (p = 6.374e-05) expression had the opposite effect on the prognosis of patients with SKCM. These results indicated that our four identified OS genes were significantly correlated with the prognosis of SKCM.
Validating the relationship between OS genes and clinicopathological features of SKCM
While investigating the association between expression of hub OS genes and SKCM clinical features, we discovered that patients with metastatic melanoma had significantly increased expression of AKAP9, VPS13C, and ACSL4 (Fig. 8I–K), while HMOX2 demonstrated higher expression in patients with primary melanoma (Fig. 8L). Therefore, we further investigated connections between AKAP9, VPS13C, ACSL4, and HMOX2 expression and cancer metastasis. Results indicated that the four hub genes were all significantly related to metastasis (p < 0.05; Fig. 8E–H); AKAP9, VPS13C, and ACSL4 were positively associated with metastatic ability, while HMOX2 was negatively associated with cancer metastasis. Additionally, T stage of SKCM was also significantly associated with VPS13C, ACSL4, and HMOX2 transcription levels (p < 0.05; Fig. 8M–O), and patients with T3 or T4 stages had significantly enhanced HMOX2 expression and decreased VPS13C and ACSL4 expression. These results suggested that our identified hub genes played a vital role in metastasis and progression of SKCM. Therefore, we considered whether these was the reason that our identified genes controlled the overall survival of patients with SKCM and investigated the relationship between clinical features and patient survival. As shown in Fig. 8P–S, overall survival was significantly associated with patient age, T stage, TNM stage, and metastatic ability of the cancer (p < 0.01). Intriguingly, patients with primary melanoma or higher tumor stage had significantly worse outcomes, which suggested that AKAP9, VPS13C, ACSL4, and HMOX2 might play a vital role in the overall survival of SKCM through mediating cancer growth and metastasis.
Associations between prognostic risk score and clinical characteristics of SKCM
As per the survival analysis indicated in Fig. 9A, overall survival of patients with SKCM was significantly decreased with an increased risk score in TCGA cohort. In addition, ROC analysis indicated that our prediction model was more credible than the clinicopathological characteristics at both 1, 3, and 5 years in TCGA cohort (Fig. 9C). Similar conclusions were also validated in the GSE65904 cohort (Fig. 9B, 9D), demonstrating the moderate specificity and sensitivity of our prognostic model. Further, univariate and multivariate Cox regression analysis determined that risk score could be identified as an independent prognostic feature associated with SKCM prognosis (Fig. 10A–B). Moreover, we evaluated the connection between risk score and each clinicopathological characteristic, revealing that patients in T0-2 stage or those with metastatic melanoma were significantly associated with lower risk scores (Fig. 10E–F). Meanwhile, risk score was also significantly associated with the age of patients with SKCM (Fig. 10D). Additionally, we discovered that risk score was negatively associated with cancer metastasis (Fig. 10G), indicating that our constructed risk model was significantly associated with SKCM prognosis possibly by predicting the cancer’s metastatic ability. A heatmap revealed the expression of the four hub OS genes in the high- and low-risk groups (Fig. 10C), and their expression level in the high-risk group was remarkably consistent with their respective prognostic value in patients with SKCM. Meanwhile, the two risk groups differed significantly with respect to T stage, TNM stage, and metastasis in TCGA cohort. A nomogram plot based on risk score and other clinical characteristics was modeled to calculate the clinical outcomes of patients with SKCM (Fig. 11A); calibration plots at 3 and 5 years demonstrated good conformity with our prognostic model (Fig. 11B). A nomogram plot was also developed based on expression of the four hub OS genes, allowing us to calculate the survival probabilities of each patient with SKCM at 1, 3, and 5 years (Fig. 11C). Calibration plots also indicated good conformity between the predicted and observed outcomes at 3 and 5 years (Fig. 11D). These results indicated that our prognostic model showed great promise for predicting SKCM outcomes and clinical features.
GSEA analysis
GSEA analysis indicated that our selected four hub genes were significantly associated with several GO functions, especially death in response to oxidative stress, Golgi vesicle transport, proteasomal protein catabolic process, protein modification by small protein removal, regulation of cellular protein catabolic process, regulation of the ERAD pathway, regulation of intracellular protein transport, regulation of response to endoplasmic reticulum stress, ubiquitin like protein ligase binding, and vesicle targeting (Fig. 12A). In KEGG pathway analysis, these genes were also significantly associated with apoptosis, chronic myeloid leukemia, the ERBB signaling pathway, long term potentiation, the neurotrophin signaling pathway, pancreatic cancer, regulation of actin cytoskeleton, renal cell carcinoma, the Toll-like receptor signaling pathway, and ubiquitin mediated proteolysis (Fig. 12B).