Association between metabolic phenotype and KRAS mutations in PC
It’s shown in FireBrowse Database (http://www.firebrowse.org/) that KRAS mutation was the most common type of mutation in PC, followed by mutation frequency of TP53, MAMLD1, MAGEC1, CDKN2A, and SMAD4 (Figure 1A). Next, we utilized mRNA expression data and corresponding clinical information of PC samples in TCGA and cBioportal to investigate metabolic processes linked to KRAS mutation status. GSEA was conducted to determine the difference of metabolic pathways between 58 PC samples of KRAS WT and 109 PC samples of KRAS MUT. The results showed that 4 metabolic biological processes were significantly enriched in KRAS MUT group, which were Glycine Serine and Threonine Metabolism (NES=1.63, size=31 and p-value<0.05), Tryptophan Metabolism (NES=1.48, size=40 and p-value<0.05), Type II Diabetes Mellitus (NES=1.41, size=47 and p-value<0.05), and Primary Bile Acid Biosynthesis (NES=1.67, size=16 and p-value<0.05) (Figure 1B).
Identification and enrichment analysis of differentially expressed metabolic genes based on KRAS mutation status
We performed differential expression analyses between 58 PC samples of KRAS WT with 109 PC samples of KRAS MUT in TCGA PC cohort. Among 1724 metabolic genes, 54 genes were significant differentially expressed and considered as DEGs (|log2-fold change (FC)|>1.5 and FDR<0.05) (Figure 2A, B). Then these 54 DEGs were integrated to DAVID 6.7 to perform GO analysis and KEGG pathway analysis. GO analysis showed that the top 10 highly enriched functions in the metabolic process were “Oxidation Reduction, Carboxylic Acid Catabolic Process, Hormone Metabolic Process, Alcohol Catabolic Process, Glucose Metabolic Process, Lipid Catabolic Process, Cellular Amino Acid Catabolic Process, Vitamin Metabolic Process, Steroid Metabolic Process, and Glutamine Family Amino Acid Metabolic Process” (Figure 2C). In KEGG analysis (Figure 2D), DEGs was found to be mainly enriched in “Adipocytokine Signaling Pathway, Retinol Metabolism, Metabolism of Xenobiotics by Cytochrome P450, Type II Diabetes Mellitus, Drug Metabolism, Endocytosis, and Fatty Acid Metabolism”.
Construction and validation of a KRAS-associated metabolic risk model
We performed univariate Cox proportional regression and found that 21 out of 54 DEGs were significantly related to OS and considered as prognosis-associated DEGs (PAGs) (Figure 3A). Further, with the help of the R packages survival, a multivariate Cox regression analysis for these 21 PAGs was performed and it revealed that 6 of them were independently associated with OS (Figure 3B). Therefore, a 6-PAGs based KRAS-associated metabolic risk model was developed by weighting the normalized expression of these 6 PAGs multiplied by corresponding coefficients derived from the multivariate Cox regression analysis: risk score = (-0.29328 * normalized expression of CYP2S1) + (-0.45614 * normalized expression of GPX3) + (-0.57665 * normalized expression of FTCD) + (0.54899 * normalized expression of ENPP2) + (0.19472 * normalized expression of UGT1A10) + (0.34727 * normalized expression of XDH) (Table 1). Patients were divided into high-risk and low-risk groups by using the median cut-off value of the risk score (Figure 3C).
Table 1
6 KRAS-associated metabolic genes to establish the risk model
Genes
|
coef
|
HR
|
HR.95L
|
HR.95H
|
P-value
|
CYP2S1
|
-0.29328
|
0.74582
|
0.60422
|
0.92060
|
0.00633
|
GPX3
|
-0.45614
|
0.63372
|
0.47754
|
0.84098
|
0.00158
|
FTCD
|
-0.57665
|
0.56178
|
0.44136
|
0.71504
|
2.80E-06
|
ENPP2
|
0.54899
|
1.73150
|
1.2754
|
2.3506
|
0.000432
|
UGT1A10
|
0.19472
|
1.21498
|
1.0496
|
1.4064
|
0.00911
|
XDH
|
0.34727
|
1.41520
|
1.1314
|
1.7702
|
0.00236
|
The predictive accuracy of the risk model was further assessed through the ROC and C-index analysis. The results showed the AUC of the risk model for OS in TCGA cohort was 0.773 at 1 years and 0.704 at 3 years (Figure 4A). Besides, C-index of the risk model in TCGA cohort was 0.733 (95% CI, 0.675 - 0.761). The calibration curves of the risk model matched well, which indicated that it could accurately predict the 1- and 3‐year OS in TCGA cohort (Figure 4B). GSE57495 and GSE79668 datasets were used for the external validation for the risk model. The AUC for 1‐, and 3‐year OS in GSE57495 datasets were 0.627, and 0.698 (Figure 4A), while the AUC for 1‐, and 3‐year OS are 0.727 and 0.617 in GSE79668 datasets (Figure 4E). The C-index of the risk model in GSE57495 and GSE79668 datasets were 0.683 (95% CI, 0.655 - 0.723) and 0.625 (95% CI, 0.598 - 0.675). And the corresponding 1-year and 3-year calibration curves are respectively shown in Figure 4D and 4F.
Association between the risk model with patients’ survival and clinicopathological characteristics in PC
To further evaluate the prognostic power of the risk model, Kaplan-Meier analyses were performed and we found that all patients in high-risk group had a shorter OS (p-valve=4.234e-07) and Disease-free survival (DFS) (p-valve=1.83e-06) than those in low-risk group in TCGA cohort (Figure 5A-B). Besides, similar results for KM analyses of OS were observed in GSE57495 dataset (p-value=8.637e-04) and GSE79668 dataset (p-valve=1.833e-02) (Figure 5C-D). Further we calculated the risk scores in different groups based on clinicopathological characteristics in TCGA PC cohort and the results showed that patients who had advanced stage, higher histologic grade or diabetes history had higher risk scores (p-valve<0.05) (Figure 5E). The data in Figure 4 and Figure 5 suggest the KRAS associated metabolic risk model has effective value in predicting patients' survival and is associated with advanced tumor characteristics.
Association between the risk model with metabolic characteristics of PC
We next explored the activities of 6 PAGs (i.e. CYP2S1, GPX3, FTCD, ENPP2, UGT1A10, and XDH) by analyzing its potential metabolic pathways in PC. The co-expression analyses for 6 PAGs were performed by using cBioPortal dataset (Spearman’s correlated coefficient>0.6 or <−0.6, p-valve<0.05). We found 131 co-expression genes for CYP2S1, 163 co-expression genes for ENPP2, 387 co-expression genes for FTCD, 521 co-expression genes for GPX3, 189 co-expression genes for UGT1A10, and 213 co-expression genes for XDH, all of which were enrolled into DAVID 6.7 and subjected to functional and pathway enrichment analyses.
The potential metabolic pathways involved were shown in Figure 6A. CYP2S1 and its neighboring genes were mainly enriched in “Oxidation-reduction process, Lipid metabolic process, Protein glycosylation, Lysophospholipase activity, Steroid metabolic process, Regulation of glucuronidation, and Response to insulin stimulus”. GPX3 may act a vital role in “Lipid metabolic process, Glutamate receptor activity, and Regulation of insulin secretion”. FTCD may play an important role in “Glucose metabolic process and Regulation of insulin secretion”. ENPP2 may be involved in “L-amino acid transport, Glutamate receptor activity, Regulation of insulin secretion, and Response to insulin stimulus”. UGT1A10 was mainly associated with “Lipid metabolic process, Steroid metabolic process, Regulation of glucuronidation, CoA succinyl transferase activity, Endocytosis” and XDH was involved in “Protein glycosylation, Phagocytosis, and Endocytosis”.
To further explore the potential metabolic targets influenced by our risk model, we compared the expression levels of several KRAS-driven metabolic genes between high-risk with low-risk group in TCGA cohort. We found higher mRNA expressions of PKM (p=1.86e-05), GLUT1 (p=3.168e-05), HK2 (p=3.056e-04), LDHA (p=2.989e-06), and VDR (p=0.012) in the high-risk group (Figure 6B).
Association between the risk model with Gemcitabine chemoresistance in PC
We found that 5 of 6 PAGs were enriched in “Drug metabolism or Response to drug” in Functional and pathway enrichment analyses (Figure 7A). In order to explore their relationship with Gemcitabine associated chemoresistance in PC, we calculated metabolic risk scores in 2 parental and Gemcitabine-resistant cell lines (CFPAC-1 and HPAFII) in GSE140077 and GSE106336 datasets (Figure 7B). The results revealed that Gemcitabine-resistant group had significantly higher metabolic risk scores than parental group (p-valve<0.001).
To investigate how the risk model leads to Gemcitabine resistance, we compared the expression of previous reported genes regulating Gemcitabine drug metabolism and efficacy (i.e. CDA, DCK, hENT1, hCNT1, RMM1, RMM2) between high‐risk with low-risk group in TCGA cohort. The results showed that the mRNA levels of CDA (p-valve=0.001) and RMM2 (p-valve=2.517e-12) were significantly up-regulated in the high-risk group (Figure 7C).