Tumor vs. normal tissue comparisons
The datasets were downloaded and processed as described in methods, and the comparison of tumor vs. normal samples for the levels of activity of the pathways and functions analyzed were applied accordingly. The comparison of the different activities at the subpathway and functional level between tumor and healthy tissue samples returned the number of up- and down-activated significant features in each project shown in Table 1.
Number of significant results per project and feature
|
Paths
|
Gene Ontology
|
Uniprot
|
UP
|
DOWN
|
TOTAL
|
UP
|
DOWN
|
TOTAL
|
UP
|
DOWN
|
TOTAL
|
BRCA
|
483
|
819
|
1302
|
388
|
848
|
1236
|
32
|
93
|
125
|
KIRC
|
805
|
635
|
1440
|
804
|
541
|
1345
|
51
|
65
|
116
|
LUAD
|
386
|
925
|
1311
|
242
|
1165
|
1407
|
27
|
96
|
123
|
Table 1: Number of significant features per project and analyzed feature. For each feature type, the number of significant up-activated, down-activated and total significant features is shown.
The significant subpathways as defined in methods for each project are depicted as heatmaps in Figure 1. Samples and features were clusterized, and clear patterns emerged from the clusterization in all three projects, allowing easily the separation between tumor and normal samples just from the values of pathway activity.
The clear pattern between tumor and normal samples is also perceptible in the heatmaps of the functional activities, proving a notable capacity of the functional data to discern between both groups, see Figure 2.
Common & specific features
After comparing tumor and normal samples separately, we looked for the features with a common pattern of significant up- or down-activation across the three cancers. We found 431 common subpathways, 400 common Gene Ontology functions and 52 common Uniprot functions, representing 23%, 24% and 36% of the total number of analyzed features, respectively.
Common pathways include subpathways from the Cell cycle (up-activated subpathways ending in RB1 and protein complexes including MCM and ORC families), Toll-like receptor signaling pathway (up-activated subpathways ending in proteins CXCL9, CXCL10, CXCL11, IFNB1, related to angiogenesis), Hippo signaling pathway (down-activated subpathways ending in ID1, NKD1 or CTGF), MAPK signaling pathway (down-activation of subpathways ending in NR4A1 and MAP3K4, which are reportedly tumor suppressors, and up-activation of subpathways ending in ELK1, TP53 and CDC25B, with oncogenic properties), PPAR signaling pathway (down-activation of subpathways ending in proteins AQP7, GK, PCK1, ACAA1, CPT1C, ACSL1, LPL, SLC27A4, strongly related to lipid and fatty acid metabolisms), ERBB signaling pathway (subpathways ending in proteins CDKN1A, CDKN1B, BAD, GSK3B, EIF4EBP1 are up and those ending in RPS6KB1, STAT5A and PRKCA are down) and AMPK signaling pathway. Supplementary Tables S1, S2 and S3 in supplementary material show the list of common pathways, GO terms and Uniprot keywords, respectively, their common status and the p-values of the comparisons between tumor and normal samples in each of the projects, ordered by the sum of the three p-values.
Interestingly, when analyzing differential expression of the genes involved in those paths, specific cancer patterns arise. As an example, Figure 3A (top) shows the boxplots representing the distribution of the subpathway AMPK signaling pathway: CCNA2 (the subpathway from the KEGG AMPK signaling pathway with effector protein CCNA2) in tumor and normal samples for each of the projects. A common pattern of up-activation is clear. Figure 3B shows the Hipathia visualization for the same subpathway for the tumor vs. normal comparisons in BRCA (top), LUAD (center) and KIRC (bottom), including gene differential expression. Notice that just two of the twelve involved nodes present a common differentially expressed pattern among all three cancers and yet the joint subpathway activity presents the same behaviour in all of them.
Going further the expected GO terms usually found in cancer and that we found in our analysis, such as the histones H3 and H4 methylation and acetylation, DNA replication and recombination, we can see an interesting mix of up and down regulated functions that can be related to cancer hallmarks. Common upregulated GO functions, such as the regulation of adaptive immune response, the leukocyte migration, and the ones related to T cell activation and B cells apoptosis hint the complex relation that these kind of immune system cells have with tumors [16] and the cancer hallmark of evading immune suppression. Also, we can observe an upregulated JAK-STAT cascade, strongly related to cell survival, migration and proliferation, making this signaling pathway an important indicator of tumorigenesis and, by definition, an important indicator of hallmarks of cancer such as the activation of invasion and metastasis and sustaining proliferative signaling.
On the other hand, with respect to downregulated GO terms, we find certain functions, such as glucose homeostasis DNA repair processes, that could be related to the hallmarks deregulation of cell energetics and genome instability and mutation respectively. Also, a great number downregulated GO terms are related to the ion levels of the cell, such as sodium export and import from cell, response to calcium ion, regulation of delayed rectifier potassium channel activity or the regulation of intracellular pH. The varying levels of different kinds of ions is oftenly related to changes in the expression levels of ion channeling proteins, which can be related to identify different kinds of cancer and their severity [17].
With respect to the common Uniprot keywords, significant functions include Mitosis (up-activated, see Figure 3A, bottom), Growth arrest (down-activated), Lipid degradation (down-activated), calcium transport (down-activated), Porin (up-activated) and Chromosome partition (up-activated), which, respectively, can be related to hallmarks Enabling replicative immortality, Evading growth suppressors, Deregulating cellular energetics, Sustaining proliferative signaling, Activating invasion and metastasis and Genome instability and mutation.
Also an interesting number of cancer-specific features arise from the analysis. Concretely, Figure 4A shows the number of specific subpathways and functions for each of the projects. The most specific cancer seems to be KIRC, with the greatest number of specific subpathways and functions differentially activated.
Specific subpathways related to KIRC include the up-activated Hippo signaling pathway (ending in protein BBC3) and also the up-activated TNF signaling pathway (ending in proteins CASP7, CASP3, CEBPB, CHUK, MAPK14 and PGAM5). Specific Uniprot functions include the up-activation of Ubl conjugation pathway and complement pathway and specific Gene Ontology terms include down-activation of carnitine transport, up-activation of the regulation of cell adhesion mediated by integrin and up-activation of substrate adhesion-dependent cell spreading.
With respect to BRCA, specific Gene Ontology terms include the up-activated longevity regulating pathway (ending in proteins SOD2, CAT and ATCG5) and the up-activated adherens junction pathway (ending in proteins LEF1, CTNNB1, and CTNND1). Just one Uniprot function has been found to be specific to BRCA: the down activation of B-cells. And for the part of specific Gene Ontology terms, we could see functions such as the up-activation of the hydrogen peroxide biosynthetic process and the down-activation of apoptotic cell clearance.
Finally, LUAD specific pathway results include the down-activated apoptosis pathway (ending in protein PTPN13) and the up-activated thyroid hormone signaling pathway (ending in the protein CASP9). Again, just one Uniprot function specific to LUAD has been found, the up-activation of necrosis. Finally, specific Gene Ontology terms for LUAD include the up-activation of cytokine biosynthetic process and the up-activation of cation transport.
Supplementary Tables S4, S5 and S6 include the cancer specific subpathways, GO terms and Uniprot keywords, together with the sign of the comparison and the project in which they were significant.
Survival-related pathways and functions
After applying the survival pipeline explained in Methods, we found the number of pathways and functions related to survival depicted in Table 2.
Number of significant survival-related features per project
|
Paths
|
Gene Ontology
|
Uniprot
|
BRCA
|
14
|
0
|
2
|
KIRC
|
953
|
894
|
96
|
LUAD
|
29
|
10
|
1
|
Table 2: Number of significant survival-related features per project and analyzed feature.
Surprisingly, the amount of significant features is clearly unbalanced between KIRC and the other two cancers. Notice that statistics show that KIRC has generally a better prognosis than another types of cancer, contributing with a meager 1.1% deaths related by cancer worldwide in comparison with the 18.4% related to LUAD [1].
Unfortunately, we found no common survival-related features significant in all three cancers at the same time. However, a number of survival-related features common to two of the three cancers were found: 31 subpathways, 6 Gene Ontology functions and 3 Uniprot keywords. Figure 4B shows the number of survival-related paths shared by each pair of cancer projects by means of an UpSet plot [18].
Among the pairwise common survival-related paths we find subpathwayAMPK signaling pathway: CCNA2, which was commonly up-regulated along the three cancers (see Section Common & Specific features and Figure 3). This subpathwayhas been significantly related to survival in KIRC and LUAD. In both cancers, a higher activity of this pathway is related to a poorer outcome, and a lower activity of the pathway is linked to a better outcome. Figure 4C shows the Kaplan-Meier curves for the three groups (20% of higher values in red, 20% of lower values in blue and remaining 60% of values in orange) in KIRC (top) and LUAD (bottom).