Pathway Analysis of Breast Cancer Genome-Wide Association Study Highlights Three Pathways and One Canonical Signaling Cascade

Introduction

Breast cancer is a complex disease with a well-established genetic component (1–4). Different genetic methodologies have been used in the last 20 years to identify multiple susceptibility loci that vary in population frequency, relative risk, and potential functional role. For example, familial linkage and positional cloning studies found that rare mutations in the genes BRCA1, BRCA2, TP53, and PTEN are associated with a 10-fold to 20-fold increase in breast cancer risk (5–8). Variations in these genes are thought to contribute to breast cancer susceptibility through different cellular mechanisms. Whereas BRCA1 and BRCA2 belong to the DNA repair mechanism in cells (9, 10), TP53 and PTEN are tumor suppressor genes that participate in processes related to cell cycle control and cell proliferation (11, 12). Further interrogation of candidate genes associated with these cellular processes has led to the discovery of additional rare genetic variants conferring moderate relative risks (2–3-fold) of breast cancer (13–16).

Recently, genome-wide association studies (GWAS) have become a key paradigm in genetic studies of complex diseases. These studies are particularly powerful in identifying common-low penetrance risk alleles. Indeed, several novel markers with low (<2) relative risk for breast cancer have been detected by this approach (17–21). However, these reported loci are only those that reached a stringent statistical “genome-wide” significance criterion, whereas less attention has been given to the other hundreds of thousands of genotyped markers. Using new methods to study existing GWAS data may provide additional biological insights and highlight new candidate loci. To this end, a pathway-based approach is particularly appealing. This method examines whether the cumulative contribution of genes with a common biological denominator is greater than expected by chance. This approach has recently been applied to GWAS of several noncancer complex diseases (22–25).

In this study, we applied pathway analysis to the breast cancer GWAS of the Cancer Genetic Markers of Susceptibility (CGEMS) project of the National Cancer Institute (NCI). This study has recently identified a significant association of single nucleotide polymorphisms (SNPs) in the FGFR2 gene with breast cancer susceptibility (18). We used the modified gene-set enrichment analysis of Wang and colleagues (22) to identify an excess of genotype-phenotype association signals in pathways from different resources. Finally, we examined whether the excess of association signals in different pathways is driven by the same subset of genes comprising a common biological module. These analyses illuminated three pathways and one canonical cascade that are possibly involved in genetic susceptibility to breast cancer.

Results

Overall, 421 pathways and 3,962 genes (164/996, 155/3,088, and 102/1,317 pathways/genes from BioCarta, KEGG, and PID, respectively) were included in our study. These were represented by 69,525 of the 528,173 SNPs (13.2%) that were originally genotyped in this GWAS. A significant variation was seen between pathway resources. More than 90% of the pathways from BioCarta in our database had <30 genes, whereas pathways from KEGG and PID were generally larger and more variable in size (mean gene count, 44 and 36; SD, 24 and 16, respectively). This variation between databases was evident even in pathways nominally representing the same biological processes. For example, the mammalian target of rapamycin signaling pathway included 19, 24, and 49 genes in BioCarta, PID, and KEGG, respectively. Interestingly, pathways from BioCarta tended to be ranked higher (higher ES) than KEGG pathways but not than PID pathways (Kruskal-Wallis test, P = 0.013 and P = 0.151, respectively). No rank differences were seen between pathways from PID and KEGG.

Of the 421 examined pathways, 21 were significantly enriched with association signals at the P < 0.05 level (Table 1). Of these, “syndecan-1–mediated signaling events” from the PID database (Fig. 1A; P_ES = 0.00053), “signaling of hepatocyte growth factor receptor” from BioCarta (Fig. 1B; P_ES = 0.00063), and “growth hormone signaling” from BioCarta (Fig. 1C; P_ES = 0.00084) had distinctly smaller P values than the rest of the pathways, with a noteworthy FDR value of 0.118. For brevity, we will further refer to these three pathways as “syndecan-1 signaling,” “c-Met signaling,” and “growth hormone signaling,” respectively.

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Figure 1.

Schematic illustration of the three top ranked pathways in this study. A, syndecan-1–mediated signaling events from PID. B, signaling of hepatocyte growth factor (c-Met) from BioCarta. C, growth hormone signaling pathways from BioCarta. Genes (ellipses) are color-coded according to their minP value, which assesses the significance of the most strongly associated SNP in each gene, using a permutation-based resampling procedure (10,000 permutations) that takes into account the number of SNPs in a gene and their underlying linkage disequilibrium structure. Arrows indicate interaction or positive regulation, and perpendicular lines indicate negative regulation. Blue diamonds represent the cellular processes induced by these pathways. Red broken boxes outline the genes of the RAS/RAF/MAPK canonical signaling cascade.

Further examination of the gene content of these three pathways revealed some overlap. For example, the gene mitogen-activated protein kinase 3 (MAPK3) is involved in all three pathways but does not seem to contribute to their significant ES due to its nonsignificant association with breast cancer risk in this study (P = 0.35). In contrast, the gene hepatocyte growth factor (HGF) was the strongest associated gene in the syndecan-1 signaling and c-Met signaling pathways (P_trend = 0.0007, for rs975360) and therefore a major contributor for their high ES. Another two genes [MAPK1 and met proto-oncogene (hepatocyte growth factor receptor; MET)] were shared by these two pathways. Similarly, six other genes [growth factor receptor-bound protein 2 (GRB2); v-Ha-ras Harvey rat sarcoma viral oncogene homologue (HRAS1); mitogen-activated protein kinase kinase 1 (MAP2K1); phosphoinositide-3-kinase, catalytic, β polypeptide (PIK3CB); v-raf-1 murine leukemia viral oncogene homologue (RAF1); and son of sevenless homologue 1 (SOS1)] were shared by the c-Met signaling and growth hormone signaling pathways.

In light of the considerable overlap between the three most significant pathways in our study, we hypothesized that common biological modules may underlie the enrichment signals in multiple biological pathways. To explore this hypothesis, we used hierarchical clustering to identify pathways with overlapping genes likely to constitute common biological process. Consequently, 16 clusters of highly related pathways were characterized (Fig. 2A). Of these, cluster #1 (Fig. 2B) was significantly more likely to include pathways with high ES than expected by chance (ARTP P = 0.0045, FDR = 0.07). Further examination of this cluster revealed that it contained two of three of the top three pathways (signaling of hepatocyte growth factor receptor and growth hormone signaling) and was dominated by the genes GRB2, SOS1, HRAS, RAF1, MAP2K1, and MAPK3, which are major components of the RAS/RAF/MAPK canonical signaling cascade (Fig. 1B and C).

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Figure 2.

Clustering analysis of pathway overlap. Pairwise pathway overlap scores are depicted in a matrix color coded in grayscale. A, clusters of highly significantly related pathways are delineated by black triangles. B, details of the pathways included in cluster #1.

To assess the power of our analysis to identify true susceptibility pathways, we constructed an artificial positive control pathway from 11 genes that were previously associated with breast cancer risk in different GWAS of postmenopausal women of European ancestry (see details in Supplemental Table S2). Applying the pathway analysis to this pseudo-pathway yielded a P_ES = 0.0126, which ranked it 8th out of all pathways in the study. It is noteworthy that most of the individual genes in the pseudo-pathway had unremarkable significance (Supplemental Table S2, last column) in the CGEMS study itself. Thus, the high rank of this artificial positive control pathway shows the ability of the gene-set enrichment analysis to detect pathways containing multiple genes that individually have weak association signals and may be missed by standard single-SNP analysis of GWAS data.

Discussion

Three pathways and one signaling cascade are highlighted in this study. The top ranked pathway in this study, syndecan-1 signaling, contains 13 genes involved in different cellular processes that are mediated by syndecan-1 (SDC1). This gene encodes a transmembrane heparan sulfate proteoglycan that mediates signal transduction cascades leading to cell proliferation, cell migration, and cell adhesion processes following interactions with extracellular matrix proteins. There are multiple lines of evidence for a potential role of syndecan-1 in breast cancer development. For example, altered syndecan-1 expression has been detected in several different tumor types and has been linked with unfavorable breast cancer prognosis (34). Additionally, expression of syndecan-1 by stromal fibroblasts has been shown to promote breast carcinoma growth in vivo and stimulates tumor angiogenesis (35). In this GWAS, SDC1 was only moderately associated with breast cancer susceptibility (P_trend = 0.019 for rs7563245); however, it is a key mediator of different pathways illuminated in this study (e.g., c-Met signaling and fibroblast growth factor signaling; see discussion below). Therefore, it can modulate breast cancer susceptibility through different biological mechanisms.

The second highest ranked pathway in our analysis, c-Met signaling, consists of 33 genes participating in signal transduction mechanisms induced by the tyrosine kinase proto-oncogene c-Met (MET). Stimulation of the c-Met pathway can lead to several different cellular processes related to tumor growth and progression, such as proliferation, enhanced cell motility, invasion, and apoptosis (36). Moreover, both c-Met and its ligand, hepatocyte growth factor, have been shown to be dysregulated and correlated with poor prognosis in a number of human malignancies, including breast cancer (37). Consequently, this pathway has served as an important therapeutic target for human cancers, particularly through the development of a small molecule that inhibit the c-Met/hepatocyte growth factor–dependent signaling (37). Notably, coexpression of SDC1 and MET, the key players in the top two ranked pathways in our study, have been established as a marker signature associated with poor prognostic factors in ductal carcinoma in situ of the breast (38).

The third ranked pathway in our study, growth hormone signaling, contains 22 genes participating in cellular mechanisms induced by either growth hormone or insulin receptors. These two receptors as well as the insulin-like growth factor (IGF) receptor are all transmembrane tyrosine kinase receptors that induce cell growth and proliferation. Alteration in the activity of these receptors or their related pathways may lead to hyperplasia and eventually to the development of a tumor (39). Naturally, the growth hormone signaling pathway had considerable overlap with both the IGF-I signaling and insulin signaling pathways from BioCarta (32% and 41%, respectively); however, only the latter had a significant ES in our study (P_ES = 0.0064). Examining the genes of these three closely related pathways revealed that what differentiates their respective ES is a SNP in the insulin receptor gene (INSR) with a small P value (P_trend = 0.0019 for rs12460755) that is absent from the IGF-I signaling pathway.

Interestingly, the fibroblast growth factor signaling pathway was ranked 10th in our study (Table 1; P_ES = 0.0219) despite the exclusion of the FGFR2 SNPs from our analyses (see Materials and Methods). Adding the FGFR2 SNPs to this pathway improved its enrichment signal (P_ES = 0.0053) and ranked it 4th in our study. This finding, in combination with the FGFR2 signal in CEGEMS (18) and elsewhere (17), suggests that variations in other genes involved in FGFR2 signaling may modulate breast cancer susceptibility.

An important extension to the gene-set enrichment analysis is the clustering analysis that highlighted the RAS/RAF/MAPK canonical signaling cascade as the common denominator of pathways associated with breast cancer risk in this study. This cascade plays an essential role in transmitting extracellular signals from growth factors to promote the growth, proliferation, differentiation, and survival of cells, and modification in its activity has been linked to multiple human malignancies (40). Notably, this cascade plays an important role in all three top pathways in this study. It is an integral component in the signaling of hepatocyte growth factor receptor and growth hormone signaling pathways, and a transducer for signals initiated by the syndecan-1 pathway.

An important limitation of the pathway-based approach for GWAS analysis is the incomplete annotation of the human genome. At present, the function of many human genes is unknown and therefore they cannot be assigned to known pathways. Moreover, susceptibility loci in intergenic regions are also not included in a study of this kind. As a result, when using this approach, only a small portion of the human genome variation can be studied and therefore it should only be used as a supplementary method to the standard single-SNP analysis of GWAS. Additionally, there is no gold standard or pathway definition, and different databases have different guidelines for their pathway construction and curation. Consequently, the gene content of pathways representing the same biological process may vary substantially between different databases, and this may have a major impact on the sensitivity and specificity of this approach. We aimed at minimizing this effect by selecting pathways from three commonly used and manually curate resources. Still, considerable differences were observed between similar pathways from different resources. For example, the c-Met signaling pathway from BioCarta contained 33 genes and was ranked second in our analysis, whereas a pathway with the same name and presumably the same cellular function in the PID database included 52 genes and was ranked only 69th. Although 30 of the 33 genes in the BioCarta pathway were also included in the PID pathway, the remaining 22 genes likely attenuated the signal in the PID pathway. These differences emphasize the importance of further synthesizing the results of such pathway-based approach. The clustering analysis we applied in this study is one way to do so, as it helps in finding the common genes underlying enrichment signals in multiple pathways and allows one to focus on a limited number of candidate genes. Other methods aiming to improve the characterization of pathways and their overlap might be useful.

A second limitation of our study is that it does not include validation of the results using a completely independent data set. The experiment we conducted using a positive control pathway with known breast cancer susceptibility genes provides us validation that our approach has high power to detect pathways containing multiple weakly associated susceptibility genes. Therefore, the top ranked pathways in our study should be prime targets for future analyses in independent data sets.

In conclusion, our results suggest that genetic alterations associated with the top three pathways and one canonical signaling cascade in our study may contribute to breast cancer susceptibility. Ultimately, additional studies would be needed to confirm and further explore the genetic variations underlying the association of these pathways with breast cancer. Moreover, this study highlights the potential insight that could be gained by using a pathway-based approach as a complimentary method to the primary single-SNP analysis of GWAS. Particularly, the organization of multiple association signals according to underlying biological processes may improve our understanding of the cellular mechanisms underlying this too common malignancy.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.