In this comprehensive study of genetic variability in EGFR signaling across the continuum of colorectal carcinogenesis, we observed several associations between SNPs in EGFR and Src and risk of colorectal cancer; specifically, the EGFR 142572T>C CC genotype was associated with an approximately 40% increase in both colon and rectal cancer risk, compared to the wildtype. This replication of findings across these two tumor locations is interesting and potentially important. However, because we did not have access to an independent study population for replication, we performed multiple comparison adjustment which attenuated the p-values. We also observed several SNP-SNP interactions, indicating that these tagSNPs may need to be considered within a wider context.
Several polymorphisms in EGFR have previously been associated with cancer outcomes. The Arg521Lys polymorphism has been associated with colorectal cancer survival in a small study (n=318) [23
]. In that study, opposite associations were observed for men and women, indicating that EGFR polymorphisms may interact with hormone status in colorectal cancer, although this pattern of findings could also have been the result of small numbers. The Arg521Lys polymorphism has also been associated with response to the EGFR inhibitor cetuximab in a study of 32 colorectal cancer patients [24
]. In the present study, we observed no association between Arg521Lys and colorectal neoplasia risk overall or when stratified by sex (data not shown), indicating that this EGFR variant may be more important for colorectal cancer progression or response to treatment than for cancer development.
Another EGFR SNP, -216G>T, has been associated with changes in promoter activity [54
] and with overall survival among non-small cell lung cancer patients [25
], especially among those treated with the EGFR inhibitor gefitinib [26
]. This SNP did not pass QC measures in our study. A third polymorphism, a CA-repeat polymorphism in intron 1, in which higher numbers of repeats results in lower EGFR expression, has been associated with risk of breast cancer risk [27
] and with survival in head and neck [28
], lung [29
], colorectal [23
] and non-small cell lung cancer [26
]. Taken together, these results suggest that genetic variability in EGFR may be associated with increased risk of several cancer types and may predict response to EGFR inhibitors. Given that several EGFR inhibitors have been approved for cancer treatment, it will be important in the future to determine whether the observed associations with cancer survival or response to treatment can be confirmed, and whether a pattern of polymorphisms can identify those most or least likely to benefit from treatment with EGFR inhibitors.
As KRAS mutation is a predictor of response to anti-EGFR antibody therapy in colorectal cancer [55
], the association of EGFR -759C>A with increased risk of KRAS-mutated rectal tumors is intriguing and further studies are needed to elucidate the role of common variants in EGFR in relation to KRAS mutations.
To our knowledge, no study has investigated the role of HER2 polymorphisms in colorectal neoplasia risk. However, several studies have investigated the role of IIe655Val in breast cancer development, with mixed results. Several studies have shown increased risk with the variant genotypes [30
] [reviewed in [34
]], whereas others have shown decreased risk [33
], or no association [35
], reviewed in [34
], indicating that the role of this polymorphism, if any, requires further elucidation. A small study of gastric cancer patients and hospital-based controls in Japan found an increased risk associated with the variant genotypes [56
], but this requires confirmation in a larger study. We observed no association with this polymorphism in our study populations; thus genetic variability HER2 may be less important for colorectal neoplasia. An exploratory analysis indicated HER2 may influence risk differentially in men and women. In Src, we observed a decreased risk of colorectal cancer with -34985A>G. No other epidemiologic studies of Src polymorphisms have been conducted to date. In one small study, a rare coding mutation in Src was observed in a subset of advanced colorectal tumors [57
]; however, two subsequent studies did not observe this mutation [58
Due to the number of statistical tests that were performed in this study, the likelihood of false positives is high. However, because we conducted a parallel investigation of identical candidate and tagSNPs in three independent study populations, we decided to present uncorrected as well as corrected p-values and evaluate whether the associations with any of these SNPs were consistent across the three study populations. SNPs that are associated with colorectal neoplasia risk in multiple study populations are more likely to be true positives. Further, this is the first study that has comprehensively examined common EGFR, HER2, and Src genetic variability in relation to colorectal adenoma and cancer risk; thus, we report any observed associations to inform future studies.
This study has several strengths. By using both tagSNPs and candidate polymorphisms in this pathway, we achieved comprehensive coverage of genetic variability in the EGFR signaling. For EGFR and HER2, we had access to resequencing data, ensuring that we captured all common variants that are likely to exist among Caucasians. Further, we have comprehensively assessed genetic variation in EGFR, Src, and HER2 in three independent case-control studies of colorectal adenoma or cancer (including tumor markers), using identical genotyping methods, thus allowing us to replicate any promising findings. It is, however, important to consider that the outcomes in the three study populations were not identical: colon and rectal cancer share some, but not all epidemiologic risk factors, and adenoma are precursors that occur years prior to invasive cancer.
In summary, our study provides a thorough investigation, not only of genetic variability in EGFR signaling, but also of the spectrum of the colorectal carcinogenic process. We provide some evidence that EGFR 142575T>C may be important in both colon and rectal cancer, but this should be confirmed in future studies. Given that genetic variants in EGFR may be related to colorectal cancer survival or responsiveness to treatment with EGFR inhibitors, further study of the impact of genetic variability in the EGFR signaling pathway, specifically replication of the tumor subtype analyses, should be conducted.