Our findings suggest that disease-specific survival varies according to SMAD7 genotype, but only in patients who were regularly using NSAIDs before diagnosis. Among women who were using NSAIDs regularly prior to diagnosis, each minor allele of rs4939827 was associated with reduced survival, and each minor allele of rs4464148 was associated with improved survival. No association with survival for either SNP was observed among women who had never used NSAIDs or had discontinued use earlier than two years prior to diagnosis. We observed poorer survival for the minor variant previously shown to be associated with a reduced risk of developing CRC, and improved survival for the minor variant previously shown to be associated with an increased risk of developing CRC.
The contrast between the genetic effects on incidence and those on survival may be a consequence of the two faces of TGFB, which shifts from acting as a tumor suppressor to become an accelerator of metastatic activity after tumor formation (Pardali and Moustakas, 2007
variants that more completely inhibit TGFB may decrease its tumor suppressing activity (resulting in increased risk), but may also decrease its ability to promote metastatic activity (resulting in slower tumor progression and improved survival). Likewise, SMAD7
variants that result in up-regulation of TGFB may increase its tumor suppressing activity (resulting in decreased risk), but may also increase its ability to promote metastatic activity (resulting in accelerated tumor progression and reduced survival). Consistent with the role of SMAD/TGFB signaling in facilitating the EMT, this association was specific to women diagnosed with tumors that exhibited some direct tumor extension or lymph node involvement and not specifically local disease, although it should be noted that survival estimates for localized stage are based on only a small number of deaths.
Slattery et al. (2010)
noted a more prominent reduction in the risk of developing CRC for those who regularly used NSAIDs two years before diagnosis for rs4939827 (odds ratio comparing CC to TT was 0.60; CI: 0.43-0.85 for recent users and 0.86; CI: 0.68-1.09 for those who were not recent users), and have additionally reported significant risk modifications with pre-diagnostic NSAID use for other SNPs in the SMAD/TGFB pathway (Slattery et al., 2011a
). We observed that the association of these SMAD7
polymorphisms with survival was limited to women who were still using NSAIDs up to two years prior to diagnosis, and was not observed among women who had discontinued use earlier.
plays an essential role in maintaining the inflammatory gastrointestinal state observed in patients with inflammatory bowel disease (IBD) (Monteleone et al., 2004
). In animal models for IBD, direct inhibition of SMAD7
via application of antisense oligonucleotides in vivo resulted in lower production of inflammatory cytokines and less injury to intestinal tissues (Monteleone et al., 2008
). Gene expression studies have observed SMAD7
underexpression in colonic mucosa following treatment with COX2 inhibitors (Glebov et al., 2006
), and expression of members of the TGFB superfamily, including NSAID-activated gene 1/growth differentiation factor 15 (NAG1/GDF15), is known to be influenced by COX2 (Iguchi et al., 2009
). The molecular mechanisms by which anti-inflammatory medications would interact directly with SMAD7
, specifically, are not yet apparent, and the ability of COX2 overexpression to alter TGFB functionality may very well take place through SMAD-independent pathways (Enders, 2007
). While not yet fully understood, our findings underscore the fact that inflammatory processes and related genetic variability may have important consequences for both disease incidence and progression. For instance, it is possible that use of NSAIDs shortly before clinical presentation could produce tumors with less aggressive characteristics (Masferrer et al., 2000
), which are more likely to be impacted by genetic alterations in growth factor signaling.
Our findings may reflect differences in patterns of NSAID use. Recent users were more likely to have used regularly for a longer duration and may be more likely to continue use after diagnosis. It is possible that the distinction we observed between recent and non-recent users actually reflects the impact of post-diagnostic NSAID use. Information on post-diagnostic NSAID use was not available for this study, and so it is unclear whether the potential interaction that we observed reflects an association with pre- or post-diagnostic behavior or both. Further research is needed to consider whether SMAD7 genotype can inform a more specific recommendation of post-diagnosis NSAID use for improved CRC survival. Clinically, it may be advantageous to target anti-inflammatory medications only to those who will benefit, and an opposition between the effects on incidence and the effects on survival suggests that the use of NSAIDs may require careful orchestration before and after a diagnosis of CRC in a manner that may depend, in part, on SMAD7 genotype.
Our population-based cohort of post-menopausal women diagnosed with incident CRC was well-characterized. The SEER records provided standardized ascertainment of several clinically-relevant covariates that could account for differential prognosis including stage, subsite, and treatment. Complete, standardized, and up-to-date vital-status information was obtained from the National Death Index, which has been shown to be valid for research purposes (Calle and Terrell, 1993
). Our analyses adjusted for important risk factors for the disease and those associated with survival after diagnosis.
This study has several limitations. Our study included only post-menopausal women. Case-control studies undertaken to extend the findings from GWASs have sometimes been inconsistent by sex. Slattery and colleagues found that the association between rs4939827 and incidence of colon cancer did not differ between men and women (P
for interaction = 0.97) (Slattery et al., 2010
). However, Thompson et al. (2009)
noted a more marked decrease in risk for rs4939827 (P
for interaction = 0.02) and an increase in risk for rs4464148 (P
for interaction = 0.08) among women compared to men. Accordingly, it is not clear if the associations we observed are sex-specific or if they can be generalized to men.
Because regular NSAID use is generally recommended for those with cardiovascular risk factors, we were not able to exclude the possibility that an excess of deaths from cardiovascular causes accounts for the lack of CRC deaths for certain SMAD7 genotypes, although among the 186 women using NSAIDs two years prior to CRC diagnosis, only 7 of the 24 deaths attributed to causes other than CRC were from cardiovascular-related conditions. A limited number of deaths, in general, prevented us from specifically examining whether the association between these SMAD7 polymorphisms and pre-diagnostic NSAID use was related to usage characteristics such as frequency, timing, duration of use, type of medication, as well as whether the association differed by anatomic tumor subsite.
The susceptibility loci in SMAD7
suggested from GWASs are intronic, and may only be associated with disease risk by being in LD with yet to be discovered functional variants of 18q21. Pittman et al. (2009)
identified a polymorphism of 18q21 that may alter SMAD7
expression by modifying transcription-factor binding. Other such SNPs may also be discovered in subsequent investigations. Additional variants of 18q21 were not available from the parent study, nor were other SNPs in the SMAD/TGFB pathway. Polymorphisms in TGFB1
, and SMAD3
, for example, have recently been linked to CRC survival (Försti et al., 2010
; Slattery et al., 2011b
), and a more comprehensive analysis encompassing additional genes in this pathway may lead to further insights about the roles of inflammation, growth factor signaling, and survival outcomes.
In our study, we focused specifically on loci in SMAD7
which, unlike variants in TGFB1
, and SMAD3
, have been modestly, yet consistently, linked to CRC risk in previous GWASs. Additionally, SMAD7’s
involvement in colorectal carcinogenesis is thought to be independent of its association with other genes in the SMAD/TGFB pathway, based on observations that adenocarcinoma cases have significantly lower SMAD7
expression than cases with adenomatous polyps, regardless of 18q21 copy number (Pittman et al., 2009
). Lastly, we did not have information on any expression status in tumor samples. Future investigations should evaluate whether differential survival for NSAID users according to SMAD7 genotype can be explained protein expression or mRNA levels of SMAD7, TGFB, or COX2 in tumor samples. If replicated, our findings should motivate further research on the functional role of these genetic variants of SMAD7
, and, more broadly, on the mechanism by which anti-inflammatory medications could influence disease prognosis.