The present study suggests a possible protective effect of green tea against breast cancer among women with high-activity genotypes of the MTHFR and TYMS genes. This effect was even stronger among those who were low consumers of dietary folate. To our knowledge, this is the first epidemiologic study that examines a gene–environment interaction between MTHFR and TYMS gene polymorphisms and green tea intake on its potential preventive activity against breast cancer development. The findings of the present study support the hypothesis that green tea polyphenols may influence the folate pathway and that genetic polymorphisms of enzymes in the folate pathway may modify the tea–breast cancer association.
EGCG inhibits dihydrofolate reductase, which catalyzes the conversion of dihydrofolate to tetrahydrofolate in the folate pathway. The folate pathway is vital for nucleotide synthesis, especially for rapidly replicating cells such as cancer cells. In fact, cancer chemopreventive agents such as antifolates induce apoptosis and thus affect the steady cell population. The mechanism of anticarcinogenic activity of EGCG was recently found to be similar to antifolate chemotherapeutic agents in vivo
Epidemiologic studies have not shown consistent results regarding the association between tea intake and breast cancer risk. Majority of studies in the USA or Europe, where black tea is preferred to green tea, have reported no association between tea intake and breast cancer risk, probably because of the extremely small amount of tea polyphenol intake (31
). Although a few studies have evaluated the association in an Asian population, where green tea consumption is relatively high, study findings have again been inconsistent (6
). One possible explanation is that gene–nutrient interaction may modify the anticarcinogenic activity of green tea polyphenols in humans. Numerous studies have investigated the effect of MTHFR
gene polymorphisms on risk of breast cancer; however, results have been discordant (35
). In the present study, the risk of breast cancer was not associated with MTHFR
as a regulating enzyme in the folate pathway, the other possible explanation for inconsistency among previous study results is the nutrient–nutrient interaction between folate and tea polyphenols. We found a lower risk of breast cancer associated with regular green tea intake only among women with low folate intake. Furthermore, the inverse association between risk of breast cancer and regular green tea intake was the strongest among women possessing high-activity MTHFR
genotypes whose dietary folate intake was low. Our results correspond with a limited number of previous studies that examined the interaction among folate intake, genotype and breast cancer risk (47
). Folate depletion increases the sensitivity of cancer cell lines to EGCG in vitro
). Thus, EGCG does not have to compete with folate and can effectively inhibit dihydrofolate reductase. Findings from the present study lend support to the hypothesis that green tea polyphenols may have anticarcinogenic properties through competing with folate in the folate pathway.
The present study has several strengths. Singapore is a small city-state where all citizens have good access to specialized medical care. The nation-wide cancer registry has been available since 1968 and has been shown to be comprehensive in its recording of cancer cases (24
). Thus, the ascertainment of incidence cases of breast cancer can be assumed to be comprehensive. Secondly, our study population is Asian with a relatively high frequency of green tea intake compared with the USA or European study cohorts. Consequently, we were able to collect a relatively large number of green tea drinkers for cases and controls. Thirdly, green tea intake was assessed prior to breast cancer diagnosis and therefore can be presumed to be free of recall bias. Furthermore, average folate intake among the current study population was significantly lower compared with the average daily dietary folate intake for adult women in the USA (300–350 μg/day) (50
), which offered the opportunity to evaluate antifolate effects of EGCG in relative absence of opposition from folate.
The major limitation of the present study is lack of detailed data on green tea intake. Although green tea drinking is more popular in Singapore compared with Western countries, Singapore, having been a major port for foreign trade, has developed the unique mixture of Asian and Western cultures, which has also had impacts on beverage drinking habits. As shown in , the number of daily green tea drinkers was relatively small in the present study population. Therefore, daily green tea drinkers were combined into the category of weekly or more frequently drinkers in the final analyses. Our findings require confirmation in other large cohorts with high exposures to green tea. Moreover, the EGCG content in green tea varies depending on how it is prepared; in the current study, information on the methods used to brew the tea (infusion time and strength) was unavailable. In addition, we assessed tea intake only at a single time point (baseline). However, non-differential misclassification of exposure status tends to minimize the underlying relative risk toward the null. It is unlikely that such exposure misclassification leads to a spurious association between exposure and disease risk. Next, biospecimens for genotype determination were donated by only 64% of controls and 51.6% of cases. However, refusal to donate biospecimens occurred before genotypes were determined; therefore, it is unlikely that genotypes could affect subjects’ decisions to donate their biospecimens. Thus, it is unlikely that refusal to donate biospecimens led to selection bias regarding genotype. Lastly, our result that dietary folate intake was significantly higher among women with breast cancer who drank green tea more frequently suggests that green tea drinking habits may be associated with a healthier lifestyle. If so, there might be some unmeasured factors that is reflected by green tea intake and decreased breast cancer risk.
In summary, the present study provides supportive epidemiologic evidence to the previously suggested hypothesis that EGCG stimulates apoptosis and cell cycle arrest through inhibition of the folate cycle. Genetic polymorphisms in MTHFR and TYMS were found to modify the preventive activities of green tea polyphenols against breast cancer. Our findings were demonstrated in a population with low average folate intake. These results need to be confirmed by future studies with large sample sizes, considering folate intake as a potential effect modifier in the interaction among tea polyphenols, MTHFR/TYMS genotypes and breast cancer risk.