similar to those reported for women of similar age in the United States (median, 0.19 µg/g creatinine) (Gallagher et al. 2011). A slightly higher concentration has been reported for an Asian population (median, 0.59 µg/L), which is possibly due to dietary exposure through rice, which easily takes up cadmium from soil (Kippler et al. 2007). Thus, if the observed association between cadmium and hypomethylation reflects a causal relation, effects may be common and stronger in groups with higher exposures, such as smokers and occupationally exposed workers. However, further studies in other populations, including occupationally exposed individuals, are warranted. CpG sites in LINE-1 promoters are usu- ally heavily methylated, and genome-wide methylation loss from these sites has been regarded as a common epigenetic event in malignancies that may play a role in carcino- genesis (Cho et al. 2010; Choi et al. 2009; Wilhelm et al. 2010). There is increasing con- cern that cadmium acts as a metalloestrogen (Fechner et al. 2011) and that it might increase the risk of hormone-related cancers in humans (Åkesson et al. 2008; Gallagher et al. 2010; Siewit et al. 2010). The association found in the present study between cadmium and LINE-1 hypomethylation in women sug- gests that cadmium can alter DNA methyla- tion, which could be another mechanism for cadmium carcinogenesis. In vitro data lend some support to this hypothesis: Takiguchi et al. (2003) reported that cadmium expo- sure of nontransformed rat cells resulted in the inhibition of methyltransferase activity and, at higher exposures, hypomethylation. They also showed that after 10 weeks of exposure the cells became malignantly transformed and had increased methyltransferase activity. This phe- nomenon has also been shown in malignantly transformed prostate epithelial cells: cadmium exposure increased DNMT enzymatic activity and overexpression of DNMT3B accompa- nied by hypermethylation (Benbrahim-Tallaa et al. 2007). Little is known about the potential effects of environmental pollutants on LINE-1 methylation. In a recent U.S. bladder cancer case–control study, higher toenail concen- trations of arsenic (> 75th percentile) were associated with reduced LINE-1 methylation (Wilhelm et al. 2010). In contrast, urinary arsenic was not associated with LINE-1 in the present study. Possibly, this can be explained by specific genetic polymorphisms in the AS3MT [arsenic (+3 oxidation state) methyl- transferase] gene that previously were found in the present study population to be associated with a very efficient methylation and detox- ification of inorganic arsenic (Engström K et al. 2011). Other factors may also influ- ence LINE-1 methylation. Exposure to coarse particulate matter (particulate matter with aerodynamic diameter ≤ 10 µm) (Tarantini et al. 2009) and black carbon (Baccarelli et al. 2009) were found to be inversely associ- ated with LINE-1 methylation in peripheral blood cells from humans. Maternal prenatal smoking was associated with lower LINE-1 methylation in buccal cells of preschool chil- dren with glutathione S-transferase mu 1 (GSTM1)-null genotype, whereas there was higher methylation in the GSTM1-present children (Breton et al. 2009). The maintenance of methylation in somatic cells is largely carried out by DNMT1 (Chen and Li 2006), and DNMT1 expression in normal peripheral blood has previously been reported to be higher than DNMT3B expression (Mizuno et al. 2001), consistent with our findings. DNMT1 and DNMT3B colocalize in the nucleus (Kim et al. 2002), and it has been shown that in cancer cells DNMT1 and DNMT3B work cooperatively to maintain DNA methylation (Rhee et al. 2002). Of note, the allelic frequencies of DNMT3B single-nucleotide polymorphisms were low in the present study, limiting the possibility to detect modest gene–environment interactions. Further, because the levels of DNMT3B expression were lower than those of DNMT1, these findings should be interpreted cautiously. There was no association between blood cadmium concentrations, which represent a measure of short-term exposure, and our mea- sure of global DNA methylation, although cadmium levels in blood and urine were moderately correlated. If a causal relation exists between cadmium exposure and DNA methylation, one may speculate that the cad- mium effect is chronic in nature or occurred a long time before the analyses, even early in life. Furthermore, we found no effect of blood hemoglobin, selenium, or zinc status on the epigenetic markers. The lack of association between selenium concentrations and global DNA methylation in our study population is in contrast to the results of Pilsner et al. (2011), who measured plasma selenium in relation to global leukocyte DNA methyla- tion. The difference in results may reflect differences in selenium status; the women in the present study all had adequate blood selenium concentrations (138–250 µg/L), whereas the Bangladeshi individuals studied by Pilsner et al. (2011) showed plasma con- centrations of 45–149 µg/L (plasma concen- trations are somewhat lower than selenium in whole blood), indicating lower selenium status. Other differences include the meth- ods used to measure global DNA methyla- tion: Pilsner et al. (2011) used a [3H]-methyl incorporation assay assessing methylation at all genomic CpG sites, whereas we used bisulfite- PCR pyrosequencing to quantitate LINE-1 Environmental Health Perspectives • volume 120 | number 6 | June 2012 Cadmium influences DNA methylation methylation at four CpG sites, which served as a surrogate for global DNA methylation. We conducted this study on a fairly homo- geneous population in which subjects differed very little in terms of diet, air pollution, and other lifestyle factors, including smoking and alcohol consumption. This homogene- ity may explain the small variation in degree of LINE-1 methylation we found compared with other studies (Choi et al. 2009; Tarantini et al. 2009). Also, we adjusted for other essen- tial and toxic elements, such as arsenic, but these elements did not appear to modify the association between cadmium and LINE-1 methylation. A limitation of our study is that we measured methylation in the DNA isolated from peripheral blood only and not from indi- vidual cell populations or other tissues. DNA methylation patterns may differ among blood cell types (Wu et al. 2011). Conclusion We found that low-level environmental cadmium exposure was associated with a marker of global DNA hypomethylation in peripheral blood. This association was modified in relation to DNMT1 genotypes. Future studies are necessary to confirm our findings in other populations and determine whether cadmium-associated epigenetic modification is present in other tissues, and if epigenetic effects of cadmium may play a role in cadmium- associated diseases. Ref Rencs Åkesson A, Julin B, Wolk A. 2008. Long-term dietary cadmium intake and postmenopausal endometrial cancer incidence: a population-based prospective cohort study. Cancer Res 68(15):6435–6441. Baccarelli A, Wright RO, Bollati V, Tarantini L, Litonjua AA, Suh HH, et al. 2009. 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