Promoter of SLC6A4 in relation to work stress using bisulfite sequencing. Using t-test, 15900046 we saw significantly lower methylation levels in the high work stress group across all five CpG esidues (CpG1: 5.562.7, CpG2:5.963.5, CpG3: 9.863.7, CpG4: 3.762.8, CpG5: 4.263.0) in comparison to low work stress (CpG1: 1366.7, CpG2: 1265.7, CpG3: 1565.4, CpG4: 8.864.3, CpG5: 7.863.5) (p,0.01 for all residues, Figure 2). Methylation results were then validated in 9 pairs of nurses matched by age from high- and low work stress environments using the Human Methylation 450k BeadChip (Illumina Inc.). While variation in methylation was smaller as compared to the bisulfite Epigenetics sequencing in all three sites (CpG1, CpG4 and CpG5) included in the BeadChip, differences in methylation levels remained in the same direction in most cases of the matched pairs (7 out of 9 for CpG1 and CpG4 and 9 out of 9 for CpG5), evidencing for a good coherence between these two methods (p = 0.998 from goodness of fit using chi-squared test). We then tested correlations between individual methylation levels in all five CpG residues. All methylation levels were highly correlated (b.0.5, p,0.01 for all residues). Using a structural equation measurement model, we tested whether these correlations could be explained by a single latent factor. The single factor Table 1. Demographic data of the nurse groups by work stress.Results Burnout and Depression Scores in High and Low Work Stress GroupsOur study sample comprised of female nurses from contrasting work environments (high and low work stress, n = 24 and 25 respectively) initially derived from a large cohort of 5615 female health care professionals [26]. Works stress was defined according to Karasek’s Model [27] so that high work stress implicated high demands and low control, while low stress was defined by a combination of low demands and high control. Division into high and low work stress environments was 15755315 based on grouping the wards based on mean scores of all responses, using median split to identify high and low stress extremes in stress distribution. To increase contrast between the comparison groups, the nurses who belonged to the quartile with least work stress in the high stress group and most stress in the low stress group were excluded.Work Stress Environment High work stress Low work stressN 24AgeMBI-GSBDI49 (66.02) 1.33 (60.760) 8.36 (66.31) 48 (66.78) 0.723 (60.487) 4.37 (63.36)doi:10.1371/journal.pone.0045813.tStress Affects 
Serotonin Transporter MethylationFigure 1. Representation of the SLC6A4 gene promoter region for bisulfite sequencing analysis. Analyzed CpG sites are bolded and numbered from 1 through 5. Coordinates are based on the GRCh37 build (NCBI Reference Sequence: NC_000017.10). doi:10.1371/journal.pone.0045813.gFigure 2. Methylation in the promoter of SLC6A4 at five CpG sites in high (grey) and low (white) work stress environments. Coordinates for each residue are 28 563 120 (CpG5), 28 563 109 (CpG4), 28 563 107 (CpG3), 28 563 102 (CpG2), and 28 563 090 (CpG1) as per GRCh37 build (NCBI Reference Sequence: NC_000017.10). Differences between work stress environments were significant as per t-test (p = 7.10E?6, p = 2.50E?5, p = 0.000292, p = 4.37E?6, and p = 0.000289 respectively). Standard errors are indicated. doi:10.1371/journal.pone.0045813.gmodel was statistically Epigenetics accepted (df = 5; x2 = 4.1; p = 0.537). This was considered as a justification for constructing a single weighted sum score for methylation lev.Promoter of SLC6A4 in relation to work stress using bisulfite sequencing. Using t-test, 15900046 we saw significantly lower methylation levels in the high work stress group across all five CpG esidues (CpG1: 5.562.7, CpG2:5.963.5, CpG3: 9.863.7, CpG4: 3.762.8, CpG5: 4.263.0) in comparison to low work stress (CpG1: 1366.7, CpG2: 1265.7, CpG3: 1565.4, CpG4: 8.864.3, CpG5: 7.863.5) (p,0.01 for all residues, Figure 2). Methylation results were then validated in 9 pairs of nurses matched by age from high- and low work stress environments using the Human Methylation 450k BeadChip (Illumina Inc.). While variation in methylation was smaller as compared to the bisulfite sequencing in all three sites (CpG1, CpG4 and CpG5) included in the BeadChip, differences in methylation levels remained in the same direction in most cases of the matched pairs (7 out of 9 for CpG1 and CpG4 and 9 out of 9 for CpG5), evidencing for a good coherence between these two methods (p = 0.998 from goodness of fit using chi-squared test). We then tested correlations between individual methylation levels in all five CpG residues. All methylation levels were highly correlated (b.0.5, p,0.01 for all residues). Using a structural equation measurement model, we tested whether these correlations could be explained by a single latent factor. The single factor Table 1. Demographic data of the nurse groups by work stress.Results Burnout and Depression Scores in High and Low Work Stress GroupsOur study sample comprised of female nurses from contrasting work environments (high and low work stress, n = 24 and 25 respectively) initially derived from a large cohort 
of 5615 female health care professionals [26]. Works stress was defined according to Karasek’s Model [27] so that high work stress implicated high demands and low control, while low stress was defined by a combination of low demands and high control. Division into high and low work stress environments was 15755315 based on grouping the wards based on mean scores of all responses, using median split to identify high and low stress extremes in stress distribution. To increase contrast between the comparison groups, the nurses who belonged to the quartile with least work stress in the high stress group and most stress in the low stress group were excluded.Work Stress Environment High work stress Low work stressN 24AgeMBI-GSBDI49 (66.02) 1.33 (60.760) 8.36 (66.31) 48 (66.78) 0.723 (60.487) 4.37 (63.36)doi:10.1371/journal.pone.0045813.tStress Affects Serotonin Transporter MethylationFigure 1. Representation of the SLC6A4 gene promoter region for bisulfite sequencing analysis. Analyzed CpG sites are bolded and numbered from 1 through 5. Coordinates are based on the GRCh37 build (NCBI Reference Sequence: NC_000017.10). doi:10.1371/journal.pone.0045813.gFigure 2. Methylation in the promoter of SLC6A4 at five CpG sites in high (grey) and low (white) work stress environments. Coordinates for each residue are 28 563 120 (CpG5), 28 563 109 (CpG4), 28 563 107 (CpG3), 28 563 102 (CpG2), and 28 563 090 (CpG1) as per GRCh37 build (NCBI Reference Sequence: NC_000017.10). Differences between work stress environments were significant as per t-test (p = 7.10E?6, p = 2.50E?5, p = 0.000292, p = 4.37E?6, and p = 0.000289 respectively). Standard errors are indicated. doi:10.1371/journal.pone.0045813.gmodel was statistically accepted (df = 5; x2 = 4.1; p = 0.537). This was considered as a justification for constructing a single weighted sum score for methylation lev.