We extracted probes located within the OXTR CpG island (n = 12), as this area has been previously investigated by Dadds et al (10; see Figure 1, Panel A) and others (13, 21, 22), and shown to play a key role in modulating the transcriptional activity of OXTR (23). For each probe, methylation levels were indexed by beta values (corresponding to the ratio of methylated signal divided by the sum of the methylated and unmethylated signal). Factor analysis was used to reduce the 12 OXTR probes into a smaller set of factors, which accounted for shared variance between them (24). A 3-factor solution (see Figure 1A) showed the best fit to the data as well as good temporal stability. Full details of the factor analysis procedure and results are provided as an online supplement (SI 2–5). We present findings relating specifically to Factor 2 (3 probes located on Exon 2), because (i) Factor 1 did not associate with CU at any time point in either youth with low or high internalizing problems, and (ii) Factor 3 variance was not significant at any time point, thereby precluding the examination of associations with environmental risk and CU traits.
