DNA methylation as a risk factor in the effects of early life stress

Early life stress has been linked with poor stress adaptation and related illnesses, such as cardiovascular disease, metabolic disorders, and psychiatric illness later in life (Wegman and Stetler, 2009). DNA methylation patterning, the addition of methyl groups to cytosine-guanine dinucleotides (CpG methylation; Bird, 1986) within gene regulatory regions, may moderate or even mediate the effects of early life stress on later stress adaptation. However, the developmental programming of the vast majority of the epigenome has not yet been characterized, and its role in the impact of early stress largely unknown. There are over 22 million genomic CpG sites that may be differentially methylated in each cell of the central (brain) and peripheral (heart, adrenal gland, blood) organs that regulate stress response. While methylation patterns in some gene regulatory regions may be sensitive to environmental cues during postnatal development (Weaver et al., 2004; Champagne et al., 2006; Roth et al., 2009; Murgatroyd et al., 2009), methylation patterns within other gene regulatory regions may be set before birth. They may be partially inherited (Jones and Takai, 2007; Schalkwyk et al., 2010), set during prenatal development, (Kaminsky et al., 2009) or dependent on DNA structure (Brandeis et al., 1994; Schalkwyk et al., 2010). The role of these marks in stress adaptation is largely unknown. Epigenetic patterns of all developmental origins within stress pathway candidate genes may play a role in stress adaptation. Structural variation in the 5’ serotonin transporter (5-HTT) regulatory region has been reported by some investigators to moderate the effects of early life stress in humans and non-human primates (eg, 5-HTTLPR, Champoux et al., 2002; Bennett et al., 2002; Caspi et al., 2003; Barr et al., 2004; Coplan et al., 2011; although see Risch et al., 2009). We have recently shown that epigenetic variation in the 5-HTT regulatory region also moderates the effects of early stress. 5-HTT 5mC content was associated with enhanced behavioral reactivity to stress in infant rhesus macaques that had experienced early life maternal deprivation (Kinnally et al., 2010). Yet there was no association between early life stress and 5-HTT methylation patterns. We concluded that whatever the developmental origins of 5-HTT methylation patterns, taken as a whole, they compounded the risk of early life stress. Thousands of genes are involved with stress response, however (Alter et al., 2008). We will better understand the role of candidate gene DNA methylation patterns if we compare them with individual differences in 5-methylcytosine across the genome. Genome-wide methylation is a potential confound of candidate gene methylation-stress associations because some important stress-related processes may occur at the epigenomic level. For example, pharmacological interventions that globally hypo- or hypermethylate the genome influence risk for stress susceptibility (Covington et al., 2009; Weaver et al., 2005). In the present study, we therefore tested whether candidate gene and global DNA methylation was associated with stress adaptation across the lifespan, even in tissue that is only secondarily involved in the stress response (e.g. blood, which includes cell subsets that have a lifespan of hours to years). We compared the roles of 5-HTT and/or whole-genome 5mC in the effects of experimental variable foraging demand (VFD) stress on behavioral stress adaptation in twenty adult (3–13 years of age) female bonnet macaques (Macaca radiata). Experimentally-induced variable foraging demand (VFD) conditions strain mother-infant relationships because dyads cannot predict whether food will be easily accessible on a given day (Coplan et al., 1996). Consequently, stress response is re-programmed in developing VFD infants and remains disrupted years after the manipulation, similar to maltreated humans. This experimental approach mimics early human adversity, while eliminating genome-environment correlations that can confound naturalistic human studies.