Background & Aims
Adverse childhood experiences (ACEs) are intensely stressful experiences, such as abuse and neglect, which are associated with increased chronic pain later in life (1). However, concurrence of ACEs and deprivation may confound assessment of potential associations (2). Furthermore, while epigenetic changes associated with ACEs have been reported including increased DNA methylation age in females (3), causal relationships mediating the impact of childhood adversity on adult health are difficult to establish in human studies.
An alternative approach is to use a preclinical model of early life adversity. One such model, the limited bedding and nesting (LBN) paradigm, was developed to mimic aspects of early neglect (4). The LBN paradigm causes fragmented maternal care (FC). We have previously reported that FC changes gene expression, pain vulnerability, and opioid tolerance (5).
This study examined FC-induced changes in DNA methylation in the CNS and spinal cord.
Methods
Male and female C57BL6/J mice were reared in either control or FC cages (with limited bedding and nesting material) between postnatal day (PD) 2 and 9. Once the mice had matured to PD 60, brain and spinal cord tissue samples were collected for DNA methylation analysis using the Infinium Mouse Methylation BeadChip and R packages – SeSAMe, limma and ENmix.
The analysis involved a) identifying differentially methylated CpGs (DMCs), b) identifying differentially methylated regions (DMRs) consisting of 2 or more CpGs and c) determining the DNA methylation age of the samples. DMCs and DMRs with an FDR < 0.05 were considered significant. Corresponding changes in gene expression were established using quantitative PCR.
Results
No individual DMCs were identified as significantly different between controls and fragmented care mice. Nevertheless, in the spinal cord, a location within the transcription start site of the gene encoding fibroblast growth factor-binding protein 3 (FGFBP3) met the DMR criteria.
Additionally, FC exposure increased the expression of mRNA encoding FGFBP3 in the spinal cord without affecting other brain regions or the expression of the gene encoding fibroblast growth factor 2 gene (FGF-2) in the spinal cord, the target of FGFBP3.
While our analysis was able to estimate the ages of samples from brain and spinal cord, there was no significant difference in the DNA methylation age of FC exposed mice relative to control mice regardless of the tissue origin.
Conclusions
Exposure to fragmented care increased the methylation and expression of the gene encoding FGFBP3 in the spinal cord, but not in the other regions of the CNS tested in this study. This positive association between methylation and expression was unexpected as an increase in methylation is most often associated with decreased gene expression. However, bi-direction effects of DNA methylation on gene expression have been previously reported (6). FGFBP3 modulates FGF-2 signalling and FGF-2 causes mechanical hypersensitivity and is implicated in stress responses (7,8). Therefore, enhanced FGFBP3 may contribute to increased vulnerability to persistent inflammatory pain seen in mice exposed to FC (5).
Exposure to FC did not increase DNA methylation age in this study of mixed male and female mice. However, we cannot rule out a sex dependent effect like that observed in a recent epidemiological study of the impact of ACEs on DNA methylation (3).
References
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