BPD and epigenetic mechanisms
The influence of environmental factors, such as childhood trauma, has been suggested to occur through epigenetic mechanisms, which may underlie gene-environment associated vulnerability to develop stress-related disorders [
74] including BPD where childhood trauma history occurs in most of the patients (with a range between 30 and 90%) [
7,
9].
Among the most investigated epigenetic mechanisms there are: (i) DNA methylation, which occurs at CG dinucleotides (CpG) and can influence the spatial structure of the DNA and the binding or the repression of specific DNA-binding proteins to the DNA [
75], (ii) histone modifications, which influence the condensation of the DNA around histone proteins and regulate the accessibility of functional regions to transcriptional factors [
76] and (iii) post-transcriptional regulation by non-coding RNAs such as microRNAs (miRNAs) [
77].
All these epigenetic processes and, in particular, changes in DNA methylation have been widely investigated in the context of long-term negative effects of early life stressful events. In non-human primates and in rodents, several paradigms of stress early in life, including maternal separation or prenatal stress have been associated with epigenetic alterations via DNA methylation [
78,
79]. For example, non-stressed dams during pregnancy showed increased frequency of licking and grooming in the first week of the puppies’ life that were associated with changes in DNA methylation within the promoter of genes, such as glucocorticoid receptor gene (NR3C1), known to be involved in behavior and neurodevelopment.
The hypothesis is that the quality of maternal care, affected by stress or depression in pregnancy and post-partum [
80,
81] could impact, through epigenetic mechanisms, on gene expression and behavioral traits that are maintained throughout life [
78].
Recently, McGowan and colleagues [
79] examined DNA methylation, histone acetylation and gene expression in a 7 million base pair region of chromosome 18 containing the NR3C1 gene in the hippocampus of adult rat offspring, whose mothers differed in the frequency of maternal care. The authors found that the adult offspring of high compared to low maternal care showed a pattern of regions spanning the NR3C1 gene which were differentially methylated and acetylated, highlighting the idea that epigenetic changes, in the context of early life stress, involve alterations in gene-networks rather than in a single or few genes.
Similarly, studies in humans reported similar results as those found in rodents, including the increased methylation levels within the NR3C1 promoter region in subjects who reported a history of early life adverse events [
82‐
84]. For example, in another interesting study, McGowan and collaborators [
82] found that in humans the cytosine methylation levels of the NR3C1 promoter were significantly increased in the postmortem hippocampus obtained from suicide victims with a history of childhood abuse as compared with those from suicide victims with no childhood abuse or with control samples. Decreased levels of NR3C1 mRNA were also identified, suggesting an effect of childhood abuse on NR3C1 methylation status and gene expression, independently from suicide.
Several epigenetic studies have been also conducted in control subjects characterized for a history of childhood trauma compared to those with no childhood trauma. In this context, Suderman and colleagues [
85] have demonstrated, by using a genome-wide promoter DNA methylation approach, an abuse-associated hypermethylation in 31 miRNAs in a sample of control adult males exposed to childhood abuse. The hypermethylated state for 6 of these miRNAs was consistent with an hypomethylation status of their target genes.
Reduced methylation levels of FKBP5 gene within regions containing functional glucocorticoid responsive elements (GRE) were also found in the blood of control individuals exposed to childhood abuse when compared to subjects without a history of trauma [
86]. This demethylation was linked to increased stress-dependent gene transcription followed by a long-term dysregulation of the stress hormone system and a global effect on the function of immune cells and brain areas associated with stress regulation. Thus, according to the authors, the changes in FKBP5 methylation levels might increase the differential responsiveness of FKBP5 to GR activation that can remain stable over time. Moreover, Labontè and colleagues [
87] have conducted a genome-wide study of promoter methylation in the hippocampus of individuals with a history of severe childhood abuse and control subjects. Methylation profiles were then compared with corresponding genome-wide gene expression profiles. Among all the differentially methylated promoters, 248 showed hypermethylation whereas 114 demonstrated hypomethylation and genes involved in cellular/neuronal plasticity were among the most significantly differentially methylated.
Despite the contribution of DNA methylation has been extensively investigated in association with childhood trauma in the context of pathologies related to stress, studies on the possible involvement of epigenetic mechanisms in BPD vulnerability are only at their birth. Indeed, only few studies are available. In particular, Martin-Blanco and colleagues, investigated the association between NR3C1 methylation status, history of childhood trauma and clinical severity in blood samples of BPD subjects, showing an association between NR3C1 methylation and childhood trauma, in the form of physical abuse, and a trend towards significance for emotional neglect [
88]. Regarding NR3C1 methylation and clinical severity, the authors also found a significant association with self injurious behavior and previous hospitalizations. All these findings support the hypothesis that alterations in NR3C1 methylation can occur early in life as consequence of stress exposure and can persist up to adulthood where subjects with higher NR3C1 methylation levels are also those with enhanced vulnerability to develop BPD.
Above to DNA methylation changes within NR3C1, hypo- or hyper-methylation within other genes have been found to play a key role in mediating the impact of early life stress on the development of stress-related disorders, including BPD [
89‐
92]. For example, in a study conducted by Dammann and colleagues [
89] DNA methylation pattern of 14 genes, selected because previously associated with BPD and other psychiatric disorders, (COMT, Dopamine Transporter 1 (DAT1), Gamma-Aminobutyric Acid Type A Receptor Alpha1 Subunit (GABRA1), G Protein Subunit Beta 3 (GNB3), Glutamate Ionotropic Receptor NMDA Type Subunit 2B (GRIN2B), 5-Hydroxytryptamine Receptor 1B (HTR1B), 5-Hydroxytryptamine Receptor 2A (HTR2A), Serotonin Transporter 1 (5-HTT), Monoamine Oxidase A (MAOA), Monoamine Oxidase B (MAOB), Nitric Oxide Synthase 1 (NOS1), NR3C1, Tryptophan Hydroxylase 1 (TPH1) and Tyrosine Hydroxylase (TH)), was analyzed in the whole blood of BPD patients and controls. An increase in the methylation levels of HTR2A, NR3C1, MAOA, MAOB and COMT was observed in BPD patients as compared to controls, suggesting that an increased methylation of CpG sites within these genes may contribute to BPD aetiopathogenesis. Recently, Perroud and colleagues [
91] investigated the role of childhood trauma on the methylation status of the Serotonin 3A Receptor (5-HT
3AR), including several CpGs located within or upstream this gene. They analyzed its association with clinical severity outcomes, also in relation with a functional genetic SNP (rs1062613) within 5-HT
3AR in adult patients with Bipolar Disorder, BPD, and Attention Deficit Hyperactivity Disorder (ADHD). The results showed that differential 5-HT
3AR methylation status was dependent on the history of childhood maltreatment and the clinical severity of the psychiatric disorder; this association was not specifically restricted to one specific psychiatric disorders investigated by the authors, but was found in patients who reported the higher severity indexes of childhood maltreatment, mainly represented by BPD patients. In particular, childhood physical abuse was associated with higher 5-HT
3AR methylation levels, whereas childhood emotional neglect was inversely correlated with CpG1 I methylation levels. As suggested by the authors, these results highlight the need to search for history of childhood maltreatment in patients suffering from psychiatric disorders as these events could be associated with the worse negative outcomes. Moreover, the authors found a modulation of the 5HT
3AR methylation status by rs1062613 at CpG2 III, where patients carrying the risk CC genotype showed the highest levels of methylation at CpG2 III. Since C allele has been also associated with a lower expression levels of 5HT
3AR, the authors suggested that increased methylation, due to exposure to childhood maltreatment, could lead to a further decrease in the expression of 5HT
3AR mRNA.
Aiming to identify novel genes that may exhibit aberrant DNA methylation frequencies in BPD patients, Teschler and collaborators [
93] performed a genome-wide methylation analysis in the blood of BPD female patients and female controls. The authors reported increased methylation levels of several genes, including neuronal adaptor proteins (Amyloid Beta Precursor Protein Binding Family A Member 2 (APBA2) and Amyloid Beta Precursor Protein Binding Family A Member 3 (APBA3)), zinc-finger transcription factors (GATA Binding Protein 4 (GATA4)), voltage-gated potassium channel gene (Potassium Voltage-Gated Channel Subfamily Q Member 1 (KCNQ1)), guanine nucleotide exchange factors (Proto-Oncogene MCF-2 (MCF2)), adhesion molecules (Ninjurin 2 (NINJ2)) and G protein-coupled receptors (Trace Amine Associated Receptor 5 (TAAR5)) in BPD samples compared to controls. Similarly, using a whole-genome methylation approach, Prados and colleagues [
94] analyzed the global DNA methylation status in the peripheral blood leukocytes of BPD patients with a history of childhood adversity and also in patients with MDD characterized by a low rate of childhood maltreatment. Contrary to Teschler [
93], who used control subjects as reference group, in this study the authors used MDD subjects, most of them suicide attempters, thus controlling not only for MDD but also for a history of suicide. The authors also assessed possible correlations between methylation signatures and the severity of childhood maltreatment. Data showed that several CpGs within or near genes involved in inflammatory processes (Interleukin 17 Receptor A (IL17RA)), regulation of gene expression (miR124–3) and neuronal excitability and development/maintenance of the nervous system (Potassium Voltage-Gated Channel Subfamily Q Member 2 (KCNQ2)) were differentially methylated, either in BPD compared with MDD or in relation to the severity of childhood maltreatment.
In a more recent study, Teschler and collaborators [
95] have analyzed also DNA methylation patterns of the ribosomal RNA gene (rDNA promoter region and 5′-external transcribed spacer/5′ETS) and the promoter of the proline rich membrane anchor 1 gene (PRIMA1) in peripheral blood samples of female BPD patients and controls. The authors have identified a significant aberrant methylation of rDNA and PRIMA1 in the group of BPD patients. Specifically, the average methylation of 6 CpG sites in the promoter of PRIMA1 was 1.6-fold higher in BPD patients compared to controls. In contrast, the methylation levels of the rDNA promoter region and the 5′ETS were significantly lower (0.9-fold) in patients with BPD compared to controls. Furthermore, decreased methylation levels were found for nine CpGs located in the rDNA promoter region and for 4 CpGs at the 5′ETS in peripheral blood of patients compared to controls. These results suggest that aberrant methylation of rDNA and PRIMA1 could be associated with the pathogenesis of BPD.
Taken together, all these studies reveal a complex interplay between BPD, early-life stressful adversities and epigenetic signatures.
BPD and neuroplasticity (the role of BDNF)
Neuroplasticity refers to brain-related mechanisms associated with the ability of the brain to perceive, adapt and respond to a variety of internal and external stimuli [
96,
97], including stress.
The exposure to acute stressful challenges can induce several beneficial and protective effects for the body, which responds to almost any sudden, unexpected events by releasing chemical mediators – i.e. catecholamines that increase heart rate and blood pressure – and help the individual to cope with the situation [
20,
98‐
101]. However, a chronic exposure to stress and thus a chronic exposure to glucocorticoids can have negative and persistent effects on the body, including altered metabolism, altered immunity, enhanced inflammation, cognitive deficits, and also an enhanced vulnerability for psychiatric disorders and for medical conditions such as cardiovascular disease, metabolic disorders and cancer [
102,
103].
Neurotrophic factors, and in particular the neurotrophin Brain-Derived Neurotrophic Factor (BDNF), have been identified as key mediators of stress on neuronal connectivity, dendritic arborization, synaptic plasticity and neurogenesis [
104‐
107]. Since its crucial role in brain development and brain plasticity, BDNF has been widely investigated also in several psychiatric diseases, including BPD [
108].
For example, Koenigsberg and colleagues [
109] found a decrease of Protein Kinase C (PKC) isoenzyme, which is a molecule downstream the activation of BDNF, and BDNF protein levels in the blood of BPD patients, suggesting an alteration of BDNF signaling and consequently of neuroplasticity-related mechanisms in BPD. In another study, Tadic and collaborators [
49] investigated the association between BPD and genetic variants within HTR1B and BDNF genes. Although data showed no significant differences in genotype or haplotype distribution for both HTR1B and BDNF variants between BPD patients and controls, logistic regression analyses revealed an association between the HTR1B A-161 variant and the functional BDNF 196A allele in BPD.
Importantly, several findings have also documented epigenetic modifications on BDNF gene in patients with BPD
, suggesting that childhood maltreatment in BPD patients can cause long term epigenetic alterations of genes crucially involved in brain functions and neurodevelopment, including BDNF, and that these alterations may contribute to enhanced vulnerability to develop BPD pathology. In this regard, Perroud and collaborators [
90] measured the percentage of methylation at BDNF CpG exons I and IV and also plasma BDNF protein levels in subjects with BPD and controls. The authors reported significantly higher methylation status in both CpG regions in patients than in controls, with the number of childhood trauma exposures associated with the high levels of BDNF methylation. Moreover, BPD patients had significantly higher BDNF plasma protein levels than controls, but this increase was not associated with changes in BDNF methylation status. More recently, Thaler and collaborators [
92] analyzed DNA methylation patterns in the promoter region of BDNF gene in women with bulimia nervosa and with history of BPD and/or trauma events. They reported that bulimia nervosa was associated per se with an hypermethylation within BDNF promoter region sites. This was particularly evident when co-occurring with childhood abuse or BPD.
Overall, these studies support the hypothesis that childhood trauma could be associated with changes in BDNF epigenetic signature, that in turn could contribute to alter cognitive functions in BPD patients. Indeed, higher levels of gene methylation are commonly accompanied by a reduced gene expression. Thus higher BDNF methylation levels should determine reduced expression of BDNF gene and reduced BDNF mRNA levels are widely observed in patients with psychiatric diseases [
110‐
112].