Schizophrenia in the Spectrum of Gene–Stress Interactions: The FKBP5 Example

Abstract

Many studies have demonstrated that genotype (G) interacts with adverse life experiences (E) to produce individual differences in vulnerability and resilience to mental disorders, including schizophrenia. Genetic susceptibility to stress and the timing of the environmental exposure(s) are relevant for these interactions and represent common risk factors. We take the example of the FKBP5 gene to illustrate G × E interactions that predict pleiotropic psychiatric outcomes, including schizophrenia.

The 3-Hit Hypothesis of Psychopathology

Individual differences in behavior are partially inherited, and thus determined by genetic variation, but also influenced by exposure to environmental factors. For mental symptoms and disorders, the most influential environmental factors are adverse life events. The diathesis–stress model of behavior distinguishes between vulnerable individuals, who are sensitive to the negative effects of adverse environments, and resilient individuals, who are less sensitive. In addition, genetic variants likely interact with positive environments. It is therefore important to distinguish between negative (eg, childhood trauma) and positive environmental influences (eg, supportive environment) as susceptible individuals may be at a disadvantage when exposed to adversity, but thrive under positive conditions.^1,2 In addition, the impact of environmental exposures is strongly determined by their timing during the lifespan.³

Developmental stressors may have an adaptive or maladaptive value for the organism, depending on the later life context; this concept has been described under the heading of predictive adaptive response, developmental origins of health/disease, and developmental match/mismatch hypotheses.^4–6 To account for these complexities, a 3-hit hypothesis was formulated as follows: “the interaction of genetic factors (hit-1) with early-life environmental factors (hit-2), as reflected in altered endocrine regulations and epigenetic modifications, programs gene expression patterns during brain development, relevant for an evolving phenotype. Such developmentally programmed phenotypes may then be moderated by later life environments (hit-3). Mental functions may be compromised by one type of environmental exposure leading to a higher risk for mental symptoms (vulnerability), but when exposed to another type of environment the same individual may become more resistant to mental dysfunction (resilience)”.⁷

Epigenetic mechanisms are considered key mediators of effects of environmental factors in interaction with genetic variation. Epigenetic modifications (ie, DNA methylation, histone modifications, and noncoding RNAs) can effectively alter transcription of the genome in the absence of changes in nucleotide sequence. There is an evidence that environmental events can directly modify the epigenetic state of the genome both during sensitive developmental periods^8,9 and in adulthood.¹⁰ Interestingly, genotype can influence the extent of epigenetic changes caused by the environment.¹¹ In addition, environmentally induced epigenetic changes (in DNA methylation,¹² histones,¹³ noncoding RNAs)^14,15 may be transferred through successive generations.¹⁶

Gene × Environment Interactions in Schizophrenia

A diathesis–stress model for schizophrenia was originally proposed by Manfred Bleuler¹⁷ and David Rosenthal.¹⁸ This model described schizophrenia as a result of a complex interaction between multiple genes and adverse environmental risk factors, none of which causes schizophrenia on their own. It was summarized by Andreasen as follows: “schizophrenia probably occurs as a consequence of multiple hits, which include some combination of inherited genetic factors and external, nongenetic, factors that affect the regulation and expression of genes governing brain function or that injure the brain directly”.¹⁹

Epidemiological studies have shown that both common genetic variants with low individual odds ratios (OR) and environmental factors with higher individual ORs, between 2 and 4, play an important role in the development of schizophrenia. The heritability estimates for schizophrenia are high in twin studies (81%)^20–22 but not in genetic studies of nonrelated subjects (28%) suggesting a role of shared environmental effects. Concordance for schizophrenia between identical twins is almost 50%, suggesting an additional important role for nonshared environmental and stochastic influences.

Nongenetic pre- and perinatal factors that exert a strong influence on early brain development have been identified as risk factors for schizophrenia. These factors include infections or obstetric complications during pregnancy or delivery, prenatal exposure to toxins or radiation, maternal stress and malnutrition during pregnancy, birth during the winter, and increased paternal age among others.²³ Furthermore, stressful experiences in childhood and even later exposures during adolescence and early adulthood (eg cannabis use) can increase the risk for psychosis through cross-sensitization.^24,25 The effects of these environmental factors are likely moderated by genetic variation. Indeed, several studies have described gene-by-environment interactions relevant to the risk for schizophrenia, including interactions between genetic liability and prenatal exposure to infection, urban birth, maternal depression as well as cannabis use.^26–30 Finally, several researchers have concluded that epigenetic alternations induced by environmental and genetic factors, contribute to the development of schizophrenia.^31,32

HPA-axis Dysregulation in Schizophrenia

Some of the genetic and environmental risk factors implicated in the development of schizophrenia contribute to hypothalamic–pituitary–adrenal (HPA) axis dysregulation.³³ Consistently, patients with schizophrenia or other psychotic disorders manifest HPA-axis dysregulation³⁴ (figure 1). It is, however, not clear if this neuroendocrine phenotype is part of the core disease phenotype or associated with comorbid depression,³⁴ commonly observed in patients with schizophrenia (50%).³⁵

Fig. 1.

Hypothalamus–pituitary–adrenal axis (HPA axis) (re)activity in healthy individuals (A, B) and patients with schizophrenia (C, D). This HPA axis model is adapted from a published model³ and based on rodent anatomy). When the brain detects a threat (physiological or psychological), a coordinated physiological stress response involving autonomic, neuroendocrine, metabolic, and immune system components is activated. A key system in the stress response is the HPA axis. Neurons in the medial parvocellular region of the paraventricular nucleus (PVN) of the hypothalamus release corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP). This triggers the subsequent secretion of adrenocorticotropic hormone (ACTH) from the pituitary gland, leading to the production of glucocorticoids (GCs) by the adrenal cortex. The responsiveness of the HPA axis to stress is in part self-regulated by the ability of GCs to control ACTH and CRH release by binding to 2 corticosteroid receptors, the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR) in the brain. Following HPA axis activation, and once the perceived stressor has subsided, GC feedback loops are triggered at various levels of the system. A direct negative feedback occurs at the hypothalamus and pituitary level (continuous inhibitory lines) and indirect through activation of the hippocampus and the mPFC (discontinuous inhibitory lines). However, GCs exert feed-forward influence on HPA axis activation as well by increasing CRH production in the amygdala, and ultimately amygdala stimulatory influence on PVN neurons activity. In low-stress conditions (A), HPA axis activity and the subsequent activation of the feedback back loops is lower than in high-stress conditions (B). HPA axis dysregulation in individuals with schizophrenia involves limbic structures that are smaller in volume and display altered function. At the same time, hypothalamus and pituitary are larger in volume possibly because of inefficient negative feedback (GC resistance). In low-stress conditions (C), the amygdala, mediating stress anticipation of psychosocial stimuli, is activated to a lesser extent (than in healthy individuals), but in parallel the GC negative feedback (reduced expression and/or function of GR and MR) is less efficient, resulting in detectable difference in hormonal levels between patients with schizophrenia and healthy individuals. However, when stress exceeds a certain threshold (high-stress conditions; D), the amygdala can get activated and so that difference in HPA axis output will be even greater.

Elevated basal cortisol and adrenocorticotropic hormone secretion can be present in schizophrenia patients at different phases of their illness.³⁶ In drug-naïve first episode patients, elevated basal HPA-axis activity has been consistently reported. This may be related to altered stress perception accompanying the psychotic symptoms experienced by the patients. Furthermore, the administration of antipsychotic medication has been shown to influence basal HPA-axis activity.^34,37,38 Finally, illnesses associated with elevated basal cortisol (eg, Cushing’s syndrome) or the exogenous administration of glucocorticoids (GCs) are often associated with the presentation of psychotic or manic symptoms, further supporting a connection between basal HPA-axis activity and psychosis.^39,40

Importantly, patients with schizophrenia respond with more negative emotions to everyday stressors.³³ Physiologically, this is reflected by enhanced cortisol responses to physical stressors and blunted cortisol responses to psychosocial stressors. In a subset of patients with schizophrenia (those with polydipsia), the levels of the hypothalamic peptide arginine vasopressin (AVP) have also been reported to be elevated in response to psychological stress. This is likely related to hippocampal-mediated impairment in the regulation of the HPA axis, and a deficit in central oxytocin activity.⁴¹ Potential differences in the stress appraisal and coping between patients and controls are thus important when interpreting findings related to disease-related differences in HPA axis activation.^34,37,42

Key brain regions involved in stress regulation include the amygdala, the hippocampus and the prefrontal cortex (PFC). Interestingly, all these regions display volumetric alterations in schizophrenia. Reductions in the volume of the hippocampus and the amygdala are consistent findings in imaging studies of schizophrenia.^43–45 The hippocampus volume negatively correlates with basal cortisol⁴⁶ or daily stress sensitivity.⁴⁷ In amygdala, the reported volumetric alterations are dependent on the disease phase.^48,49 Remarkably, early-life stress or later chronic stress can induce similar changes in the volume of hippocampus and amygdala.^50–52 A decrease of PFC—and increases of hypothalamus—and pituitary volumes have also been reported.^53,54

At the molecular level, downregulation of the receptors for GCs, the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR), has been reported in the limbic areas that regulate the central part of the HPA axis (PFC,⁵⁵ hippocampus,⁵⁶ and amygdala).⁵⁷ This is hypothesized to be a compensatory mechanism to protect against chronic hypercortisolism. The expression pattern of GR coregulators in schizophrenia further supports a downregulation in GR signaling.⁵⁸ Such decreased GR function, caused by reduced GR expression and/or sensitivity, likely underlies the impaired GC negative feedback observed in schizophrenia, leading to an excessive and prolonged stress response.⁵⁹

GR is an ubiquitously expressed receptor that can also be found in the dopaminergic and dopaminoceptive circuitry mediating psychotic symptoms. Stress sensitization and the resulting GC dysregulation may trigger a cascade of events resulting in neural circuit dysfunction leading eventually to a dopamine system dysregulation, under the influence of additional genetic and environmental factors.^33,60,61

In summary, both imaging as well as postmortem molecular studies support a dysregulation of stress-sensitive circuits in schizophrenia.

FKBP5 and Schizophrenia

FK506-binding protein 51 (FKBP51) is an interesting molecule in the context of stress liability in schizophrenia. FKBP51, encoded by the FKBP5 gene located on chromosome 6, is a heat shock protein 90 (Hsp90) associated cochaperone that is also part of the GR complex. When bound to the complex, FKBP51 functions as an inhibitor of GC binding to the GR. When it is released, the ligand-bound receptor can then translocate to the nucleus and act as a transcription factor. Importantly the FKBP5 gene contains multiple DNA enhancer motifs, glucocorticoid response elements (GRE), where the GR can strongly bind to and quickly induce FKBP5 gene expression. Thus, FKBP5 induction forms an ultra-short negative feedback for GR activation.⁶² We have identified a single nucleotide polymorphism (SNP), rs1360780, in intron 2 that leads to a differential 3D structure of the gene, with the intron 2 GRE only coming into direct contact with the transcription start site in carriers of the risk allele.^11,63 This is accompanied by higher induction of FKBP5 expression by a stress-induced increase in GCs that leads to a reduction in GR sensitivity and GC negative feedback.⁶⁴ Additionally, the increased activation of the GR following early trauma is followed by a demethylation of another GRE in intron 7 and a further increase in GC-stimulated FKBP5 transcription. In this case, the long term epigenetic response to childhood abuse is linked to the individual’s genetic predisposition via subsequent systemic changes in the HPA-axis.¹¹ This gene × early trauma interaction, but also the genetic variation itself or the haplotype tagged by it, have been linked to a plethora of mental disorders and phenotypes depending on the later life context and/or additional genetic factors: bipolar disorder,^65–67 depression,^68–73 psychosis,^74,75 PTSD,^72,76,77 aggression,⁷⁸ and suicide attempt.⁷⁹ In the case of psychosis, 2 studies have reported effects of rs1360780 on risk. The first study⁷⁵ reports an interaction of SNPs within the FKBP5 risk haplotype with child abuse on psychotic symptoms in a twin sample from the general population as well as in unaffected siblings and patients with psychosis. These findings are supported by Ajnakina et al⁷⁴ who report a higher prevalence of the rs1360780 risk allele in patients with first onset psychosis, but only after adjusting for 2 environmental risk factors: parental separation and cannabis use. Both studies suggest that FKBP5 variants contribute to psychosis risk only in the presence of specific environmental factors. Finally, FKBP5 mRNA levels were shown to be elevated in postmortem PFC of patients with schizophrenia.⁵⁸

One could thus hypothesize that a disinhibition of FKBP5 gene expression by genetic and epigenetic factors is associated with an impaired negative feedback of the stress hormone response and possibly risk for a number of mental disorders and traits for which adverse life events have been shown to be risk factors. The altered risk for mental disorders by FKBP5 could thus be mediated by the effects of a prolonged cortisol response following even minor stressors^64,80–82 on GR-responsive pathways. The affected pathways may be different depending on the individual’s risk for a specific disorder. In schizophrenia, such pathways could include dopamine neurotransmission³³ and the immune system.⁸³ Through these molecular changes, FKBP5 variants, by themselves or in interaction with early trauma, may alter the activity or structure of stress-relevant brain regions. For example, carriers of the FKBP5 risk allele have a higher bias toward threat and this is associated with an increased activation of the hippocampus in that task.⁸⁴ Differences in the structural connectivity of posterior cingulum, a white matter tract proximal to the hippocampus that facilitates communication between the entorhinal and cingulate cortices, have been also reported depending on rs1360780 allele status.⁸⁵ Two studies have now also shown an increased activation of the amygdala with risk allele carrier status and exposure to childhood trauma, with both main and interactive effects reported.^86,87 One study shows effects of FKBP5 variants within a composite genetic risk score to interact with early life stress to predict left hippocampal and left amygdala volumes in 3–5-year-old children.⁸⁸ However, the GxE effects on overall hippocampus volume are more controversial as 2 studies report negative findings for FKBP5 × early-trauma on hippocampus volume.^88,89 As summarized above, differences in the volume and activity of these brain regions have been associated with schizophrenia.^43–45 In addition, environmental risk factors for schizophrenia, such as urbanicity have been shown to also increase amygdala activity in stress tasks.⁹⁰ It is thus possible that FKBP5 could moderate the effects of these environmental factors on disease risk by either diminishing or enhancing their effect on stress-related brain circuits.

Overall, FKBP5 may serve as an example of a general stress-moderating factor that contributes to more generic risk and resilience to a number of mental disorders by altering the negative effects of adverse life events.

Conclusions

Risk for schizophrenia is impacted by both environmental and genetic factors. One important area of risk is the domain of stressful life events, especially early in life. Neural circuits involved in the regulation of the stress response, including the amygdala and the hippocampus, show alterations in schizophrenia, and endocrine and molecular changes support a dysregulation of the stress system in this disorder. Genetic and epigenetic factors moderating the long term consequences of adverse life events may thus also moderate the risk for schizophrenia. The first examples at the candidate gene level suggest that this may be possible. These investigations require follow-up in larger cohorts as well as in hypothesis-free genome-wide approaches.⁹¹