Glucocorticoids (GCs) may play a significant role in the etiology of several neuropsychiatric illnesses. A model for evaluating the role of GCs in mood disorders is Cushing’s syndrome (CS), due to an endogenous increase in cortisol secretion, or secondary to iatrogenic GC treatment. Remarkably, up to 60~90% of patients with endogenous CS develop depression [1
], and the depressive symptoms often disappear with resolution of hypercortisolemia [2
]. In addition, non-CS patients suffering from major depressive disorder (MDD) often present with elevated levels of plasma cortisol and glucocorticoid resistance [5
], further implicating glucocorticoids in mood disorder biology.
Effects of glucocorticoids are mediated in part by the glucocorticoid receptor (GR) and its associated chaperone protein complex consisting of FKBP5, HSP70, and HSP90. Upon activation by GC binding, GR dissociates from the chaperone complex, homodimerizes with other GR molecules, and translocates into the nucleus by interactions with FKBP4 and members of the dynein family of motor proteins [6
]. In the nucleus, the GR dimer acts as a potent activator of transcription by binding glucocorticoid response elements (GREs) and recruiting transcription factors to gene promoters [7
]. Importantly, FKBP5 is one of the immediate-early target genes of GC action, and GC induction of FKBP5 provides a short intracellular negative feedback loop, where FKBP5 reduces its own transcription by impeding further translocation of the GR complex. As a result, FKBP5 has been shown to be a strong modulator of GC-signaling in vitro
, as levels of FKBP5 reduce GR’s affinity for GCs in a dose-dependent manner [6
]. This event, along with GC-induced changes in levels of GR [8
], forms the molecular basis for glucocorticoid resistance.
An animal model of hypercortisolemia and GC resistance provides additional in vivo
support for FKBP5 as a key mediator of GC sensitivity. GC resistance in squirrel monkeys, New World primates of the genus Saimiri
, has been attributed to elevated levels of FKBP5, and transfection assays implicate species- and sequence-specific FKBP5 as the key modulator of GC resistance in this primate [10
]. In humans, several single nucleotide polymorphisms (SNPs) in FKBP5
have been associated with depression, altered HPA-axis function, and rapid response to antidepressants. MDD patients that carry these SNPs exhibit symptoms of GC resistance in that they often fail to properly suppress their endogenous cortisol levels following dexamethasone suppression test [12
]. Specifically, SNP rs1360780 confers decreased sensitivity to GCs by causing elevated FKBP5 protein levels [12
], consistent with symptoms of GC resistance often comorbid with MDD.
Recently, results from several candidate gene association studies have linked FKBP5
to MDD, suicide, or PTSD, only
in the context of previous stressors such as early-life trauma or child abuse events, and have begun to highlight gene-environment interactions as crucial factors for disease development [15
]. These studies suggest that the HPA-axis may become dysregulated during early-life traumatic events and that this alteration persists through many years, even decades, to help precipitate mental illnesses later in life. These findings also suggest that long-term consequences of these early-life traumatic events may be potentiated by DNA sequence-independent, non-mutational events that chronically alter function of HPA-axis genes. To comprehend the long-term consequences of GC exposure on the HPA-axis and mood disorder biology, an epigenetic approach may be useful.
During our efforts to identify GC-induced epigenetic alterations on candidate HPA-axis genes, we made several discoveries in Fkbp5
: (i) GC administration decreases brain and blood DNA methylation (DNAm) in the Fkbp5
gene; (ii) DNAm alterations in Fkbp5
are associated with behavioral deficits in an animal model of Cushing’s syndrome; and (iii) there is a persistence of GC-induced DNAm change in both blood and brain for up to 4 weeks following GC withdrawal [20
To address whether the observed loss of DNAm is confined to cells of lymphocytic and neuronal origins, we chose the murine AtT-20 anterior pituitary corticotroph cells [22
], as these cells synthesize adrenocorticotropic hormone (ACTH), and its transcription is suppressed by GCs [23
]. We also tested this cell line for GC-induced changes in mRNA levels of DNA methyltransferase Dnmt1
, hypothesizing that GCs would decrease Dnmt1