The epigenetic landscape of alcoholism.

Schematic representation of the epigenetic mechanisms of acetylation and methylation of lysine (K) residues at histone H3 and H4 tails that mediate the switching between ‘open’ (relaxed) and ‘closed’ (condensed) chromatin structures. Acetylation of histone H3 at K9, K14, K18 and K56 and of H4 at K5, K8, K12 and K16 are believed to relax the chromatin structure making it more accessible to transcription factors and chromatin remodelers, thereby promoting transcription, whereas methylation at these lysine residues condenses the chromatin followed by reduced gene transcription. Histone deacetylases (HDACs) remove acetyl groups and histone methyltransferases (HMTs) attach methyl groups at lysine residues. HDACs are classified into four different classes (class I–IV), of which the class III, sirtuins, are NAD+ dependent and comprise of seven different isoforms (SIRT 1–7). HDACs classes I (HDAC 1, 2, 3, and 8), II (HDAC 4–7, 9, and 10) and IV (HDAC11) are Zn+ dependent enzymes. Histone methyltransferases such as G9a, GLP and SETDB1 can catalyze the methylation of H3-K9 contributing to the restrictive state of the chromatin. Histone demethylases (HDMs) (e.g. KDM family) remove the methyl groups from lysine residues associated with repressive marks (e.g. H3K9) in the tails of histones facilitating the relaxation of chromatin. Histone acetyltransferases (HATs) such as GNAT and MYST families of enzymes and CREB-binding protein (CBP)-p300 complex transfer acetyl groups leading to relaxed chromatin (Allis et al., 2007; Kouzarides, 2007).

Schematic representation of the histone acetylation mechanisms in the amygdala of Sprague-Dawley (SD) and alcohol preferring (P) and –non-preferring (NP) rats, that are operative in the modulation of ethanol drinking and anxiety-like behaviors. Acute ethanol exposure of SD rats inhibits the histone deacetylase (HDAC) activity and increases CBP expression, which has intrinsic histone acetyltransferase activity, leading to the hyperacetylation of histones. This relaxes chromatin and increases expression of activity-regulated cytoskeleton-associated protein (Arc), brain-derived neurotrophic factor (BDNF) and neuropeptide Y (NPY) resulting in acute ethanol’s anxiolytic effects. On the contrary, withdrawal from chronic ethanol treatment increases HDAC activity and decreases CBP levels leading to hypoacetylation of histones and a condensed state of chromatin that reduces BDNF, Arc and NPY, which precipitate anxiety-like behaviors. Treatment with trichostatin A (TSA), a pan-HDAC inhibitor, in alcohol-withdrawn SD rats was able to inhibit HDAC activity thereby increasing histone acetylation and expression of Arc, BDNF and NPY and further attenuated the withdrawal-related anxiety-like behaviors. Similarly, innately higher HDAC activity and HDAC2 expression in the amygdala of P rats compared to NP rats is associated with hypoacetylation of histones and low levels of Arc, BDNF and NPY and high levels of anxiety-like and alcohol-drinking behaviors. Inhibition of HDAC activity by treatment with TSA or knockdown of HDAC2 mRNA levels by using HDAC2 siRNA was able to correct the deficits in innate hypoacetylation and expression of Arc, BDNF and NPY thereby attenuating the high anxiety-like behavior and alcohol-preference in P rats (Pandey, Ugale et al., 2008; Moonat et al., 2013; You et al., 2014; Sakharkar, Zhang, et al., 2014).

Hypothetical representation of chromatin remodeling driven by histone acetylation mechanisms in the amygdala that may be operative in the process of alcohol addiction. The first drinking episode (comparable to an acute ethanol treatment in animal models) may cause more acetylation of the histone tails due to HDAC inhibition that results in an ‘open’ or relaxed chromatin structure leading to euphoric effects e.g. anxiolysis. Repeated exposure to alcohol (chronic drinking) may lead to neuroadaptive changes in these epigenetic mechanisms translating into the development of chronic tolerance to these euphoric effects of ethanol. Consequentially, cessation from drinking may precipitate withdrawal-related anxiety due to a rebound phenomenon and underlying epigenetic causes, e.g. hypoacetylation due to an increase in HDAC activity further ‘closing’ or condensing the chromatin structure, which manifests in the negative affective state or the ‘dark side of addiction’ (Koob & Le Moal, 2005). This leads to the development of dysphoric symptoms such as anxiety and depression and subjects tend to self-medicate with ethanol to relieve these dysphoric symptoms, resulting in alcohol abuse and dependence. As seen from preclinical data (Figure 2), this cycle of withdrawal leading to self-medication can be broken with HDAC inhibitors, which by promoting histone acetylation can revert a ‘closed’ chromatin structure to an ‘open’ configuration and can lead to a normal behavioral state (Pandey, Ugale et al., 2008; Arora et al., 2013; Pandey, Ugale, et al., 2008; You et al., 2014). Innately condensed chromatin due to higher HDAC2 expression and deficits in histone acetylation in the amygdala may also be involved in anxiety and alcoholism (Moonat et al., 2013; Sakharkar, Zhang, et al., 2014).