**3. Epigenetics and the brain – foetal programming**

The observed foetal basis of some adult onset diseases requires both epigenetic and genetic factors to be involved in regulating developmental biology outcomes, as emphasized by Skinner (2011), who cites the now classical example of insulin resistance and obesity. Developmental studies of metabolic 'programming' suggest that insulin resistance may appear during early development in individuals born small for gestational age. Insulin resistance can promote obesity, which in turn, could sustain the state of insulin resistance in later life. Skinner et al. (2011) stresses that "the current paradigm of DNA mutational events promoting evolution is accurate, but the inclusion of epigenetics allows for a much higher degree of variability in the biological system to facilitate an adaptation event and epigenetic transgenerational inheritance is a novel concept with considerable experimental support in plant and mammalian studies. This insight, therefore, does not modify the fundamental Darwinian evolutionary paradigm, but adds a neo-Lamarckian component allowing a more diverse molecular mechanism" (Skinner 2011). The role of epigenetics in controlling neuronal functions that may ultimately underlie behavioural adaptations represents a strong emerging research theme (Nelson & Monteggia, 2011).

Stress Shaping Brains: Higher Order DNA/Chromosome

behaviours and neurodevelopment.

than originally envisaged.

hippocampus (Guterman et al., 2006).

**driver** 

Mechanisms Underlying Epigenetic Programming of the Brain Transcriptome 353

and affective disorders show sex differences (Crews, 2008). A comprehensive review of over 100 cases of transgenerational epigenetic inheritance have now reported the phenomena in a wide range of organisms including prokaryotes, plants, and animals (Jablonka et al., 2009). It appears that RNA plays a major role in germline dependent epigenetic modifications (Ashe & Whitelaw, 2007), an aspect which is relevant to links suggested to exist between chromosomal rearrangement at fragile regions, transgenerational transmission of certain

Twin research findings clearly indicate how an appreciation of epigenetics is missing from an understanding of how different phenotypes can originate from the same genotype. Although epigenetically indistinguishable during the early years of life, monozygotic twins exhibit remarkable differences later in life in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression profiles (Fraga et al., 2005). Epigenetic changes can result in a normal genotype suddenly being associated with a disease phenotype and genotypes associated with susceptibility to certain disorders may have reduced penetrance and subsequently develop as a normal phenocopy (Singh et al., 2004). Due to the indirect relationship between phenotype and genotype in epigenetics, finding a distinct set of genes that will be consistently linked with a particular neurobehavioural disorder phenotype on a worldwide basis may prove to be more difficult

**4. The brain genome - environment interface: – stress as an evolutionary** 

Following Waddington's early epigenetic concept, biological stress could be stated to represent a dyshomeostatic influence which produces a diversifying biological response following which a novel variant may have a survival advantage, making it an essential driver of evolution. Evolutionary processes are strongly influenced by the competition for available energy, with the required physical or mental skills being passed to offspring of the most able competitors. Diversity is clearly an asset in this process. A broader repertoire of cognitively linked, novelty stress-based learning associated with a complex range of emotions and increased cognitive integration through higher interneuronal density in humans is suggested to have diversified novelty information management. Stress hormones participate in modulation of memory consolidation processes in both the amygdala and the

It increasingly appears possible that stress management systems operating within nonpathological parameters are utilised to deal with 'novelty'. The physiological activity of stress hormones has been shown to play an important role in modulation of memory consolidation processes in both the amygdala and the hippocampus (Turner et al., 2008). Severe psychosocial stress in early life crossing the proposed physiological stress management system boundaries, can adversely impact brain development itself, and the literature on stress suggests that these changes also occur largely through the hypothalamic pituitary adrenocortical (HPA) axis (Loman and Gunnar, 2010). Steroid receptors function by binding to specific structural elements in the regulatory regions of target genes by recruitment of cofactors that modify histones and chromatin structure (Trapman & Dubbink, 2007). Global changes in epigenetic markers in response to fear conditioning have been demonstrated

**3.3 Caveats of twin studies in disorders where epigenetics are important** 

### **3.1 Context-dependent epigenetic modifications**

Context-dependent epigenetic modifications refer to transmission within a generation (within an individual's own lifetime, including the interaction of parent and young), while germlinedependent epigenetic modifications deal with transmission across generations (Crews, 2008). The best examples of context-dependent epigenetic modifications are those that either have an effect early in life, such as exposure to endocrine disrupting compounds *in utero* or smoking during childhood and adolescence (known collectively as the foetal basis of adult disease, or foetal programming also alluded to above. In the first instance the onset of disease manifests later or the deleterious effects decline with time. However, "the extent to which the modification is perpetuated is by simple persistence of the environmental factors that bring about the epigenetic modification; that is, in each generation individuals are exposed to the same conditions. Hence, the environment can induce epialleles, but this environmentally induced epigenetic state can be reversed by a different environmental factor" (Crews, 2008). An example supplied by Crews (2008) of a context-dependent epigenetic modification on behaviour is considered to be exemplified by the study of Meaney and colleagues (Meany, 2001; Meaney & Syzf, 2005; Champagne, 2008). In a series of studies in rats, this group demonstrated that the nature and amount of care a pup receives from the mother modulates its reaction to stress in later life, largely through effects on the glucocorticoid receptor (GR) in the hippocampus. This maternal effect can cross generations, but its heritability depends upon the pup's experience in the first week of life. Crews (2008) mentions in his review that Meaney's group also recently documented that being reared by a high quality mother results in the expression of the transcription factor A (NGFI-A), a nerve growth factor-inducible protein, that binds to the first exon of the GR gene, resulting in increased expression of GR. High quality maternal care during this critical period demethylates NGFI-A and the acetylation of histones. Just as cross-fostering can reverse these molecular and behavioural changes, infusion of methionine, a histone deacetylase inhibitor, into the hippocampus can also reverse these events (Weaver et al., 2006).

Selective breeding cannot stabilize these brain–behaviour differences and, the effects of high and low quality mothering disappear after five generations indicating that it is not a germline-dependent epigenetic modification but a context-dependent epigenetic modification. The implications of the work are, however, still regarded as important: in humans it has been reported that rearing environment can overcome the influence of a polymorphism in the gene encoding the neurotransmitter-metabolizing enzyme monoamine oxidase A in the aetiology of violent behaviour (Caspi et al., 2003).

#### **3.2 Germline-dependent epigenetic modifications**

Germline-dependent epigenetic modifications "are fundamentally different from contextdependent epigenetic modification in that the epigenetic imprint has become independent of the original causative agent" (Crews, 2008). Here the epigenetic modification is transferred to subsequent generations because the change in the epigenome has been incorporated into the germline. Thus, the effect is manifest each generation without the need for re-exposure. The inheritance of environmentally induced phenotypes is the origin of the concept of epigenetics as conceptualized by Conrad Waddington in 1934. (Costa et al., 2004; Morange, 2009). In such instances the DNA methylation imprints of heritable epialleles are passed through to subsequent generations rather than being erased as occurs normally during gametogenesis and shortly after fertilization. Germline-dependent epigenetic modifications tend to be associated with one sex, an important aspect as many behaviours

Context-dependent epigenetic modifications refer to transmission within a generation (within an individual's own lifetime, including the interaction of parent and young), while germlinedependent epigenetic modifications deal with transmission across generations (Crews, 2008). The best examples of context-dependent epigenetic modifications are those that either have an effect early in life, such as exposure to endocrine disrupting compounds *in utero* or smoking during childhood and adolescence (known collectively as the foetal basis of adult disease, or foetal programming also alluded to above. In the first instance the onset of disease manifests later or the deleterious effects decline with time. However, "the extent to which the modification is perpetuated is by simple persistence of the environmental factors that bring about the epigenetic modification; that is, in each generation individuals are exposed to the same conditions. Hence, the environment can induce epialleles, but this environmentally induced epigenetic state can be reversed by a different environmental factor" (Crews, 2008). An example supplied by Crews (2008) of a context-dependent epigenetic modification on behaviour is considered to be exemplified by the study of Meaney and colleagues (Meany, 2001; Meaney & Syzf, 2005; Champagne, 2008). In a series of studies in rats, this group demonstrated that the nature and amount of care a pup receives from the mother modulates its reaction to stress in later life, largely through effects on the glucocorticoid receptor (GR) in the hippocampus. This maternal effect can cross generations, but its heritability depends upon the pup's experience in the first week of life. Crews (2008) mentions in his review that Meaney's group also recently documented that being reared by a high quality mother results in the expression of the transcription factor A (NGFI-A), a nerve growth factor-inducible protein, that binds to the first exon of the GR gene, resulting in increased expression of GR. High quality maternal care during this critical period demethylates NGFI-A and the acetylation of histones. Just as cross-fostering can reverse these molecular and behavioural changes, infusion of methionine, a histone deacetylase inhibitor, into the hippocampus can

Selective breeding cannot stabilize these brain–behaviour differences and, the effects of high and low quality mothering disappear after five generations indicating that it is not a germline-dependent epigenetic modification but a context-dependent epigenetic modification. The implications of the work are, however, still regarded as important: in humans it has been reported that rearing environment can overcome the influence of a polymorphism in the gene encoding the neurotransmitter-metabolizing enzyme monoamine

Germline-dependent epigenetic modifications "are fundamentally different from contextdependent epigenetic modification in that the epigenetic imprint has become independent of the original causative agent" (Crews, 2008). Here the epigenetic modification is transferred to subsequent generations because the change in the epigenome has been incorporated into the germline. Thus, the effect is manifest each generation without the need for re-exposure. The inheritance of environmentally induced phenotypes is the origin of the concept of epigenetics as conceptualized by Conrad Waddington in 1934. (Costa et al., 2004; Morange, 2009). In such instances the DNA methylation imprints of heritable epialleles are passed through to subsequent generations rather than being erased as occurs normally during gametogenesis and shortly after fertilization. Germline-dependent epigenetic modifications tend to be associated with one sex, an important aspect as many behaviours

**3.1 Context-dependent epigenetic modifications** 

also reverse these events (Weaver et al., 2006).

oxidase A in the aetiology of violent behaviour (Caspi et al., 2003).

**3.2 Germline-dependent epigenetic modifications** 

and affective disorders show sex differences (Crews, 2008). A comprehensive review of over 100 cases of transgenerational epigenetic inheritance have now reported the phenomena in a wide range of organisms including prokaryotes, plants, and animals (Jablonka et al., 2009). It appears that RNA plays a major role in germline dependent epigenetic modifications (Ashe & Whitelaw, 2007), an aspect which is relevant to links suggested to exist between chromosomal rearrangement at fragile regions, transgenerational transmission of certain behaviours and neurodevelopment.

### **3.3 Caveats of twin studies in disorders where epigenetics are important**

Twin research findings clearly indicate how an appreciation of epigenetics is missing from an understanding of how different phenotypes can originate from the same genotype. Although epigenetically indistinguishable during the early years of life, monozygotic twins exhibit remarkable differences later in life in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression profiles (Fraga et al., 2005). Epigenetic changes can result in a normal genotype suddenly being associated with a disease phenotype and genotypes associated with susceptibility to certain disorders may have reduced penetrance and subsequently develop as a normal phenocopy (Singh et al., 2004). Due to the indirect relationship between phenotype and genotype in epigenetics, finding a distinct set of genes that will be consistently linked with a particular neurobehavioural disorder phenotype on a worldwide basis may prove to be more difficult than originally envisaged.
