From animal to human epigenetics

A rat story: Behavioral epigenetics beginnings Human neurodevelopment is a dynamic and protracted process. It starts in the pre-natal life, driven by genetic information, and continues unfolding following region-specific pathways up to early adulthood (Gogtay et al., 2004; Koenderink & Uylings, 1995; Petanjek et al., 2011). Es pecially during the pre-natal and early post-natal life, the developing brain depends on and is sensitive to external inputs that shape its architecture and fine-tune neural connectivity patterns according to environmental requirements (Branchi & Cirulli, 2014; Fox, Levitt, & Nelson, 2010; Hensch, 2005; Takesian & Hensch, 2013; Teicher, Samson, Anderson, & Ohashi, 2016). Environ mental inputs are therefore critical for a normative development. On the other hand, adverse conditions occurring during sensitive periods for the nervous system maturation can interact with genetic make-up and bias developmental trajectories toward maladaptive outcomes, as dem onstrated by increased occurrence of psychopathology and psychiatric conditions following childhood neglect, maltreatment and abuse (Benjet, Borges, & Medina-Mora, 2010; Bick & Nelson, 2016; Cohen, Brown, & Smaile, 2001; Green et al., 2010; Kessler et al., 2010; Widom, 1999). It has been suggested that modifications of adult brain function and behavior changes induced by early experiences can be determined by changes in the epigenetic status of specific genes (Bale et al., 2010; Fraga et al., 2005; Maccari, Krugers, Morley-Fletcher, Szyf, & Brunton, 2014; Tsankova, Renthal, Kumar, & Nestler, 2007). In fact, epigenetic mechanisms, including DNA methylation, histone modifications, and non-coding RNAs regulation can be affected by various extrinsic factors, providing a molecular link between external cues and gene expression (Kang et al., 2011; Maze et al., 2014; Nord, Pattabiraman, Visel, & Rubenstein, 2015; Shibata, Gulden, & Sestan, 2015). Human studies evidence that individuals exposed to adversity in early post-natal life (Romens, McDonald, Svaren, & Pollak, 2015; Tyrka, Price, Marsit, Walters, & Carpenter, 2012; van der Knaap et al., 2014), or during pre-natal life (Mulligan, D’Errico, Stees, & Hughes, 2012; Perroud et al., 2014) exhibit altered methylation of genes involved in hypothalamic-pituitary-adrenal (HPA) axis functionality, as the NR3C1 gene coding glucocorticoid receptors (GR), a key element for the ho meostasis of stress response system (Herman et al., 2016; Sapolsky, Meaney, & McEwen, 1985). In turn, altered NR3C1 methylation levels have been associated to emotional and behavioral problems, externalizing and internalizing symptoms (Cicchetti & Handley, 2017; Dadds, Moul, Hawes, Mendoza Diaz, & Brennan, 2015; Parade et al., 2016; Perroud et al., 2014; van der Knaap, van Oort, Verhulst, Oldehinkel, & Riese, 2015). Further, reduced levels of NR3C1 messenger RNA (mRNA) and mRNA transcripts, as well as increased cytosine methylation of the NR3C1 promoter were found in suicide victims with a history of childhood abuse compared to suicide victims without childhood trauma and controls (McGowan et al., 2009). Finally, longitudinal studies on very preterm infant admitted to neonatal intensive care unit, and thus subjected to pain-related stress and maternal separation, evidence an altered serotonin transporter gene (SLC6A4) methylation status, predictive of enhanced socio-emotional stress reactivity and asso ciated with less-than-optimal score at Personal-Social scale of Griffith Mental Development Scales at 12 months of age (Fumagalli et al., 2018; Montirosso et al., 2016; Provenzi, Guida, & Montirosso, 2018). Taken together, these findings corroborate an association between environmental experiences, epigenetic modifications and behavioral outcomes. Nevertheless, the cascade of biochemical events through which the environment is embedded in the individual biology, affecting physiology and behavior remains unclear. In addition, not all individuals exposed to early life adversity develop health issues, psychopathology or psychiatric disorders (Collishaw et al., 2007; Yehuda & LeDoux, 2007). Though, the genetic make-up, epigenetic charac teristics, and risk and protective factors that render individuals differently sensitive to environmental influences are not yet understood (Belsky et al., 2009; Belsky & Pluess, 2009; Branchi, 2011). Several aspects hamper the possibility to draw firm conclusions from human studies. Retrospective designs rely on indirect information about the conditions of the participant, and even when information is available or directly collected within prospective studies, it is virtually impossible to disentangle the contribution of multiple factors occurring in pre-natal and post-natal life on specific physiological and behavioral outcomes. Moreover, both retrospective and prospective human studies depend on availability and access to appropriate tissues for epigenetic analysis and are based primarily on saliva, blood and buccal cells samples. Nonetheless, epigenetic patterns appear to be tissue and gene specific (Forest et al., 2018; Smith et al., 2015) and there is little consensus on how much changes observed in peripheral tissues may correlates each other and resemble changes in nervous tissue (Di Sante et al., 2018; Thompson et al., 2013; Walton et al., 2016). Animal models have strongly stimulated and complemented human studies (Phillips & Roth, 2019)(Box 1). Indeed, the animal models allow to prospectively manipulate the onset, quality, du ration and predictability of environmental exposures under controlled conditions and to evaluate im mediate, long-term and trans-generational consequences on candidate gene expression and behavior. Laboratory animals can be exposed to aversive or permissive environments at different develop mental time points and both genomic and non-genomic inheritance can be systematically investigated (Bohacek & Mansuy, 2015, 2017; Francis, Diorio, Liu, & Meaney, 1999; Jirtle & Skinner, 2007; Mitchell et al., 2016; Richards, 2006). In addition, since epigenetic reactions are bidirectional and po tentially reversible (Cervoni & Szyf, 2001; Roth, Denu, & Allis, 2001) the causal relationship between different epigenetic identities and behavioral outcomes can be addressed in animal models by admin istering specific molecular compounds able to promote or inhibit epigenetic mechanisms as DNA methylation (Keller, Doherty, & Roth, 2018, 2019; Weaver et al., 2004, 2005; Weaver, Meaney, & Szyf, 2006). Here, we report the contribution of animal studies, in particular, laboratory rats (Rattus norvegicus) and mice (Mus musculus), to the field of human behavioral epigenetics. The chapter focuses on the role of maternal environment as one of the most studied vectors in inducing epigenetic modifications and enduring phenotype in the offspring. In addition, studies on the interplay among genetic polymor phisms, epigenome and aversive environments in contributing to psychiatric disorders and, as a bright note, on the effect of the exposure to permissive environment (with attention to environmental enrich ment) will be reported.