transgenerational epigenetic effects on brain functions

REVIEW

Transgenerational Epigenetic Effects onBrain Functions
Johannes Bohacek, Katharina Gapp, Bechara J. Saab, and Isabelle M. Mansuy
Psychiatric diseases are multifaceted disorders with complex etiology, recognized to have strong heritable components. Despite intenseresearch efforts, genetic loci that substantially account for disease heritability have not yet been identified. Over the last several years,epigenetic processes have emerged as important factors for many brain diseases, and the discovery of epigenetic processes in germcells has raised the possibility that they may contribute to disease heritability and disease risk. This review examines epigeneticmechanisms in complex diseases and summarizes the most illustrative examples of transgenerational epigenetic inheritance inmammals and their relevance for brain function. Environmental factors that can affect molecular processes and behavior in exposedindividuals and their offspring, and their potential epigenetic underpinnings, are described. Possible routes and mechanisms oftransgenerational transmission are proposed, and the major questions and challenges raised by this emerging field of research areconsidered.

Key Words: Disease risk, DNA methylation, epigenetics, germline,histone, inheritance, mechanisms, mental illness, sperm,transgenerational

Many complex diseases including cancer, diabetes, andpsychiatric and neurological disorders have strong heri-table components (1–3). Multiple genome-wide associa-

tion studies have been carried out in the past decades to tryto identify the genetic basis of these disorders, but despitethese studies, the factors responsible for the diseases andtheir heritability are still obscure (2–6). This “missing diseaseheritability” is a significant obstacle for the clinic because itcomplicates characterization of diseases and confounds predic-tion of risk and susceptibility (2). Missing heritability may resultfrom genetic heterogeneity of patients, overestimated heritabil-ity, or the involvement of multiple, possibly rare gene variants ofsmall effect that are difficult to detect (2,7). Furthermore, it hasrecently been recognized that nongenetic components, specifi-cally epigenetic factors, may also contribute to disease heritability(8,9).

Epigenetics is the ensemble of mechanisms that concurrentlymodify the chromatin to stably or dynamically modulate geneexpression, without affecting the DNA sequence itself. Thesemechanisms primarily involve DNA methylation (DNAme), his-tone posttranslational modifications (HPTMs), and small noncod-ing RNAs (sncRNAs). DNAme is a covalent modification of DNAinduced by addition of a methyl residue to cytosine, usually indinucleotide CpG sequences (10). HPTMs, like DNAme, are alsocovalent chromatin modifications, but unlike DNAme, they occuron the histone proteins around which DNA is wrapped. Sixteenknown HPTMs exist to date and include acetylation, methylation,phosphorylation, ubiquitylation, and sumoylation (11,12). Together,DNAme and HPTMs alter chromatin structure and serve asdocking sites for specialized binding proteins and partners thatultimately regulate gene expression. SncRNAs are short RNAsequences including microRNAs, small interfering RNAs (siRNAs),

From the Brain Research Institute, University of Zurich/Swiss Federal

Institute of Technology (ETH), Zurich, Switzerland.

Address correspondence to Isabelle Mansuy, Ph.D., Brain Research

Institute, University of Zurich/ETH Zurich, Winterthurerstrasse 190,

8057 Zurich, Switzerland; E-mail: [email protected].

Received Feb 17, 2012; revised Aug 7, 2012; accepted Aug 19, 2012.

0006-3223/$36.00http://dx.doi.org/10.1016/j.biopsych.2012.08.019

Piwi-interacting RNAs (piRNAs), and small nucleolar RNA (snoRNA)that regulate transcriptional and/or translational processes (13). Theensemble of these epigenetic processes allows cells to acquire andmaintain a molecular fingerprint in response to internal or externalfactors.

Epigenetic regulation is ubiquitous in the nervous system andis essential for complex neuronal processes, such as memoryformation (14), the persistent remodeling of the stress axis inresponse to adverse or stressful experiences in early life (15–17),and the lasting cascade of events underlying drug addiction (18).Epigenetic alterations induced by salient environmental stimuli orevents often persist a lifetime and, in certain conditions, can betransmitted to subsequent generations (9,19–21). The fact thatepigenetic marks can be inherited provides a plausible explana-tion for the missing heritability of complex disorders. Here, wereview current evidence for transgenerational inheritance ofcellular or behavioral traits induced by environmental conditionsin mammals and outline some of the potential underlyingmolecular mechanisms.

Routes for Transgenerational Transmission ofEnvironmental Effects

In classical genetics, individual traits and features are inheritedby transfer of chromosomal DNA sequences through the germ-line. However, this concept is now recognized to be incomplete,because trait inheritance also occurs via nongenetic factors, inparticular, through epigenetics (8). Epigenetic marks can propa-gate across generations via two routes: one that is independentof the germline and implicates behavioral/social transmission andanother that fully depends on the germline (22,23). In general,behavioral/social transmission is experimentally clearly tractable,for example, by using cross-fostering designs. In contrast,epigenetic germline inheritance has proven more challengingto study, because in experimental setups it is difficult to excludethe influence of maternal care or intrauterine effects as a sourceof transmission (24).

Behavioral TransferNongermline transmission of behavioral and physiological

features generally involves environmental and social factors.The most influential of these are external conditions in early life,particularly interactions between parent/caregiver and offspring.In mammals, perturbation of maternal care during early postnatal

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life can severely and persistently impact adolescent and adult life(16). In rat, it is associated with epigenetic changes involvingDNAme in the brain of the offspring. Thus, good or poormaternal care, as assessed by the level of maternal licking andgrooming, alters DNAme throughout the genome (25). Inparticular, good care decreases DNAme at a transcriptionfactor-binding site within the glucocorticoid receptor (GR) genein the hippocampus, increasing GR expression (26,27). Thischange is associated with predisposition to stress resilience inadult animals, and the effect can be passed to the followinggeneration through altered maternal behaviors: daughters receiv-ing good maternal care during postnatal life become goodmothers themselves, thus reestablishing a similar DNAme profilein the brain of their offspring. When pups are cross-fostered tolow licking/grooming mothers, DNAme is shaped by poormaternal care, and GR in the hippocampus becomes hypermethy-lated (16), proving that the transmission is entirely germline-independent. Other genomic areas including transcriptional andintragenic sequences are also differentially methylated by maternalcare, for example, the chromosomal region containing protocad-herin-a, -b, and -g (Pcdh) genes, which are noticeable because oftheir involvement in synaptogenesis (25). Epigenetic reprogram-ming through behavioral transfer reflects the strong influence ofmaternal care in early life. However, although it can be perpetuatedfrom mother to offspring, it is not permanent and needs to bereinstated at each generation by maternal behaviors.

Germline TransmissionGermline transmission of behavioral and physiological traits

induced by environmental conditions does not require reinstate-ment at each generation and is the focus of this review. In ratsand mice, studies based on paternal lines (or patrilines; when atreated male is paired with a naive female to derive a line) haveshown that such transmission is associated with epigeneticchanges, in particular DNAme, in sperm cells (19,21,22,28). Thisdemonstration has, however, been challenged because demon-strating the sole contribution of epigenetic factors experimentallyis not trivial. First, although in both rats and mice, malesminimally contribute to the rearing of pups, which in theorydisqualifies paternal social/behavioral factors, it is difficult tofirmly exclude other nongermline factors. For example, malesmay indirectly affect rearing because their fitness can modify thedam’s maternal investment toward pups (29). This particular biascan be mostly avoided by cross-fostering after birth, but in uteroconditions may still influence the pups and contribute totransmission. An ultimate control therefore may be in vitrofertilization (IVF), although assisted reproductive methods canalso alter the epigenome on their own, potentially interferingwith germline transmission (30). In conclusion, a careful combi-nation of pre- and postnatal manipulations is a requisite toalleviate such experimental biases and draw firm conclusions.

Potential Mechanisms for Germline EpigeneticTransmission

If acquired traits and associated epigenetic marks can beinherited via the germline, epigenetic changes must first beestablished in germ cells, then transferred to the embryo.However, most epigenetic marks are reset during sexual repro-duction. Most of DNAme is erased by reprogramming duringgametogenesis, presumably to confer totipotency to the embryo,then reestablished after fertilization (31,32). Similarly, in the male

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germline, most HPTMs are lost when histones are replaced byprotamines during spermatogenesis (33). Recently, however,molecular “loopholes” allowing the transmission of both DNAmeand HPTMs have been revealed. Furthermore, many sncRNAshave been detected in sperm and may also contribute totransmission (34,35). Together, these epigenetic mechanismsare likely vectors of epigenetic inheritance (Figure 1).

DNA MethylationDNAme is a strong candidate for the transgenerational

inheritance of acquired traits because it is already known to beinvolved in one form of epigenetic inheritance: genomic imprint-ing. Imprinting is a process of gene silencing by DNAme thatallows the selective expression of only one parental allele(maternal or paternal) (36,37). At most imprinted loci, silencingis mediated by imprint control regions that are hypermethylatedin the inactive allele (HPTMs and sncRNAs are also involved). Sex-specific DNAme imprints are established during oogenesis andspermatogenesis, and are protected from global demethylationfollowing fertilization (31,36,38,39). Such DNAme-dependentregulation affects approximately 100 genes and modulates theirexpression in adulthood (38,40). DNAme can also escape repro-gramming at certain nonimprinted loci, in particular at genesactive in the male germline (41). These genes keep theirpromoter methylation profile in the early preimplantationembryo, implying inheritance of this profile. Finally, anotherimportant feature of DNAme in germ cells is that it can bealtered by environmental factors such as toxins (42), stress (43), oraging (44) at specific genes and remains altered across genera-tions. DNAme therefore represents a conceivable means for themaintenance and perpetuation of acquired epigenomic changes.

Histone Posttranslational ModificationsHPTMs influence chromatin structure and are essential for

gene regulation in somatic tissues. Their role in sperm cells is lessclear, given that histones are not preserved in these cells but arelargely replaced by protamines (up to 98% in mice and 85% inhuman) (35,45). The remaining histones, however, likely play animportant role in gene regulation because they are retained atgenetic loci essential for embryogenesis (46). For instance, inhumans and mice, dimethylated lysine 4 on histone 3 (H3K4me2)and trimethylated lysine 27 on H3 (H3K27me3)—marks thatactivate and repress gene transcription, respectively—are presentat genes involved in spermatogenesis and developmental reg-ulation (45,47). Interestingly, genes enriched for H3K27me3 attranscription start sites in sperm are generally transcriptionallyrepressed in the early embryo (47). This suggests that main-tenance of H3K27me3 in sperm may contribute to paternaltransmission of epigenetic information across generations. Ulti-mately, the possibility that protamines also acquire posttransla-tional modifications and contribute to the epigenetic profile ofsperm cells is conceivable and needs to be investigated.

RNAsAlthough initially thought to be absent in transcriptionally

quiescent germ cells, complex populations of RNAs have beendetected in mature sperm and oocytes. Recent deep sequencinganalyses have estimated that mature sperm cells contain upto 100 fg of spermatozoal RNA in rat and 10 to 400 fg in human(48). Some of these RNAs are fragmented remnants of ribosomalRNAs cleaved to prevent spurious translation (49), and othersare mRNAs and sncRNAs including microRNAs, piRNAs, and

The processes by which epigenetic modifications can be passed from one generation to the next are complex and likely engage several mechanisms that may operate independently or in synergy. The present model elaborates on findings in animals and postulates their existence in human.

One of the mechanisms potentially implicated in epigenetic inheritance involves methylation of sperm DNA (DNAme). DNAme (5-methyl-cytosine) in sperm can persist on certain genes/loci even after the widespread demethylation of the genome occurring in the embryo during reprogramming prior to implantation. Another epigenetic mechanism involves posttranslational modifications (PTMs) of histones (e.g. H3K4me2 or H3K27me3) and protamines in sperm chromatin. DNAme and histones/protamines are likely carried from sperm to the oocyte upon fertilization, and together, establish a specific epigenetic profile in the preimplantation embryo. They may be maintained (and/or continuously re-instated in mitotic cells) during development and adulthood. In the developing and adult organism, DNAme and histone PTMs participate in the epigenetic regulation of gene transcription in various tissues and cells including neurons in the brain. Further to these epigenetic marks at the chromatin, mRNAs and small non-coding RNAs (sncRNA) also exist in sperm cells, and can be transferred to the oocyte upon fertilization. They may directly affect transcriptional and translational processes, modify the epigenetic profile of the early embryo, and ultimately, alter development and behavior.

Major life events such as traumatic stress (represented by a yellow thunderbolt) experienced by an individual in postnatal life (parent generation) may alter epigenetic marks and mechanisms throughout life (represented by red crosses). These epigenetic alterations may be passed to the next generation (offspring) through the germline. While this figure represents potential mechanisms in the male germline, comparable mechanisms in the oocyte may also contribute.

Figure 1. Model for transmission of epigenetic marks through the male germline.

J. Bohacek et al. BIOL PSYCHIATRY 2013;73:313–320 315

snoRNAs (50). Oocytes also contain miRNAs, piRNAs, and a largepopulation of endo-siRNAs (51). The role of RNAs in sperm andoocytes is not clear, but they may contribute to chromatinremodeling and posttranscriptional regulation. Some miRNAsand siRNAs can bind to promoter regions and recruit the genesilencing machinery (52–54). miRNAs might also prevent thetransition from histone to protamine during sperm nuclearremodeling (35).

Further to affecting chromatin remodeling in sperm cells,sperm RNAs are also delivered to the oocyte upon fertilizationand are retained in the developing embryo (55). A fertilizinghuman sperm cell is estimated to transfer 10 to 20 fg of RNAs tothe oocyte (35). This constitutes a direct vector to transmitinformation from father to offspring. However, the functions ofpaternal RNAs delivered to the oocyte are still unknown andlikely vary depending on the type of RNA. Recently, sperm-borne

miR-34c and miR-134A were shown to regulate key genes in thezygote and contribute to embryo development (56,57). They mayrepress translation by targeting the 30UTR of specific mRNAs,which is a typical function of miRNAs (13). piRNAs are alsoimportant for germ cell development and have been implicatedin the silencing of transposons during spermatogenesis (58,59).They may act as guides for DNAme and help establish methyla-tion profiles. Their functions in transmission have not beeninvestigated but may reveal to be important.

Major Examples of Germline TransgenerationalEpigenetic Effects

Although the molecular mechanisms for transgenerationaltransmission of epigenetic marks exist in the germline, solid



evidence for transgenerational epigenetic inheritance in mam-mals is still scarce. The following section summarizes thebest examples relevant to brain development and psychiatricdisorders to date. Other examples related to metabolismor environmental chemicals have recently been reviewed else-where (9,60).

Transposable ElementsEarly evidence that transgenerational epigenetic inheritance

can occur in mammals has been provided by the study ofmetastable epialleles in mice. These are alleles that are variablyexpressed in genetically identical individuals due to epigeneticmodifications established in gametes. Viable yellow agouti (Avy)and axin-fused (AxinFu) are two such alleles which expression inmice correlates with the level of DNAme (61,62). Both genes carrytransposable elements that, when hypomethylated, are activeand interfere with the endogenous gene. This induces a specificphenotype (i.e., yellow instead of brown coat for Avy and kinkedtail for AxinFu) (63). Intermediate levels of methylation on theseloci produce a graded effect with varying yellow coat patches,and different severity of kinked tails, respectively (62,63,64).The differential methylation profile of these metastable epiallelesis persistent and can be passed to the offspring by mothers in thecase of Avy (61), and by both mothers and fathers for AxinFu

(62,65), suggesting transgenerational maintenance and transferof epigenetic information at these loci. It can, however, becorrected in vivo when gestating dams are fed a diet rich inmethyl donors (64). Evidence that such diet-induced transgenera-tional epigenetic changes can also be transferred to subsequentgenerations has indeed accumulated ([66–68], but see Waterlandet al. [69]).

The way such transmission occurs is not fully understood butlikely involves germ cells. In sperm, DNAme is altered at the AxinFu

locus similar to somatic cells (62). Its level in sperm cellsdetermines the penetrance of the kinked tail phenotype, withhigh DNAme being associated with a normal tail and hypomethy-lation with a kinked tail (65). However, DNAme at AxinFu is lost atimplantation because of epigenetic reprograming, suggesting thatDNAme may not be involved in transmission of the phenotype(65). Instead, H3K4me2 and H3K9ac, histone marks associated withactive chromatin, persist in the blastocyst at the AxinFu locus (65)and may prevent the reestablishment of DNAme at AxinFu, thusperpetuating the kinked tail phenotype in the progeny (70). Thesefindings in the Avy and AxinFu models are interesting but couldhave limited cross-species relevance because no comparableretroviral inserts exist at the human orthologs of the Agouti-related protein and Axin1 genes (63). Nevertheless, the Avy andAxinFu epialleles constitute valuable model systems to studygeneral epigenetic mechanisms in mammals.

StressStressful and traumatic experiences, particularly in early life,

are major risk factors for the development of psychiatric disorderslater in life (71). Many stress-induced conditions, includingaffective and psychotic disorders, have a strong heritable com-ponent, but this component cannot be explained solely bygenetic factors. Studies in animal models have examined thepossibility that epigenetic factors contribute to this heritability.An important model developed in this respect has used amanipulation based on unpredictable maternal separation andmaternal stress that mimics traumatic events in early postnatallife. This manipulation severely affects behavior across several


generations (43). Pups subjected to this form of stress (F1generation), and their offspring (F2 generation obtained bybreeding F1 animals to naive controls), develop depressive-likebehaviors when adult, and have deficits in novelty response, riskassessment, and social behaviors (43,72,73). These alterations areaccompanied by persistent molecular changes, in particular,defects in components of the stress pathway and serotonergicsignaling. Transmission occurs through both females (73) andmales (43) and is observed down to the third generation (F3). Itinvolves germ cells and does not depend on maternal/socialfactors, because symptoms can be transmitted by both femalesand males and persist irrespective of whether pups are reared bybiological or surrogate mothers. In sperm of stressed males (F1),DNAme is modified (increased or decreased) at the promoter ofseveral candidate genes, for instance genes involved in the stressresponse or in epigenetic regulation, which may underlie thebehavioral phenotype. Similar alterations are detected in thebrain of the F2 progeny and correlate with altered expression ofthe candidate genes. Although still correlative, these data providestrong initial evidence for a relationship between DNAme andinherited behavioral traits induced by postnatal stress in mice.

Transgenerational effects have also been reported in a modelof maternal abuse in rats. Pups exposed to a stressed dam withpoor and abusive maternal behaviors show DNA hypermethyla-tion in the brain-derived neurotrophic factor (BDNF) gene andhave reduced BDNF mRNA expression in the prefrontal cortex(74). This effect is passed to the offspring by the mother and isonly partially reversed by cross-fostering, suggesting the germ-line contributes to the transmission. Besides postnatal stress, fetalstress can also have a transgenerational impact on offspring (75).Exposure of pregnant dams to mild chronic stress during the firstweek of gestation increases stress-sensitivity and alters hormonalregulation resulting in dysmasculinized male offspring (75). Themale offspring shows a female-like shift in the expression profileof neurodevelopmental genes, and several miRNAs are signifi-cantly reduced in the brain. Progeny of these males havecomparable anomalies, suggesting germline transmission of thephenotype.

Stress has a transgenerational impact not only when experi-enced in early life but is also detrimental in adulthood. In adultmales, chronic social defeat, a severe form of social stress,induces marked depressive-like and anxiety behaviors in theprogeny (76). Transmission may also involve the germlinebecause some (but not all) symptoms can be propagated byIVF (30).

In summary, some evidence suggests that highly stressfulexperiences at different stages in life can markedly affectbehaviors across generations and might constitute heritable riskfactors for affective disorders. DNAme in germ cells likely plays arole, but the precise mechanisms of transmission remain to beidentified.

Endocrine Disruptors, Environmental ToxicantsChemicals and xenobiotics can have strong and persistent

negative effects on physiological and biological functions inhumans and animals, and some of these effects can be trans-mitted. Vinclozolin, an antiandrogenic chemical used as afungicide for agricultural fruit crops, is one such compound.In mice, exposure to high-dose vinclozolin during midgesta-tion increases the risk of infertility, tumor formation, kidneydisease, immune abnormalities, and anxiety behavior acrossfour generations (22,28). The effect of vinclozolin on behavior is


sex-dependent, increasing anxiety in females and reducinganxiety in males (F3 generation). Further, vinclozolin predisposesmales to the detrimental behavioral effects of chronic stresswhen adolescent (F3 generation), suggesting a transgenerationalimpact of ancestral exposure. Vinclozolin also alters gene expres-sion across the genome in hippocampus and amygdala anddifferently activates gene networks in these brain structures(77,78). This correlates with aberrations in DNAme in thepromoters of several genes in sperm that persist in the offspringdown to F3 (28,42,79). Many of the differentially methylatedregions across the genome carry a specific consensus DNAsequence (42), suggesting that certain sequences may behypersensitive to epigenetic changes and/or highly resistant toerasure during development.

Another endocrine disruptor, the environmental estrogenbisphenol A (BPA), present in many plastics and dental cement,and detected in human urine samples, also exerts transgenera-tional effects. Administration of BPA to gestating mice at dosessimilar to those experienced by humans decreases sociability inthe immediate offspring (F1) (80), while increasing sociability insubsequent generations (F2–F4). BPA also transgenerationallydecreases gene expression, in particular, the expression ofoxytocin and vasopressin receptor genes. The mechanismsunderlying these effects remain unknown. Other endocrinedisruptors such as the synthetic nonsteroidal estrogen diethyl-stilbestrol (81–85) and dioxin (86) are also detrimental, andpromote tumorigenesis across generations, suggesting a wide-spread influence of endocrine disruptors on the epigenome.

Enriched EnvironmentIn animals, enriched environmental conditions, such as social

housing in a spacious cage containing running wheels and toys,provide stimulating and engaging conditions that are beneficialfor behavior across generations (87,88). Juvenile mice (2–4 weeksold) raised in enriched conditions have enhanced memoryperformance and increased synaptic plasticity as adults, an effectthat is also observed in their offspring (89). Transmission occursthrough females but not males and is independent of maternalcare because it persists after cross-fostering (89). The contributionof epigenetic mechanisms to this effect has yet to be investi-gated, but these results highlight the potential lasting benefit ofstimulating environmental conditions early in life.

RNA-Dependent EffectsRecent findings have suggested that sncRNAs, in particular

RNAs present in sperm (34,35), may represent an alternativemode of transgenerational transmission of information. The Kitgene, which encodes a tyrosine kinase receptor, is one of the firstgenes shown to be subjected to epigenetic modulation bysncRNAs (90). Kit deficiency in heterozygous knockout mice alterstail and foot pigmentation (white instead of pink). This alteredpigmentation also affects the wildtype offspring of heterozygousanimals and correlates with a reduced level of Kit expression,despite the presence of two intact alleles. This effect is associatedwith higher levels of abnormal, possibly degraded Kit RNA insperm in heterozygous animals. Injection of total RNA extractedfrom the sperm of Kit mutants or of microRNAs targeting theKit mRNA (miR-221 or miR-222) into wild-type fertilized eggsreproduces the pigmentation phenotype in the resulting off-spring (90), strongly supporting a causal role for RNAs intransmission. Another example is miR-1 and its mRNA targetCdh9, a key regulator of cardiac growth. Microinjection of miR-1

in fertilized eggs increases Cdh9 mRNA expression and leads totransmission of cardiac hypertrophy across three generationsthrough both matriline and patriline (91). No study on brainfunctions or behavior has yet been carried out, but the findingssuggest that RNAs, RNA fragments, or miRNAs in germ cells orfertilized eggs likely contribute to the transmission of informationacross generations.

Conclusions and Outlook

Recognizing the potential involvement of epigenetic pro-cesses in the expression and inheritance of behaviors representsa major step forward in the understanding of complex brainfunctions. A transgenerational dimension to how environmentalfactors may influence epigenetic processes in both brainand germ cells adds an important layer of complexity togene � environment interactions. An obvious evolutionaryadvantage of epigenetic inheritance over classical inheritance isthat adaptive responses to environmental challenges can berapidly acquired and passed across generations (21,92), thusbetter preparing the offspring for potential exposure to similarchallenges (60). At the same time, a mismatch between expectedand encountered environments can also lead to maladaptiveresponses (93), as suggested by the Barker hypothesis (94), whena salient environment does not persist as long as the epigeneticeffects. Furthermore, detrimental challenges such as exposure totoxicants or drugs may disturb epigenetic mechanisms andthereby affect disease risk across generations. This offers aplausible explanation for the contribution of epigenetic changesinduced by the environment to the missing heritability ofcomplex diseases.

Research on transgenerational epigenetics is still in its infancyand will require major efforts in the future to uncover its fullpotential. The complexity of experimental designs, long timelinesnecessary to breed multiple generations, and the demand forinterdisciplinary expertise make this research particularly challen-ging. However, development and improvement of powerful andsensitive epigenome-wide methods of analyses hold great promisefor rapid progress, in particular in the identification of novelepigenetic components and pathways in disease etiology and risk.A critical milestone in this respect will be to determine, with highresolution, how DNAme and HPTMs are modified by environmentalfactors across the genome in developing and mature germ cellsand in the brain (95), and which alterations persist and havefunctional relevance. Similarly, identifying and characterizing thefunctions of RNAs in germ cells is needed to clarify their role intransgenerational inheritance and determine whether and howtheir composition is altered by environmental factors.

A final critical step will be to explore the use of epigeneticdrugs as potential therapeutics for brain diseases. Unlike geneticmutations or SNPs that cannot be reversed without gene therapy,epigenetic marks can be manipulated and possibly correctedthrough classical pharmacology. Epigenetic drugs targetingDNAme and HPTMs can reverse altered DNAme and histoneacetylation in vivo and already show therapeutic benefit incancer (96). For brain diseases, histone deacetylase inhibitorscan mimic the effects of antidepressants and alleviate cognitiveand neurological defects in animals (97,98). Although still at apreclinical stage, derivatives of these drugs could in the futurerelieve or cure symptoms of complex neuropsychiatric disordersin human patients (99,100). Before such treatments can beseriously envisaged, however, major progress in basic research



is necessary to identify epigenetic targets more precisely anddevelop more selective drugs.

The lab of I.M. Mansuy is supported by the University Zurich, theSwiss Federal Institute of Technology Zurich, the Swiss NationalScience Foundation, Roche, and the National Center of Competencein Research Neural Plasticity and Repair. JB is supported by apostdoc fellowship from the ETH Zurich and a Roche fellowship. KGis supported by a DOC-fFORTE Fellowship from the AustrianAcademy of Science. BS is a National Alliance for Research onSchizophrenia and Depression Young Investigator.

The authors report no biomedical financial interests or potentialconflicts of interest.

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