Review Article Epigenetics and Autismdownloads.hindawi.com/journals/aurt/2013/826156.pdfReview Article Epigenetics and Autism TafariMbadiweandRichardM.Millis Department of Physiology

Hindawi Publishing CorporationAutism Research and TreatmentVolume 2013 Article ID 826156 9 pageshttpdxdoiorg1011552013826156

Review ArticleEpigenetics and Autism

Tafari Mbadiwe and Richard M Millis

Department of Physiology amp Biophysics The Howard University College of Medicine Washington DC 20059 USA

Correspondence should be addressed to Richard M Millis rickmillisaolcom

Received 9 April 2013 Revised 17 July 2013 Accepted 1 August 2013

Academic Editor Jean-Louis Adrien

Copyright copy 2013 T Mbadiwe and R M MillisThis is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

This review identifies mechanisms for altering DNA-histone interactions of cell chromatin to upregulate or downregulate geneexpression that could serve as epigenetic targets for therapeutic interventions in autism DNA methyltransferases (DNMTs) canphosphorylate histone H3 at T6 Aided by protein kinase C1205731 the DNMT lysine-specific demethylase-1 prevents demethylationof H3 at K4 During androgen-receptor-(AR-) dependent gene activation this sequence may produce AR-dependent geneoveractivation which may partly explain the male predominance of autism AR-dependent gene overactivation in conjunctionwith a DNMT mechanism for methylating oxytocin receptors could produce high arousal inputs to the amygdala resulting inaberrant socialization a prime characteristic of autism Dysregulation of histone methyltransferases and histone deacetylases(HDACs) associated with low activity of methyl CpG binding protein-2 at cytosine-guanine sites in genes may reduce the capacityfor condensing chromatin and silencing genes in frontal cortex a site characterized by decreased cortical interconnectivity inautistic subjects HDAC1 inhibition can overactivate mRNA transcription a putative mechanism for the increased number ofcerebral cortical columns and local frontal cortex hyperactivity in autistic individuals These epigenetic mechanisms underlyingmale predominance aberrant social interaction and low functioning frontal cortex may be novel targets for autism prevention andtreatment strategies

1 Introduction

Autism spectrum disorders (ASDs) are a range of neurode-velopmental disorders typically characterized by repetitiveand stereotyped behavior limited social development andimpaired language skills Autistic disorder Asperger syn-drome and pervasive development disorder not otherwisespecified (PDD-NOS) are the most commonly diagnosedASDs and there are also a large number of cases thatare considered idiopathic because the etiology is unclear[1] Notably the incidence of ASD diagnoses has increasedsubstantially in the past twenty years growing by as muchas 5- or 10-fold although the approximate 4 1 ratio ofaffected males to female has been maintained [2] Some ofthis increase has been driven by shifting diagnostic criteriaheightened awareness and improved diagnostic techniques[3] However some of this increase could be attributable toan authentic increase in the frequency of ASDs The identityof the factors fueling this increase remains somewhat elusiveand the precise causes of ASD diagnoses remain unknown

[4] Although its basis is thought to be multifactorial autismis known to be a highly heritable disorder [5] To a certainextent the observed inheritance patterns can be explained bytypical genetic processes but especially in light of studies not-ing discordance among monozygotic twins this is unlikelyto be the whole story [6] Recently the idea that epigeneticinfluences may be partly responsible for the development ofASDs in many patients has gained popularity

Epigenetics refers to processes notably the methylationof genes and modification of histones that affect geneexpression without altering the genetic code a related termepigenomics concerns the study of the epigenome which isa catalog of the heritable chemical changes made to DNA andhistonesThe effects of environment on phenotype are gener-allymediated through epigenetic processes [7] Typical epige-netic mechanisms include the formation of 5-methylcytosineand acetylation of histones therebymodifying the chromatin[8] These epigenetic mechanisms can result in the silencingof particular genes and will ultimately impact the expressedphenotype [9] Each individualrsquos unique epigenomemdashthe

2 Autism Research and Treatment

genome plus any epigenetic modificationsmdashdevelops as aconsequence of a variety of factors The first and probablymost important is the various effects exerted by the environ-ment on the epigenome [10] Second the epigenome is itselfheritable a mother for instance can pass a methylated geneto her offspring [11] Third the epigenomemdashmuch like thegenomemdashis subject to replication errors however whereasthe typical error rate for gene replication is 1 1000000 theusual error rate for replicating epigenetic elements is closer to1 1000 [12] Fourth spontaneous changes to the epigenomeapart from changes driven by environmental causes arethought to occur known as epigenetic drift [13]The latter twoelements account for the stochastic nature of the epigenomethat is the tendency of epigenomes to diverge despite havingidentical starting conditions

This review examines the current state of knowledgeconcerning the impact of epigenetic mechanisms of thedevelopment of ASD and is organized in the followingsections

2 Epigenetic Protein-DNA InteractionsProteins Mediating Epigenetic Signaling

21MeCP2 Methyl CpG binding protein 2 (MeCP2) is activein CNS regulation and development of synaptic contacts[14] MeCP2 is known to be involved in gene silencing andas a consequence in its role epigenetic regulation has beenthe focus of a significant amount of investigation [15] TheMeCP2 gene is located on the q arm of the X chromosomeSince the MeCP2 gene is located on the X chromosomeit is X-linked and subject to X inactivation Until recentlyit was widely thought that MeCP2 was only responsiblefor the silencing of genes [16] However gene silencing isinconsistent with the mode of action of the MeCP2 proteinproduct Like other members of the methyl-CpG bindingdomain (MBD) familyMeCP2 binds tomethylatedDNA [17]and after binding MeCP2 forms a complex with the enzymehistone deacetylase 1 (HDAC1) that removes acetyl groupsfrom histones thereby causing the chromatin structure tocondense The condensation of the chromatin is critical togene inactivation However recent investigations suggest thatMeCP2 may also be capable of acting as an activator ofa variety of genes [18] Although the mechanism of thisactivating action is not totally clear the dual functionality ofMeCP2 is amply demonstrated by studies showing that 63ofMeCP2-bound promoters are actively expressed [17] It is notclear whether the role of MeCP2 in the epigenetic regulationof autism is related to its role as a gene silencer or promoterNevertheless a correlation between reduced expression ofMeCP2 and ASD is noted

Using immunofluorescence Nagarajan et al quantifiedMeCP2 in the frontal cortex (Brodmann area 9) and fusiformgyrus (Brodmann area 37) [19] The frontal cortex had previ-ously been linked to autism and associated with high levels ofMeCP2 expression whereas the fusiform gyrus is associatedwith face processing [20 21]MeCP2 expression at these brainsites was measured for 14 autistic brains each of which wascompared to three age-matched controls In 11 of the 14 cases

the autism brain samples showed significantly decreasedMeCP2 expression compared to age-matched controls insome cases the reduction was as much as twofold Similarlythe proportion of cells that expressed high levels of MeCP2was reduced in 11 out of the 14 autistic samples Of thesix fusiform gyrus samples examined by Nagarajan et alfive showed decreased expression of MeCP2 in the fusiformcortex and each of those five were among the samples thatexhibited decreased MeCP2 expression in the frontal cortexThis concordance suggests that whatever is responsible fordecreased MeCP2 expression in the brains of ASD subjects islikely exerting a generalized nonlocalized effect The autismpatients whose brains were examined byNagarajan et al wereclassified as idiopathic indicating that there was no knowngenetic cause for their ASD or for the decreased MeCP2expression that appears to be linked to the ASD Epigeneticregulation might help explain these findings In order to testfor methylation of the promoter region associated with theMeCP2 gene Nagarajan et al conducted bisulfite sequencingThe MeCP2 is on the X chromosome and all the studysubjects were males and thus actively expressing the Xchromosome Methylation of the 51015840 portion of the MeCP2regulatory region was observed for most autism samples andthe autism group showed a statistically significant increasein methylation when compared to similarly aged controlgroup samples As expected an inverse correlation wasfound between promoter region methylation and MeCP2expression These findings suggest that aberrant methylationmay have resulted in decreased expression of MeCP2 whichwas associated with autism It is also worth noting that thereis a well-established relationship betweenMeCP2 defects andRett syndrome (Rett is definitively diagnosed by evidenceof such a defect) since from a clinical standpoint Rett isclassified as an ASD

3 Epigenetic DNA-Protein Interactions

31 Protein Kinase C Beta Recent evidence suggests thata correlation exists between downregulation of the pro-tein kinase C beta gene (PRKCB1) in the temporal lobeand ASDs [22] In particular this association appears tobe linked to the alternative splicing of PRKCB1 isozymesfsI and betaII In addition PRKCB1 haplotypes are (sta-tistically) significantly associated with autism Moreoverwhole genome expression analysis showed less coordinatedexpression of PKCB1-driven genes [22] Phosphorylation ofhistone H3 at threonine 6 (H3T6) by protein kinase c beta-1 protein appears to prevent lysine-specific demethylase 1(LSD1) from demethylatingH3K4 during androgen receptor-dependent gene activation [23] This finding may partlyexplain the male predominance of ASDs and might sup-port the hypothesis that the higher fetal androgen levelsin males than females produce greater arousal inputs tothe amygdala which might sensitize boys to environmentalstressors Girls lack such androgen-facilitated arousal inputsto the amygdala and are protected from such high arousalinputs by estrogens oxytocin and the oxytocin receptor[24] A role for an oxytocin receptor polymorphism in

Autism Research and Treatment 3

ASDs is also reported in Chinese and Japanese cohorts[25 26] and dysregulation of DNA methylation in thepromoter region of the oxytocin receptor gene has beenobserved after acute psychosocial stress in an elderly Germancohort [27]

32 Oxytocin Receptor Epigenetic regulation of the oxytocinreceptor gene (OXTR) has been implicated in the etiologyof ASDs [28] Oxytocin along with vasopressin has alsobeen determined to have a prosocial function [29] Inselwas the first to suggest a link between oxytocin and ASDs[30] Some evidence for this link comes from animal studiesOXTR and oxytocin-knockout mice have been shown tohave limited social memory and a diminished ability torecognize other individuals both of which are common ASDsymptoms [31 32] Interestingly the effect ofOXTR-knockouton social functioning is thought to be sex-specific both devel-opmental compensation and the effects of vasopressin havebeen posited as possible explanations for the normal socialdevelopment in female OXTR knockout mice [33] Thesefindings suggest that any defect of the oxytocin pathwayincluding a deficiency of oxytocin receptors may in somecases contribute to the development of ASDs A diminishednumber of oxytocin receptors can have a variety of causesincluding both genomic and epigenetic A study by Gregoryet al looked at a family inwhich themother had a hemizygousdeletion of the OXTR gene which she passed down to oneof her sons but not the other however both sons werediagnosed with autism [34] Kimura et al hypothesized thatthe promoter region of the OXTR gene of the affected siblingwithout the deletion was hypermethylated Prior studies hadidentified two CpG island regions of the OXTR gene thatas a consequence of variable methylation are reported tobe associated with differential OXTR expression in liver andmyometrium [35] The first CpG island overlaps with exons1 2 and 3 of OXTR gene and the second CpG island waslocalized to the third intron The second CpG island withinintron 3 was found to be heavily methylated in all threefamily members studied the mother and her two affectedsons On the other hand the other CpG islandmdashoverlappingexons 1 2 and 3mdashwas methylated differently in each ofthe family members specifically the affected sibling withoutthe deletion showed significantly more methylation thanhis brother or mother at three sites within the intron Thishypermethylation occurred at locations that have previouslybeen shown to impact OXTR expression Since both siblingswere autistic even though one had a genomic deletion andthe other displayed hypermethylated promoter regions theGregory et al study stands as an elegant demonstration ofthe idea that epigenetic and genetic mechanisms can haveequivalent effects on phenotypeTheGregory et al studywentone step further in an attempt to demonstrate that OXTRgene silencing is not unique to the highlighted case and isin fact a common contributor to autism Five differentiallymethylated CpG islands were examined in a group of 20autistic and 20 phenotypically normal individuals and asexpected the autism group showed a statistically significantlyhigher level of methylation at several examined loci These

observations were made in samples of both blood andcerebral cortex Additionally low levels of OXTR expressionwere found to be associated with increased methylation ata statistically significant level This finding strengthens theidea that promoter region methylation causes gene silencingMoreover when the data were stratified by sex two ofthe loci showed significant differences in methylation formales only thereby implying that the different frequencies ofautism in males and females might be driven by epigeneticmechanisms

33 Bcl-2 Bcl-2 (B-cell lymphoma 2) is a protein responsiblefor the regulation of apoptosis [36] The Bcl-2 gene hasbeen implicated in the etiology of several cancers and theabnormal expression of the gene has also been linked todiseaseswith social impacts such as schizophrenia and autism[37]The Bcl-2 protein is reported to be decreased in both thecerebellum and frontal cortex of autistic subjects compared toage and gender-matched controls [38 39]

Evidence linking Bcl-2 gene expression to the develop-ment of ASD is still being assembled and thus the extentto which a causative relationship exists remains largely amatter of speculation That said a study by Nguyen etal [40] examining lymphoblastoid cell lines from sets ofmonozygotic twins that were discordant for autism and alsocomparing the twinsrsquo cell lines to those of their nonautisticnontwin siblings provides a basis for preliminary discussion

34 RORA The proposition that epigenetic regulation of theretinoic acid-related orphan receptor alpha (RORA) mightcause autism is relatively new Although the functions ofRORA are largely unknown RORA regulation of circadianrhythm and neuroprotection against oxidative stress andinflammation is reported [41 42] A link between RORA andautism makes intuitive sense because autism is thought tobe associated with increased levels of oxidative stress andinflammation [43 44] The Nguyen et al study was thefirst to give scientific grounding to this intuition by notingthatmdashas was the case with Bcl-2mdashthere were statisticallysignificant differences in both RORA gene promoter regionmethylation and protein product expression between autisticsubjects and their (non-twin) unaffected siblings [40] Thiswas the case in both lympoblastoid cell line and postmortembrain tissue Interestingly when the population was stratifiedby the various ASD subtypes it turned out that reducedRORA expression was only observed in ASD subjects withsevere language impairment As a consequence reducedRORA expression was not observed in all ASD subjects Thenotion that RORA methylation might be largely responsiblefor the language deficits sometimes associated with ASDis a useful finding that helps clarify the etiology of autismand an important first step in determining the particularmechanism responsible for these symphtoms Moreover theconnection between promoter regionmethylation andRORAexpressionwas confirmed by treatmentwith global inhibitionof methylation using 5-Aza which increased gene expressionin autistic subjects but not in unaffected subjects Howeveras with Bcl-2 the effects of 5-Aza on undiagnosed cotwins


and unaffected subjects were not found to be statistically sig-nificant This finding must be interpreted cautiously becausethere could be a variety of possible explanations

35 120573-Catenin The 120573-catenin gene has been the subject ofmuch investigation related to its potential as an oncogenebut it can also act as the fulcrum in at least two relevantepigenetic processes related to the development of ASD Oneepigenetic process of 120573-catenin involves estrogens Estrogensare critical players in the sexual differentiation of the brainand it is likely that brain estrogen levels are increased inautistic subjects [45] Estrogens being steroid hormones andtheir receptorsmdashincluding estrogen receptor alpha (ER120572)mdashare located in the nucleus and in the cytosol of target cellsOne of the targets of cytosolic ER120572 is GSK3B which is knownto form a complex with 120573-catenin for degradation of 120573-catenin ER120572 activation by estradiol is reported to release120573-catenin from this complex thereby increasing 120573-cateninavailability [46] An increase in the cytosolic concentrationof estrogens is thought to result in increases in cytosolicand nuclear 120573-catenin during critical periods of prenatalandor neonatal development wherein 120573-catenin binding tothe LECTCF promoter has positive effects on Wnt pathwaygene transcription Such increased transcription in the Wntpathway is strongly associatedwith the development of ASDsThe effect of ER120572 in this process is to cause the dissociation of120573-catenin from a complex whose integral members includethe proteins GSK3120573 axin and adenomatous polyposis colitumor suppressor (APC) GSK3120573 axin and APC are negativeregulators of the Wnt signaling pathway and the complexrequires all of these constituents to initiate the destructionof 120573-catenin The absence or downregulation of any of thesecomponents may increase the availability of cytosolic 120573-catenin as well as in the various knockin effects discussedpreviouslymdashincreased nuclear 120573-catenin with greater Wntpathway transcription Lithium usedmostly as amood stabi-lizing drug exerts an inhibitory effect onGSK3120573 both directlyand indirectly by interrupting the dephosphorylation ofphospo-GSK3120573 [47] In either case the effect is the sameand also the same as that of increased estrogen levels thatis the complex responsible for initiating the degradation of120573-catenin is made nonfunctional and the concentration ofcytosolic 120573-catenin increases

36 The Neurexin-Neuroligin Pathway SHANK3 is a scaf-folding protein in the neurexin-neuroligin pathway thatinteracts with synaptic proteins Recent research suggeststhat copy number variations or mutations of either ofthese proteins may be associated with the development ofASDs [48] It appears that epigenetic mechanisms are usedto control the expression of this gene For instance Beriet al identified five CpG islands in the SHANK3 genethe posttranslational methylation of which determines geneexpression [49]One specific locusmdashCpG island 2mdashappearedto particularly impact tissue SHANK expression Furthermore taking advantage of the fact that the SHANK3 geneis well conserved between humans and rodents Uchino andWaga demonstrated that the neonatal expression of certain

SHANK3 transcripts in mice temporarily decreases as themethylation of CpG island 2 peaks two weeks after birth [50]This suggests that the expression of SHANK3 (and thus itseffect on the development of ASD) is regulated by epigeneticmechanisms though this connection has yet to be directlyestablished in humans Additionally two genes responsiblefor the production of cell adhesionmolecules in this pathwayNLGN3 and NLGN4 have also been associated with thedevelopment of ASD [48] However to date epigeneticregulation of this gene is still unproven [51]

4 Role of Maternal Hypomethylationin Autism

Thus far discussion of the contribution made by epigeneticmechanisms to the development of ASDs has focused onthe genes and proteins of autistic patients There is goodreason for this ASDs are known to possess a variety ofgenetic determinants so investigations into the effects of genesilencing or promotion naturally focus on the silencing orpromotion of the genes of the patient However epigeneticmechanisms do not exert their influence only bymechanismsof gene manipulation For example epigenetic influences onthe maternal genome could alter the intrauterine environ-ment such that the probability of the offspring developingASDs is increased or decreased DNA hypomethylationlinked to variants in the maternal folate pathway has beenlinked to aberrant fetal development [52 53] Also and moreimportantly folate is the primary one carbon donor criticalfor methylation reactions Because epigenetic mechanismstypically exert their influences by methylation of DNAinvestigation of the folate pathway should provide insight intothe availability of methylation precursors and also the extentof genomic methylation in mothers of both their autistic andtheir unaffected children

A study by James et al was performed in part tobolster the findings that mothers of autistic children oftenpresented with aberrant DNA methylation [54] Mothersof autistic children exhibited significantly lower levels ofmethylfolate and methioninemdashessential precursors for DNAmethylationmdashthan their counterparts in the control groupIn addition levels of the methylation-inhibiting proteins S-adenosylmethionine adenosine and homocysteine were allelevated in autism mothers S-adenosylmethionine (SAM) isthe primary methyl donor for the DNA methyltransferasereaction which produces S-adenosylhomocysteine (SAH)and methylated DNA Because SAM and SAH are linkedby the transferase reaction the SAMSAH ratio is generallyconsidered to be a good indicator of DNA methylationpotential Mothers of autistic children displayed a lowerSAMSAH ratio than the control group which is indicativeof a diminished capacity for methylation Going one stepfurther in addition to having a lower capacity for methyla-tion the DNA of autism mothers is in fact less methylatedthan that of mothers of unaffected children the ratio of5-methylcytosine to total cytosinemdasha measure of overallgenomicmethylationmdashwas significantly lower in themothersof autistic children Taken together this evidence strongly


suggests that hypomethylation of maternal DNA may belinked to ASDs The significance of these findings about theinfluences of epigenetics on the development of autism isnot entirely clear However it is not even clear that whatis being observed is the effect of epigenetics For examplealthough the ratio of 5-methycytosine to total cytosine wasstatistically correlated to the SAMSAH ratio it was moreclosely linked to the presence of an uncommon recessiveallele in the gene that codes for reduced folate carrier proteinIt is generally assumed that genes subjected to an atypicallevel of methylation are manifesting the effects of epigeneticinfluences However for this particular case of the SAMSAHratio this assumption should be questioned If as the cor-relation data suggest the differential methylation exhibitedby mothers of autistic children is largely a consequence of agenetic polymorphism then it is not likely to be epigeneticsat work If intrauterine conditions are altered by maternalhypomethylation such changed conditions must impact fetaldevelopment pathways by specific mechanisms that remainto be elucidated

5 Epigenetics and Nutritional Factorsin Autism

The period of development in which the nutritional imbal-ance occurs is very important in determining which disease-related genes will be affected Different organs have criticaldevelopmental stages and the time point at which they arecompromised will predispose individuals to specific diseasesEpigeneticmodifications that occur during developmentmaynot be expressed until later in life depending on the functionof the gene While the majority of studies implicate prenatalperiods as critical time windows some research has shownthat nutritional intake during adulthood can also affect theepigenome

Genetic polymorphisms of cytochrome P450 enzymeshave been linked to ASD specifically the cytochrome P450family 27 subfamily B gene (CYP27B1) that is essentialfor proper vitamin D metabolism Epigenetic regulation ofcytochrome P450 genes for hydroxylation and activationof vitamin D in has been shown in prostate cancer cells[55] Vitamin D is important for neuronal growth andneurodevelopment and defects in metabolism or deficiencyhave also been implicated in ASDs [56] Mutations of MeCP2associated with impaired methylation are known to be asso-ciated with ASDs and the related neurological disorder Rettsyndrome One component of Rett syndrome is abnormalbone formation wherein abnormal vitamin D metabolism isassociated with epigenetic dysregulation of cytochrome P450genes [57] which could be a conceptual model for epigeneticinteractions between MeCP2 vitamin D and cytochromeP450 genes [56] Abnormal folic acid metabolism may alsoplay a role in the decreased capacity for methylation andDNA hypomethylation associated with significantly greaterthan normal levels of plasma homocysteine adenosine andSAH in mothers of subjects diagnosed with ASDs [58]Changes in autism-related behaviors are reported to bestrongly associated with vitamin-supplementation associated

changes in plasma levels of biotin and vitamin K [59] andalthough biotin is a known cofactor in bioavailability ofmethyl groups for DNA methylation a vitamin-K-relatedepigenetic mechanism has not been described

6 Epigenetics and Toxic Factors in Autism

61 Valproic Acid Valproic acid (VPA) is a therapeutic anti-convulsant and mood stabilizing drug that gained attentionin the 1980s as a potential teratogen VPA exposure ishighly correlated with autism as many as 60 of infantswho exhibit the suite of symptoms associated with VPAteratogenicity also display two ormore autistic characteristics[60] Autism has also been shown to occur in 9 of cases ofprenatal exposure to VPA [61] The mechanisms underlyingthe pharmacological actions of VPA are also suggestive of acorrelation betweenVPA and autism [62] VPA is responsiblefor inhibiting two enzymes myo-inositol-1-phosphate (MIP)synthase and the class 1 and 2 histone deacetylase (HDAC)HDAC1 is an important inhibitor of DNA transcription thatworks by associating with the LECTCF transcription factorWhen HDAC1 is removed from the LECTCF complex itleaves behind a primed (but inactive) promoter of genetranscription The primed promoter then forms a complexwith 120573-catenin thus activating the promoter and increasingtranscriptions rates of a variety of genes in the Wnt signalingpathway including cyclin D1 required for the transition fromthe G1 to S phases of mitosis and MYC a transcriptionenhancer for many genes throughout the genome [63]Accordingly the consequence of VPA-mediated inhibition ofHDAC1 is to upregulate the transcription of Wnt pathwaygenes In addition VPA increases cellular levels of 120573-cateninpresumably in response to the increased availability of primedLECTCF promoters [64] The effect of VPA on Wnt genetranscription is well understood but fails to explain the con-nection between VPA and autism In order to complete thislink an increase in the number of neocortical minicolumnsis highly correlated with autism [65]This observation is sup-ported by fMRI studies that report differences in how autismbrains coordinate the processing of information [66] It isreasonable to assume that processes which upregulate genesof the Wnt signaling pathwaymdashsuch as prenatal exposure toVPAmdashmay result in poorly regulatedmitosis and cellular pro-liferation one manifestation of which could be an increase inthe number of neocortical minicolumns and macrocephalyThis mechanism has been observed at work in a slightlydifferent context Recall that MeCP2 inactivates genes byforming complexes with a variety of different molecules Oneof these molecules is HDAC1 [67] and in the absence ofHDAC1 or even if HDAC1 has merely been downregulatedthe gene inactivating properties of MeCP2 is expected tohave a diminished effect One of the promoters on whichMeCP2 typically exerts its regulatory effect is the LECTCFpromoter which as mentioned previously ultimately regu-lates the transcription rates of the Wnt signaling pathwayAlthough MeCP2 has effects on gene methylation the func-tion of HDAC1 concerns acetylation of histones However totranscriptionally deactivate genes they must often be both


(4) Histone H3 phosphorylation by protein kinase C beta

HMT LSD1 prevents H3K4 demethylation

Failure of AR-gene activation increases high arousal inputs to amygdala

(3) Oxytocin receptor gene methylation

Low oxytocin and estrogen activity fail to protect against oxytocin receptor gene

receptor mediated high arousal inputs to amygdala and antisocial behaviors

(1) Low MeCP2 activity fails to silence genes in frontal cortex

MeCP2 does not form a complex with HDAC1 for condensing chromatin and silencing

(5) Maternal hypomethylation SAMSAH ratio indicator of potential for DNA methylation

mothers of autistic children

HMT

Antiautism factorsFemale genderEstrogens OxytocinFeverBrain norepinephrine

Vitamin K

Proautism factorsMale genderAndrogensValproic acidLithiumSAM deficiencyOXTR polymorphisms Low OXTR expressionLow RORA expressionWnt pathway upregulationFolatemethionine deficiency

Role of nutritional factorsPlasma homocysteine adenosine and S-adenosylhomocysteine are higher in mothers of autistic children consistent with DNA hypomethylation and improved autism-related negative behavior was associated with multivitamin supplementation-induced increases in plasma biotin a cofactor in a cohort of autistic children and adults

HDAC

(2) HDAC1 inhibitionby valproic acid and GSK3B inhibition by lithium upregulate Wnt pathway transcription DNMT

HMT

HDACGSK

rarr SAMSAH ratio decreased in

genes rarr low capacity for gene silencing

rarr macrocephaly

methylation and silencing rarr androgen for androgen receptor-gene activation rarr

Biotin (vitamin B7)

Figure 1 Main mechanisms of epigenetic alterations in autism Each alteration involves many enzymes but the main players to causemethylation or acetylation are shown by arrows These are not separate mechanisms and the enzymes do not act alone Several enzymesact at a promoter simultaneously (1) Low methyl CpG binding protein-2 (MeCP2) at CpG islands of frontal cortex reduces capacity forcomplexing with histone deacetylase 1 (HDAC1) for gene silencing (2) HDAC1 inhibition by valproic acid exposure and glycogen synthetasekinase-3B (GSK3B) inhibition by lithium upregulate Wnt signaling pathway and activate transcription associated with macrocephalywith increased number of cerebral cortical column (3) DNA methyltransferase (DNMT) methylates oxytocin receptor gene produces lowoxytocin and estrogen activity necessary for androgen receptor mediated high-arousal inputs to amygdala (4) Histone H3 phosphorylationby protein kinase C beta activates the histone methyltransferase (HMT) lysine demethylase 1 (LSD1) which prevents demethylation oflysine-4 site of histone-3 (H3K4) that is also necessary for androgen receptor (AR) mediation of high arousal inputs to amygdala (5)Maternal hypomethylation by dietary folic acid deficiency decreases availability of S-adenosyl methionine (SAM) associated with abnormalintrauterine growth

methylated and deacetylated Thus VPA-induced inhibitionof HDAC1 interferes with the functionality of MeCP2 whichappears to increase the risk of developing ASDs

7 Epigenetics and Other Illnesses Associatedwith Autism

Epigenetic effects may also manifest through aberrantmethylation patterns of imprinted genes The expression of

imprinted genes which are mostly found in clusters onchromosomes 6 7 11 14 and 15 is controlled by a seriesof epigenetic marks (DNA methylation and histone modi-fication) Imprinting defects may be primary or secondaryPrimary imprinting defects cause changes in methylationpatterns but leave the DNA sequences unaltered and thusmay be classified as an epigeneticmechanism [68] Angelmansyndrome which is caused by an absence of active maternalgenes in the 15q11-1q13 region may result from a primaryimprinting defect (though the syndrome is more commonly


caused by a deletion on the maternal chromosome or apaternal uniparental disomy)There is some basis to suspect alink between Angelman syndrome and ASDs For instance astudy by Steffenburg et al sought to ascertain the frequency ofASDs among children diagnosed with Angelman syndrome[69]The study screened a series ofmentally retarded childrenfor Angelman syndrome and subsequently evaluated thechildren with Angelman syndrome for evidence of autismFour out of the approximately 49000 screened children werediagnosed with Angelman syndrome and each of those fourwere found to demonstrate autistic behaviors However otherstudies place the rate of cooccurrence of ASDs andAngelmansyndrome at a rate of as low as 2 Taken together itseems reasonable to assert that to the extent that Angelmansyndrome and ASDs are linked the condition of somepercentage of these patients will be related to an epigeneticprimary imprinting defect That said the available evidencedoes not establish whether the epigenetic defect causingAngelman syndrome leads directly to autistic symptoms orif instead the relationship between Angelman syndrome andASDs is merely correlative and not causative

On the other hand secondary imprinting defects occurwhen a gene mutation results in improper epigenetic regu-lation Such a defect may occur in Prader-Willi syndromewhich is characterized by the lack of a paternal contributionat the 15q11-q13 locus The specific mutation most commonlyresponsible for secondary imprinting Prader-Willi syndromeis a cis-acting defect of the imprinting regulatory center ofthe Prader-Willi gene [70] Prader-Willi patients present withautistic behavior more frequently than Angelman syndromepatients studies suggest that the frequency of ASDs co-occurrence with Prader-Willi syndrome is between 18 and38 [71] though the causative nature of this relationship hasnot been established

Fragile X syndrome is the leading single-gene cause ofautism accounting for as many at 5 of all cases [72]As with Prader-Willi and Angelman syndromes epigeneticmechanism can contribute to the development of the fragileX which is characterized by the presence of 200 or moreCGG repeats in the 51015840 untranslated region of the FMR1 gene[73] The resulting increased concentration of cytosine andguanine nucleotides causes the global methylation of notonly the CGG-repeat region but also adjacent regions whichhappen to include FMR1 promoter elements

8 Conclusions

Figure 1 summarizes the main epigenetic mechanisms thatmay play roles in ASD Low activity of methyl CpG bindingprotein 2 (MeCP2) at CpG islands in genes of frontal cortexis shown to reduce the capacity for inhibiting HDAC1 andchromatin condensation for gene silencing HDAC1 inhi-bition by valproic acid and GSK3120573 inhibition by lithiumare shown to upregulate the Wnt signaling pathway whichcauses accumulation of 120573-catenin in the cytoplasm andits translocation to the nucleus acting as an activator oftranscription and resulting in macrocephaly with increasednumbers of cerebral cortical columns DNMT is shown to

methylate the oxytocin receptor gene and silence it resultingin the low oxytocin and estrogen activity necessary forandrogen receptor mediation of high arousal inputs to theamygdala associated with antisocial behaviors after exposureto environmental stressors Histone H3 phosphorylation byprotein kinase C beta is shown to activate the LSD1 an HMTthat prevents demethylation of H3K4 that is also necessaryfor androgen receptor mediation of high arousal inputs to theamygdala Hypomethylation by decreased availability of S-adenosyl methionine (SAM) is shown to occur in mothers ofautistic children Environmental and nutritional conditionsacting as pro- or antiautism factors by epigeneticmechanismssuggest strategies for decreasing the prevalence of ASD Thisknowledge of putative epigenetic targets should motivateclinical practitioners and educators to develop novel treat-ment strategies based on the environment-gene interactionswhich could contribute to the core symptoms of ASD

Conflict of Interests

The authors have no conflict of interests to declare

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[2] B Kadesjo C Gillberg and B Hagberg ldquoBrief report autismand asperger syndrome in seven-year-old children a total pop-ulation studyrdquo Journal of Autism and Developmental Disordersvol 29 no 4 pp 327ndash331 1999

[3] S E Levy D SMandell and R T Schultz ldquoAutismrdquoTheLancetvol 374 no 9701 pp 1627ndash1638 2009

[4] C Stoltenberg S Schjoslashlberg M Bresnahan et al ldquoTheautism birth cohort a paradigm for gene-environment-timingresearchrdquoMolecular Psychiatry vol 15 no 7 pp 676ndash680 2010

[5] V W Hu B C Frank S Heine N H Lee and J QuackenbushldquoGene expression profiling of lymphoblastoid cell lines frommonozygotic twins discordant in severity of autism revealsdifferential regulation of neurologically relevant genesrdquo BMCGenomics vol 7 article 118 2006

[6] C Ptak and A Petronis ldquoEpigenetic approaches to psychiatricdisordersrdquo Dialogues in clinical neuroscience vol 12 no 1 pp25ndash35 2010

[7] J T Bell and T D Spector ldquoA twin approach to unravelingepigeneticsrdquo Trends in Genetics vol 27 no 3 pp 116ndash125 2011

[8] B F Vanyushin ldquoEnzymatic DNA methylation is an epigeneticcontrol for genetic functions of the cellrdquo Biochemistry vol 70no 5 pp 488ndash499 2005

[9] A P Feinberg ldquoEpigenomics reveals a functional genomeanatomy and a new approach to common diseaserdquo NatureBiotechnology vol 28 no 10 pp 1049ndash1052 2010

[10] R Jaenisch and A Bird ldquoEpigenetic regulation of gene expres-sion how the genome integrates intrinsic and environmentalsignalsrdquo Nature Genetics vol 33 pp 245ndash254 2003

[11] L M Hjelmeland ldquoDark matters in AMD genetics epigeneticsand stochasticityrdquo Investigative Ophthalmology and Visual Sci-ence vol 52 no 3 pp 1622ndash1631 2011


[12] A D Riggs Z Xiong L Wang and J M LeBon ldquoMethylationdynamics epigenetic fidelity and X chromosome structurerdquoNovartis Foundation Symposium vol 214 pp 214ndash232 1998

[13] A Petronis I I Gottesman P Kan et al ldquoMonozygotictwins exhibit numerous epigenetic differences clues to twindiscordancerdquo Schizophrenia Bulletin vol 29 no 1 pp 169ndash1782003

[14] S Luikenhuis E Giacometti C F Beard and R JaenischldquoExpression of MeCP2 in postmitotic neurons rescues Rettsyndrome in micerdquo Proceedings of the National Academy ofSciences of theUnited States of America vol 101 no 16 pp 6033ndash6038 2004

[15] M Chahrour Y J Sung C Shaw et al ldquoMeCP2 a keycontributor to neurological disease activates and repressestranscriptionrdquo Science vol 320 no 5880 pp 1224ndash1229 2008

[16] S Cohen Z Zhou and M E Greenberg ldquoActivating a repres-sorrdquo Science vol 320 no 5880 pp 1172ndash1173 2008

[17] D H Yasui S Peddada M C Bieda et al ldquoIntegratedepigenomic analyses of neuronal MeCP2 reveal a role for long-range interaction with active genesrdquo Proceedings of the NationalAcademy of Sciences of the United States of America vol 104 no49 pp 19416ndash19421 2007

[18] M F Mehler ldquoEpigenetic principles and mechanisms underly-ing nervous system functions in health and diseaserdquo Progress inNeurobiology vol 86 no 4 pp 305ndash341 2008

[19] R P Nagarajan A R Hogart Y Gwye M R Martin and JM LaSalle ldquoReduced MeCP2 expression is frequent in autismfrontal cortex and correlates with aberrant MECP2 promotermethylationrdquo Epigenetics vol 1 no 4 pp e1ndashe11 2006

[20] J M LaSalle J Goldstine D Balmer and CM Greco ldquoQuanti-tative localization of heterogeneous methyl-CpG-binding pro-tein 2 (MeCP2) expression phenotypes in normal and Rett syn-drome brain by laser scanning cytometryrdquo Human MolecularGenetics vol 10 no 17 pp 1729ndash1740 2001

[21] K Pelphrey R Adolphs and J P Morris ldquoNeuroanatomicalsubstrates of social cognition dysfunction in autismrdquo MentalRetardation and Developmental Disabilities Research Reviewsvol 10 no 4 pp 259ndash271 2004

[22] C Lintas R Sacco K Garbett et al ldquoInvolvement of thePRKCB1 gene in autistic disorder significant genetic asso-ciation and reduced neocortical gene expressionrdquo MolecularPsychiatry vol 14 no 7 pp 705ndash718 2009

[23] EMetzger A Imhof D Patel et al ldquoPhosphorylation of histoneH3T6 by PKCbeta(I) controls demethylation at histone H3K4rdquoNature vol 464 no 7289 pp 792ndash796 2010

[24] D W Pfaff I Rapin and S Goldman ldquoMale predominancein autism neuroendocrine influences on arousal and socialanxietyrdquo Autism Research vol 4 no 3 pp 163ndash176 2011

[25] S Wu M Jia Y Ruan et al ldquoPositive association of theoxytocin receptor gene (OXTR)with autism in theChineseHanpopulationrdquo Biological Psychiatry vol 58 no 1 pp 74ndash77 2005

[26] X Liu Y Kawamura T Shimada et al ldquoAssociation of theoxytocin receptor (OXTR) gene polymorphisms with autismspectrum disorder (ASD) in the Japanese populationrdquo Journalof Human Genetics vol 55 no 3 pp 137ndash141 2010

[27] E Unternaehrer P Luers J Mill et al ldquoDynamic changes inDNA methylation of stress-associated genes (OXTR BDNF)after acute psychosocial stressrdquo Translational Psychiatry vol 2article e150 2012

[28] S Jacob C W Brune C S Carter B L Leventhal C Lordand E H Cook Jr ldquoAssociation of the oxytocin receptor gene

(OXTR) in Caucasian children and adolescents with autismrdquoNeuroscience Letters vol 417 no 1 pp 6ndash9 2007

[29] J-P Gouin C S Carter H Pournajafi-Nazarloo et al ldquoMaritalbehavior oxytocin vasopressin and wound healingrdquo Psy-choneuroendocrinology vol 35 no 7 pp 1082ndash1090 2010

[30] T R Insel ldquoOxytocinmdasha neuropeptide for affiliation evidencefrom behavioral receptor autoradiographic and comparativestudiesrdquo Psychoneuroendocrinology vol 17 no 1 pp 3ndash35 1992

[31] J N Ferguson L J Young E F Hearn M M Matzuk T RInsel and J T Winslow ldquoSocial amnesia in mice lacking theoxytocin generdquo Nature Genetics vol 25 no 3 pp 284ndash2882000

[32] Y Takayanagi M Yoshida I F Bielsky et al ldquoPervasive socialdeficits but normal parturition in oxytocin receptor-deficientmicerdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 102 no 44 pp 16096ndash16101 2005

[33] L Sun L Huang P Nguyen et al ldquoDNA methyltrans-ferase 1 and 3B activate BAG-1 expression via recruitment ofCTCFLBORIS and modulation of promoter histone methyla-tionrdquo Cancer Research vol 68 no 8 pp 2726ndash2735 2008

[34] S G Gregory J J Connelly A J Towers et al ldquoGenomic andepigenetic evidence for oxytocin receptor deficiency in autismrdquoBMCMedicine vol 7 article 62 2009

[35] T Kimura F Saji K Nishimori et al ldquoMolecular regulation ofthe oxytocin receptor in peripheral organsrdquo Journal ofMolecularEndocrinology vol 30 no 2 pp 109ndash115 2003

[36] Y Tsujimoto L R Finger and J Yunis ldquoCloning of thechromosome breakpoint of neoplastic B cells with the t(1418)chromosome translocationrdquo Science vol 226 no 4678 pp1097ndash1099 1984

[37] L A Glantz J H Gilmore J A Lieberman and L FJarskog ldquoApoptotic mechanisms and the synaptic pathology ofschizophreniardquo Schizophrenia Research vol 81 no 1 pp 47ndash632006

[38] S H Fatemi J M Stary A R Halt and G R Realmuto ldquoDys-regulation of reelin and Bcl-2 proteins in autistic cerebellumrdquoJournal of Autism and Developmental Disorders vol 31 no 6pp 529ndash535 2001

[39] S Hossein Fatemi and A R Halt ldquoAltered levels of Bcl2 and p53proteins in parietal cortex reflect deranged apoptotic regulationin autismrdquo Synapse vol 42 no 4 pp 281ndash284 2001

[40] A Nguyen T A Rauch G P Pfeifer and V W Hu ldquoGlobalmethylation profiling of lymphoblastoid cell lines reveals epi-genetic contributions to autism spectrum disorders and anovel autism candidate gene RORA whose protein productis reduced in autistic brainrdquo FASEB Journal vol 24 no 8 pp3036ndash3051 2010

[41] M Akashi and T Takumi ldquoThe orphan nuclear receptor ROR120572regulates circadian transcription of the mammalian core-clockBmal1rdquoNature Structural ampMolecular Biology vol 12 no 5 pp441ndash448 2005

[42] F Boukhtouche G Vodjdani C I Jarvis et al ldquoHuman retinoicacid receptor-related orphan receptor 1205721 overexpression pro-tects neurones against oxidative stress-induced apoptosisrdquo Jour-nal of Neurochemistry vol 96 no 6 pp 1778ndash1789 2006

[43] C A Pardo D L Vargas and A W Zimmerman ldquoImmunityneuroglia and neuroinflammation in autismrdquo InternationalReview of Psychiatry vol 17 no 6 pp 485ndash495 2005

[44] A Chauhan and V Chauhan ldquoOxidative stress in autismrdquoPathophysiology vol 13 no 3 pp 171ndash181 2006


[45] N J MacLusky A S Clark F Naftolin and P S Goldman-Rakic ldquoEstrogen formation in the mammalian brain possiblerole of aromatase in sexual differentiation of the hippocampusand neocortexrdquo Steroids vol 50 no 4-6 pp 459ndash474 1987

[46] P Cardona-Gomez M Perez J Avila L M Garcia-Segura andFWandosell ldquoEstradiol inhibits GSK3 and regulates interactionof estrogen receptors GSK3 and beta-catenin in the hippocam-pusrdquoMolecular andCellularNeuroscience vol 25 no 3 pp 363ndash373 2004

[47] R S Jope ldquoLithium and GSK-3 one inhibitor two inhibitoryactions multiple outcomesrdquo Trends in Pharmacological Sci-ences vol 24 no 9 pp 441ndash443 2003

[48] Y Liu YDuW Liu et al ldquoLack of association betweenNLGN3NLGN4 SHANK2 and SHANK3 gene variants and autismspectrum disorder in a Chinese populationrdquo PLoS ONE vol 8Article ID e56639

[49] S Beri N Tonna G Menozzi M C Bonaglia C Sala and RGiorda ldquoDNA methylation regulates tissue-specic expressionof Shank3rdquo Journal of Neurochemistry vol 101 no 5 pp 1380ndash1391 2007

[50] S Uchino and C Waga ldquoSHANK3 as an autism spectrumdisorder-associated generdquo Brain Development vol 35 pp 106ndash110 2013

[51] Y Yasuda R Hashimoto H Yamamori et al ldquoGene expressionanalysis in lymphoblasts derived from patients with autismspectrum disorderrdquo Molecular Autism vol 2 no 1 article 92011

[52] A E Beaudin C A Perry S P Stabler R H Allen and P JStover ldquoMaternal Mthfd1 disruption impairs fetal growth butdoes not cause neural tube defects in micerdquo American Journalof Clinical Nutrition vol 95 no 4 pp 882ndash891 2012

[53] T O Scholl and W G Johnson ldquoFolic acid influence on theoutcome of pregnancyrdquo American Journal of Clinical Nutritionvol 71 no 5 pp 1295Sndash1303S 2000

[54] S Jill James S Melnyk S Jernigan A Hubanks S Rose andD W Gaylor ldquoAbnormal transmethylationtranssulfurationmetabolism and DNA hypomethylation among parents ofchildren with autismrdquo Journal of Autism and DevelopmentalDisorders vol 38 no 10 pp 1966ndash1975 2008

[55] W Luo A R Karpf K K Deeb et al ldquoEpigenetic regulation ofvitaminD 24-hydroxylaseCYP24A1 in human prostate cancerrdquoCancer Research vol 70 no 14 pp 5953ndash5962 2010

[56] S A Currenti ldquoUnderstanding and determining the etiology ofautismrdquo Cellular and Molecular Neurobiology vol 30 no 2 pp161ndash171 2010

[57] R D OrsquoConnor M Zayzafoon M C Farach-Carson and NC Schanen ldquoMecp2 deficiency decreases bone formation andreduces bone volume in a rodent model of Rett syndromerdquoBone vol 45 no 2 pp 346ndash356 2009

[58] S J James S Melnyk S Jernigan et al ldquoA functional polymor-phism in the reducedfolate carrier gene and DNA hypomethy-lation in mothers of children with autismrdquo American Journal ofMedical Genetics Part B vol 153 no 6 pp 1209ndash1220 2010

[59] J B Adams T Audhya S McDonough-Means et al ldquoEffectof a vitaminmineral supplement on children and adults withautismrdquo BMC Pediatrics vol 11 article 111 2011

[60] S J Moore P Turnpenny A Quinn et al ldquoA clinical studyof 57 children with fetal anticonvulsant syndromesrdquo Journal ofMedical Genetics vol 37 no 7 pp 489ndash497 2000

[61] A D Rasalam H Hailey J H G Williams et al ldquoCharac-teristics of fetal anticonvulsant syndrome associated autistic

disorderrdquoDevelopmental Medicine and Child Neurology vol 47no 8 pp 551ndash555 2005

[62] J A Shimshoni E C Dalton A Jenkins et al ldquoThe effects ofcentral nervous system-active valproic acid constitutional iso-mers cyclopropyl analogs and amide derivatives on neuronalgrowth cone behaviorrdquo Molecular Pharmacology vol 71 no 3pp 884ndash892 2007

[63] A N Billin H Thirlwell and D E Ayer ldquo120573-Catenin-histonedeacetylase interactions regulate the transition of LEF1 from atranscriptional repressor to an activatorrdquoMolecular andCellularBiology vol 20 no 18 pp 6882ndash6890 2000

[64] ZWang L Xu X Zhu et al ldquoDemethylation of specificWnt120573-catenin pathway genes and its upregulation in rat brain inducedby prenatal valproate exposurerdquoAnatomical Record vol 293 no11 pp 1947ndash1953 2010

[65] E LWilliams andM F Casanova ldquoAutism or autisms Findingthe lowest common denominatorrdquo Boletın de la AsociacionMedica de Puerto Rico vol 102 no 4 pp 17ndash24 2010

[66] N J Minshew and D L Williams ldquoThe new neurobiologyof autism cortex connectivity and neuronal organizationrdquoArchives of Neurology vol 64 no 7 pp 945ndash950 2007

[67] X Nan H-H Ng C A Johnson et al ldquoTranscriptionalrepression by themethyl-CpG-binding proteinMeCP2 involvesa histone deacetylase complexrdquo Nature vol 393 no 6683 pp386ndash389 1998

[68] M Gos ldquoEpigenetic mechanisms of gene expression regulationin neurological diseasesrdquo Acta Neurobiologica Experimentalisvol 73 pp 19ndash37 2013

[69] S Steffenburg C L Gillberg U Steffenburg andM KyllermanldquoAutism in Angelman syndrome a population-based studyrdquoPediatric Neurology vol 14 no 2 pp 131ndash136 1996

[70] E Dykens and B Shah ldquoPsychiatric disorders in Prader-Willisyndrome epidemiology andmanagementrdquo CNS Drugs vol 17no 3 pp 167ndash178 2003

[71] MWM Veltman R JThompson S E Roberts N SThomasJ Whittington and P F Bolton ldquoPrader-Willi syndrome astudy comparing deletion and uniparental disomy cases withreference to autism spectrum disordersrdquo European Child andAdolescent Psychiatry vol 13 no 1 pp 42ndash50 2004

[72] Y McLennan J Polussa F Tassone and R Hagerman ldquoFragileX syndromerdquoCurrent Genomics vol 12 no 3 pp 216ndash224 2011

[73] R Willemsen J Levenga and B Oostra ldquoCGG repeat in theFMR1 gene size mattersrdquo Clinical Genetics vol 80 no 3 pp214ndash225 2011

Submit your manuscripts athttpwwwhindawicom

Stem CellsInternational

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


MEDIATORSINFLAMMATION

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Behavioural Neurology

EndocrinologyInternational Journal of



Disease Markers


BioMed Research International

OncologyJournal of



Oxidative Medicine and Cellular Longevity


PPAR Research

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Immunology ResearchHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

ObesityJournal of



Computational and Mathematical Methods in Medicine

OphthalmologyJournal of


Diabetes ResearchJournal of



Research and TreatmentAIDS


Gastroenterology Research and Practice


Parkinsonrsquos Disease

Evidence-Based Complementary and Alternative Medicine

Volume 2014Hindawi Publishing Corporationhttpwwwhindawicom


genome plus any epigenetic modificationsmdashdevelops as aconsequence of a variety of factors The first and probablymost important is the various effects exerted by the environ-ment on the epigenome [10] Second the epigenome is itselfheritable a mother for instance can pass a methylated geneto her offspring [11] Third the epigenomemdashmuch like thegenomemdashis subject to replication errors however whereasthe typical error rate for gene replication is 1 1000000 theusual error rate for replicating epigenetic elements is closer to1 1000 [12] Fourth spontaneous changes to the epigenomeapart from changes driven by environmental causes arethought to occur known as epigenetic drift [13]The latter twoelements account for the stochastic nature of the epigenomethat is the tendency of epigenomes to diverge despite havingidentical starting conditions

This review examines the current state of knowledgeconcerning the impact of epigenetic mechanisms of thedevelopment of ASD and is organized in the followingsections

2 Epigenetic Protein-DNA InteractionsProteins Mediating Epigenetic Signaling

21MeCP2 Methyl CpG binding protein 2 (MeCP2) is activein CNS regulation and development of synaptic contacts[14] MeCP2 is known to be involved in gene silencing andas a consequence in its role epigenetic regulation has beenthe focus of a significant amount of investigation [15] TheMeCP2 gene is located on the q arm of the X chromosomeSince the MeCP2 gene is located on the X chromosomeit is X-linked and subject to X inactivation Until recentlyit was widely thought that MeCP2 was only responsiblefor the silencing of genes [16] However gene silencing isinconsistent with the mode of action of the MeCP2 proteinproduct Like other members of the methyl-CpG bindingdomain (MBD) familyMeCP2 binds tomethylatedDNA [17]and after binding MeCP2 forms a complex with the enzymehistone deacetylase 1 (HDAC1) that removes acetyl groupsfrom histones thereby causing the chromatin structure tocondense The condensation of the chromatin is critical togene inactivation However recent investigations suggest thatMeCP2 may also be capable of acting as an activator ofa variety of genes [18] Although the mechanism of thisactivating action is not totally clear the dual functionality ofMeCP2 is amply demonstrated by studies showing that 63ofMeCP2-bound promoters are actively expressed [17] It is notclear whether the role of MeCP2 in the epigenetic regulationof autism is related to its role as a gene silencer or promoterNevertheless a correlation between reduced expression ofMeCP2 and ASD is noted

Using immunofluorescence Nagarajan et al quantifiedMeCP2 in the frontal cortex (Brodmann area 9) and fusiformgyrus (Brodmann area 37) [19] The frontal cortex had previ-ously been linked to autism and associated with high levels ofMeCP2 expression whereas the fusiform gyrus is associatedwith face processing [20 21]MeCP2 expression at these brainsites was measured for 14 autistic brains each of which wascompared to three age-matched controls In 11 of the 14 cases

the autism brain samples showed significantly decreasedMeCP2 expression compared to age-matched controls insome cases the reduction was as much as twofold Similarlythe proportion of cells that expressed high levels of MeCP2was reduced in 11 out of the 14 autistic samples Of thesix fusiform gyrus samples examined by Nagarajan et alfive showed decreased expression of MeCP2 in the fusiformcortex and each of those five were among the samples thatexhibited decreased MeCP2 expression in the frontal cortexThis concordance suggests that whatever is responsible fordecreased MeCP2 expression in the brains of ASD subjects islikely exerting a generalized nonlocalized effect The autismpatients whose brains were examined byNagarajan et al wereclassified as idiopathic indicating that there was no knowngenetic cause for their ASD or for the decreased MeCP2expression that appears to be linked to the ASD Epigeneticregulation might help explain these findings In order to testfor methylation of the promoter region associated with theMeCP2 gene Nagarajan et al conducted bisulfite sequencingThe MeCP2 is on the X chromosome and all the studysubjects were males and thus actively expressing the Xchromosome Methylation of the 51015840 portion of the MeCP2regulatory region was observed for most autism samples andthe autism group showed a statistically significant increasein methylation when compared to similarly aged controlgroup samples As expected an inverse correlation wasfound between promoter region methylation and MeCP2expression These findings suggest that aberrant methylationmay have resulted in decreased expression of MeCP2 whichwas associated with autism It is also worth noting that thereis a well-established relationship betweenMeCP2 defects andRett syndrome (Rett is definitively diagnosed by evidenceof such a defect) since from a clinical standpoint Rett isclassified as an ASD

3 Epigenetic DNA-Protein Interactions

31 Protein Kinase C Beta Recent evidence suggests thata correlation exists between downregulation of the pro-tein kinase C beta gene (PRKCB1) in the temporal lobeand ASDs [22] In particular this association appears tobe linked to the alternative splicing of PRKCB1 isozymesfsI and betaII In addition PRKCB1 haplotypes are (sta-tistically) significantly associated with autism Moreoverwhole genome expression analysis showed less coordinatedexpression of PKCB1-driven genes [22] Phosphorylation ofhistone H3 at threonine 6 (H3T6) by protein kinase c beta-1 protein appears to prevent lysine-specific demethylase 1(LSD1) from demethylatingH3K4 during androgen receptor-dependent gene activation [23] This finding may partlyexplain the male predominance of ASDs and might sup-port the hypothesis that the higher fetal androgen levelsin males than females produce greater arousal inputs tothe amygdala which might sensitize boys to environmentalstressors Girls lack such androgen-facilitated arousal inputsto the amygdala and are protected from such high arousalinputs by estrogens oxytocin and the oxytocin receptor[24] A role for an oxytocin receptor polymorphism in



































HMT


Vitamin K



HDAC


HMT

HDACGSK



rarr macrocephaly


Biotin (vitamin B7)










8 Conclusions





References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of


















































HMT


Vitamin K



HDAC


HMT

HDACGSK



rarr macrocephaly


Biotin (vitamin B7)










8 Conclusions





References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of











































HMT


Vitamin K



HDAC


HMT

HDACGSK



rarr macrocephaly


Biotin (vitamin B7)










8 Conclusions





References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of



































HMT


Vitamin K



HDAC


HMT

HDACGSK



rarr macrocephaly


Biotin (vitamin B7)










8 Conclusions





References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of



























HMT


Vitamin K



HDAC


HMT

HDACGSK



rarr macrocephaly


Biotin (vitamin B7)










8 Conclusions





References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of




















8 Conclusions





References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of























































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of




















































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of





















of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of
















Documents

Review Article Epigenetics and Autismdownloads.hindawi.com/journals/aurt/2013/826156.pdfReview Article Epigenetics and Autism TafariMbadiweandRichardM.Millis Department of Physiology