Epigenetics and ADHD: Toward an Integrative Approach of the Disorder Pathogenesis

Abstract

Objective: Epigenetic hypothesis is one of the research pathways used to explain the complex etiology of neurodevelopmental disorders. This review highlights the findings of recent studies in the field of epigenetics in ADHD. Methods: An electronic literature search using Medline. Results: In the Gene × Environment interaction model, several clinical, genetic and molecular arguments support the epigenetic hypothesis in ADHD etiology. Environmental ADHD risk factors including toxic, nutritional factors and stressful life events lead to changes in DNA methylation and in histone modification levels. One critical CpG site located in the promoter of the DRD4 gene exhibited a specific pattern in ADHD children. A methylome wide exploration of DNA showed decreased methylation in vasoactive intestinal peptide receptor 2 gene, which was not replicated by further research. Conclusion: Current data require consolidation and could lead to the identification of biomarkers and the introduction of new modalities of treatment.

Keywords

ADHD epigenetics DNA methylation histone modification

Introduction

The behavioral and cognitive patterns of ADHD have been well described over the years, and defined diagnostic criteria have been established (American Psychiatric Publishing, 2013). However, its exact etiology has not yet been identified, and several factors have been implicated. Studies of twins, families, and adopted children have already documented the importance of genetic factors in the development of the disorder, and have shown substantial heritability and a higher ADHD risk in the first-degree biological relatives of individuals with the disorder (Li, Chang, Zhang, Gao, & Wang, 2014). Nevertheless, genetic linkage and hypothesis-driven candidate gene association studies have related ADHD to several genes especially those involved in neurotransmission but failed to identify specific genes significantly related to ADHD. Similarly, genome-wide association studies were unable to find any significant association to common variation in ADHD (Franke, Neale, & Faraone, 2009; Neale et al., 2010; Schuch, Utsumi, Costa, Kulikowski, & Muszkat, 2015). Besides the genetic component of ADHD, epidemiological studies have highlighted several pre and perinatal environmental risk factors predisposing to the disorder including toxic exposure, maternal stress during pregnancy, low birth-weight, and psychosocial adversities (Mill & Petronis, 2008). Currently, researchers suggest that genetic and environmental factors do not act independently but in synergy in a way that leads to the emergence of the disorder. The processes underlying this interaction are still poorly known, but epigenetic modifications are becoming one of the most explored research pathways that could explain the complex etiology of neurodevelopmental disorders where the environmental insults can lead to gene expression modulation without any modification of the DNA sequence (Miller, 2010). Epigenetic mechanisms have been involved in schizophrenia, depression (Tsankova, Renthal, Kumar, & Nestler, 2007), and autism spectrum disorders (Grafodatskaya, Chung, Szatmari, & Weksberg, 2010). Recent studies link them to ADHD. In this review, we present a summary of current knowledge and highlight the findings of recent studies in this field.

Epigenetics: Definition and Mechanisms

The term epigenetics derives from the fusion of the terms genetic and epigenesis. It has been highlighted from the question: “If the individual’s characters are determined by genes then why aren’t all the organism’s cells the same?” (Morgan, 1926).

The invention of the term epigenetics is attributed to the British embryologist and geneticist Conrad Waddington (2012), who described it in 1942 as “the interactions of genes with their environment, which bring the phenotype into being.”

As a whole, it refers to the molecular processes that might modulate gene expression without any modification of the nucleotide sequence of DNA. In fact, the prefix “epi” from Greek, literally means “on, above.” To illustrate this phenomenon, two metaphors are often used. Genetics would be similar to a book’s writing and epigenetics to the reader’s interpretation. Or, if the DNA is a “hard disk,” epigenetics is the “software” that dictates gene behavior.

The set of epigenetic marks constitutes the cell’s epigenome which is different from one cell to another, even though they share a common gene pool. These marks are usually inherited through mitosis and meiotic inheritance (transgenerational), is possible through germ cells (Bohacek & Mansuy, 2015; Skinner, 2014; Yahyavi, Zarghami, & Marwah, 2014). They are considered as flexible factors and therefore reversible. However, the consequences of epigenetic modifications occurring at key stages of development may be irreversible (Scheen & Junien, 2011).

The most studied epigenetic mechanisms are those that modify the chromatin structure. Chromatin is a complex formed by nucleosomes: units of DNA wrapping around a histone octamer (two copies each of the core histones H2A, H2B, H3, and H4). The nucleosome is the basic unit of DNA packaging in eukaryotes. Euchromatin is the lightly packed form of chromatin that allows access to transcriptional factors leading to gene expression, while the heterochromatin represents the tightly packed form of chromatin where genes are generally silenced.

There are two main epigenetic mechanisms involved in chromatin remodeling. The first one consists in DNA methylation of cytosine residues within CpG dinucleotides ensured by DNA methyltransferases (DNMTs). These enzymes catalyze the addition of a methyl group to the cytosine residue. DNA hypermethylation usually leads to chromatin condensing and gene silencing, whereas DNA hypomethylation promotes DNA transcription and hence gene expression (Portela & Esteller, 2010). The second mechanism is the post-translational histone modifications including phosphorylation, acetylation, methylation, and ubiquitination. The most comprehensively studied modifications are histone methylation and acetylation. Although hypoacetylation, catalyzed by histone deacetylases (HDACs), is known for its repressive effect on gene expression, histone methylation can lead to transcriptional activation or repression, depending on the position of the lysine acceptor (Bannister & Kouzarides, 2011).

Other epigenetic regulation mechanisms involve RNA molecules including non-coding RNA and small interfering RNA (siRNA). Micro RNAs (miRNAs) are small non-coding RNAs that participate in post-transcriptional regulation of gene expression by pairing with complementary sequences within messenger RNA (mRNA) molecules. siRNAs, meanwhile, play a role in the RNA interference (RNAi) pathway where they interfere with specific genes expression by causing the destruction of specific mRNA molecules (Agrawal et al., 2003).

Epigenetic Paradigm and ADHD: What Arguments for?

In the Gene × Environment interaction model, several clinical, genetic, and molecular arguments support the epigenetic hypothesis in ADHD etiology.

The developmental course of ADHD may also be related to this mechanism. In fact, longitudinal studies have shown that over the years, the hyperactive–impulsive symptoms decrease significantly from early childhood to adolescence whereas attention problems are more stable in late childhood and adolescence (Lahey & Willcutt, 2010). This decrease in hyperactivity noted with aging could potentially be due to epigenetic factors decreasing the expression of genes involved in activity levels (Elia, Laracy, Allen, Nissley-Tsiopinis, & Borgmann-Winter, 2012).

Therapeutic effects of methylphenidate in ADHD are known to be mediated by catecholamine reuptake inhibition. However, animal studies highlighted other potential molecular mechanisms involving epigenetics. In one study, distinct expression profiles of long non-coding RNAs (a type of non-coding RNAs) were found in the normal rat prefrontal cortex after exposure to methylphenidate pointing toward a new therapeutic target in ADHD treatment (Wu et al., 2015).

Genetic studies in ADHD, especially twin studies, have demonstrated high heritability of the disorder (76%; Faraone et al., 2005). However, the concordance rate between monozygotic twins remains incomplete suggesting that besides genetic factors, environmental exposure contributes to the emergence of the disorder and its effects may be mediated by epigenetic mechanisms.

Genetic association studies have identified several ADHD candidate genes involved in neurotransmission, receptor function, and neuronal plasticity (Gizer, Ficks, & Waldman, 2009). Among them, the dopamine transporter gene (DAT1 or SLC6A3) was significantly associated with the disorder (Gizer et al., 2009). Its genomic attributes are of special interest since they could support the epigenetic hypothesis in ADHD (Elia et al., 2012). In fact, in their report, Shumay, Fowler, and Volkow (2010) revealed pronounced sensitivity of DAT1 gene to epigenetic factors due to some of its features including the abundance of variable number of tandem repeats (VNTRs) (indicating a tendency for open chromatin structure and increased accessibility to chromatin modifiers) and high CpG density throughout the gene (Shumay et al., 2010).

Furthermore, family-based studies and genetic association studies have reported that some alleles at ADHD candidate genes have preferential paternal transmission to the affected offspring (Hawi et al., 2005). This parent-of-origin effects could be explained, among other genetic mechanisms, by genomic imprinting (Zayats, Johansson, & Haavik, 2015). This process represents a form of epigenetic regulation where only one of the two alleles inherited from parents is expressed while the other is being silenced by DNA methylation and histone modifications (Delaval & Feil, 2004).

The expression of several genes interfering with the epigenetic process was also found to be disrupted in ADHD. Methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms (C677T and A1298C) were associated with the disorder (Gokcen, Kocak, & Pekgor, 2011). The MTHFR enzyme encoded by this gene plays a central role in folate metabolism and in S-adenosylmethionine generation, a major source of methyl groups in the brain (Blom & Smulders, 2011). Thus, decreased MTHFR enzyme activity in case of nucleotide polymorphisms (Weisberg, Tran, Christensen, Sibani, & Rozen, 1998) alters histone and DNA methylation, disturbing the expression of genes regulated by epigenetic mechanisms.

The methyl CpG binding protein 2 gene (MeCP2) expression was also found to be altered in ADHD. In frontal cortex samples, significantly reduced MeCP2 levels were described (Nagarajan, Hogart, Gwye, Martin, & LaSalle, 2006). This protein interferes with epigenetic regulation. In fact, MeCP2 binds to methylated DNA and forms a complex with the HDAC 1 enzyme which removes acetyl groups from histone, causing chromatin condensation and thus gene transcriptional repression (Du, Luu, Stirzaker, & Clark, 2015).

Additional data provided by Kandemir et al. (2014) study could even support the epigenetic theory in ADHD pathogenesis. The authors evaluated several miRNA levels in 52 ADHD patients versus a matched control group, and significant dysregulation of circulating miRNA levels were found in the ADHD group (Kandemir et al., 2014). These molecules, as previously seen, are part of epigenetic regulation mechanisms.

Environmental Factors, Epigenetic Marks, and ADHD Risk

Several studies have reported changes in global and gene-specific DNA methylation and in histone modification levels following exposure to a range of environmental agents (Arita & Costa, 2011). The results of those relevant to ADHD involving environmental factors already linked to the disorder are presented in this section.

Nutritional Factors

Lower maternal folate status in early pregnancy was associated with childhood hyperactivity in offspring (Schlotz et al., 2010). Folate is an essential component required for the formation of S-adenosyl methionine, a principal methyl-doner. A diet lacking folate during pregnancy can cause a genome-wide hypomethylation and increase the susceptibility to numerous conditions including ADHD.

In an animal study, it has been shown that offspring of mice given a diet deficient in proteins throughout pregnancy and lactation developed hyperactivity and had altered reward processing. Across these animal’s brain regions, hypomethylation of the promoter of cyclin-dependent kinase inhibitor 1C gene was found concomitant with an increase of its mRNA levels. This gene plays a crucial role in dopaminergic neuronal development, neurons known to be involved in ADHD etiology (Vucetic et al., 2010).

Toxic Factors

Several chemical agents were associated with an increased ADHD risk following a prenatal exposure. That does include the following: tobacco smoke, alcohol, glucocorticoids, and polychlorinated biphenyls (PCBs; Mill & Petronis, 2008).

Maternal smoking during pregnancy was associated with higher ADHD risk in several studies (Cho et al., 2010). Neurobiological effects might explain this association since nicotine leads to abnormalities in cell proliferation, synaptic plasticity, as well as cholinergic and catecholaminergic transmission (Ernst, Moolchan, & Robinson, 2001). However, epigenetic events could underpin this relationship. In fact, changes in the DNA methylation profile were well highlighted in tobacco-related cancers studies and were correlated with tobacco consumption (Arita & Costa, 2011).

Gestational alcohol exposure was also associated with ADHD (Bhatara, Loudenberg, & Ellis, 2006). It was reported that alcohol consumption induces alterations in DNA methylation at both global and gene-specific levels (Bleich & Hillemacher, 2009). This can be explained by the fact that alcohol affects folate absorption metabolism and S-adenosyl methionine bioavailability causing disturbances in DNA methylation (Tollefsbol, 2010). Kim et al. (2013) have also highlighted another possible mechanism as they reported a significantly decreased MeCP2 expression in the prefrontal cortex and striatum of mouse and rat offspring born from dam exposed to alcohol during pregnancy. These animals displayed hyperactive, inattentive, and impulsive behavior (Kim et al., 2013). In other animal studies, germ line epigenome changes were described after paternal chronic alcohol consumption. Bielawski, Zaher, Svinarich, and Abel (2002) reported decreased cytosine methyltransferase mRNA levels in paternal sperm after 9 weeks of alcohol exposure (Bielawski et al., 2002). The findings of Kim et al. (2014) study suggested that preconceptional exposure to ethanol through the paternal route induces ADHD-like hyperactive inattentive and impulsive behaviors in offspring, possibly mediated by epigenetic changes. In fact, the expression of MeCP2 and DNMT1, proteins involved in epigenetic regulation, was found markedly decreased in offspring cortex and striatum. In these brain regions, increased methylation of the promoter of the DAT gene with decreased DAT protein expression was also found. Interestingly, the same DAT methylation profile was observed in the sperm of sire mice, which could represent additional data to the meiotic inheritance hypothesis of epigenetic marks (Kim et al., 2014).

Repeated antenatal exposure to synthetic glucocorticoids was identified as a risk factor of ADHD (French, Hagan, Evans, Mullan, & Newnham, 2004). Animal studies have shown that gestational exposure to synthetic glucocorticoids had effects on DNA methylation status in fetus and offspring and altered the expression of several genes encoding proteins involved in epigenetic regulation (Crudo et al., 2012). Some reports demonstrated that these medications given prenatally alter, via epigenetic mechanisms, the hypothalamic–pituitary–adrenal (HPA) axis function (Elia et al., 2012) which was associated with increased risk of psychiatric disorders (Mill & Petronis, 2008).

Neurological effects of PCBs have been well investigated, and changes in neurotransmitter levels including dopamine have been described, providing a potential explanation for the link between ADHD risk and PCBs exposure (Eubig, Aguiar, & Schantz, 2010; Mill & Petronis, 2008). In animal studies, several models have been developed to explore the physiopathology of ADHD. The PCB-exposed rat model is one of the well-characterized environmental models of ADHD (Berger et al., 2001; DasBanerjee et al., 2008). To identify mechanisms through which these rat models mimic ADHD, DasBanerjee et al. (2008) examined the expression profile of genes considered relevant to ADHD and to epigenetic regulation. Significant changes in expression of four epigenetic genes in PCB-exposed rats were observed, suggesting that disturbances in the epigenetic pathway might play a role in the pathogenesis of ADHD (DasBanerjee et al., 2008).

Lead exposure was also considered as a chemical environmental risk factor for the development of ADHD (Eubig et al., 2010). In animal models, ADHD-like symptoms after chronic lead exposure were described in many studies. Several authors tried to establish whether the link between the environmental factor and the disorder was ensured by epigenetic factors or not. Schneider, Kidd, and Anderson (2013) showed that lead exposure might alter the dynamic modulation of DNA methylation with changes in DNMT and MeCP2 expression levels in the hippocampus of exposed dams (Schneider et al., 2013).

Luo et al. (2014), meanwhile, reported an increase of histone acetylation in the hippocampus of rats exposed to various doses of lead highlighting another possible epigenetic mechanism underlying the hyperactivity induced by this heavy metal (Luo et al., 2014).

Psychosocial Factors

Recent evidence has indicated that not only exposure to chemical or nutritional environmental factors can alter the epigenome, but psychosocial factors can also lead to long-lasting changes in epigenetic marks and thus modulate gene expression (Gudsnuk & Champagne, 2012). This may be relevant to ADHD since some reports link psychosocial adversity to an increased ADHD risk (Biederman, 2005).

In an animal study, Mueller and Bale (2008) have shown that offspring exposed to chronic stress during gestation displayed elevated stress sensitivity and an increased HPA axis responsivity. In the hippocampus and the amygdala of these animals, hypomethylation of the corticotropin-releasing factor gene was found to be associated with an increased secretion level providing evidence of epigenetic changes during early prenatal stress (Mueller & Bale, 2008). Maternal stress during pregnancy especially in the third trimester was associated to an increased risk of ADHD (Class et al., 2014) and has been correlated with symptoms severity (Grizenko, Shayan, Polotskaia, Ter-Stepanian, & Joober, 2008). Also, it should be noted that abnormalities in the cortisol response to stress have been described in children with severe or persistent ADHD (Fairchild, 2012).

Furthermore, lack of maternal care and early attachment deprivation have been linked to more externalized behavioral problems and ADHD later on. This was outlined by studies on adopted children which support the hypothesis that ADHD symptoms are a characteristic outcome of early deprivation (Roskam et al., 2014). Weaver et al. (2004) have demonstrated that postnatal maternal deprivation in rats affects glucocorticoid receptor gene (Nr3C1) expression in the hippocampus of offspring leading to increased corticosterone secretion level, which is due to glucocorticoid negative feedback lifting. This was attributed to hypermethylation of the promoter region of the glucocorticoid receptor gene inhibiting the binding of the transcriptional factor NGF1A to this site, thus affecting gene expression (Weaver et al., 2004).

The environmental ADHD risk factors reported above have all been described to induce epigenome modifications (Arita & Costa, 2011), and several studies have clearly demonstrated the involvement of epigenetic mechanisms in neurodevelopmental regulation (Gräff, Franklin, & Mansuy, 2011). However, no study has been able to establish a causal linear link between environmental agent exposure - specific epigenetic modification - and the subsequent emergence of ADHD.

In humans, investigating this link is made complicated by numerous confounding limitations:

Isolating a single environmental risk factor to which exposure can be studied is difficult as they are often concomitant. For example, the use of tobacco during pregnancy can be associated to other risk factors such as stress, alcohol, and synthetic glucocorticoids intake (Elia et al., 2012; Knopik et al., 2006; Rodriguez & Bohlin, 2005). Even though some recent researches have focused on studying a unique factor (prenatal unhealthy diet) while attempting controlling other variables (stressful events, parental substance use, and maternal smoking during pregnancy) (Cecil, Walton, & Barker, 2016; Rijlaarsdam et al., 2017), results have been criticized questioning the accountability of epigenetic changes and ADHD onset to the studied factor by showing that another uncontrolled factor (exposure to air pollution) could lead to the same modifications in DNA methylation either directly or indirectly through changing in eating behavior (Fluegge, 2017).

More than studying a one-targeted factor, the time at which exposure occurs is also important to consider given the dynamism of cerebral growth, influencing neurodevelopmental outcomes (Olney, Farber, Wozniak, Jevtovic-Todorovic, & Ikonomidou, 2000). Thus, epigenetic changes may differ depending on the timing of exposure making interpretation of the studies results more complicated.

Many epigenetic modifications tend to be transient and may be reversible with age (Weaver, 2011). Current studies are taking this into account by conducting longitudinal and prospective researches, performing molecular analysis at birth and then later at pre-school or school age (Rijlaarsdam et al., 2017). However, in such studies, even if the methodology is more rigorous, it is more difficult to attribute the epigenetic modifications to a specific environmental factor given the multitude of agents to which individual is exposed throughout his life course that cannot be controlled. Nevertheless, epigenetic mechanisms could be attractive candidates to explain long-lasting, and potentially even permanent, alterations in neuronal function following risk factors exposure, but it is still unclear, as it was studied in drug addiction (Nestler, 2014) how long after the changes in chromatin structure could persist.

Because epigenetic changes can be inherited across mammalian generations, it is difficult to discern which ones were acquired as a result of exposure to an environmental factor from those that were inherited.

In animals, certainly some of these limitations can be overcome (controlling a single factor, time of exposure, and recurrent analysis) but raises the question about extrapolation of the results to humans.

ADHD Genes and Epigenetics

Candidate genes studies conducted in ADHD have initially targeted genes involved in neurotransmission pathways based on the mechanisms of action of stimulant medication, recognized to be effective in ADHD (Schachar, 2014). Results have highlighted associations with several dopaminergic genes (especially the DAT 1 gene and the dopamine receptors genes DRD4 and DRD5), genes related to the noradrenergic system (such as ADRA2A and ADRA2C genes), and genes involved in the serotonergic pathway (in particular the serotonin receptor gene HTR1B and the serotonin transporter gene SLC6A4) (Gizer et al., 2009; Schachar, 2014). Furthermore, other genes playing a role in neuronal plasticity and synaptic transmission were also associated to ADHD including the synaptosomal-associated protein 25 (SNAP-25) gene and the brain-derived neurotrophic factor (BDNF) gene (Gizer et al., 2009).

Lately, a number of reports have focused on the study of the methylation profile of some of these genes emphasizing the role of epigenetic mechanisms in ADHD (Schuch et al., 2015).

Wong et al. (2010) have quantitatively measured DNA methylation across the promoter regions of DRD4, SLC6A4, and the X-linked monoamine oxidase A genes through DNA sampled in 46 monozygotic twin pairs and 45 dizygotic twin pairs at both age 5 and 10 years. The results suggested, as outlined by the authors, that differences in DNA methylation are apparent in early childhood, even between genetically identical individuals and that individual differences in methylation are not stable over time (Wong et al., 2010).

van Mil et al. (2014) have, across a prospective study, investigated whether methylation patterns, observable at birth, of seven selected neuronal genes were associated with the occurrence of ADHD symptoms at the age of 6 years, identified using the child behavior checklist. The DNA was extracted from umbilical cord blood cells, sampled at birth. In the overall analysis of the 11 regions assessed, lower DNA methylation levels were associated with higher ADHD symptoms scores. The effect was largely explained by associations with DNA methylation levels in DRD4 and 5-HTT regions. However, the authors could not identify the underlying mechanisms of these findings as several factors including pre and postnatal environment were not taken into account (van Mil et al., 2014).

In a more recent study, conducted among Chinese ADHD children, Xu et al. (2015) have demonstrated that the methylation of one critical CpG site, located in the promoter of the DRD4 gene, exhibited a considerably different pattern in ADHD children compared to healthy controls. Moreover, the methylation change affected negatively the transcription levels of DRD4 gene leading the authors to conclude that CpG site 1 is a critical site for DRD4 gene expression and might play a role in ADHD development (Xu et al., 2015).

Next to studying the methylation profile of dopaminergic genes, a recent report investigated changes in DNA methylation of the serotonin 3A receptor gene among ADHD adult subjects who had a history of childhood maltreatment. Differential methylation status was observed depending on history of child adversity and the clinical severity of the disorder (Perroud et al., 2016).

Besides studies that have examined DNA methylation in candidate genes in ADHD, there is to date, to our knowledge, only two studies that carried out a methylome wide exploration of DNA methylation among ADHD children. Wilmot et al. (2016) study evaluated 42 ADHD boys aged 7 to 12 years and matched non-ADHD controls (Wilmot et al., 2016). DNA was extracted from salivary samples, and 5-methylcytosine levels were quantified using the Illumina 450 k Human methylation array. Results showed decreased CpG methylation level in the vasoactive intestinal peptide receptor 2 gene (VIPR2) in ADHD children compared to the control group. VIPR2 plays a role in neuronal function, and duplications in the gene have been associated with an increased risk of schizophrenia (Yuan et al., 2014). VIPR2 is not commonly known as a candidate gene for ADHD. However, in a recent study that explored epigenetic changes in 50 individuals, who suffered from malnutrition during childhood, a moderate correlation between VIPR2 methylation at specific CpG site and ADHD index on the Conners’ Adult ADHD Rating Scales (CAARS) questionnaire was found (Peter et al., 2016).

The second methylome wide study conducted by Walton et al. (2016) in general population and on a larger sample identified 13 probes that were differentially methylated at birth between high and low trajectories of ADHD symptoms assessed via maternal ratings at age of 7 years using the Developmental and Well-Being Assessment interview. The probes were located in genes implicated in peroxisomal processes (PEX2), neural tube development (SKI), mental retardation (ST3GAL3), as well as one gene previously linked to ADHD (ZNF544). However, none of these probes maintained an association with ADHD trajectories at age of 7 years (Walton et al., 2016). Moreover, contrasting results regarding methylation in VIPR2 and DRD4 genes (that were associated with ADHD in previous epigenetic studies) were reported in this work. Methodological differences could explain the divergent results; therefore, to consolidate these findings, further epigenome wide association studies in ADHD are required to identify epigenetic signatures associated with the disorder.

For all these studies, many methodological biases limit the extent of their results and make their interpretation difficult. In fact, the epigenome, unlike the genome, is tissue specific, and using peripheral cell samples such as blood and buccal cells in epigenetic studies raises the question of whether their methylome mirrors that of nerve cells. Some studies have concluded that the correlation rate between the DNA methylome of blood cells and brain cells was “high” in certain cerebral regions and could lead to the identification of peripheral biomarkers reflecting cerebral epigenetic modifications (Aberg et al., 2013). Others attested that buccal epithelial cells better reflect the epigenome of the brain and thus would be a better substitute than blood cells in these studies (Lowe et al., 2013; Smith et al., 2015).

Moreover, blood samples contain different cell types that vary in proportion from one individual to another. Therefore, differences found in DNA methylation could be due to the samples’ cell heterogeneity (Miller, 2010).

Besides the methodological limitations linked to the type of cells used, some specificities at molecular level should be taken into account. In fact an inter-individual variability is noted in the arrangement of CpG dinucleotides inside the DNA sequence and should be considered when interpreting differences in methylation profiles (Chadwick et al., 2015).

The National Institutes of Health has come up with guidelines on how to conduct research in epigenetics (Chadwick et al., 2015). This will limit the shortcomings of these studies and lead to a better understanding of the role played by epigenetics in neurodevelopmental disorders.

Future Prospects

The aforementioned data do not allow a synthetic approach, premature at this stage of knowledge. Although preclinical studies have allowed the development of epigenetic models in the genesis of somatic diseases, this is more complicated in psychiatric illnesses, even neurodevelopmental ones. For instance, disruption of the epigenome was described as a fundamental mechanism in cancer, and epigenetic drugs including DNA methylation inhibitors and HDAC inhibitors have shown promising results. Moreover, emerging data describe molecular determinants of clinical responses (Chen, Zhu, Li, & Meng, 2014).

The epigenetic action of certain psychotropic drugs has been demonstrated as for valproate (Dong, Chen, Gavin, Grayson, & Guidotti, 2010). More recently, based on preclinical researches, epigenetic drugs have received increased attention for the management of psychiatric diseases like schizophrenia, depression, and bipolar disorders (Gräff et al., 2011; Karsli-Ceppioglu, 2016).

In ADHD, no preclinical study of the effect of epigenetic drugs was performed. However, the link between epigenetic modifications of dopamine gene, and ADHD treatments were studied. In humans, DAT1 methylation profile was proven to modulate the response to methylphenidate treatment on oppositional and hyperactive–impulsive symptoms in ADHD (Ding et al., 2016).

In animal model, methylphenidate was associated with an epigenetic activation, in the prefrontal cortex of rats, of the UBE2B gene, a gene probably involved in neurite outgrowth and axonal regeneration (Wu et al., 2015).

These discoveries necessarily give to the epigenetic field a very strong interest and place a new hope for discovering powerful environmental influences in the emergence of the disorder to develop more effective prevention in addition to new treatments.

Conclusion

In conclusion, although the importance of epigenetic dysfunctions in ADHD begins to be appreciated, a more thorough understanding of epigenetic processes in this disorder is still required.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Author Biographies

Meriem Hamza, MD, child and adolescent psychiatrist at the department of child psychiatry at Mongi Slim Hospital. Assistant professor at the faculty of medicine of Tunis, University of Tunis El Manar. Fields of research: Cognitive approach in the treatment of children with ADHD, epigenomics, genetics.

Soumeyya Halayem, MD, child and adolescent psychiatrist at the department of child psychiatry at Razi Hospital. Associate professor at the faculty of medicine of Tunis, University of Tunis El Manar. University degree in pedagogy and neuropsychopharmacology. Fields of research: Autism spectrum disorders, genetics.

Soumaya Bourgou, MD, child and adolescent psychiatrist at the department of child psychiatry at Mongi Slim Hospital. Assistant professor at the faculty of medicine of Tunis, University of Tunis El Manar.

Mona Daoud, Resident in child and adolescent psychiatry.

Fatma Charfi, MD, child and adolescent psychiatrist at the department of child psychiatry at Mongi Slim Hospital. Associate professor at the faculty of medicine of Tunis, University of Tunis El Manar.

Ahlem Belhadj, MD, head of the department of child and adolescent psychiatry at Mongi Slim Hospital. Professor at the faculty of medicine of Tunis, University of Tunis El Manar. Member of the ethics committee of the faculty of medicine of Tunis. President of the Tunisian college of child psychiatry. Master of advanced studies in genetics. Numerous actions in the voluntary sector. Fields of research: Autism spectrum disorders, genetics, gender violence and language.

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