A fundamental question is how prenatal THC or nicotine exposure causes alterations in gene expression that are maintained into adulthood, long after the initial drug exposure period in utero. As discussed above, both THC and nicotine causes substantial changes in gene expression levels with abnormalities persisting long into the life of the individual. One potential mechanism by which prenatal drug exposure could lead to long-lasting changes in behavior is through inducing alterations in epigenetic gene regulation mechanisms in the brain, which would be propagated throughout development stably enough to cause enduring phenotypical abnormalities. ‘Epigenetic’ refers to mechanisms that modulate gene expression without altering the genetic code. The epigenome is influenced by the environment and thus is a highly relevant biological candidate to maintain aberrant neuronal processing as a result of drug exposure during prenatal development. Despite the vulnerability of the developing brain to drugs, most epigenetic addiction studies have focused on the adult brain so limited information currently exists as to epigenetic effects associated with developmental drug exposure. In this section, we discuss several epigenetic processes that will merit future investigation as the most likely candidate mechanisms involved in mediating the long-term effects of prenatal cannabis and cigarette exposure.
Epigenetic modifications that can regulate gene expression levels include microRNAs, DNA methylation and post-translational modifications of nucleosomal histones () (Saetrom et al., 2007
; Ooi & Wood, 2008
; Guil & Esteller, 2009
). The role of microRNAs in neuronal development has been widely documented and a number of observations have indicated, for example, that cocaine addiction can change the normal composition of striatal microRNAs in adult animals (Eipper-Mains et al., 2011
). Interestingly, ethanol treatment during the prenatal period has been associated with teratogenesis and changes in microRNA composition in the mouse fetal brain, leading to mental retardation later in the life of the offspring (Wang et al., 2009
). It will be interesting to assess whether prenatal cannabis and nicotine exposure affects neuronal microRNA populations and related gene regulatory processes.
A few possible epigenetic regulatory mechanisms that can be disrupted by maternal cannabis and nicotine u se, leading to persistent abnormal gene expression levels
DNA methylation is an epigenetic mark that is known to be particularly stable throughout development. Maternal stress or nutritional deficiencies have been reported to cause long-lasting impairments in DNA methylation (Ronald et al., 2010
; Jousse et al., 2011
; Tarantino et al., 2011
), and it is conceivable that exposure to psychoactive drugs during prenatal development could induce similar abnormalities. In fact, maternal cocaine administration has indeed been shown to trigger changes in DNA methylation that are detectable in hippocampal neurons of neonatal and adolescent mice (Novikova et al., 2008
). Several groups have identified atypical DNA methylation patterns within the placenta (Suter et al., 2010
) of cigarette smoke-exposed infants and altered methylation of CpG islands at specific gene loci has been reported as long-term effects associated with prenatal nicotine exposure in somatic tissues (Toro et al., 2008
; Breton et al., 2009
; Suter et al., 2010
; Toledo-Rodriguez et al., 2010
). These observations raise the important question whether similar epigenetic disturbances occur in the brain that could possibly explain the development of abnormal behavioral tendencies.
Most studies related to drug abuse in the adult brain have focused on histone modification, which is broadly implicated in the dynamic regulation of transcriptionally repressive (inactive) and permissive (active) states (). Covalent modifications of histones play a major part in epigenetic regulation and histone acetylation, methylation and phosphorylation have been implicated in gene regulation and neurobiological disturbances related to drug abuse (Li et al., 2007
; Nestler, 2009
). Published data thus far has indicated that increases in acetylation and phosphorylation are transient and appear to be associated with the quick activation of genes in response to drug exposure rather than the maintenance of an altered transcription state (Kumar et al., 2005
). Histone methylation is known to maintain stable gene expression alterations. In addition, the regulation of histone H3 modification is unique because methylation of distinct residues can have the opposite effect on transcription (Bannister & Kouzarides, 2005
). Especially interesting is the developmental role of H3K9 dimethylation and H3K27 trimethylation in the maintenance of life-long tissue-specific gene silencing patterns (Horn & Peterson, 2006
; Swigut & Wysocka, 2007
). This raises the possibility of histone methylation playing a role in the propagation of perturbed gene transcription states induced by developmental cannabis and cigarette exposure.
Our studies of offspring from the prenatal THC rat model have begun to uncover such disturbances in histone modification in the adult brain. The reduction of Drd2 mRNA transcript levels in the NAc of cannabis-exposed human fetuses and in neonatal rats with prenatal THC exposure, together with the persistence of the change into adulthood in rats, indicated that the observed downregulation of Drd2 gene expression may be achieved via epigenetic processes. Examining various marks with antagonistic roles in histone H3 regulation revealed increased levels of dimethylation of lysine 9 on histone H3 (2meH3K9), a repressive mark, as well as decreased RNA polymerase II association with the gene in the NAc which are consistent with reduction of the Drd2 gene expression (). The methylation profile of histone H3 on the Drd2 gene was specific since there was no alteration of 2meH3K9 observed at the Drd1 gene (Dinieri et al., 2011
). These findings suggest that maternal cannabis use alters the developmental regulation of mesolimbic D2
R in offspring through epigenetic mechanisms, specifically histone lysine methylation, and the ensuing reduction of D2
R may contribute to increased vulnerability to drug abuse later in life. To date, no other epigenetic studies to our knowledge have been published regarding the effects of prenatal cannabis exposure on the brain.