Epigenetic Interaction Between Mecp2 and Organic Pollutants in Murine Development

Background: Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene MECP2 that encodes methyl CpG binding protein 2. MeCP2 is a known epigenetic modulator of gene expression required for postnatal neuronal maturation. In previous studies, Mecp2-deficient mice showed delayed neuronal maturation exhibited by alterations to the level and localization of histone H3K9 acetylation, an epigenetic marker of gene activity.  Also, 80% of autism brain samples showed decreased MeCP2 expression and exhibited similar alterations to H3K9 acetylation patterns in cortical neurons.  RTT is one of several neurodevelopmental disorders with a known genetic cause of epigenetic abnormalities, highlighting the importance of epigenetic mechanisms in postnatal brain development.  However, neurodevelopmental disorders which have unknown genetic etiologies such as autism and mental retardation are likely to be caused by a combination of genetic and environmental factors.  The recent increase in the incidence of autism cases suggests that environmental factors may play a role.  The widespread use of persistant organic polybrominated diphenyl ethers (PBDEs) as commercial flame retardants over the past decade has raised concern about human exposure to this new pollutant and potential effects on the developing brain.

Objectives: This study aims to test the hypothesis that perinatal exposure to brominated diphenyl ether 47 (BDE-47) affects the development of social and cognitive behavior through epigenetic changes in neurons during development in a mouse model genetically susceptible to an autistic phenotype.

Methods: Heterozygous Mecp2^308/+ dams were exposed orally via cornflake to either vehicle control (corn-oil), 0.03 mg BDE-47/kg/day, or 0.1 mg BDE-47/kg/day for a 10-week perinatal period (4 weeks prior to mating, 3 weeks in utero, 3 weeks lactation).  Dams were bred to wildtype males in order to yield four possible genotypes per treatment group – Mecp2^+/+, Mecp2^308/+, Mecp2^+/y, Mecp2^308/y. The pups undergo behavioral testing throughout development until approximately pnd70 when they are sacrificed and their tissues are collected.  A tissue microarray was constructed consisting of triplicate 6um cores from each of the various genotypes and treatment groups.  Immunofluorescence staining was performed and laser scanning cytometry (LSC) was used to quantify changes in histone acetylation and MeCP2.

Results: The 0.1 mg BDE-47/kg/day exposure negatively impacted fertility and litter survival specifically in Mecp2-mutant but not wild-type C57Bl6/J mice, suggesting an increased genetic susceptibility of Mecp2­-mutant mice to BDE-47 in reproductive success.  Preliminary behavioral studies on pups from 0.03 mg BDE-47/kg/day exposed Mecp2^308/+ dams indicate significant defects in at least one measurement of social behavior (ultrasonic vocalizations) but no changes in growth, reflex, and motor skills.  Preliminary immunofluorescence and LSC data have shown changes in the levels of MeCP2 and histone acetylation in cerebral cortical neuronal nuclei in BDE-47 exposed mice influenced by Mecp2 genotype.

Conclusions: Perinatal exposure to low doses of the organic pollutant BDE-47 combined with genetic susceptibility alters epigenetic patterns in maturing neurons and ultrasonic vocalizations in mice.