The Epigenetic and Neurodevelopmental Consequences of Maternal Tobacco Smoke Exposure

Abstract

Maternal smoking is a deleterious and preventable risk to fetal health. Maternal tobacco smoke (TS) exposure in humans has been linked to impaired fetal growth, preterm birth, sudden infant death syndrome, and neurobehavioral disorders including cognitive dysfunction, attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). In the United States, nearly 10% of pregnant women smoke despite ongoing public health efforts to reduce the incidence of smoking. Of additional concern is the steady rise of electronic nicotine delivery systems (ENDS) use among pregnant women over the past decade. Further, ENDS are often used in conjunction with tobacco cigarettes, compounding exposure effects. In animal models, both maternal TS and nicotine exposure lead to adverse neurodevelopmental outcomes, including increased anxiety and ADHD-like behavior, that are transmitted to subsequent generations. A likely explanation for this phenomenon lies in early developmental epigenetic programming. Epigenetic markers that are established early in development, like DNA methylation, can persist throughout somatic cell division and gametogenesis. During early development, zygotic DNA methylation is reprogrammed following a wave of global demethylation, and only re-established during the peri-implantation period. Environmental perturbations during these critical phases of reprogramming have been associated with persistent, and even transgenerationally-inherited effects, underscoring the importance of examining these associations in the context of human health and disease.The broader goals of this dissertation were to identify alterations in DNA methylation patterning in the brain as a result of maternal TS exposure, and assess their neurodevelopmental significance. In an effort to better understand the effects of nicotine alone, especially given the increasing usage of ENDS during pregnancy, we chose to examine developmental nicotine exposure in addition to TS exposure. Finally, we evaluated the translational significance of these alterations by examining correlations in humans developmentally exposed to TS. Using a rat model for gestational nicotine exposure, DNA methylation levels were measured in the brains of neonatal and adult rats to determine the persistence of exposure effects. Specific brain regions, including the rat preoptic area (POA), hippocampus and cortex were targeted for evaluation based on their neurobehavioral significance. A rat gestational exposure model for tobacco smoke extract (TSE) was further employed to determine potential overlap with methylomic regions affected by developmental nicotine exposure. Finally, to derive translational implications, DNA methylation was examined on both epigenome-wide and targeted levels in human cord blood from newborns exposed to maternal TS.In neonatal rats developmentally exposed to nicotine, DNA methylation was reduced in regions implicated in masculinization of the preoptic area (POA), a region of the brain that requires epigenetic reprogramming events to sexually differentiate. Subsequent behavioral analyses in adulthood revealed that these alterations may have contributed to the developmental masculinization of the POA in nicotine-exposed females. In adults males developmentally exposed to nicotine, alterations to DNA methylation were observed in the hippocampus and cortex, two brain regions that are also associated with ADHD- and ASD-like behaviors, respectively. Further, a comparison of differentially-methylated regions (DMRs) between the brains of animals exposed to developmental nicotine and TSE revealed significant overlap, indicating that nicotine is largely driving the developmental alterations to DNA methylation observed in TSE-exposed animals. Examination of DNA methylation alterations in human infant cord blood as a result of maternal TS exposure indicated significant overlap with those revealed in rats, supporting common impacts on developmental epigenetic reprogramming across species. Moreover, nearly half of these common regions were implicated in neurodevelopmental disorders, namely ASD and ADHD. Alterations to DNA methylation at human metastable epialleles, or regions for which DNA methylation is stochastically established during early development, were observed in the cord blood of infants exposed to TS in utero, supporting the ability of TS-exposure to alter vulnerable regions of the epigenome during early developmental reprogramming.