The role of DMDs in the maintenance of epigenetic states

Mechanisms of transgenerational inheritance of addictive-like behaviors

Genetic factors are implicated in the heritability of drug abuse. However, even with advances in current technology no specific genes have been identified that are critical for the transmission of drug-induced phenotypes to subsequent generations. It is now evident that epigenetic factors contribute to disease heritability and represent a link between genes and the environment. Recently, epigenetic mechanisms have been shown to underlie drug-induced structural, synaptic, and behavioral plasticity by coordinating the expression of gene networks within the brain. Therefore, the epigenome provides a direct mechanism for drugs of abuse to influence the genetic events involved in the development of addiction as well as its heritability to subsequent generations. In this review we discuss the mechanisms underlying intergenerational epigenetic transmission, highlight studies that demonstrate this phenomenon with particular attention to the field of addiction, and identify gaps for future studies.

Transgenerational epigenetic effects on brain functions

Psychiatric diseases are multifaceted disorders with complex etiology, recognized to have strong heritable components. Despite intense research efforts, genetic loci that substantially account for disease heritability have not yet been identified. Over the last several years, epigenetic processes have emerged as important factors for many brain diseases, and the discovery of epigenetic processes in germ cells has raised the possibility that they may contribute to disease heritability and disease risk. This review examines epigenetic mechanisms in complex diseases and summarizes the most illustrative examples of transgenerational epigenetic inheritance in mammals and their relevance for brain function. Environmental factors that can affect molecular processes and behavior in exposed individuals and their offspring, and their potential epigenetic underpinnings, are described. Possible routes and mechanisms of transgenerational transmission are proposed, and the major questions and challenges raised by this emerging field of research are considered.

Epigenetic inheritance of disease and disease risk

Epigenetic marks in an organism can be altered by environmental factors throughout life. Although changes in the epigenetic code can be positive, some are associated with severe diseases, in particular, cancer and neuropsychiatric disorders. Recent evidence has indicated that certain epigenetic marks can be inherited, and reshape developmental and cellular features over generations. This review examines the challenging possibility that epigenetic changes induced by environmental factors can contribute to some of the inheritance of disease and disease risk. This concept has immense implications for the understanding of biological functions and disease etiology, and provides potential novel strategies for diagnosis and treatment. Examples of epigenetic inheritance relevant to human disease, such as the detrimental effects of traumatic stress or drug/toxic exposure on brain functions, are reviewed. Different possible routes of transmission of epigenetic information involving the germline or germline-independent transfer are discussed, and different mechanisms for the maintenance and transmission of epigenetic information like chromatin remodeling and small noncoding RNAs are considered. Future research directions and remaining major challenges in this field are also outlined. Finally, the adaptive value of epigenetic inheritance, and the cost and benefit of allowing acquired epigenetic marks to persist across generations is critically evaluated.

Epigenetic regulation in neurodevelopment and neurodegenerative diseases

From fertilization throughout development and until death, cellular programs in individual cells are dynamically regulated to fulfill multiple functions ranging from cell lineage specification to adaptation to internal and external stimuli. Such regulation is of major importance in brain cells, because the brain continues to develop long after birth and incorporates information from the environment across life. When compromised, these regulatory mechanisms can have detrimental consequences on neurodevelopment and lead to severe brain pathologies and neurodegenerative diseases in the adult individual. Elucidating these processes is essential to better understand their implication in disease etiology. Because they are strongly influenced by environmental factors, they have been postulated to depend on epigenetic mechanisms. This review describes recent studies that have identified epigenetic dysfunctions in the pathophysiology of several neurodevelopmental and neurodegenerative diseases. It discusses currently known pathways and molecular targets implicated in pathologies including imprinting disorders, Rett syndrome, and Alzheimer's, Parkinson's and Hungtinton's disease, and their relevance to these diseases.

The role of MeCP2 in the brain

Methyl-CpG binding protein 2 (MeCP2) was first identified in 1992 as a protein that binds specifically to methylated DNA. Mutations in the MECP2 gene were later found to be the cause of an autism spectrum disorder, Rett syndrome. Despite almost 20 years of research into the molecular mechanisms of MeCP2 function, many questions are yet to be answered conclusively. This review considers several key questions and attempts to evaluate the current state of evidence. For example, is MeCP2 just a methyl-CpG binding protein? Is it a multifunctional protein or primarily a transcriptional repressor? We also consider whether MeCP2, as a chromosome-binding protein, acts at specific sites within the genome or more globally, and in which cell types it is functionally important. Finally, we consider two alternative views of MeCP2 in the brain: as a regulator of brain development or as a factor that helps maintain neuronal/glial function.

Effect of tamoxifen treatment on global and insulin-like growth factor 2-H19 locus-specific DNA methylation in rat spermatozoa and its association with embryo loss

OBJECTIVE

To determine the effect of tamoxifen treatment on global and insulin-like growth factor 2-H19 imprinting control region (Igf2-H19 ICR)-specific DNA methylation in rat spermatozoa and analyze its association with postimplantation loss.

DESIGN

Experimental prospective study.

SETTING

Animal research and academic research facility.

SUBJECT(S)

Male and female 75-day-old Holtzman rats.

INTERVENTION(S)

Global and Igf2-H19 ICR-specific DNA methylation was analyzed in an epididymal sperm sample in control and tamoxifen-treated rats at a dose of 0.4 mg tamoxifen/kg/day. DNA methylation status was correlated to postimplantation loss in females mated with tamoxifen-treated males.

MAIN OUTCOME MEASURE(S)

Global sperm DNA methylation level, methylation status of Igf2-H19 ICR in sperm, postimplantation loss.

RESULT(S)

Tamoxifen treatment significantly reduced methylation at Igf2-H19 ICR in epididymal sperm. However, the global methylation level was not altered. A mating experiment confirmed a significant increase in postimplantation loss upon tamoxifen treatment and showed significant correlation with methylation at Igf2-H19 ICR.

CONCLUSION(S)

Reduced DNA methylation at Igf2-H19 ICR in rat spermatozoa upon tamoxifen treatment indicated a role of estrogen-associated signaling in the acquisition of paternal-specific imprints during spermatogenesis. In addition, association between DNA methylation and postimplantation loss suggests that errors in paternal imprints at Igf2-H19 ICR could affect embryo development.

Maternal cocaine administration in mice alters DNA methylation and gene expression in hippocampal neurons of neonatal and prepubertal offspring

Previous studies documented significant behavioral changes in the offspring of cocaine-exposed mothers. We now explore the hypothesis that maternal cocaine exposure could alter the fetal epigenetic machinery sufficiently to cause lasting neurochemical and functional changes in the offspring. Pregnant CD1 mice were administered either saline or 20 mg/kg cocaine twice daily on gestational days 8–19. Male pups from each of ten litters of the cocaine and control groups were analyzed at 3 (P3) or 30 (P30) days postnatum. Global DNA methylation, methylated DNA immunoprecipitation followed by CGI² microarray profiling and bisulfite sequencing, as well as quantitative real-time RT-PCR gene expression analysis, were evaluated in hippocampal pyramidal neurons excised by laser capture microdissection. Following maternal cocaine exposure, global DNA methylation was significantly decreased at P3 and increased at P30. Among the 492 CGIs whose methylation was significantly altered by cocaine at P3, 34% were hypermethylated while 66% were hypomethylated. Several of these CGIs contained promoter regions for genes implicated in crucial cellular functions. Endogenous expression of selected genes linked to the abnormally methylated CGIs was correspondingly decreased or increased by as much as 4–19-fold. By P30, some of the cocaine-associated effects at P3 endured, reversed to opposite directions, or disappeared. Further, additional sets of abnormally methylated targets emerged at P30 that were not observed at P3. Taken together, these observations indicate that maternal cocaine exposure during the second and third trimesters of gestation could produce potentially profound structural and functional modifications in the epigenomic programs of neonatal and prepubertal mice.

Gender differences in germ-cell mutagenesis and genetic risk

Current international classification systems for chemical mutagens are hazard-based rather than aimed at assessing risks quantitatively. In the past, germ-cell tests have been mainly performed with a limited number of somatic cell mutagens, and rarely under conditions aimed at comparing gender-specific differences in susceptibility to mutagen exposures. There are profound differences in the genetic constitution, and in hormonal, structural, and functional aspects of differentiation and control of gametogenesis between the sexes. A critical review of the literature suggests that these differences may have a profound impact on the relative susceptibility, stage of highest sensitivity and the relative risk for the genesis of gene mutation, as well as structural and numerical chromosomal aberrations in male and female germ cells. Transmission of germ-cell mutations to the offspring may also encounter gender-specific influences. Gender differences in susceptibility to chemically derived alterations in imprinting patterns may pose a threat for the health of the offspring and may also be transmitted to future generations. Recent reports on different genetic effects from high acute and from chronic low-dose exposures challenge the validity of conclusions drawn from standard methods of mutagenicity testing. In conclusion, research is urgently needed to identify genetic hazards for a larger range of chemical compounds, including those suspected to disturb proper chromosome segregation. Alterations in epigenetic programming and their health consequences will have to be investigated. More attention should be paid to gender-specific genetic effects. Finally, the database for germ-cell mutagens should be enlarged using molecular methodologies, and genetic epidemiology studies should be performed with these techniques to verify human genetic risk.