The Role of Epigenetics in Addiction: Clinical Overview and Recent Updates.

Addiction is an international public health problem. It is a polygenic disorder best understood by accounting for the interplay between genetic and environmental factors. A recent way of perceiving this interaction is through epigenetics, which help grasp the neurobiological changes that occur in addiction and explain its relapsing-remitting nature. It is now known that every cell has a different way of expressing its phenotype, despite a universal DNA sequence. This is particularly true in the central nervous system where environmental factors influence this expression. Three major epigenetic processes have been found to participate in the perpetuation of addiction by changing the state of the chromatin and the degree of gene transcription: histone acetylation and methylation, DNA methylation, and noncoding RNAs. In the animal model literature, substantial evidence exists about the role of these epigenetic changes in the different phases of substance use disorders. This book chapter is a non-systematic literature review of the recent publications tackling the topic of epigenetics in addiction. Even though this evidence remains scarce and relatively poorly systematized, it is a promising foundation for future research of molecules that target specific brain regions and their functions to address core behavioral changes seen in addiction.

Mouse REC114 is essential for meiotic DNA double-strand break formation and forms a complex with MEI4.

Programmed formation of DNA double-strand breaks (DSBs) initiates the meiotic homologous recombination pathway. This pathway is essential for proper chromosome segregation at the first meiotic division and fertility. Meiotic DSBs are catalyzed by Spo11. Several other proteins are essential for meiotic DSB formation, including three evolutionarily conserved proteins first identified in Saccharomyces cerevisiae (Mer2, Mei4, and Rec114). These three S. cerevisiae proteins and their mouse orthologs (IHO1, MEI4, and REC114) co-localize on the axes of meiotic chromosomes, and mouse IHO1 and MEI4 are essential for meiotic DSB formation. Here, we show that mouse Rec114 is required for meiotic DSB formation. Moreover, MEI4 forms a complex with REC114 and IHO1 in mouse spermatocytes, consistent with cytological observations. We then demonstrated in vitro the formation of a stable complex between REC114 C-terminal domain and MEI4 N-terminal domain. We further determine the structure of the REC114 N-terminal domain that revealed similarity with Pleckstrin homology domains. These analyses provide direct insights into the architecture of these essential components of the meiotic DSB machinery.

Tex19 paralogs are new members of the piRNA pathway controlling retrotransposon suppression.

Tex19 genes are mammalian specific and duplicated to give Tex19.1 and Tex19.2 in some species, such as the mouse and rat. It has been demonstrated that mutant Tex19.1 males display a variable degree of infertility whereas they all upregulate MMERVK10C transposons in their germ line. In order to study the function of both paralogs in the mouse, we generated and studied Tex19 double knockout (Tex19DKO) mutant mice. Adult Tex19DKO males exhibited a fully penetrant phenotype, similar to the most severe phenotype observed in the single Tex19.1KO mice, with small testes and impaired spermatogenesis, defects in meiotic chromosome synapsis, persistence of DNA double-strand breaks during meiosis, lack of post-meiotic germ cells and upregulation of MMERVK10C expression. The phenotypic similarities to mice with knockouts in the Piwi family genes prompted us to check and then demonstrate, by immunoprecipitation and GST pulldown followed by mass spectrometry analyses, that TEX19 paralogs interact with PIWI proteins and the TEX19 VPTEL domain directly binds Piwi-interacting RNAs (piRNAs) in adult testes. We therefore identified two new members of the postnatal piRNA pathway.

Tex19 and Sectm1 concordant molecular phylogenies support co-evolution of both eutherian-specific genes.

BACKGROUND: Transposable elements (TE) have attracted much attention since they shape the genome and contribute to species evolution. Organisms have evolved mechanisms to control TE activity. Testis expressed 19 (Tex19) represses TE expression in mouse testis and placenta. In the human and mouse genomes, Tex19 and Secreted and transmembrane 1 (Sectm1) are neighbors but are not homologs. Sectm1 is involved in immunity and its molecular phylogeny is unknown. METHODS: Using multiple alignments of complete protein sequences (MACS), we inferred Tex19 and Sectm1 molecular phylogenies. Protein conserved regions were identified and folds were predicted. Finally, expression patterns were studied across tissues and species using RNA-seq public data and RT-PCR. RESULTS: We present 2 high quality alignments of 58 Tex19 and 58 Sectm1 protein sequences from 48 organisms. First, both genes are eutherian-specific, i.e., exclusively present in mammals except monotremes (platypus) and marsupials. Second, Tex19 and Sectm1 have both duplicated in Sciurognathi and Bovidae while they have remained as single copy genes in all further placental mammals. Phylogenetic concordance between both genes was significant (p-value < 0.05) and supported co-evolution and functional relationship. At the protein level, Tex19 exhibits 3 conserved regions and 4 invariant cysteines. In particular, a CXXC motif is present in the N-terminal conserved region. Sectm1 exhibits 2 invariant cysteines and an Ig-like domain. Strikingly, Tex19 C-terminal conserved region was lost in Haplorrhini primates while a Sectm1 C-terminal extra domain was acquired. Finally, we have determined that Tex19 and Sectm1 expression levels anti-correlate across the testis of several primates (ρ = -0.72) which supports anti-regulation. CONCLUSIONS: Tex19 and Sectm1 co-evolution and anti-regulated expressions support a strong functional relationship between both genes. Since Tex19 operates a control on TE and Sectm1 plays a role in immunity, Tex19 might suppress an immune response directed against cells that show TE activity in eutherian reproductive tissues.

The mammalian-specific Tex19.1 gene plays an essential role in spermatogenesis and placenta-supported development.

STUDY QUESTION: What is the consequence of Tex19.1 gene deletion in mice? SUMMARY ANSWER: The Tex19.1 gene is important in spermatogenesis and placenta-supported development. WHAT IS KNOWN ALREADY: Tex19.1 is expressed in embryonic stem (ES) cells, primordial germ cells (PGCs), placenta and adult gonads. Its invalidation in mice leads to a variable impairment in spermatogenesis and reduction of perinatal survival. STUDY DESIGN, SIZE, DURATION: We generated knock-out mice and ES cells and compared them with wild-type counterparts. The phenotype of the Tex19.1 knock-out mouse line was investigated during embryogenesis, fetal development and placentation as well as during adulthood. PARTICIPANTS/MATERIALS, SETTING, METHODS: We used a mouse model system to generate a mutant mouse line in which the Tex19.1 gene was deleted in the germline. We performed an extensive analysis of Tex19.1-deficient ES cells and assessed their in vivo differentiation potential by generating chimeric mice after injection of the ES cells into wild-type blastocysts. For mutant animals, a morphological characterization was performed for testes and ovaries and placenta. Finally, we characterized semen parameters of mutant animals and performed real-time RT-PCR for expression levels of retrotransposons in mutant testes and ES cells. MAIN RESULTS AND THE ROLE OF CHANCE: While Tex19.1 is not essential in ES cells, our study points out that it is important for spermatogenesis and for placenta-supported development. Furthermore, we observed an overexpression of the class II LTR-retrotransposon MMERVK10C in Tex19.1-deficient ES cells and testes. LIMITATIONS, REASONS FOR CAUTION: The Tex19.1 knock-out phenotype is variable with testis morphology ranging from severely altered (in sterile males) to almost indistinguishable compared with the control counterparts (in fertile males). This variability in the testis phenotype subsequently hampered the molecular analysis of mutant testes. Furthermore, these results were obtained in the mouse, which has a second isoform (i.e. Tex19.2), while other mammals possess only one Tex19 (e.g. in humans). WIDER IMPLICATIONS OF THE FINDINGS: The fact that one gene has a role in both placentation and spermatogenesis might open new ways of studying human pathologies that might link male fertility impairment and placenta-related pregnancy disorders. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by the Centre National de la Recherche Scientifique (CNRS), the Institut National de la Santé et de la Recherche Médicale (INSERM) (Grant Avenir), the Ministère de l'Education Nationale, de l'Enseignement Supérieur et de la Recherche, the Université de Strasbourg, the Association Française contre les Myopathies (AFM) and the Fondation pour la Recherche Médicale (FRM) and Hôpitaux Universitaires de Strasbourg.The authors have nothing to disclose.

TRIM28 repression of retrotransposon-based enhancers is necessary to preserve transcriptional dynamics in embryonic stem cells.

TRIM28 is critical for the silencing of endogenous retroviruses (ERVs) in embryonic stem (ES) cells. Here, we reveal that an essential impact of this process is the protection of cellular gene expression in early embryos from perturbation by cis-acting activators contained within these retroelements. In TRIM28-depleted ES cells, repressive chromatin marks at ERVs are replaced by histone modifications typical of active enhancers, stimulating transcription of nearby cellular genes, notably those harboring bivalent promoters. Correspondingly, ERV-derived sequences can repress or enhance expression from an adjacent promoter in transgenic embryos depending on their TRIM28 sensitivity in ES cells. TRIM28-mediated control of ERVs is therefore crucial not just to prevent retrotransposition, but more broadly to safeguard the transcriptional dynamics of early embryos.