X chromosome inactivation and autoimmunity

Epigenetic alterations and autoimmune disease

Recent advances in epigenetics have enhanced our knowledge of how environmental factors (UV radiation, drugs, infections, etc.) contribute to the development of autoimmune diseases (AID) in genetically predisposed individuals. Studies conducted in monozygotic twins discordant for AID and spontaneous autoimmune animal models have highlighted the importance of DNA methylation changes and histone modifications. Alterations in the epigenetic pattern seem to be cell specific, as CD4+ T cells and B cells are dysregulated in systemic lupus erythematosus, synovial fibroblasts in rheumatoid arthritis and cerebral cells in multiple sclerosis. With regard to lymphocytes, the control of tolerance is affected, leading to the development of autoreactive cells. Other epigenetic processes, such as the newly described miRNAs, and post-translational protein modifications may also be suspected. Altogether, a conceptual revolution is in progress, in AID, with potential new therapeutic strategies targeting epigenetic patterns.

Epigenetic regulation of the immune system in health and disease

Epigenetics comprises various mechanisms that mold chromatin structures and regulate gene expression with stability, thus defining cell identity and function and adapting cells to environmental changes. Alteration of these mechanisms contributes to the inception of various pathological conditions. Given the complexity of the immune system, one would predict that a higher-order, supragenetic regulation is indispensable for generation of its constituents and control of its functions. Here, we summarize various aspects of immune system physiology and pathology in which epigenetic pathways have been implicated. Increasing knowledge in this field, together with the development of specific tools with which to manipulate epigenetic pathways, might form a basis for new strategies of immune function modulation, both to optimize immune therapies for infections or cancer and to control immune alterations in aging or autoimmunity.

Epigenetic deregulation in rheumatoid arthritis

In this chapter, we discuss the current understanding of the possible epigenetics changes that occur in rheumatoid arthritis. In particular, we describe that deregulation ofDNA methylation and histone modifications can occur in the immune system and lead to rheumatoid arthritis. In addition, we discuss the role of rheumatoid arthritis synovial fibroblasts in autoimmunity. Examples of changes in DNA methylation and histone modification occurring in synovial fibroblasts during the disease process are reviewed in this chapter. In conclusion, we discuss the possible use of epigenetic therapy and describe future experiments that can elucidate further the epigenetic changes observed in the disease.

O-GlcNAc cycling: emerging roles in development and epigenetics

The nutrient-sensing hexosamine signaling pathway modulates the levels of O-linked N-acetylglucosamine (O-GlcNAc) on key targets impacting cellular signaling, protein turnover and gene expression. O-GlcNAc cycling may be deregulated in neurodegenerative disease, cancer, and diabetes. Studies in model organisms demonstrate that the O-GlcNAc transferase (OGT/Sxc) is essential for Polycomb group (PcG) repression of the homeotic genes, clusters of genes responsible for the adult body plan. Surprisingly, from flies to man, the O-GlcNAcase (OGA, MGEA5) gene is embedded within the NK cluster, the most evolutionarily ancient of three homeobox gene clusters regulated by PcG repression. PcG repression also plays a key role in maintaining stem cell identity, recruiting the DNA methyltransferase machinery for imprinting, and in X-chromosome inactivation. Intriguingly, the Ogt gene resides near the Xist locus in vertebrates and is subject to regulation by PcG-dependent X-inactivation. OGT is also an enzymatic component of the human dosage compensation complex. These 'evo-devo' relationships linking O-GlcNAc cycling to higher order chromatin structure provide insights into how nutrient availability may influence the epigenetic regulation of gene expression. O-GlcNAc cycling at promoters and PcG repression represent concrete mechanisms by which nutritional information may be transmitted across generations in the intra-uterine environment. Thus, the nutrient-sensing hexosamine signaling pathway may be a key contributor to the metabolic deregulation resulting from prenatal exposure to famine, or the 'vicious cycle' observed in children of mothers with type-2 diabetes and metabolic disease.