A Tale of Two Epiphenomena: The Complex Interplay of Epigenetics and Epilepsy

Abstract

Epilepsy is a disorder primarily characterized by the spontaneous recurrence of unprovoked seizures. Seizures can be triggered by multiple factors including genetic mutations, head injury, toxins, a fever, high or low blood sugar, a tumor, electrolyte imbalance, drug withdrawal; and are also a core component of developmental and degenerative disorders (Loscher and Brandt 2010). However, not all patients that have seizures go on to develop epilepsy, and the mechanisms of epileptogenesis are still poorly understood. Only a small number of genetic mutations identified in ion channels or proteins associated with these channels have been directly linked to causing epilepsy (Greenberg and Pal 2007). In fact, complex epidemiological studies indicate that the interplay of environmental factors with relatively minor genetic alterations may contribute to the difference between the susceptibility to suffer seizures, and the development of epilepsy (Ottman et al. 1996). Evidence is emerging that epileptogenesis involves changes in the expression patterns of several classes of functionally or genomically-grouped genes that coordinate neural development, homeostasis and stress responses, and neural network formation (Lukasiuk et al. 2006 and references therein). This has led to speculation that minor and modifiable changes outside the open reading frames of affected genes could alter the course of epilepsy. How entire groups of genes may be co-regulated with precision during different stages of neural development and function could be the result of epigenetic changes in histone and chromatin structure and DNA methylation that accompany shifts in neural “state.” Chromatin structure and function can be altered to silence gene expression by DNA methylation leading to the recruitment of methyl-DNA binding proteins and histone deacetylation. Histones can also be modified at their N-terminus by phosphorylation, acetylation, methylation, ubiquitination, ADP ribosylation, carbonylation, SUMOylation, glycosylation and biotinylation. Here we will focus on DNA methylation and histone acetylation, and discuss how these epigenetic modifications could regulate developmental alterations that may contribute to the process of epileptogenesis. We will summarize how epigenetic changes may both regulate and be regulated by activity-dependent synaptic plasticity, and how involvement of common mechanisms underlying glial-neuronal interactions could lead to epileptogenesis. Finally, we will discuss how intervening in epilepsy by treating with widely-used drugs that themselves can alter chromatin state (like Valproic Acid) may further affect ongoing epileptogenesis, and discuss which specific epigenetic modifications may be novel therapeutic targets for the treatment of epilepsy.