Abstracts

Rationale: Juvenile myoclonic epilepsy (JME) is clinically distinctive, being characterized by adolescent onset, myoclonic jerks on awakening, tonic-clonic seizures, and sometimes absence. Several disease-related genes have been proposed, but only BRD2 and EFHC1 were identified by linkage analysis of large data sets. The Brd2 mouse model provides consistent evidence substantiating Brd2 as a major susceptibility gene for JME including male-female differences, behavioral changes, and a profound effect on the brain GABAergic system. Brd2 is an essential gene involved in cell cycle regulation and neuronal differentiation. It is crucial for neural development. Homozygous Brd2 knockout mice are embryonic lethal with a dysmorphic nervous system but heterozygous (het) knockouts are viable without overt differences from wildtype (wt) mice. Histologically, het mice have a reduction in GABAergic neuron counts in a number of brain regions including the prefrontal cortex (PFC), striatum (ST), and substantia nigra reticulata but no reduction in the hippocampus. This deficit in inhibitory neurons likely underlies the decreased threshold for induced seizures, as well as the occurrence of spontaneous seizures and abnormal EEGs observed in hets. JME patients exhibit a nearly 3:1 female to male prevalence ratio; similarly, female het mice are more susceptible to seizures. We investigated the role of Brd2 in sex-dependent seizure susceptibility by examining Brd2 expression levels in male and female mice. Methods: Total RNA was isolated from microdissected brain regions (PFC, ST, motor and somatosensory cortex; CTX, and hippocampus; hippo) from adolescent (P50) Brd2 het and wt mice. Brd2, parvalbumin (Pvalb), and calbindin (Calb1) gene expression were determined using quantitative PCR. Results: Female het mice exhibited the expected 50% reduction in Brd2 gene expression in all brain regions tested. However, the expression pattern in male het mice showed some regions with wt-level Brd2 mRNA. CTX and hippo showed a reduction in expression but there was no het-wt expression difference in the PFC or ST. In a surprise finding, there was a similar sex-based expression pattern for the GABAergic neuron subtype marker Pvalb. A reduction in Pvalb was detected in the female but not in the male het mice. There was no change in Calb1 expression, a marker for a different neuron sub-population whose numbers do not change in the Brd2 het mouse. Conclusions: These surprising findings support a direct role of Brd2 in sex differences in seizure susceptibility in the JME mouse model, potentially through a direct regulation of GABAergic system development. The sex differences in both JME patients and the Brd2 mouse model suggest a role for sex hormones in controlling Brd2 expression and consequently the changes in seizure susceptibility. Furthermore, the ability of the male Brd2+/- mouse to compensate its expression of Brd2, despite possessing only one functional copy of the gene, highlights a possible therapeutic avenue for JME through manipulation of Brd2 expression.