Juvenile Myoclonic Epilepsy (JME) is a common epilepsy syndrome that starts in adolescence or young adulthood and requires lifelong treatment. BRD2 is a gene that, in some populations, is a causal factor in JME and in related electroencephalographic phenomena, which is demonstrated by replicated linkage and association studies. Based on family and population genetic analysis, it is currently the best candidate gene for a common epilepsy. This evidence is further strengthened by our studies of a Brd2 knockout mouse. The knockout data show that absence of BRD2 is incompatible with life. In heterozygote knockout mice (Brd2), the neural substrate is altered: there is a deficit of inhibitory neurons in two brain regions we have studied. Most compellingly, the heterozygotes show increased seizure susceptibility with a differential male/female pattern reminiscent of the pattern for JME in humans. We hypothesize that, in humans, mis- expression results in a slight insufficiency of BRD2 protein, leading to subtle changes in brain organization. Several lines of human imaging data report observing disruptions of neural substrate in JME patients, supporting the population genetic and mouse studies. We propose collaborative, multidisciplinary studies to determine the effect of altered BRD2 expression on brain substrate and function, and to identify the mechanisms by which such disturbances lead to brain hyperexcitability and seizures.
Our aims are: 1) To fully characterize changes to the neural substrate, behavior, seizure susceptibility, and electrophysiology caused by insufficiency of murine Brd2 protein in Brd2 mice. This includes using brain- slice experiments to study the behavior of neural circuits in Brd2 mice compared to wild type, as well as investigating the responses of individual neurons. 2) To identify the specific change(s) in the expression of the human BRD2 gene that lead to brain hyperexcitability. We will express human BRD2 alleles associated with JME in tissue culture, study changes in their transcription, splicing, and translation, and correlate those changes with specific polymorphisms. We will examine polymorphisms in the BRD2 promoters, 3'UTR and 5'UTR, and other gene regions. In particular, we identified a highly conserved, alternately-spiced exon located in intron 2. We hypothesize that CA-repeat polymorphisms, found in intron 2 and associated with JME, affect the balance of alternatively spliced forms. New preliminary experiments show that the composition of the inter-exon DNA alters the splicing characteristics of the alternate exon. We will insert varying sizes of CA-repeat polymorphisms at that locus to observe the effect on levels of alternatively spliced BRD2 transcripts, eventually studying the fate of both the normal and alternately spliced RNA.
Project Narrative We propose to define the genetic, neuroanatomical, and circuitry mechanisms by which the gene BRD2 leads to susceptibility to Juvenile Myoclonic Epilepsy, a common epilepsy of adolescent onset and one that requires life-long medication to suppress seizures. The BRD2 gene, which we identified through studies of human epilepsy, has a profound effect on neural structure and circuitry. BRD2 is one of the few proven genes for a common epilepsy. This work could, at last, lead to understanding the basic cause and mechanism of an epilepsy that affects millions of Americans and places a difficult burden on the patient, their families, and the health care system.
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