Abstracts

Rationale: The BRD2 gene has been identified as the EJM1 gene for JME, located at chromosome 6p21. The locus was originally located and replicated by linkage analysis [1-4], and identified and replicated by association studies [5-7], in some European populations. While the statistical genetic evidence for BRD2’s involvement is strong, we now present biological evidence that reveal its profound effect on brain development and structure. Mutational evidence in the mouse model supports the population human genetic results showing polymorphisms of BRD2 that influence seizure susceptibility. Methods: We examined brain development, anatomy, brain structure, and seizure susceptibility in a Brd2-null (Brd2-/-) mouse and the null/wild-type heterozygote (Brd2+/-).Results: The knock out of the mouse Brd2 gene is an embryonic lethal: At embryonic day 9.5, the homozygous Brd2-/- embryos are noticeably smaller and exhibit profound neural tube abnormalities (fig. 1). In situ hybridization shows the highest expression levels of Brd2 are in the developing nervous system. Although heterozygous Brd2+/- mice appear grossly normal, closer inspection reveals selective anatomical and behaviorial abnormalities. Brd2+/- females appear to more susceptible to pentylenetetrazole-induced (50mg/kg; IP) seizures than same age wild-type mice. Anatomically, the Brd2+/- mice have a notably lower density of parvalbuimin-positive GABAergic interneurons in prefrontal cortical regions (anterior cingulate, prelimbic and infralimbic cortex) (fig. 2) without alteration in the density of calbindin-positive GABAergic interneurons in the same brain regions. Conclusions: The data from the null mutation show that mouse Brd2 has a profound influence on brain structure, supporting a role for human BRD2’s influence on brain structure. The observations from the heterozygous Brd2+/- mice further suggest that a decrease in BRD2 protein levels may be the mechanism underlying the dominant inheritance of JME, indicated by the linkage data. That is, small decreases in BRD2 protein levels during development could lead to subtle changes in frontal lobe structure that predispose to JME and brain hyperexcitability when other seizure-related genes are present. [1] Greenberg, DA et al. Am J Med Genet 1988; 31:185-192. [2] Weissbecker, K A et al. Am J Med Genet 1991; 38:32-36. [3] Durner, M et al. Ann Neurol 2001; 49:328-335. [4] Greenberg, DA et al. American Journal of Human Genetics 2000; 66:508-516. [5] Pal, DK, et al. Am J Hum Genet 2003; 73:261-270 Epub 2003 Jun 2025. [6] Lorenz, S Neurosci Lett 2006; 400:135-139. [7] Cavalleri, GL et al. Epilepsia 2007; 48:706-712.