Abstracts

Rationale: The Chd2 gene encodes a member of the chromodomain helicase DNA-binding (CHD) family of proteins, a chromatin modifier and transcription factor with relatively unknown function in the developing or mature brain. Increasing evidence suggests de novo loss of function mutations in Chd2 contribute to a broad spectrum of human neurodevelopmental disorders, including epilepsy. Individuals with Chd2 mutation and epilepsy often develop seizures before 3 years of age, suggesting this gene might be critical for early brain development and function. Here, we generated Chd2 mutant mice to model Chd2 haploinsufficiency in human, and evaluated the animals at different stages of cortical development.  Methods: To establish a mouse line with a heterozygous loss of function mutation in Chd2 (i.e., Chd2+/- mice), we crossed transgenic mice containing loxP-flanked exon 3 of Chd2 (i.e., Chd2tm1c(EUCOMM)Hmgu mice) with a b-actin Cre line. Wild-type and Chd2+/- littermates were evaluated in a series of immunostaining experiments at E14.5, P7 and P30. Whole-cell patch-clamp recordings and long-term EEG monitoring experiments were performed in young-adult animals at P30-P65. Results: At E14.5, Chd2 was expressed strongly in the subventricular zone and cortical plate of both the dorsal and ventral telencephalon and remained highly expressed throughout the brain in adult animals. At P30, Chd2 co-localized in the nucleus of nearly all mature neurons and oligodendrocytes, but not astrocytes. Chd2+/- mice exhibited reduced body weight compared to wild-type littermates and a subtype-specific decrease in the density of GABAergic interneurons in hippocampus and somatosensory neocortex. However, we did not observe a difference in cortical lamination between genotypes at P30. Patch-clamp recordings obtained from CA1 pyramidal neurons showed increased action potential firing in Chd2+/- mice compared to wild-type littermates, as well as a dramatic shift in the balance between synaptic excitation and inhibition. Mutant animals displayed aberrant cortical rhythmogenesis and spiking, but overt spontaneous electrographic seizures were not detected. At E14.5, immunostaining analysis revealed decreases in Ki67+ cells in the ventricular zone of Chd2+/- mice as well as reductions in Nkx2.1+ progenitors in MGE and GAD67+ progenitors in neocortex.  Conclusions: We identified a broad dysregulation of cell proliferation, GABAergic neurogenesis and neuronal function in Chd2+/- mice. Our findings support a relatively direct role of chromatin modification in cortical circuit function that may contribute to human disorders of brain development when disrupted.  Funding: LGS FoundationNIH / NINDS R00 NS085046NIH / NINDS R01 NS096012NIH / NINDS T32 NS045540