Chd8 haploinsufficiency leads to molecular layer heterotopias and age-dependent cortical expansion

Mutations in the chromatin remodeler CHD8 are associated with autism and macrocephaly. While mouse models of Chd8 haploinsufficiency recapitulate brain overgrowth, the specific cellular mechanisms and developmental timing that lead to these anatomical abnormalities remain poorly understood. Moreover, 2D microscopy of physically sectioned tissues that are often used to assess brain structural deficits may miss region-specific differences that are associated with Chd8 brain overgrowth. Tissue clearing followed by light-sheet microscopy enables cellular resolution imaging of intact brain structure, allowing quantitative analysis of structural changes caused by genetic or environmental perturbations. Whole brain imaging results in more accurate quantification of cells and the study of region-specific differences that may be missed with commonly used microscopy of physically sectioned tissue. Here in collaboration with two other graduate students, I developed a protocol and conducted 3D imaging of Chd8V986*/+ mouse brains using magnetic resonance imaging followed by tissue clearing and cellular resolution light-sheet microscopy across embryonic and postnatal developmental stages. We describe MRI to measure brain volume prior to shrinkage caused by tissue clearing dehydration steps, tissue clearing using the iDISCO+ method including immunolabeling, followed by light-sheet microscopy using a commercially available platform to image mouse brains at cellular resolution. We found that brain overgrowth occurs postnatally, driven by an expansion of oligodendrocytes and microglia. Unexpectedly, we identified prevalent molecular layer heterotopias (MLH) within the frontal cortex of Chd8V986*/+ mice composed of neurons breaking through the pial surface during embryonic development and persisting throughout life. These MLH are covered by astrocytic caps in adulthood, vascularized, and integrated with cortical circuitry through myelinated axons. MLH were previously identified in post-mortem brains from individuals with autism and other neurodevelopmental disorders, suggesting functional significance in human patients.