MOLECULAR LAYER HETEROTOPIAS AND AGE-DEPENDENT CORTICAL EXPANSION RESULTING FROM CHD8 HAPLOINSUFFICIENCY

Autism spectrum disorder is a neurodevelopmental condition linked to hundreds of genetic risk factors. Among these, mutations in the chromatin remodeler CHD8 are highly penetrant and also lead to intellectual disability 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. Here, we conducted high-throughput 3D imaging of intact Chd8V986*/+ mouse brains using gross-brain structure MRI followed by cellular resolution tissue clearing and light-sheet microscopy across embryonic and postnatal developmental stages. We found that brain overgrowth occurs postnatally driven by an expansion of non-neuronal cells. Unexpectedly, we also identified prevalent molecular layer heterotopias within the frontal cortex during embryonic development, which may have been missed in previous studies using slice immunohistochemistry or low resolution brain imaging. These malformations break through the pial boundary, where Chd8 is highly expressed, leading to neurons that overmigrate, persist through the lifespan, are capped by astrocytes later in life, and are connected to adjacent cortex through myelinated axons and vasculature. Molecular layer heterotopias have also been identified in post-mortem brains from individuals with autism, and other neurodevelopmental disorders, suggesting functional significance in human patients.