Postnatal day 6 cerebellum of Ts65Dn and TcMAC21 Down syndrome mouse models

Dysregulation of Sonic hedgehog (SHH) signaling may contribute to multiple Down syndrome-associated phenotypes, including cerebellar hypoplasia, congenital heart defects, craniofacial and skeletal dysmorphologies, and Hirschsprung disease. Granule cell precursors isolated from the developing cerebellum of Ts65Dn mice are less responsive to the mitogenic effects of SHH than euploid cells, and a single postnatal dose of the SHH pathway agonist SAG rescues cerebellar morphology and performance on learning and memory tasks in Ts65Dn mice. SAG treatment also normalizes expression levels of OLIG2 in neural progenitor cells derived from human trisomy 21 iPSCs. However, despite evidence that activating SHH signaling rescues Down syndrome-associated phenotypes, chromosome 21 does not encode any canonical components of the SHH pathway. Here, we screened 163 chromosome 21 cDNAs in a series of SHH-responsive cell lines to identify chromosome 21 genes that modulate SHH signaling and confirmed overexpression of trisomic candidate genes using RNA-seq in Ts65Dn and TcMAC21 cerebellum. Our study indicates that some chromosome 21 genes, including DYRK1A, activate SHH signaling while others, such as HMGN1 and MIS18A, inhibit SHH signaling. Moreover, overexpression of genes involved in chromatin structure and mitosis, but not genes previously implicated in ciliogenesis, regulate the SHH pathway. Our data suggest that cerebellar hypoplasia and other phenotypes related to aberrant SHH signaling arise from the net effect of trisomy for multiple chromosome 21 genes rather than the overexpression of a single trisomic gene. Identifying which chromosome 21 genes modulate SHH signaling may also suggest new therapeutic avenues for ameliorating Down syndrome phenotypes. Overall design: Comparative gene expression profiling analysis for 4 Ts65Dn cerebella, 4 Ts65Dn euploid littermate cerebella, 4 TcMAC21 cerebella, and 4 TcMAC21 euploid littermate cerebella