A hyperactivating mutation in formin-like 2 causes brain and vertebrae defects
Formins are a group of proteins that nucleate and polymerize actin monomers into filaments to promote cell morphology and migration. Mutations in formin-encoding genes cause a broad spectrum of human diseases, ranging from kidney disease to non-syndromic intellectual disability. Through a forward genetics screen, we identified a mouse strain (nmf418) with a homozygous mutation in a splice acceptor in Fmnl2 that causes hindlimb motor dysfunction, enlarged midbrain colliculi, cerebellar hypoplasia, and vertebral abnormalities in the thoracolumbar spinal cord. The nmf418 mutation causes skipping of exon 10, which encodes a portion of the diaphanous inhibitory domain (DID) required for FMNL2 autoinhibition, and results in dramatically lower FMNL2 protein levels. Consistent with a gain-of-function mechanism, mutant FMNL2 has altered enrichment at filopodia tips, increased actin polymerization activity, and its expression results in enhanced cell migration in vitro. Furthermore, none of the neural or skeletal phenotypes were recapitulated in Fmnl2 knockout mice. Transcriptomic analysis of the embryonic cerebellum suggests that complete loss of Fmnl2 triggers broad compensatory upregulation of actin regulatory pathways, while expression of mutant FMNL2 is associated with dysregulation of cell migration, motility pathways, and calcium homeostasis pathways. Taken together, these findings suggest that Fmnl2 is largely dispensable for overall mouse development but that its autoregulation is essential for proper formation of the mouse nervous and skeletal systems.