Disrupting fzd9b in Zebrafish Recapitulates Stress- and Anxiety-Like Behaviours Relevant to Williams Syndrome.

Williams syndrome (WS) is a multifaceted developmental disorder characterized by a spectrum of physical and intellectual traits. Individuals with WS exhibit friendly, impulsive, and hyper-social behaviours, often coupled with anxiety. WS is caused by a hemizygous microdeletion on chromosome 7q11.23. Among the affected genes, FZD9 is implicated in neural development and synaptic function. Thus, we postulated that disruption of FZD9 might contribute to the behavioural features of WS, and that pharmacological modulation of Wnt signalling, particularly the canonical β-catenin pathway, could offer therapeutic potential. We generated two zebrafish lines carrying predicted loss-of-function alleles of fzd9b, a zebrafish orthologue of FZD9. Our results demonstrated that mutant fish exhibited altered expression of hypothalamic-pituitary-interrenal axis-related genes, increased neuronal branching, impaired habituation to acoustic startle, and increased anxiety-like behaviour in adulthood. Canonical Wnt/β-catenin pathway gene expression was also altered, together with the expression of other frizzled receptors; however, acute GSK-3 inhibition failed to rescue behavioural phenotypes under the conditions tested. These findings indicate that the selected acute treatment paradigm was insufficient to reverse the phenotype but do not exclude a contribution of canonical Wnt signalling. Expression of fzd9b, wnt5b, and tafa5l was upregulated, further supporting transcriptional dysregulation. Together, our results highlight a conserved role for Fzd9b in modulating stress reactivity and anxiety-like behaviour in zebrafish and support FZD9 as a contributor to WS-associated neurobehavioural alterations.