Identification and functional characterization of a novel PAK3 missense mutation associated with intellectual disability

Background: The Rho GTPases effector PAK3 is a serine-threonine kinase regulating multiple signaling pathways, including the MAPK cascade. PAK3 is predominantly expressed in the brain, contributing to neurogenesis [1,2]. Kinase activity-independent PAK3 function has been reported [3,4].In families with non-syndromic X-linked intellectual disability (NS-XLID), pathogenic PAK3 mutations have been shown to impair catalytic activity of the kinase [5]. We previously functionally characterized the PAK3K389N mutant implicated in NS-XLID [6]. Here, we biochemically profile a novel variant (p.Y439C) identified in a child affected with severe developmental delay, dystonic movements, severe feeding difficulties and seizures. Material and methods: The novel missense PAK3 mutation was identified by exome sequencing. Protein levels, activation, and PAK3 downstream signaling events (i.e. MAPK activation) were analyzed upon the stimulation of the Rho GTPase CDC42 in cells overexpressing PAK3 wild type (WT), and the p.K389N and p.Y439C mutants. Results: Biochemical studies show that p.Y439C decreases the full activity of the kinase, concomitantly reducing the negative feedback-induced degradation of CDC42 and PAK3 itself. Similarly to the p.K389N variant, p.Y439C enhances the downstream MAPK signaling pathway with respect to the WT protein. Conclusion: Our study expands the molecular and clinical spectrum of PAK3 pathogenic variants and suggests that, beside the loss of function behavior, dysregulation of multiple signaling pathways contributes to the pathogenic mechanism.