Regulation of Liprin-α phase separation by CASK is disrupted by a mutation in its CaM kinase domain
CASK is a unique membrane associated guanylate kinase (MAGUK), due to its Ca2+/calmodulin-dependent kinase (CaMK) domain. We describe four male patients with a severe neurodevelopmental disorder with microcephaly carrying missense variants affecting the CaMK domain. One boy who carried the p.E115K variant and died at an early age showed pontocerebellar hypoplasia (PCH) in addition to microcephaly, thus exhibiting the classical MICPCH phenotype observed in individuals with CASK loss-of-function variants. All four variants selectively weaken the interaction of CASK with Liprin-2, a component of the presynaptic active zone. Liprin- proteins form spherical condensates in a process termed liquid-liquid phase separation (LLPS), which we observe here in Liprin-2 overexpressing HEK293T cells and primary cultured neurons. Condensate formation is reversed by interaction of Liprin-2 with CASK; this is associated with altered phosphorylation of Liprin-2. The p.E115K variant fails to interfere with condensate formation. As the individual carrying this variant had the severe MICPCH disorder, we suggest that regulation of Liprin-2-mediated LLPS is a new functional feature of CASK which must be maintained to prevent PCH.