A complex of distal appendage-associated kinases linked to human disease regulates ciliary trafficking and stability

Cilia biogenesis is a complex, multi-step process involving the coordination of multiple cellular trafficking pathways. Despite the importance of ciliogenesis in mediating the cellular response to cues from the microenvironment, we have only a limited understanding of the regulation of cilium assembly. We previously identified a kinase that acts as a key regulator of ciliogenesis, TTBK2. Here, using CRISPR kinome screening, we identify the CK2 subunit CSNK2A1 as an important modulator of TTBK2 function in cilia trafficking. Super-resolution microscopy reveals that CSNK2A1 is a centrosomal protein concentrated at the mother centriole and associated with the distal appendages where it physically interacts with TTBK2. Further, Csnk2a1 knockout partially corrects defects in cilia formation and length in Ttbk2 hypomorphic cells. Csnk2a1 mutant cilia are longer than those of control cells and exhibit instability, particularly at the tip. Csnk2a1 mutant cilia also abnormally accumulate key cilia assembly and SHH-related proteins including IFT, GLI2, KIF7, and Smoothened (SMO). De novo mutations of Csnk2a1 were recently linked to the human genetic disorder Okur-Chung neurodevelopmental syndrome (OCNDS). Consistent with the role of CSNK2A1 in cilium stability, we find that expression of OCNDS-associated Csnk2a1 variants in wild-type cells cause ciliary structural defects. Our findings provide new insights into mechanisms involved in ciliary length regulation, trafficking, and stability that in turn shed light on the significance and implications of cilia instability in human disease.