Functional characterization of 42 CK2α de novo variants associated with Okur-Chung neurodevelopmental syndrome

In 2016, CSNK2A1, the gene which encodes the catalytic α-subunit of human protein kinase CK2 was linked to an autism spectrum disorder called Okur-Chung neurodevelopmental syndrome (OCNDS) for the first time. Human protein kinase CK2 is a heterotetrameric phosphotransferase with an α₂β₂ composition. To gain more insight into genotype-phenotype relationships, 42 CK2α variants associated with OCNDS were characterized in this study in terms of enzymatic activity using a canonical CK2 peptide substrate. Out of the 42 variants tested, 13 had no detectable enzymatic activity and 12 showed less than 10% of wild-type CK2α activity. The addition of the regulatory CK2β subunit increased the activity of all active variants. Twelve variants that exhibited at least 30% wild-type enzymatic activity were chosen to determine the dissociation constants with CK2β; highlighting that none of these mutations had an impact on the interaction of CK2α with CK2β. The variants R21Q, T127M, E264D, E282K, and R333* showed no reduced activity in comparison to wild-type CK2α. Additionally, affinity to CK2β and thermostability remained unaltered. K198R is the most frequent missense variant observed in OCNDS patients; thus, we performed a site-saturation mutagenesis at position K198 to elucidate what impact other variants at this position could have on enzymatic activity. Results showed that replacement of K198 by any other amino acid resulted in similar loss of activity as the prevalent K198R variant. The presented results suggest that other parameters beyond enzymatic activity, CK2β affinity or thermostability may contribute to neurodevelopmental disorders such as OCNDS.