Molecular Mechanism of SLC6A8 Dysfunction with c.1699T>C (p.S567P) Mutation in Cerebral Creatine Deficiency Syndromes

Cerebral creatine deficiency syndromes (CCDS) are neurodevelopmental disorders caused by a decrease in creatine levels in the central nervous system (CNS) due to functional mutations in creatine synthetic enzymes or creatine transporter (CRT/SLC6A8). Although SLC6A8 mutations have been reported to be the most frequent cause of CCDS, sufficient treatment for patients with CCDS harboring SLC6A8 mutations has not yet been achieved. This study aimed to elucidate the molecular mechanism of SLC6A8 dysfunction caused by the c. 1699T>C missense mutation, which is thought to induce dysfunction through an unidentified mechanism. A study on SLC6A8-expressing oocytes showed that the c.1699T>C mutation decreased creatine uptake compared to that in wild-type (WT) oocytes. In addition, a kinetics study of creatine uptake revealed that the c.1699T>C mutation reduced the maximum uptake rate but not Michaelis-Menten constant. In contrast, the c.1699T>C mutation did not attenuate SLC6A8 protein levels or alter its cellular localization. Based on the SLC6A8 structure in the AlphaFold protein structure database, it is possible that the c.1699T>C mutation alters the interaction between the S567 and Y143 residues of SLC6A8, leading to decreased creatine transport function. These findings contribute to the understanding of the pathology of CCDS and to the development of strategies for CCDS treatment.