Understanding the pathogenic mechanisms of the ‘Spastic paraplegia, Intellectual disability, Nystagmus and Obesity’ (SINO) syndrome.

SINO is a newly described syndrome characterized by Spastic paraplegia, Intellectual Disability, Nystagmus and Obesity, caused by heterozygous mutations in the KIDINS220 gene. Kidins220 (kinase D interacting substrate of 220kDa) is a multi-functional scaffold protein abundantly expressed across the nervous system. Some KIDINS220 pathogenic mutations result in truncated proteins resembling the isoforms naturally expressed during adulthood. This work examines the molecular effect of KIDINS220 mutations on protein expression and how altered KIDINS220 impacts cell homeostasis in HEK293T and primary neurons. Immunocytochemistry revealed that truncating mutations form aggregates and colocalise with p62 and calnexin both in HEK293T cells and neurons. Investigation of mitochondrial membrane potential revealed adverse effects of the mutant proteins on mitochondrial health. Protein extraction of both soluble and insoluble fractions and western blot analysis confirmed that the mutated protein aggregates, unlike the WT, accumulate in the insoluble fraction suggesting impaired liquid-liquid phase separation (LLPS) mechanism which was further assessed by 1,6-hexanediol treatment. In conclusion, we have shown that mutated KIDINS220 aggregates in cells potentially trapping other proteins and disrupting the intracellular interactions. We hypothesise that the mechanisms behind aggregate formation represent a potential druggable target for treatment of KIDINS220-related pathology. In parallel with the experimental work, we have been working together with clinicians and patients’ families to prepare the first clinical overview of SINO syndrome, which should become the reference for newly diagnosed patients.