Molecular basis of Celia's encephalopathya fatal neurodegenerative syndrome associated to a new mutation in the Seipin-BSCL2 gene

Resumen

Summary Celia¿s Encephalopathy is a recently discovered fatal neurodegenerative syndrome associated with a new BSCL2 mutation (c.985C>T) that results in an aberrant isoform of seipin (Celia seipin). This mutation is lethal in both homozygosity and compounded heterozygosity with a lipodystrophic BSCL2 mutation, resulting in a progressive encephalopathy with fatal outcomes at age 6-8. Strikingly, heterozygous carriers are asymptomatic, conflicting with the gain of toxic function attributed to this mutation. This thesis reports key insights about the molecular pathogenic mechanism of this new syndrome. First, intranuclear inclusions containing mutant seipin were found in brain tissue from a homozygous patient (index case) suggesting a pathogenic mechanism similar to other neurodegenerative diseases featuring brain accumulation of aggregated, misfolded proteins. Moreover, sucrose gradient distribution showed that mutant seipin forms much larger aggregates as compared with wild type (wt) seipin, indicating an impaired oligomerization. On the other hand, the interaction between wt and Celia seipin confirmed by co-immunoprecipitation (CoIP) assays, together with the identification of mixed oligomers in sucrose gradient fractionation experiments can explain the lack of symptoms in heterozygous carriers. A nuclear localization of a fraction of seipin is proposed, based on immunofluorescence microscopy and subcellular fractionation assays. The existence of endoplasmic reticulum (ER) stress in cells overexpressing Celia seipin confirmed similar findings in preadipocytes from the index case. Finally, a proteomic analysis of preadipocytes from the index case showed significant changes in antioxidant activity, response to stress, cytoskeleton regulation and apoptotic pathways. A pathogenic model according to which the increased aggregation and subsequent impaired oligomerization of Celia seipin leads to cell death is proposed. The lack of symptoms in heterozygous carriers is explained by a phenotype rescue mechanism whereby wt seipin might prevent the damage caused by mutant seipin through its sequestration into harmless mixed oligomers.