Unraveling molecular pathogenesis of SCA38, a novel autosomal dominant ataxia with a possible treatment

SCA38 is a rare form of ataxia characterized by a pure cerebellar phenotype with slow progression. Gait ataxia is the presenting symptom, usually occurring in the fourth decade of life. Cerebellar atrophy without cerebral and brainstem involvement is reported. We identified SCA38 causative mutations in ELOVL fatty acid elongase 5 gene (ELOVL5). To date two missense mutations are known: c.689G>T (p.Gly230Val) in three Italian families and c.214C>G (p.Leu72Val) in a French family. ELOVL5 belongs to a multigenic family of genes encoding elongases and it is involved in the synthesis of Poly Unsaturated Fatty Acids (PUFA) of both the ω3 and ω6 series (arachidonic acid (ω6), eicosapentaenoic acid (EPA, ω3) and docosahexaenoic acid (DHA, ω3)). We demonstrated that the identified mutations caused subcellular mislocalization of ELOVL5 in four different cell types. The p.Leu72Val and p.Gly230Val ELOVL5 showed a less diffuse ER signal, with a perinuclear and polarized subcellular localization, consistent with an increase in the Golgi apparatus. We focused our study on the impact of missense ELOVL5 changes on protein misfolding and the activation of unfolded protein response (UPR). Our preliminary data demonstrated a significant increased of the transcription factor CHOP, an UPR marker, in p.Gly230Val transfected cells. The p.Leu72Val aminoacidic substitution only slightly enhanced CHOP. Overall, our data showed the p.Gly230Val substitution was causing a mislocalization of the protein and might activate UPR. We will plan to treat cells expressing aberrant ELOVL5 with MG-132, a membrane-permeable proteasome inhibitor, or 5% glycerol, a chemical chaperone known to stabilize the unfolded proteins, in order to study the impact of missense changes on the protein conformation. If missense changes cause a misfolded ELOVL5 protein, we will expect an increased protein level (detected by Wb) in MG-132 and 5% glycerol treated cells carrying the mutations. These experiments will clarify if missense mutations in ELOVL5 are engaging a toxic gain of function mechanism by an accumulation of unfolded protein. Moreover, we are interested in understanding if the ablation of Elovl5 may cause a neurological phenotype in Elovl5-/- mice by behavioral tests. To clarify the physiological role of ELOVL5 in the cerebellum, we will develop a Elovl5/L7-GFP mouse cellular model to isolated Elovl5-/- Purkinje cells. Because the main product of ELOVL5 is DHA and its level regulates ELOVL5 expression by a feedback regulatory mechanism, we have initial showed that DHA treatment reduced ELOVL5 expression in two affected subjects’ blood and ameliorated their clinical phenotype. Based on these evidences we will start a phase II clinical trial with 10 affected individuals. Overall our experiments are aimed to clarify the pathophysiological mechanisms of SCA38 and to validate DHA use in the treatment of SCA38.