Spinocerebellar ataxia type 3 (SCA3/DMJ) (ORPHA98757) is a neurodegenerative polyglutamine (polyQ) disorder, which exhibits marked clinical heterogeneity, manifested, amongst other aspects, in the wide distribution of the age at onset. SCA3 is the worldwide most frequent spinocerebellar ataxia, being particularly prevalent in the Portuguese population, namely in the Azores Islands; remaining without pharmacological intervention. At the ATXN3 gene, the size of CAG tract only explains about 50% of the variation in age at onset, reinforcing the importance of studying genetic factors underlying the remaining variance. The identification of genetic modifiers would allow the increase of onset prediction, promoting further understanding of the biological pathways disrupted in SCA3 and potentially suggesting new therapeutic targets. Given the common features underlying polyQ disorders, this knowledge would, moreover, be potentially transferable to other neurodegenerative diseases of the same group.
Previous attempts to identify such modifiers have used a candidate gene approach, which has proven to be extremely limited. In this project we will use Whole Exome Sequencing (WES) to analyze discordant and concordant pairs of affected SCA3 sibs belonging to Azorean families; discordant pairs will be matched to concordant, by SCA3 family. WES data will be filtered and filtered variants will then be screened in sib pairs. A meticulous analysis of the pathways in which these different genes are involved will then be conducted to allow prioritization of variants to be used in the subsequent stage of the project. Furthermore, we will access data on the expression levels of such genes in peripheral blood and cerebellum of SCA3 patients. Validation of the strongest resulting candidate modifiers will be performed using four distinct series (Azores, Mainland Portugal, United Kingdom and Brazil), comprising around 350 patients. A functional effect of the validated genes will be tested using a RNAi based strategy in a well established C. elegans model of SCA3.

Fig. 1 - Workflow of the main tasks of the EXOS3 project