The RNA-binding activity of SACSIN HEPN domain is connected to ARSACS

Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS) is a neurodegenerative disorder caused by mutations in the SACS gene, though the molecular function of its protein product, SACSIN, remains elusive. Therapeutic strategies for ARSACS are limited, mostly due to the exceptionally large size of SACSIN ([~]520 kDa), which precludes conventional gene therapy and standard molecular delivery methods. Over 200 pathogenic variants, including missense and nonsense mutations, have been identified. Regardless of variant type, patients uniformly exhibit cerebellar ataxia and spasticity. Among them, patients carrying missense mutations within the SACSIN higher eukaryotes and prokaryotes nucleotide-binding (HEPN) domain, suggesting that the alteration of the functional role of SACSIN HEPN domain could be linked to the manifestation of ARSACS symptoms. Here, we provide the first direct evidence that SACSIN HEPN domain binds RNA, corroborating the hypothesis that SACSIN functions as an RNA-binding protein. In silico predictions through the catRAPID algorithm as well as AlphaFold 3 revealed the propensity of SACSIN HEPN domain to specifically interact with RNAs. These interactions were validated both in vitro and in cell studies, confirming that SACSIN HEPN domain binds to RNA transcripts. Overall, our study unveils an unknown molecular function of SACSIN that, if altered, can contribute to the development of ARSACS disease. Therefore, paving the way for innovative therapeutic approaches for ARSACS patients.