Comparative Analysis and Functional Mapping of SACS Mutations Reveal Novel Insights into Sacsin Repeated Architecture

Abstract

Autosomal recessive spastic ataxia of Charlevoix–Saguenay (ARSACS) is a neurological disease with mutations in SACS, encoding sacsin, a multidomain protein of 4,579 amino acids. The large size of SACS and its translated protein has hindered biochemical analysis of ARSACS, and how mutant sacsins lead to disease remains largely unknown. Three repeated sequences, called sacsin repeating region (SRR) supradomains, have been recognized, which contribute to sacsin chaperone-like activity. We found that the three SRRs are much larger (≥1,100 residues) than previously described, and organized in discrete subrepeats. We named the large repeated regions Sacsin Internal RePeaTs (SIRPT1, SIRPT2, and SIRPT3) and the subrepeats sr1, sr2, sr3, and srX. Comparative analysis of vertebrate sacsins in combination with fine positional mapping of a set of human mutations revealed that sr1, sr2, sr3, and srX are functional. Notably, the position of the pathogenic mutations in sr1, sr2, sr3, and srX appeared to be related to the severity of the clinical phenotype, as assessed by defining a severity scoring system. Our results suggest that the relative position of mutations in subrepeats will variably influence sacsin dysfunction. The characterization of the specific role of each repeated region will help in developing a comprehensive and integrated pathophysiological model of function for sacsin.

New practical definitions for the diagnosis of autosomal recessive spastic ataxia of Charlevoix–Saguenay

“…Sacsin contains 3 sr domains (sr1-3), 14 which form the SIRPTs (Sacsin Internal RePeaTs). 12 This sr1 contains a histidine kinase-like adenosine triphosphatase, and mutations in this domain display a higher SPAX score, which evaluates phenotype severity. 12 These data support our findings that p.R272 might play an important role in the sacsin-dependent mitochondrial phenotype.…”

“…12 This sr1 contains a histidine kinase-like adenosine triphosphatase, and mutations in this domain display a higher SPAX score, which evaluates phenotype severity. 12 These data support our findings that p.R272 might play an important role in the sacsin-dependent mitochondrial phenotype. It would be interesting to analyze whether the 2 other families carrying this mutation display the same severe mitochondrial phenotype.…”

“…10,11 More than 100 mutations have been described to date, essentially leading to a premature stop codon. 9,12 The encoded protein sacsin is composed of several domains: ubiquitin-like, Hsp90-like, XPCB, DnaJ, HEPN, and the sacsin repeating region, [11][12][13][14] but its precise function remains largely unknown. Sacsin was first shown to be cytoplasmic, with 30% localized at or near the cytoplasmic face of the mitochondria in SHSY-5Y cells.…”

A Novel Homozygous <b><i>SACS</i></b> Mutation Identified by Whole-Exome Sequencing in a Consanguineous Family with Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay

“…1 D) [Romano et al, 2013]. This variant results in a truncated protein terminated at residue 3,174 and the loss of 2 highly conserved domains: DnaJ and HEPN.…”

Do not jump to easy conclusions! Lessons from pitfall in the molecular diagnosis of ARSACS

“…1), and corrected after extending the analyses to informative family members. In particular, we underscore the need for caution in interpreting molecular findings when segregation cannot be easily determined; this issue is critically related to the emerging occurrence of variants with unclear significance while sequencing SACS gene in patients with spastic ataxias (1,2), as well as to the wider use of whole-exome sequencing (WES) in clinical settings, which also increases the risk of possible drawbacks in molecular diagnosis. Case 1 ( Fig.…”