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Assessment of Sacsin Turnover in Patients With ARSACS: Implications for Molecular Diagnosis and Pathogenesis
Fabiana Longo1, Daniele De Ritis1, Annarita Miluzio1
1From the Mitochondrial Dysfunctions in Neurodegeneration Unit (F.L., D.D.R., D.F., F.M.) and Department of Neurology (M.S.), Ospedale San Raffaele, Milan, Italy; Istituto Nazionale di Genetica Molecolare (A.M., S.B.), INGM, "Romeo ed Enrica Invernizzi," Milan, Italy; Laboratory of Neuromuscular Pathology (J.B.), Institute Born-Bunge, University of Antwerp; Neuromuscular Reference Centre (J.B.), Department of Neurology, Antwerp University Hospital, Belgium; Molecular Medicine (F.M.S.), IRCCS Fondazione Stella Maris, Pisa, Italy; Department of Biosciences (S.B.), University of Milan; and Università Vita-Salute San Raffaele (F.M., D.D.R.), Milan, Italy.
Background And Objectives:
Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by variations in SACS gene encoding sacsin, a huge multimodular protein of unknown function. More than 200 SACS variations have been described worldwide to date. Because ARSACS presents phenotypic variability, previous empirical studies attempted to correlate the nature and position of SACS variations with the age at onset or with disease severity, although not considering the effect of the various variations on protein stability. In this work, we studied genotype-phenotype correlation in ARSACS at a functional level.
Methods:
We analyzed a large set of skin fibroblasts derived from patients with ARSACS, including both new and already published cases, carrying variations of different types affecting diverse domains of the protein.
Results:
We found that sacsin is almost absent in patients with ARSACS, regardless of the nature of the variation. As expected, we did not detect sacsin in patients with truncating variations. We found it strikingly reduced or absent also in compound heterozygotes carrying diverse missense variations. In this case, we excluded SACS mRNA decay, defective translation, or faster posttranslational degradation as possible causes of protein reduction. Conversely, our results demonstrate that nascent mutant sacsin protein undergoes cotranslational ubiquitination and degradation.
Discussion:
Our results provide a mechanistic explanation for the lack of genotype-phenotype correlation in ARSACS. We also propose a new and unambiguous criterion for ARSACS diagnosis that is based on the evaluation of sacsin level. Last, we identified preemptive degradation of a mutant protein as a novel cause of a human disease.
