Maltecca, F. | bioRxiv Science

bioRxiv2021

Sacsin cotranslational degradation causes autosomal recessive spastic ataxia of Charlevoix-Saguenay

Autosomal recessive spastic ataxia of Charlevoix-Saguenay is caused by more than 200 different mutations in the SACS gene encoding sacsin, a huge multimodular protein of unknown function. ARSACS phenotypic spectrum is highly variable. Previous studies correlated the nature and position of SACS mutations with age of onset or disease severity, though the effects on protein stability were not considered. In this study, we explain mechanistically the lack of genotype-phenotype correlation in ARSACS, with important consequences for disease diagnosis and treatment. We found that sacsin is almost absent in ARSACS fibroblasts, regardless of the nature of the mutation. We did not detect sacsin in patients with truncating mutations, while we found it strikingly reduced or absent also in compound heterozygotes carrying diverse missense mutations. We excluded SACS mRNA decay, defective translation, or faster post-translational degradation as causes of protein reduction. Conversely, we demonstrated that nascent mutant sacsin protein undergoes preemptive cotranslational degradation, emerging as a novel cause of a human disease. Based on these findings, sacsin levels should be included in the diagnostic algorithm for ARSACS.

genetics↗

bioRxiv2018

Mice harboring a SCA28 patient mutation in AFG3L2 develop late-onset ataxia associated with enhanced mitochondrial proteotoxicity.

Spinocerebellar ataxia 28 is an autosomal dominant neurodegenerative disorder caused by missense mutations affecting the proteolytic domain of AFG3L2, a major component of the mitochondrial m-AAA protease. However, little is known of the underlying pathogenetic mechanisms or how to treat patients with SCA28. Currently available Afg3l2 mutant mice harbour deletions that lead to severe, early-onset neurological phenotypes that do not faithfully reproduce the late-onset and slowly progressing SCA28 phenotype. Here we describe production and detailed analysis of a new knock-in murine model harbouring an Afg3l2 allele carrying the p.Met665Arg patient-derived mutation. Heterozygous mutant mice developed normally but signs of ataxia were detectable by beam test at 18 months. Cerebellar pathology was negative; electrophysiological analysis showed increased spontaneous firing in Purkinje cells from heterozygous mutants with respect to wild-type controls, although not statistically significant. As homozygous mutants died perinatally with evidence of cardiac atrophy, for each genotype we generated mouse embryonic fibroblasts (MEFs) to investigate mitochondrial function. MEFs from mutant mice showed altered mitochondrial bioenergetics, with decreased basal oxygen consumption rate, ATP synthesis and mitochondrial membrane potential. Mitochondrial network formation and morphology was also altered, in line with greatly reduced expression of Opa1 fusogenic protein L-isoforms. The mitochondrial alterations observed in MEFs were also detected in cerebella of 18-month-old heterozygous mutants, suggesting they may be a hallmark of disease. Pharmacological inhibition of de novo mitochondrial protein translation with chloramphenicol caused reversal of mitochondrial morphology in homozygous mutant MEFs, supporting the relevance of mitochondrial proteotoxicity for SCA28 pathogenesis and therapy development.

genetics↗