Reduction of sacsin levels in peripheral blood mononuclear cells as a diagnostic tool for spastic ataxia of Charlevoix–Saguenay

Autosomal recessive spastic ataxia of Charlevoix-Saguenay is a rare neurodegenerative disease caused by biallelic variants in the <i>SACS</i> gene encoding for sacsin. More than 200 pathogenic variants have been identified to date, most of which are missense. It is likely that the prevalence of autosomal recessive spastic ataxia of Charlevoix-Saguenay is underestimated due to the lack of an efficient diagnostic tool able to validate variants of uncertain significance. We have previously shown that sacsin is almost absent in fibroblasts of patients with autosomal recessive spastic ataxia of Charlevoix-Saguenay regardless of the type of <i>SACS</i> variant, because sacsin carrying missense variants is cotranslationally degraded. In this work, we aimed to establish the pathogenicity of <i>SACS</i> variants by quantifying sacsin protein in blood samples, with relevant implications for autosomal recessive spastic ataxia of Charlevoix-Saguenay diagnosis. We developed a protocol to assess sacsin protein levels by western blot using small amounts of peripheral blood mononuclear cells, which can be propagated in culture and cryopreserved. The study involves eight patients with autosomal recessive spastic ataxia of Charlevoix-Saguenay (including a novel case) carrying variants of different types and positions along the <i>SACS</i> gene and two parents who are carriers of heterozygous missense variants. We show that patients with autosomal recessive spastic ataxia of Charlevoix-Saguenay (carrying either missense or truncating variants) almost completely lacked sacsin in peripheral blood mononuclear cells. Moreover, both carriers of a <i>SACS</i> missense variant showed 50% reduction in sacsin protein levels compared to controls. We also describe a patient with uniparental isodisomy carrying a homozygous nonsense variant near the 3' end of the <i>SACS</i> gene. This resulted in a stable sacsin protein lacking the last 202 amino acids, probably due to escape of nonsense-mediated decay of mRNA. In conclusion, we have optimized a minimally invasive diagnostic tool for autosomal recessive spastic ataxia of Charlevoix-Saguenay in blood samples based on sacsin protein level assessment. Indeed, our results provide definite evidence that sacsin carrying missense pathogenic variants undergoes cotranslational degradation. The quantitative reduction in sacsin levels in the case of missense variants of uncertain significance allows defining them as pathogenic variants, something which cannot be predicted bioinformatically with high certainty.