Abstract
Parkinson disease (PD) is the second most common neurodegenerative disorder. Most cases of PD are sporadic, while 5–10% have a known genetic basis. Variants in the PARK2 gene are the most frequent cause of autosomal recessive juvenile-onset PD. PARK2 encodes parkin, a multi-domain protein that functions as an ubiquitin E3 ligase. Numerous variants spanning all parkin domains have been identified, although the pathogenic relevance for several of those remains unclear. In this study, we aimed to functionally characterize two truncating parkin variants: N52Mfs*29, which is highly prevalent in the Portuguese and Spanish populations, and L358Rfs*77, recently identified in the Portuguese population. Our results indicate that both variants are prematurely degraded by the proteasome, even though proteins levels are still moderate. We also showed that they are aggregation-prone and lead to mislocalized parkin. Interestingly, the L358Rfs*77 variant is mislocalized to the nucleus, which was never reported for parkin variants. While N52Mfs*29 impaired self-ubiquitination activity, the L358Rfs*77 variant seemed to retain it. Both variants, however, fail to ubiquitinate p62 substrate and did not relocalize to depolarized mitochondria. Therefore, we conclude that parkin truncating variants cause loss of parkin function, thus showing their causative role in PD pathogenesis.
Highlights
Parkinson disease (PD) is the second most common neurodegenerative disorder
Presence of parkin in cytosol or mitochondria fractions was analyzed by immunoblotting with anti-EGFP antibody and normalized to caspase 3 or TOM20, respectively
The autosomal recessive inheritance of parkin-related PD suggests that protein loss-of-function is at the basis of autosomal recessive juvenile-onset parkinsonism (ARJP) pathogenesis
Summary
Parkinson disease (PD) is the second most common neurodegenerative disorder. Most cases of PD are sporadic, while 5–10% have a known genetic basis. We aimed to functionally characterize two truncating parkin variants: N52Mfs*29, which is highly prevalent in the Portuguese and Spanish populations, and L358Rfs*77, recently identified in the Portuguese population. Given the complex activation process of parkin and depending on the specific residue affected, disease-associated variants can affect parkin E3 ligase activity through different mechanisms These variants can directly impair parkin activity or abolish translation of a functional protein, cause reduced solubility and enhanced aggregation, disturb protein folding and stability, and/or affect parkin ability to bind to cofactors and substrates[19,20,21]. Our study showed that each variant lead to misfolding and mislocalized parkin, being defective in the ubiquitination process and resulting in an apparent loss of parkin function
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