Large Intronic Deletion of the Fragile Site Gene PRKN Dramatically Lowers Its Fragility Without Impacting Gene Expression

Recent developments in gene therapy for Parkinson's disease

Parkinson's disease (PD) is a progressive, neurodegenerative disorder for which there is currently no cure. Gene therapy has emerged as a novel approach offering renewed hope for the development of treatments that meaningfully alter the course of the disease. In this review, we explore various gene therapy strategies currently being developed targeting key aspects of PD pathogenesis: the restoration of the dopamine system by delivering genes involved in dopamine biosynthesis, reinforcing the inhibitory signaling pathways through glutamic acid decarboxylase (GAD) delivery to increase GABA production, enhancing neuronal survival and development by introducing various neurotrophic factors, delivery of genes to complement recessive familial PD mutations to correct mitochondrial dysfunction, restoring lysosomal function through delivery of GBA1 to increase glucocerebrosidase (GCase) activity, and reducing α-synuclein levels by reducing or silencing SNCA expression. Despite promising early work, challenges remain in developing safe, effective, and long-lasting gene therapies. Key considerations include optimizing viral vectors for targeted delivery, achieving controlled and sustained gene expression using different promoters, minimizing immune responses, and increasing transgene delivery capacity. Future prospects may involve combinatory strategies targeting multiple pathways, such as multi-gene constructs delivered via high-capacity viral systems.

PRKN-linked familial Parkinson's disease: cellular and molecular mechanisms of disease-linked variants

Parkinson's disease (PD) is a common and incurable neurodegenerative disorder that arises from the loss of dopaminergic neurons in the substantia nigra and is mainly characterized by progressive loss of motor function. Monogenic familial PD is associated with highly penetrant variants in specific genes, notably the PRKN gene, where homozygous or compound heterozygous loss-of-function variants predominate. PRKN encodes Parkin, an E3 ubiquitin-protein ligase important for protein ubiquitination and mitophagy of damaged mitochondria. Accordingly, Parkin plays a central role in mitochondrial quality control but is itself also subject to a strict protein quality control system that rapidly eliminates certain disease-linked Parkin variants. Here, we summarize the cellular and molecular functions of Parkin, highlighting the various mechanisms by which PRKN gene variants result in loss-of-function. We emphasize the importance of high-throughput assays and computational tools for the clinical classification of PRKN gene variants and how detailed insights into the pathogenic mechanisms of PRKN gene variants may impact the development of personalized therapeutics.

A mutational atlas for Parkin proteostasis

Proteostasis can be disturbed by mutations affecting folding and stability of the encoded protein. An example is the ubiquitin ligase Parkin, where gene variants result in autosomal recessive Parkinsonism. To uncover the pathological mechanism and provide comprehensive genotype-phenotype information, variant abundance by massively parallel sequencing (VAMP-seq) is leveraged to quantify the abundance of Parkin variants in cultured human cells. The resulting mutational map, covering 9219 out of the 9300 possible single-site amino acid substitutions and nonsense Parkin variants, shows that most low abundance variants are proteasome targets and are located within the structured domains of the protein. Half of the known disease-linked variants are found at low abundance. Systematic mapping of degradation signals (degrons) reveals an exposed degron region proximal to the so-called "activation element". This work provides examples of how missense variants may cause degradation either via destabilization of the native protein, or by introducing local signals for degradation.