Parkinson’s Disease: The Devil Is in the Protein


Patricia F. Fitzpatrick Dimond Ph.D. Technical Editor of Clinical OMICs President of BioInsight Communications

Focus Is on the Development of Small Molecule and Monoclonal Antibody Therapies

While the prevalence of neurodegenerative disorders, including Parkinson’s disease (PD), remains daunting, recent significant progress in defining the molecular causes of these diseases has provided novel drug targets. Increased development efforts focused on molecular targets may improve the odds for discovering new treatments.

The U.S. National Institute for Neurological Disorders and Stroke estimated in 2006 that about 50,000 new PD cases are diagnosed in the U.S. each year, reaching a current total of 500,000 patients. Based on published prevalence studies, Dorsey et al. (Neurology, January 30, 2007) projected that the number of individuals over age 50 with PD in Western Europe’s 5 most and the world’s 10 most populous nations would double to between 8.7 and 9.3 million by 2030 from 4.1 to 4.6 million in 2005

The true prevalence of Parkinson’s remains difficult to assess, as diagnosis of the disease is not made until the disease process has advanced.  

In 1997, researchers at the NIH’s National Human Genome Research Institute discovered defects in a gene uniquely linked to PD in particular families. They discovered abnormalities in the gene encoding alpha synuclein (α-Synuclein), a neuronal protein that forms a major component of pathological inclusions that characterize several neurodegenerative disorders. These disorders include PD, dementia with Lewy bodies, neurodegeneration with brain iron accumulation type 1, and multiple system atrophy collectively termed synucleinopathies

This tangible discovery of a gene defect linked to PD in particular families was a game-changing finding, scientists say, as it opened the door to studies investigating the genetic base of the disorder, culminating in more recent genome-wide association studies (GWAS). But, they point out, these study results have come full circle back to the origins of the molecular genetic era of PD; one of the major genes identified as linked to sporadic PD is SNCA, the gene encoding α-synuclein.

To date, more than 13 loci and 9 genes have been identified through linkage and next-gen sequencing studies of familial PD, as well as candidate gene and GWAS in sporadic PD.  A review and analysis of these studies by Trinh and Farrer (Nat Rev Neurol., August. 2013) noted that while many of the genetic findings overlap, results of molecular studies define a sequence of pathological events whereby deficits in synaptic exocytosis and endocytosis, endosomal trafficking, lysosome-mediated autophagy, and mitochondrial maintenance increase susceptibility to PD.

Molecular Insights and Research Tools

These discoveries have provided the rationale, molecular insight, and research tools to develop neuroprotective and disease-modifying therapies, many of them directed at preventing the neurotoxic aggregation of α-synuclein into Lewy bodies, the protein clumps that are the pathological hallmark of Parkinson’s disease

Present in high concentration at presynaptic terminals and found in both soluble and membrane-associated fractions of the brain, α-Synuclein accounts for as much as 1% of the total protein in soluble cytosolic brain fractions. Its possible functions include synaptic vesicle release and trafficking, fatty acid binding, and physiological regulation of certain enzymes, transporters, and neurotransmitter vesicles, as well as roles in neuronal survival.

α-Synuclein is an intrinsically disordered protein that can adopt a number of different conformational states depending on conditions and cofactors. These conformations include the helical membrane-bound form, a partially folded state that is a key intermediate in aggregation and fibrillation. α-Synuclein may contribute to PD pathogenesis in a number of ways, but it is generally thought that its aberrant soluble oligomeric conformations, termed protofibrils, are the toxic species that mediate disruption of cellular homeostasis and neuronal death through effects on various intracellular targets, including synaptic function.

Recent work using animal models with intracellular α-synuclein and tau inclusions have added decisive evidence that misfolded protein aggregates can induce a self-perpetuating process that leads to amplification and spreading of pathological protein assemblies. When coupled with the progressive nature of neurodegeneration, recognition of such cell-to-cell aggregate spread suggests a unifying mechanism underlying the pathogenesis of these disorders. These include the helical membrane-bound protein form, a partially folded state that is a key intermediate in aggregation and fibrillation, various oligomeric species, and fibrillar and amorphous aggregates. The molecular basis of PD appears to be tightly coupled to the aggregation of α-synuclein and the factors that affect its conformation.

W. Peelaerts et al. (Nature, June 18, 2015) recently described properties of structurally well-defined α-synuclein assemblies. Their study the authors said showed that  α-synuclein strain conformation and seeding propensity lead to distinct histopathological and behavioral phenotypes.  The investigators assessed the properties of structurally well-defined α-synuclein assemblies (oligomers, ribbons, and fibrils) after injection in rat brain, and reported that α-synuclein strains amplify in vivo.

They reported that fibrils seem to be the major toxic strain, resulting in progressive motor impairment and cell death, whereas ribbons cause a distinct histopathological phenotype displaying Parkinson’s disease and multiple system atrophy traits. Additionally, they showed that α-synuclein assemblies cross the blood–brain barrier and distribute to the central nervous system after intravenous injection.

The investigators said they had demonstrated that distinct α-synuclein strains display differential seeding capacities, inducing strain-specific pathology and neurotoxic phenotypes. While different assemblies could affect neural transmission, only fibrillary α-synuclein exhibited perpetual behavior and aggravated neurotoxic phenotypes in vivo.

Pharma Sees Opportunities

Based on these and other related findings, pharma companies have been targeting synucleins in the search from effective PD drugs.  Roche and Prothena entered a collaboration in 2013 to develop and commercialize antibodies targeting alpha-synuclein, including PRX002, Prothena’s monoclonal antibody for the treatment of Parkinson’s disease.

The companies are evaluating PRX002 in a multiple ascending dose study in patients with Parkinson's, with results expected in the first half of 2016. PRX002 is designed to slow or reduce the progressive neurodegeneration associated with synuclein misfolding and/or the cell-to-cell transmission of the pathogenic forms of synuclein in Parkinson's disease and other synucleinopathies.

Prothena had demonstrated the efficacy of PRX002 in various cellular and animal models of synuclein-related disease. In transgenic mouse PD models, passive immunization with 9E4, the murine version of PRX002, reduced the appearance of synuclein pathology, protected synapses, and improved performance by the mice in behavioral testing.

In a Phase I study, 40 volunteers were randomized into five escalating dose cohorts (0.3 mg/kg, 1 mg/kg, 3 mg/kg, 10 mg/kg, or 30 mg/kg) to receive either PRX002 or placebo. No hypersensitivity reactions or drug-related serious adverse events were reported. PRX002 demonstrated favorable pharmacokinetic properties, supporting the current dosing frequency in the on-going Phase I multiple ascending dose study.

This past February, Lysosomal Therapeutics announced it had raised $20 million in Series A financing from Atlas Venture, Roche Ventures, Lilly Ventures, Hatteras Venture Partners, and Sanofi Genzyme Bioventures to develop a glucocerebrosidase activator for the treatment of Parkinson’s disease. In May 2014, the startup gained a $4.8 million seed commitment the same syndicate led by Atlas Venture. Lysosomal was co-founded by former Genzyme CEO Henri Termeer.

Lysosomal Therapeutics develops small molecule, orally available drugs to treat PD based on the work of its scientific co-founders, Dimitri Krainc, M.D., and Joseph Mazzuli, Ph.D. The drug development program rests on the premise that increasing lysosomal enzyme activity, in an attempt to prevent accumulation of α-synuclein, may provide therapeutic benefit.

Drs. Krainc and Mazzuli had demonstrated that functional loss of Gaucher’s Disease linked-GCase in cultures of human  iPS neurons compromises lysosomal protein degradation, causes accumulation of α-synuclein, resulting in neurotoxicity through aggregation-dependent mechanisms.

Accumulation of glucosylceramide (GlcCer), the GCase substrate, could be involved in amyloid formation of α-synuclein by stabilizing its oligomeric intermediates. They further found that α-synuclein inhibits the lysosomal activity of normal GCase in neurons and idiopathic PD brain, suggesting that the enzyme depletion contributes to the pathogenesis of sporadic synucleinopathies and that the bidirectional effect of α-synuclein and GCase forms a positive feedback loop that may lead to a self-propagating disease.

Kees Been, CEO of Lysosomal Therapeutics, told GEN that the recently discovered clinical, genetic, and pathological linkage between Gaucher's disease (GD) and PD offered a unique opportunity to examine lysosomal GCase for development of targeted therapies in synucleinopathies. GBA variants are now recognized as the most common genetic risk factors associated with PD, and individuals with Parkinson disease are more than five times more likely to carry mutations in GBA.

“With the enzyme replacement therapy for GD, which was very successful, observant doctors noticed an increase in incidence of PD among GD patients,” he said.   “The single allele mutation in the GBA gene is a high risk factor for Parkinson’s.”

The average risk for PD is 1-2%, he further noted, saying “If you have a severe mutation the risk goes up 13-15 fold.” Individuals with two faulty copies of the GBA gene, who are already known to have higher risk of Parkinson’s disease, develop PD at a younger age (about 10 years earlier) than those with only one or no defective copies.

“We are going after the abnormal enzyme to try to activate it with a small, orally available molecule, hoping (that enzyme activation) will provide a disease-modifying effect,” continued Been. “We also intend to eventually use the enzyme level as a biomarker to determine the risk of developing PD, then use the drug to lower that risk.”

The company’s small molecule, he points out, “is designed to cross the blood-brain barrier to activate the enzyme in the brain.”  Plans are in place to initiate a Phase I clinical trial with its drug candidate in early 2017.

In the past five years the number of drugs being developed by large drug makers for brain and nervous system disorders fell 50% to 129, according to NeuroPerspective, an industry newsletter.  But discoveries made at the gene level that have translated into protein targets for neurodegenerative diseases may reverse that trend.

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