Parkinson Diseases in the 2020s and Beyond: Replacing Clinico-Pathologic Convergence With Systems Biology Divergence

Parkinson disease has been considered for practical purposes a heterogeneous clinico-pathological entity. The operational definition requires clinical ascertainment of a levodopa-responsive parkinsonism with no "atypical" features, and pathological criteria based on the finding, usually at postmortem, of aggregates of α-synuclein in Lewy bodies and Lewy neurites. The underlying assumption has been that a molecular-biological disorder, targetable for disease modification as a whole, underlies this clinico-pathologic, convergent model of disease. The 2020s will be expected to mark the beginning of the end for this model, especially if therapeutic success in a specific molecular subtype, such as PD-GBA, is not translated to "sporadic PD". The complex and dynamic biological abnormalities of aging, which have informed the evolution of other fields in medicine into divergent, systems-biology models, will also provide the template for the development of disease modifying therapies for neurodegenerative disorders. In the 2020s and 2030s we will no longer ask whether any given molecule may be neuroprotective in early Parkinson disease but, rather, which subtype (which endophenotype) among the Parkinson diseases would be the best mechanistic recipient for such molecule and which would not. The next breakthrough in Parkinson's research will be conceptual: the recognition that discoveries in a subtype of PD will apply only or largely to that subtype and not construed to represent "a piece" that seamlessly inserts into, and helps explains, a unifying "Parkinson's puzzle". Successful neuroprotection for each PD subtype will likely require pharmacotherapeutic combinations ("drug cocktails") to harness synergistic potential benefits when more than the dominant pathogenic mechanism is targeted, as identified from forthcoming population-based unbiased biomarker discovery programs.