Therapies for Genetic Forms of Parkinson's Disease: Systematic Literature Review

Parkinson's disease (PD) is a disabling neurological condition characterized by the loss of dopaminergic neurons. Currently, the treatment for PD is symptomatic and compensates for the endogenous loss of dopamine production. In cases where the pharmacological therapy is only partly beneficial or results in major wearing-off complications, surgical interventions such as deep brain stimulation may be an alternative treatment. The disease cause often remains unknown, but in some patients, a monogenic cause can be identified. Mutations in at least six genes, LRRK2, SNCA, and VPS35 (dominant forms) or Parkin/PRKN, PINK1, and DJ1/PARK7 (recessive forms) have been unequivocally linked to PD pathogenesis. We here systematically screened 8,576 publications on these monogenic PD forms. We identified 2,226 mutation carriers from 456 papers. Levodopa was the most widely applied treatment; only 34 patients were indicated to be untreated at the time of reporting. Notably, detailed treatment data was rarely mentioned including response quantification (good, moderate, minimal) in 951 and/or dose in 293 patients only. Based on available data, levodopa showed an overall good outcome, especially in LRRK2, VPS35, Parkin, and PINK1 mutation carriers ('good' response in 94.6-100%). Side effects of levodopa therapy were reported in ∼15-40%of levodopa-treated patients across genes with dyskinesias as the most frequent one. Non-levodopa medication was indicated to be administered to <200 patients with mainly good outcome. Only a few reports were available on outcomes of brain surgery. Here, most mutation carriers showed a good response. Importantly, none of the available treatments is harmful to one genetic form but effective in another one. In the light of different medication schemes, the progressive nature of PD, and side effects, an improvement of therapeutic options for PD is warranted including a treatabolome database to guide clinicians in treatment decisions. Further, novel disease-cause-modifying drugs are needed.