The etiopathogenesis of Parkinson disease and suggestions for future research. Part II.

TLDR: The second part of this two-part, state-of-the-art review by leaders in PD research critically examines the research field to identify areas for which new knowledge and ideas might be helpful for treatment purposes.

Abstract: We are at a critical juncture in our knowledge of the etiology and pathogenesis of Parkinson disease (PD). It is clear that PD is not a single entity simply resulting from a dopaminergic deficit; rather it is most likely caused by a combination of genetic and environmental factors and, although there is extensive new information on the etiology and pathogenesis of PD that may advance its treatment, new syntheses of this information are needed. The first part of this two-part, state-of-the-art review by leaders in Parkinson research critically examines the field to identify where new knowledge and ideas might be helpful for treatment purposes. Topics reviewed in Part I include the definition of the disease, neuropathologic contributions, and epidemiologic, environmental, and demographic issues. ### Current Knowledge The definition of idiopathic Parkinson disease (PD) remains controversial. Classically, it includes a characteristic motor phenotype (1) and a distinctive neuropathology and substantial loss of dopaminergic neurons from the substantia nigra associated with the presence of α-synuclein-positive inclusions in the cell body (Lewy bodies) and processes (Lewy neurites) of specific neurons of the brainstem. ### Parkinson Syndrome Without Synucleinopathy Some genetically determined Parkinson syndromes resemble, sometimes closely, idiopathic PD, but differ from it neuropathologically, sometimes substantially. In particular, a number of juvenile-onset autosomal recessive cases of familial Parkinson syndrome do not have α-synuclein lesions (2), contrasting with the autosomal dominant forms of the disease which often have unusual types of lesions (3-5). The recently identified leucine-rich repeat kinase 2 ( LRRK2, Park-8 ) mutations produce variable pathologic phenotypes. Although the most common is Lewy body disease, nigral degeneration with ubiquitin inclusions or, in a few cases, even pathologic changes more typical of frontotemporal lobar degeneration or progressive supranuclear palsy have been described in patients with LRRK2 mutations (6). Two recent reports insist on the high prevalence of Lewy bodies in the LRRK2 mutations (7 …