Striatal and Extra-Striatal Cholinergic Terminal Density in LRRK2-PD Mutation

LRRK2 mutations are among the most common genetic risk factors for Parkinson's Disease (PD), accounting for ~1% to 36% of familial cases depending on the ancestry, and a significant fraction of sporadic cases. These mutations are not fully penetrant, with 17-85% of carriers eventually developing PD. Among LRRK2-PD patients, clinical features manifest differently compared to idiopathic PD (iPD). LRRK2-PD patients often experience milder progression, with fewer non-motor symptoms and relatively preserved cognition, but higher likelihood of postural instability and gait difficulties (PIGD). This complex profile likely reflects the pleiotropic nature of LRRK2 effects, with mutations causing pathological increases in kinase activity and hyperphosphorylation of downstream targets. A comprehensive understanding of the protective and harmful changes induced by LRRK2 mutations remains a significant research gap.

While PD has traditionally been viewed as primarily a dopaminergic disorder, it is increasingly recognized as a multi-system condition involving other neuromodulatory systems, particularly cholinergic systems. Loss of cholinergic systems integrity is linked to dopamine-refractory symptoms, including morbid gait and cognitive impairments. Recent studies, however, demonstrate that subsets of both newly diagnosed and established PD patients exhibit upregulation of the vesicular acetylcholine transporter (VAChT), associated with preserved cognitive function, in both cross-sectional and longitudinal analyses.

A comprehensive, whole-brain investigation using the specific 18F-fluoroethoxybenzovesamicol ([18F]FEOBV) radioligand will help us better understand the nuanced whole-brain system changes in cholinergic nerve terminal integrity in LRRK2-PD compared to iPD. Our primary recruitment efforts will focus on the University of Michigan's Movement Disorder clinics, allowing us to connect with individuals diagnosed with PD who have the LRRK2 mutation. Additionally, we will work closely with PDGENEration and their sites such as Cleveland and Columbus. This collaboration will provide access to a larger pool of individuals with LRRK2 mutations, improving our ability to recruit participants for our study and ensuring a thorough investigation. We will identify regional cholinergic system alterations in PD with LRRK2 mutations. This would facilitate research on understanding the pathogenic basis for cholinergic activity alterations in iPD as well. Cholinergic systems offer multiple potential targets for pharmacologic and non-invasive neurostimulation interventions. Identification of distinctive cholinergic systems abnormalities in LRRK2-PD would provide new targets for potential therapies, initiating new approaches to experimental therapeutics in PD with LRRK2 mutations and informed by novel insights into PD pathomechanisms.

Aim 1a: To assess in vivo comprehensive, whole-brain expression and activity of cholinergic nerve terminals of LRRK2-PD patients relative to iPD patients using the [18F]FEOBV vesicular acetylcholine transporter (VAChT) radioligand.

Aim 1b: To correlate in vivo striatal and extra-striatal [18F]FEOBV regional binding with cognitive and motor assessments (esp. PIGD symptoms) in LRRK2-PD and iPD.

Aim 2a: To examine differences in striatal and extra-striatal dopamine terminal density between LRRK2-PD and iPD using the complementary PET tracer N-(3-iodoprop-2E-enyl)-2β-carbomethoxy-3β-(4-methyl-phenyl)nortropane ([¹¹C]PE2i).

Aim 2b: To investigate the association between dopaminergic function, cognition, and PIGD symptoms

Inclusion criteria

• Male or Female, age 45 years and over.
• Diagnosis of PD based on the United Kingdom Parkinson's Disease Society Brain Bank Diagnostic Research Criteria (Hughes et al., 1992).
• Presence of LRRK2 mutation as confirmed by referral from UM Movement Disorders clinic, medical record review, or participation in the PDGENEration study.

Exclusion criteria

• Evidence of atypical parkinsonism.
• Contra-indications to MR imaging including but not limited to pacemakers, aneurysm clips, intraocular metal, cochlear implant, or severe claustrophobia.
• Evidence of large vessel stroke or mass lesion on MRI.
• Regular use of typical anti-cholinergic drugs or cholinesterase inhibitors.
• History of deep brain stimulation surgery.
• Pregnant or nursing.
• Suicidal ideation, as indicated by a response of 2 or 3 on question 9 of the Beck Depression Inventory.
• Cognitive impairment that results in the inability to give consent, as demonstrated by the Decision Making Capacity Tool.
• Any other condition or criterion that would preclude safe and meaningful participation in the study.