Cerebral pathological and compensatory mechanisms in the premotor phase of leucine-rich repeat kinase 2 parkinsonism

Imaging Markers in Genetic Forms of Parkinson's Disease

Parkinson's disease (PD) is a complex neurodegenerative disorder characterized by motor symptoms such as bradykinesia, rigidity, and resting tremor. While the majority of PD cases are sporadic, approximately 15-20% of cases have a genetic component. Advances in neuroimaging techniques have provided valuable insights into the pathophysiology of PD, including the different genetic forms of the disease. This literature review aims to summarize the current state of knowledge regarding neuroimaging findings in genetic PD, focusing on the most prevalent known genetic forms: mutations in the GBA1, LRRK2, and Parkin genes. In this review, we will highlight the contributions of various neuroimaging modalities, including positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI), in elucidating the underlying pathophysiological mechanisms and potentially identifying candidate biomarkers for genetic forms of PD.

Cognitive Impairment in Genetic Parkinson's Disease

Cognitive impairment is common in idiopathic Parkinson's disease (PD). Knowledge of the contribution of genetics to cognition in PD is increasing in the last decades. Monogenic forms of genetic PD show distinct cognitive profiles and rate of cognitive decline progression. Cognitive impairment is higher in GBA- and SNCA-associated PD, lower in Parkin- and PINK1-PD, and possibly milder in LRRK2-PD. In this review, we summarize data regarding cognitive function on clinical studies, neuroimaging, and biological markers of cognitive decline in autosomal dominant PD linked to mutations in LRRK2 and SNCA, autosomal recessive PD linked to Parkin and PINK1, and also PD linked to GBA mutations.

Increased neural motor activation and functional reorganization in patients with idiopathic rapid eye movement sleep behavior disorder

INTRODUCTION

Altered brain activity and functional reorganization patterns during self-initiated movements have been reported in early pre-motor and motor stages of Parkinson's disease. The aim of this study was to investigate whether similar alterations can be observed in patients with idiopathic REM-sleep behavior disorder (RBD).

METHODS

13 polysomnography-confirmed male and right-handed RBD patients and 13 healthy controls underwent a bilateral hand-movement fMRI task including internally selected (INT) and externally-guided (EXT) movement conditions for each hand. We examined functional activity and connectivity differences between groups and task-conditions, structural differences using voxel-based morphometry, as well as associations between functional activity and clinical variables.

RESULTS

No group differences were observed in fMRI-task performance or in voxel-based morphometry. Both groups showed faster reaction times and exhibited greater neural activation when movements were internally selected compared to externally-guided tasks. Compared to controls, RBD patients displayed stronger activation in the dorsolateral prefrontal cortex and primary somatosensory cortex during INT-tasks, and in the right fronto-insular cortex during EXT-tasks performed with the non-dominant hand. Stronger activation in RBD patients was associated with cognitive and olfactory impairment. Connectivity analysis demonstrated overall less interregional coupling in patients compared to controls. In particular, patients showed reduced temporo-cerebellar, occipito-cerebellar and intra-cerebellar connectivity, but stronger connectivity in fronto-cerebellar and fronto-occipital pathways.

CONCLUSION

The observed stronger activation during hand-movement tasks and connectivity changes in RBD may reflect early compensatory and reorganization patterns in order to preserve motor functioning. Our findings may contribute to a better understanding and prognosis of prodromal stages of α-synucleinopathies.

A Proposed Roadmap for Parkinson's Disease Proof of Concept Clinical Trials Investigating Compounds Targeting Alpha-Synuclein

The convergence of human molecular genetics and Lewy pathology of Parkinson's disease (PD) have led to a robust, clinical-stage pipeline of alpha-synuclein (α-syn)-targeted therapies that have the potential to slow or stop the progression of PD and other synucleinopathies. To facilitate the development of these and earlier stage investigational molecules, the Michael J. Fox Foundation for Parkinson's Research convened a group of leaders in the field of PD research from academia and industry, the Alpha-Synuclein Clinical Path Working Group. This group set out to develop recommendations on preclinical and clinical research that can de-risk the development of α-syn targeting therapies. This consensus white paper provides a translational framework, from the selection of animal models and associated end-points to decision-driving biomarkers as well as considerations for the design of clinical proof-of-concept studies. It also identifies current gaps in our biomarker toolkit and the status of the discovery and validation of α-syn-associated biomarkers that could help fill these gaps. Further, it highlights the importance of the emerging digital technology to supplement the capture and monitoring of clinical outcomes. Although the development of disease-modifying therapies targeting α-syn face profound challenges, we remain optimistic that meaningful strides will be made soon toward the identification and approval of disease-modifying therapeutics targeting α-syn.

LRRK2 in Parkinson disease: challenges of clinical trials

One of the most common monogenic forms of Parkinson disease (PD) is caused by mutations in the LRRK2 gene that encodes leucine-rich repeat kinase 2 (LRRK2). LRRK2 mutations, and particularly the most common mutation Gly2019Ser, are observed in patients with autosomal dominant PD and in those with apparent sporadic PD, who are clinically indistinguishable from those with idiopathic PD. The discoveries that pathogenic mutations in the LRRK2 gene increase LRRK2 kinase activity and that small-molecule LRRK2 kinase inhibitors can be neuroprotective in preclinical models of PD have placed LRRK2 at the centre of disease modification efforts in PD. Recent investigations also suggest that LRRK2 has a role in the pathogenesis of idiopathic PD and that LRRK2 therapies might, therefore, be beneficial in this common subtype of PD. In this Review, we describe the characteristics of LRRK2-associated PD that are most relevant to the development of LRRK2-targeted therapies and the design and implementation of clinical trials. We highlight strategies for correcting the effects of mutations in the LRRK2 gene, focusing on how to identify which patients are the optimal candidates and how to decide on the timing of such trials. In addition, we discuss challenges in implementing trials of disease-modifying treatment in people who carry LRRK2 mutations.

Altered reward-related neural responses in non-manifesting carriers of the Parkinson disease related LRRK2 mutation

Disturbances in reward processing occur in Parkinson's disease (PD) however it is unclear whether these are solely drug-related. We applied an event-related fMRI gambling task to a group of non-manifesting carriers (NMC) of the G2019S mutation in the LRRK2 gene, in order to assess the reward network in an "at risk" population for future development of PD. Sixty-eight non-manifesting participants, 32 of which were non-manifesting non-carriers (NMNC), performed a gambling task which included defined intervals of anticipation and response to both reward and punishment in an fMRI setup. Behavior and cerebral activations were measured using both hypothesis driven and whole brain analysis. NMC demonstrated higher trait anxiety scores (p = 0.04) compared to NMNC. Lower activations were detected among NMC during risky anticipation in the left nucleus accumbens (NAcc) (p = 0.05) and during response to punishment in the right insula (p = 0.02), with higher activations among NMC during safe anticipation in the right insula (p = 0.02). Psycho-Physiological Interaction (PPI) analysis from the NAcc and insula revealed differential connectivity patterns. Whole brain analysis demonstrated divergent between-group activations in distributed cortical regions, bilateral caudate, left midbrain, when participants were required to press the response button upon making their next chosen move. Abnormal neural activity in both the reward and motor networks were detected in NMC indicating involvement of the ventral striatum regardless of medication use in "at risk" individuals for future development of PD.

Network abnormalities among non-manifesting Parkinson disease related LRRK2 mutation carriers

Non-manifesting carriers (NMC) of the G2019S mutation in the LRRK2 gene represent an "at risk" group for future development of Parkinson's disease (PD) and have demonstrated task related fMRI changes. However, resting-state networks have received less research focus, thus this study aimed to assess the integrity of the motor, default mode (DMN), salience (SAL), and dorsal attention (DAN) networks among this unique population by using two different connectivity measures: interregional functional connectivity analysis and Dependency network analysis (D_EP NA). Machine learning classification methods were used to distinguish connectivity between the two groups of participants. Forty-four NMC and 41 non-manifesting non-carriers (NMNC) participated in this study; while no behavioral differences on standard questionnaires could be detected, NMC demonstrated lower connectivity measures in the DMN, SAL, and DAN compared to NMNC but not in the motor network. Significant correlations between NMC connectivity measures in the SAL and attention were identified. Machine learning classification separated NMC from NMNC with an accuracy rate above 0.8. Reduced integrity of non-motor networks was detected among NMC of the G2019S mutation in the LRRK2 gene prior to identifiable changes in connectivity of the motor network, indicating significant non-motor cerebral changes among populations "at risk" for future development of PD.

Structural and Functional MRI in Familial Parkinson's Disease

Between 10 and 15% of Parkinson disease (PD) cases can be traced to a genetically identified causative mutation which currently number over 40. This enables the study of both "at risk" populations for future development of PD and a unique sub-group of genetically determined patient population. Structural and functional magnetic imaging has the potential of assisting diagnosis, early detection and disease progression as it is relatively cheap and easy to implement. However, the large variety of imaging options and different analytical approaches hamper the pursuit of a unified imaging biomarker. This chapter details the current imaging options and summarizes the findings among both genetically determined patients with PD and their non-manifesting first degree relatives, speculating on possible compensational mechanisms while mapping future directions in order to better utilize MRI in the research of genetic PD.

Neuroimaging abnormalities in individuals exhibiting Parkinson's disease risk markers

BACKGROUND

The concept of prodromal Parkinson's disease (PD) involves variable combinations of nonmotor features and subtle motor abnormalities as a result of ongoing neurodegeneration in the brain stem including substantia nigra (SN) and abnormal findings upon transcranial sonography and nuclear imaging. Except for nuclear imaging, the predictive value of risk markers for the conversion to overt PD is low.

OBJECTIVE

The objective of this study was to determine whether PD risk markers are associated with changes in brain structure and to what extent cognitive changes are risk markers for PD.

METHODS

Diffusion-weighted imaging, voxel-based morphometry, and cortical thickness analysis was performed in 29 individuals with hyposmia and/or an increased SN hyperechogenicity (SN+) upon transcranial sonography and 28 controls without these 2 risk markers. Classical parkinsonian signs were an exclusion criterion. All of the participants underwent a neuropsychological test battery addressing executive functions, learning ability, and verbal fluency.

RESULTS

In the PD risk group, diffusion-weighted imaging mean diffusivity was increased in 4 left hemisphere clusters (posterior thalamus, inferior longitudinal fasciculus, fornix, corticospinal tract). A negative relationship of mean diffusivity and smell function was present for the posterior thalamus and the corticospinal tract. There was a significant correlation of mean diffusivity values and SN+ in all clusters. Neither voxel-based morphometry nor cortical thickness analysis revealed any group differences. No relevant group differences were observed for cognitive tests included.

CONCLUSION

PD-free individuals with PD risk markers show microstructural changes of the white matter, including areas relevant for motor and limbic processes. In addition, our study provides for the first time a neuroanatomical correlate for SN hyperechogenicity. © 2018 International Parkinson and Movement Disorder Society.

LRRK2-associated Parkinson's disease patients have better stereopsis than idiopathic Parkinson disease

OBJECTIVES

Visual dysfunctions are frequent and have several manifestations in idiopathic Parkinson's disease (PD). However, the characteristics of these complications in LRRK2 (leucine-rich kinase 2)-associated PD patients still lack systematic research. The purpose of this study is to assess visual functions of LRRK2-associated PD patients.

PATIENTS AND METHODS

Twenty-five (25) PD patients with LRRK2 R1628P and G2385R variants were included in the study and compared to 28 PD patients without these variants and 28 age-matched healthy controls. The genotypes of PD patients were kept double-blinded. Information on age, sex, disease duration, the movement disorder society-unified Parkinson's disease rating scale (MDS-UPDRS), Hoehn and Yahr staging scale (H&Y), Mini-Mental Examination (MMSE) and Montreal Cognitive Assessment (MoCA) were included. Visual functions assessment included color perception, contrast sensitivity and stereopsis.

RESULTS

PD patients with or without LRRK2 R1628P and G2385R variants have declined contrast sensitivity, diminished color discrimination and damaged stereopsis. There was no significant difference in retinal level visual deficiency (color discrimination and contrast sensitivity) between PD with LRRK2 variants and those without, but cortex level visual function, i.e. stereopsis is better in PD with LRRK2 variants than non-carrier PD patients. The associated factors of stereopsis are different. The stereopsis is associated with MoCA scores independently in non-carrier PD patients, but with UPDRSIII scores in LRRK2-associated PD patients.

CONCLUSIONS

Visual functions are similarly affected in PD patients with and without LRRK2 R1628P and G2385R variants, but LRRK2-associated PD patients have better stereopsis than idiopathic PD patients.

Neuroimaging in pre-motor Parkinson's disease

The process of neurodegeneration in Parkinson's disease begins long before the onset of clinical motor symptoms, resulting in substantial cell loss by the time a diagnosis can be made. The period between the onset of neurodegeneration and the development of motoric disease would be the ideal time to intervene with disease modifying therapies. This pre-motor phase can last many years, but the lack of a specific clinical phenotype means that objective biomarkers are needed to reliably detect prodromal disease. In recent years, recognition that patients with REM sleep behaviour disorder (RBD) are at particularly high risk of future parkinsonism has enabled the development of large prodromal cohorts in which to investigate novel biomarkers, and neuroimaging has generated some of the most promising results to date. Here we review investigations undertaken in RBD and other pre-clinical cohorts, including modalities that are well established in clinical Parkinson's as well as novel imaging methods. Techniques such as high resolution MRI of the substantia nigra and functional imaging of Parkinsonian brain networks have great potential to facilitate early diagnosis. Further longitudinal studies will establish their true value in quantifying prodromal neurodegeneration and predicting future Parkinson's.

The prodromal phase of leucine-rich repeat kinase 2-associated Parkinson disease: Clinical and imaging Studies

BACKGROUND

Asymptomatic, nonmanifesting carriers of leucine-rich repeat kinase 2 mutations are at increased risk of developing PD. Clinical and neuroimaging features may be associated with gene carriage and/or may demarcate individuals at greater risk for phenoconversion to PD.

OBJECTIVES

To investigate clinical and dopamine transporter single-photon emission computed tomography imaging characteristics of leucine-rich repeat kinase 2 asymptomatic carriers.

METHODS

A total of 342 carriers' and 259 noncarriers' relatives of G2019S leucine-rich repeat kinase 2/PD patients and 39 carriers' and 31 noncarriers' relatives of R1441G leucine-rich repeat kinase 2/PD patients were evaluated. Motor and nonmotor symptoms were assessed using specific scales and questionnaires. Neuroimaging quantitative data were obtained in 81 carriers and compared with 41 noncarriers.

RESULTS

G2019S carriers scored higher in motor scores and had lower radioligand uptake compared to noncarriers, but no differences in nonmotor symptoms scores were observed. R1441G carriers scored higher in motor scores, had lower radioligand uptake, and had higher scores in depression, dysautonomia, and Rapid Eye Movements Sleep Behavior Disorder Screening Questionnaire scores, but had better cognition scores than noncarriers. Among G2019S carriers, a group with "mild motor signs" was identified, and was significantly older, with worse olfaction and lower radioligand uptake.

CONCLUSIONS

G2019S and R1441G carriers differ from their noncarriers' relatives in higher motor scores and slightly lower radioligand uptake. Nonmotor symptoms were mild, and different nonmotor profiles were observed in G2019S carriers compared to R1441G carriers. A group of G2019S carriers with known prodromal features was identified. Longitudinal studies are required to determine whether such individuals are at short-term risk of developing overt parkinsonism. © 2017 International Parkinson and Movement Disorder Society.

Experience-dependent modulation of alpha and beta during action observation and motor imagery

Background

EEG studies investigating the neural networks that facilitate action observation (AO) and kinaesthetic motor imagery (KMI) have shown reduced, or desynchronized, power in the alpha (8–12 Hz) and beta (13–30 Hz) frequency bands relative to rest, reflecting efficient activation of task-relevant areas. Functional modulation of these networks through expertise in dance has been established using fMRI, with greater activation among experts during AO. While there is evidence for experience-dependent plasticity of alpha power during AO of dance, the influence of familiarity on beta power during AO, and alpha and beta activity during KMI, remain unclear. The purpose of the present study was to measure the impact of familiarity on confidence ratings and EEG activity during (1) AO of a brief ballet sequence, (2) KMI of this same sequence, and (3) KMI of non-dance movements among ballet dancers, dancers from other genres, and non-dancers.

Results

Ballet dancers highly familiar with the genre of the experimental stimulus demonstrated higher individual alpha peak frequency (iAPF), greater alpha desynchronization, and greater task-related beta power during AO, as well as faster iAPF during KMI of non-dance movements. While no between-group differences in alpha or beta power were observed during KMI of dance or non-dance movements, all participants showed significant desynchronization relative to baseline, and further desynchronization during dance KMI relative to non-dance KMI indicative of greater cognitive load.

Conclusions

These findings confirm and extend evidence for experience-dependent plasticity of alpha and beta activity during AO of dance and KMI. We also provide novel evidence for modulation of iAPF that is faster when tuned to the specific motor repertoire of the observer. By considering the multiple functional roles of these frequency bands during the same task (AO), we have disentangled the compounded contribution of familiarity and expertise to alpha desynchronization for mediating task engagement among familiar ballet dancers and reflecting task difficulty among unfamiliar non-dance subjects, respectively. That KMI of a complex dance sequence relative to everyday, non-dance movements recruits greater cognitive resources suggests it may be a more powerful tool in driving neural plasticity of action networks, especially among the elderly and those with movement disorders.

Nigral and striatal connectivity alterations in asymptomatic LRRK2 mutation carriers: A magnetic resonance imaging study

BACKGROUND

The study of functional connectivity by means of magnetic resonance imaging (MRI) in asymptomatic LRRK2 mutation carriers could contribute to the characterization of the prediagnostic phase of LRRK2-associated Parkinson's disease (PD). The objective of this study was to characterize MRI functional patterns during the resting state in asymptomatic LRRK2 mutation carriers.

METHODS

We acquired structural and functional MRI data of 18 asymptomatic LRRK2 mutation carriers and 18 asymptomatic LRRK2 mutation noncarriers, all first-degree relatives of LRRK2-PD patients. Starting from resting-state data, we analyzed the functional connectivity of the striatocortical and the nigrocortical circuitry. Structural brain data were analyzed by voxel-based morphometry, cortical thickness, and volumetric measures.

RESULTS

Asymptomatic LRRK2 mutation carriers had functional connectivity reductions between the caudal motor part of the left striatum and the ipsilateral precuneus and superior parietal lobe. Connectivity in these regions correlated with subcortical gray-matter volumes in mutation carriers. Asymptomatic carriers also showed increased connectivity between the right substantia nigra and bilateral occipital cortical regions (occipital pole and cuneus bilaterally and right lateral occipital cortex). No intergroup differences in structural MRI measures were found. In LRRK2 mutation carriers, age and functional connectivity correlated negatively with striatal volumes. Additional analyses including only subjects with the G2019S mutation revealed similar findings.

CONCLUSIONS

Asymptomatic LRRK2 mutation carriers showed functional connectivity changes in striatocortical and nigrocortical circuits compared with noncarriers. These findings support the concept that altered brain connectivity precedes the onset of classical motor features in a genetic form of PD. © 2016 International Parkinson and Movement Disorder Society.

A cognitive fMRI study in non-manifesting LRRK2 and GBA carriers

Mutations in the GBA and LRRK2 genes account for one-third of the prevalence of Parkinson's disease (PD) in Ashkenazi Jews. Non-manifesting carriers (NMC) of these mutations represent a population at risk for future development of PD. PD patient who carry mutations in the GBA gene demonstrates more significant cognitive decline compared to idiopathic PD patients. We assessed cognitive domains using fMRI among NMC of both LRRK2 and GBA mutations to better understand pre-motor cognitive functions in these populations. Twenty-one LRRK2-NMC, 10 GBA-NMC, and 22 non-manifesting non-carriers (NMNC) who participated in this study were evaluated using the standard questionnaires and scanned while performing two separate cognitive tasks; a Stroop interference task and an N-Back working memory task. Cerebral activation patterns were assessed using both whole brain and predefined region of interest (ROI) analysis. Subjects were well matched in all demographic and clinical characteristics. On the Stroop task, in spite of similar behavior, GBA-NMC demonstrated increased task-related activity in the right medial frontal gyrus and reduced task-related activity in the left lingual gyrus compared to both LRRK2-NMC and NMNC. In addition, GBA-NMC had higher activation patterns in the incongruent task compared to NMNC in the left medial frontal gyrus and bilateral precentral gyrus. No whole-brain differences were noted between groups on the N-Back task. Paired cognitive and task-related performance between GBA-NMC, LRRK2-NMC, and NMNC could indicate that the higher activation patterns in the incongruent Stroop condition among GBA-NMC compared to LRRK2-NMC and NMNC may represent a compensatory mechanism that enables adequate cognitive performance.

Lateral occipitotemporal cortex (LOTC) activity is greatest while viewing dance compared to visualization and movement: learning and expertise effects

The lateral occipitotemporal cortex (LOTC) is comprised of subregions selectively activated by images of human bodies (extrastriate body area, EBA), objects (lateral occipital complex, LO), and motion (MT+). However, their role in motor imagery and movement processing is unclear, as are the influences of learning and expertise on its recruitment. The purpose of our study was to examine putative changes in LOTC activation during action processing following motor learning of novel choreography in professional ballet dancers. Subjects were scanned with functional magnetic resonance imaging up to four times over 34 weeks and performed four tasks: viewing and visualizing a newly learned ballet dance, visualizing a dance that was not being learned, and movement of the foot. EBA, LO, and MT+ were activated most while viewing dance compared to visualization and movement. Significant increases in activation were observed over time in left LO only during visualization of the unlearned dance, and all subregions were activated bilaterally during the viewing task after 34 weeks of performance, suggesting learning-induced plasticity. Finally, we provide novel evidence for modulation of EBA with dance experience during the motor task, with significant activation elicited in a comparison group of novice dancers only. These results provide a composite of LOTC activation during action processing of newly learned ballet choreography and movement of the foot. The role of these areas is confirmed as primarily subserving observation of complex sequences of whole-body movement, with new evidence for modification by experience and over the course of real world ballet learning.

Intact working memory in non-manifesting LRRK2 carriers--an fMRI study

Cognitive impairments are prevalent in patients with Parkinson's disease. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common cause of genetic Parkinsonism. Non-manifesting carriers of the G2019S mutation in the LRRK2 gene were found to have lower executive functions as measured by the Stroop task. This exploratory study aimed to assess whether the cognitive impairment in non-manifesting carriers is specific for executive functions or includes other cognitive domains such as working memory. We recruited 77 non-manifesting first-degree relatives of Parkinson's disease patients (38 carriers). A block-design fMRI N-back task, with 0-back, 2-back and 3-back conditions, was used in order to assess working memory. Participants were well matched on the Montreal Cognitive Assessment, University of Pennsylvania Smell Identification Test, Unified Parkinson's Disease Rating Scale part III, digit span, age, gender and Beck Depression Inventory. The task achieved the overall expected effect in both groups with longer reaction times and lower accuracy rates with increasing task demands. However, no whole-brain or region-of-interest between-groups differences were found on any of the task conditions. These results indicate that non-manifesting carriers of the G2019S mutation in the LRRK2 gene have a specific cognitive profile with executive functions, as assessed by the Stroop task, demonstrating significant impairment but with working memory, as assessed with the N-back task, remaining relatively intact. These finding shed light on the pre-motor cognitive changes in this unique 'at risk' population and should enable more focused cognitive assessments of these cohorts.

Nonmotor symptoms in healthy Ashkenazi Jewish carriers of the G2019S mutation in the LRRK2 gene

BACKGROUND

The asymptomatic carriers of the Leucine rich repeat kinase 2 (LRRK2) G2019S mutation represent a population at risk for developing PD. The aim of this study was to assess differences in nonmotor symptoms between nonmanifesting carriers and noncarriers of the G2019S mutation.

METHODS

Two hundred fifty-three subjects participated in this observational cross-sectional multicenter study. Standard questionnaires assessing anxiety, depression, cognition, smell, nonmotor symptoms, and rapid eye movement (REM) sleep behavior were administered. Analyses were adjusted for age, sex, family relations, education, and site.

RESULTS

One hundred thirty-four carriers were identified. Carriers had higher nonmotor symptoms score on the Nonmotor symptoms (NMS) questionnaire (P = 0.02). These findings were amplified in carriers older than age 50 y, with higher nonmotor symptoms scores and trait anxiety scores (P < 0.03).

CONCLUSIONS

In this cross-section study, carriers of the G2019S LRRK2 mutation endorsed subtle nonmotor symptoms. Whether these are early features of PD will require a longitudinal study. © 2015 International Parkinson and Movement Disorder Society.

Transcranial magnetic stimulation follow-up study in early Parkinson's disease: A decline in compensation with disease progression?

BACKGROUND

A number of neurophysiological abnormalities have been described in patients with Parkinson's disease, but very few longitudinal studies of how these change with disease progression have been reported. We describe measures of motor cortex inhibition and plasticity at 6 and 12 mo in 12 patients that we previously reported at initial diagnosis. Given the well-known interindividual variation in these measures, we were particularly concerned with the within-subject changes over time.

METHODS

Patients were assessed clinically, and transcranial magnetic stimulation (TMS) was used to measure motor cortical excitability, inhibition (short interval intracortical inhibition, cortical silent period), and plasticity (response to excitatory paired associative stimulation protocol) in both hemispheres. All measurements were performed 6 mo and 12 mo after the baseline experiments.

RESULTS

Asymmetry in clinical motor symptoms was reflected in asymmetry of plasticity and inhibition. In the group as a whole, little change was seen in any of the parameters over 12 mo. However, analysis of within-individual data showed clear correlations between changes in clinical asymmetry and asymmetry of response to paired associative stimulation protocol and cortical silent period.

CONCLUSIONS

Longitudinal changes in cortical silent period and response to paired associative stimulation protocol in Parkinson's disease reflect dynamic effects on motor cortex that are related to progression of motor signs. They are useful objective markers of early disease progression that could be used to detect effects of disease-modifying therapies. The decline in heightened plasticity that was present at disease onset may reflect failure of compensatory mechanisms that maintained function in the preclinical state.

Reorganization of corticostriatal circuits in healthy G2019S LRRK2 carriers

OBJECTIVE

We investigated system-level corticostriatal changes in a human model of premotor Parkinson disease (PD), i.e., healthy carriers of the G2019S LRRK2 mutation that is associated with a markedly increased, age-dependent risk of developing PD.

METHODS

We compared 37 asymptomatic LRRK2 G2019S mutation carriers (age range 30-78 years) with 32 matched, asymptomatic nonmutation carriers (age range 30-74 years). Using fMRI, we tested the hypothesis that corticostriatal connectivity in premotor PD shifts from severely affected to less affected striatal subregions, as shown previously in symptomatic PD. Specifically, we predicted that in premotor PD, the shift in corticostriatal connectivity would follow the same gradient of striatal dopamine depletion known from overt PD, with the dorsoposterior putamen being more affected than the ventroanterior putamen.

RESULTS

The known parallel topology of corticostriatal loops was preserved in each group, but the topography of putamen connectivity shifted. In LRRK2 G2019S mutation carriers, the right inferior parietal cortex had reduced functional connectivity with the dorsoposterior putamen but increased connectivity with the ventroanterior putamen, as compared with noncarriers. This shift in functional connectivity increased with age in LRRK2 G2019S mutation carriers.

CONCLUSIONS

Asymptomatic LRRK2 G2019S mutation carriers show a reorganization of corticostriatal circuits that mirrors findings in idiopathic PD. These changes may reflect premotor basal ganglia dysfunction or circuit-level compensatory changes.

Neuropsychological performance in LRRK2 G2019S carriers with Parkinson's disease

BACKGROUND

Ashkenazi Jewish (AJ) LRRK2 carriers are more likely to manifest the postural instability gait difficulty (PIGD) motor phenotype than non-carriers but perform similarly to non-carriers on cognitive screening tests.

OBJECTIVE

To compare the cognitive profiles of AJ with Parkinson's disease (PD) with and without LRRK2 G2019S mutations using a comprehensive neuropsychological battery.

METHODS

We administered a neuropsychological battery to PD participants in the Michael J. Fox Foundation AJ consortium. Participants (n = 236) from Beth Israel Medical Center, NY, Columbia University Medical Center, NY and Tel Aviv Medical Center, Israel included 116 LRRK2 G2019S carriers and 120 non-carriers. Glucocerbrosidase mutation carriers were excluded. We compared performance on each neuropsychological test between carriers and non-carriers. Participants in New York (n = 112) were evaluated with the entire battery. Tel Aviv participants (n = 124) were evaluated on attention, executive function and psychomotor speed tasks. The association between G2019S mutation status (predictor) and each neuropsychological test (outcome) was assessed using linear regression models adjusted for PIGD motor phenotype, site, sex, age, disease duration, education, Unified Parkinson's Disease Rating Scale (UPDRS) Part III, levodopa equivalent dose, and Geriatric Depression Score (GDS).

RESULTS

Carriers had longer disease duration (p < 0.001) and were more likely to manifest the PIGD phenotype (p = 0.024). In adjusted regression models, carriers performed better than non-carriers in Stroop Word Reading (p < 0.001), Stroop Interference (p = 0.011) and Category Fluency (p = 0.026).

CONCLUSION

In AJ-PD, G2019S mutation status is associated with better attention (Stroop Word Reading), executive function (Stroop Interference) and language (Category Fluency) after adjustment for PIGD motor phenotype.

Emerging therapies for gait disability and balance impairment: promises and pitfalls

Therapeutic management of gait and balance impairment during aging and neurodegeneration has long been a neglected topic. This has changed considerably during recent years, for several reasons: (1) an increasing recognition that gait and balance deficits are among the most relevant determinants of an impaired quality of life and increased mortality for affected individuals; (2) the arrival of new technology, which has allowed for new insights into the anatomy and functional (dis)integrity of gait and balance circuits; and (3) based in part on these improved insights, the development of new, more specific treatment strategies in the field of pharmacotherapy, deep brain surgery, and physiotherapy. The initial experience with these emerging treatments is encouraging, although much work remains to be done. The objective of this narrative review is to discuss several promising developments in the field of gait and balance treatment. We also address several pitfalls that can potentially hinder a fast and efficient continuation of this vital progress. Important issues that should be considered in future research include a clear differentiation between gait and balance as two distinctive targets for treatment and recognition of compensatory mechanisms as a separate target for therapeutic intervention.

A voxel-based morphometry and diffusion tensor imaging analysis of asymptomatic Parkinson's disease-related G2019S LRRK2 mutation carriers

BACKGROUND

Patients with Parkinson's disease have reduced gray matter volume and fractional anisotropy in both cortical and sub-cortical structures, yet changes in the pre-motor phase of the disease are unknown.

METHODS

A comprehensive imaging study using voxel-based morphometry and diffusion tensor imaging tract-based spatial statistics analysis was performed on 64 Ashkenazi Jewish asymptomatic first degree relatives of patients with Parkinson's disease (30 mutation carriers), who carry the G2019S mutation in the leucine-rich repeat kinase 2 (LRRK2) gene.

RESULTS

No between-group differences in gray matter volume could be noted in either whole-brain or volume-of-interest analysis. Diffusion tensor imaging analysis did not identify group differences in white matter areas, and volume-of-interest analysis identified no differences in diffusivity parameters in Parkinson's disease-related structures.

CONCLUSIONS

G2019S carriers do not manifest changes in gray matter volume or diffusivity parameters in Parkinson's disease-related structures prior to the appearance of motor symptoms.

Parkinson disease phenotype in Ashkenazi Jews with and without LRRK2 G2019S mutations

The phenotype of Parkinson's disease (PD) in patients with and without leucine-rich repeat kinase 2 (LRRK2) G2019S mutations reportedly is similar; however, large, uniformly evaluated series are lacking. The objective of this study was to characterize the clinical phenotype of Ashkenazi Jewish (AJ) PD carriers of the LRRK2 G2019S mutation. We studied 553 AJ PD patients, including 65 patients who were previously reported, from three sites (two in New York and one in Tel-Aviv). Glucocerebrosidase (GBA) mutation carriers were excluded. Evaluations included the Montreal Cognitive Assessment (MoCA), the Unified Parkinson's Disease Rating Scale (UPDRS), the Geriatric Depression Scale (GDS) and the Non-Motor Symptoms (NMS) questionnaire. Regression models were constructed to test the association between clinical and demographic features and LRRK2 status (outcome) in 488 newly recruited participants. LRRK2 G2019S carriers (n = 97) and non-carriers (n = 391) were similar in age and age at onset of PD. Carriers had longer disease duration (8.6 years vs. 6.1 years; P < 0.001), were more likely to be women (51.5% vs. 37.9%; P = 0.015), and more often reported first symptoms in the lower extremities (40.0% vs. 19.2%; P < 0.001). In logistic models that were adjusted for age, disease duration, sex, education, and site, carriers were more likely to have lower extremity onset (P < 0.001), postural instability and gait difficulty (PIGD) (P = 0.043), and a persistent levodopa response for >5 years (P = 0.042). Performance on the UPDRS, MoCA, GDS, and NMS did not differ by mutation status. PD in AJ LRRK2 G2019S mutation carriers is similar to idiopathic PD but is characterized by more frequent lower extremity involvement at onset and PIGD without the associated cognitive impairment.