Task-dependent interactions between dopamine D2 receptor polymorphisms and L-DOPA in patients with Parkinson's disease

The correlation between genetic factors and freezing of gait in patients with Parkinson's disease

BACKGROUND

Clinical-related risk factors to freezing of gait (FOG) in Parkinson's disease (PD) have been identified. Still, the influence of genetic variations on the FOG occurrence has been poorly studied thus far.

AIM

We aimed to evaluate the association of six selected polymorphisms of DRD2, ANKK1, and COMT genes with the FOG occurrence and explore the influence of ANNK1/DRD2 haplotypes on the onset of FOG in the group of PD patients.

METHOD

PD patients (n = 234), treated with levodopa for at least two years, were genotyped for the rs4680 in COMT, rs6277, rs1076560, and rs2283265 in DRD2, and rs1800497 and rs2734849 polymorphisms in ANKK1 genes. FOG was evaluated by posing a direct question. In addition, a comprehensive set of clinical scales was applied to all patients.

RESULTS

FOG occurred in 132 (56.4%) PD patients in our cohort. Freezers were younger at PD onset, had longer disease duration, used higher levodopa daily doses and dopaminergic agents, and had higher motor and non-motor scales scores than non-freezers. FOG was more frequent among AA rs4680 COMT carriers than AG and GG rs4680 COMT carriers. Independent predictors of FOG were: disease duration of more than ten years, levodopa daily dose higher than 500 mg/day, motor status, and COMT AA genotype. AGGAA and GGAAA haplotypes were revealed as protective and vulnerability factors for FOG occurrence.

CONCLUSION

In addition to previously identified disease- and therapy-related risk factors, our results suggested a possible contribution of dopamine-related genes to the FOG occurrence.

Clustering of Parkinson subtypes reveals strong influence of DRD2 polymorphism and gender

Most classification approaches for idiopathic Parkinson’s disease subtypes primarily focus on motor and non-motor symptoms. Besides these characteristics, other features, including gender or genetic polymorphism of dopamine receptors are potential factors influencing the disease’s phenotype. By utilizing a kmeans-clustering algorithm we were able to identify three subgroups mainly characterized by gender, DRD2 Taq1A (rs1800497) polymorphism—associated with changes in dopamine signaling in the brain—and disease progression. A subsequent regression analysis of these subgroups further suggests an influence of their characteristics on the daily levodopa dosage, an indicator for medication response. These findings could promote further enhancements in individualized therapies for idiopathic Parkinson’s disease.

Polymorphisms of Dopamine Receptor Genes and Parkinson's Disease: Clinical Relevance and Future Perspectives

Parkinson’s disease (PD) is a neurodegenerative disease caused by loss of dopaminergic neurons in the midbrain. PD is clinically characterized by a variety of motor and nonmotor symptoms, and treatment relies on dopaminergic replacement. Beyond a common pathological hallmark, PD patients may present differences in both clinical progression and response to drug therapy that are partly affected by genetic factors. Despite extensive knowledge on genetic variability of dopaminergic receptors (DR), few studies have addressed their relevance as possible influencers of clinical heterogeneity in PD patients. In this review, we summarized available evidence regarding the role of genetic polymorphisms in DR as possible determinants of PD development, progression and treatment response. Moreover, we examined the role of DR in the modulation of peripheral immunity, in light of the emerging role of the peripheral immune system in PD pathophysiology. A better understanding of all these aspects represents an important step towards the development of precise and personalized disease-modifying therapies for PD.

Contribution of functional dopamine D2 and D3 receptor variants to motor and non-motor symptoms of early onset Parkinson's disease

In the present study, we focused on investigating the contribution of functional dopamine D2 and D3 receptor variants to motor and/or non-motor symptoms of early onset Parkinson's disease (EOPD). Three functional single nucleotide polymorphisms (SNPs), DRD3 rs6280, DRD2 rs2283265 and DRD2 rs1076560, were genotyped in 128 Turkish EOPD patients and then, statistical analysis was conducted for the potential impacts of SNPs on clinical parameters. All three SNPs were found to be statistically significant in terms of PD-related pain: DRD3 [rs6280; risk allele "T" for pain; p = 0.031; odds ratio (OR)=4.25], DRD2 [rs2283265; risk allele "A" for pain; p = 0.001; OR=8.47] and, DRD2 [rs1076560; risk allele "A" for pain; p = 0.022; OR=4.58]. Additionally, bilateral disease [p = 0.011; OR=5.10] and gender [risk group "female"; p = 0.003; OR=8.53] were also identified as significant univariate risk factors for PD-related pain. Based on logistic regression analysis conducted with the significant univariate risk factors, this the first report to clarify that a female patient with bilateral PD and DRD2 rs2283265 polymorphism has a significant risk for PD-related pain. Our findings might contribute to improve life quality by offering treatment options for pain in PD patients with these clinical and genetic features.

Axonal degeneration in Parkinson's disease - Basal ganglia circuitry and D2 receptor availability

Basal ganglia (BG) circuitry plays a crucial role in the control of movement. Degeneration of its pathways and imbalance of dopaminergic signalling goes along with movement disorders such as Parkinson's disease. In this study, we explore the interaction of degeneration in two BG pathways (the nigro-striatal and dentato-pallidal pathway) with D2 receptor signalling to elucidate an association to motor impairment and medication response. Included in the study were 24 parkinsonian patients [male, 62 years (± 9.3 SD)] compared to 24 healthy controls [male, 63 years (± 10.2 SD)]; each participant passed through three phases of the study (i) acquisition of metadata/clinical testing, (ii) genotyping and (iii) anatomical/diffusion MRI. We report a decline in nigro-striatal (p < .003) and dentato-pallidal (p < .0001) connectivity in the patients compared to controls, which is associated with increasing motor impairment (relating to nigro-striatal, r = -0.48; p < .001 and dentato-pallidal connectivity, r = -0.36; p = .035). Given, that variations of the ANKK1 Taq1 (rs 1,800,497) allele alters dopamine D2-dependent responses, all participants were genotyped respectively. By grouping patients (and controls) according to their ANKK1 genotype, we demonstrate a link between D2 receptor signalling and decline in connectivity in both investigated pathways for the A1- variant (nigro-striatal pathway: r = -0.53; p = .012, dentato-pallidal pathway: r = -0.62; p = .0012). In patients with the A1+ variant, we only found increased brain connectivity in the dentato-pallidal pathway (r = 0.71; p = .001) correlating with increasing motor impairment, suggesting a potentially compensatory function of the cerebellum. Related to medication response carriers of the A1+ variant had a better drug effect associated with stronger brain connectivity in the nigro-striatal pathway (r = 0.54; p < .02); the A1- group had a good medication response although nigro-striatal connectivity was diminished (r = -0.38; p < .05); these results underscore differences in receptor availability between both groups in the nigro-striatal pathway. No effect onto medication response was found in the dentato-pallidal pathway (p > .05). Interplay between basal ganglia connectivity and D2 receptor availability influence the clinical presentation and medication response of parkinsonian patients. Furthermore, while current models of basal-ganglia function emphasize that balanced activity in the direct and indirect pathways is required for normal movement, our data highlight a role of the cerebellum in compensating for physiological imbalances in this respect.

Dopaminergic polymorphisms associated with medication responsiveness of gait in Parkinson's disease

BACKGROUND

Gait dysfunction is a common symptom of Parkinson's disease that can cause significant disability and put patients at risk for falls. These symptoms show variable responsiveness to dopaminergic therapy.

OBJECTIVE

To determine whether dopaminergic (rs1076560 DRD2 G > T and rs4680 catechole-o-methyltranspherase (COMT) Val158Met) or brain derived neurotrophic factor (rs6265 BDNF Val66Met) genetic polymorphisms are associated with gait function and medication responsiveness in Parkinson's disease.

METHOD

Gait function was evaluated on two days for patients (ON and OFF medication in a counterbalanced fashion) and a single session for controls. Investigators were blinded to genotype during data collection. Associations between genotype and medication responsiveness were analyzed using mixed model ANOVAs. A priori hypotheses were tested using GAITRite^® electronic mat spatiotemporal gait parameters including step length, step width, velocity, portion of double and single support per gait cycle, and variability of these measures ON and OFF medication.

RESULTS

We found that the DRD2 polymorphism, but neither COMT nor BDNF, was consistently associated with gait function and medication responsiveness in the patients. Specifically, Parkinson's disease patients with reduced striatal D2 expression (DRD2 T allele carriers) had worse gait dysfunction and showed greater dopamine responsiveness of gait function compared to patients who were homozygous for the G allele. There was no effect of any of the genetic polymorphisms on gait for controls.

CONCLUSIONS AND RELEVANCE

The findings suggest that genetic subgrouping, in particular for DRD2, may be used to identify Parkinson's disease patient subgroups that are more dopamine responsive for gait function.

Genetic Variation and Neuroplasticity: Role in Rehabilitation After Stroke

BACKGROUND AND PURPOSE

In many neurologic diagnoses, significant interindividual variability exists in the outcomes of rehabilitation. One factor that may impact response to rehabilitation interventions is genetic variation. Genetic variation refers to the presence of differences in the DNA sequence among individuals in a population. Genetic polymorphisms are variations that occur relatively commonly and, while not disease-causing, can impact the function of biological systems. The purpose of this article is to describe genetic polymorphisms that may impact neuroplasticity, motor learning, and recovery after stroke.

SUMMARY OF KEY POINTS

Genetic polymorphisms for brain-derived neurotrophic factor (BDNF), dopamine, and apolipoprotein E have been shown to impact neuroplasticity and motor learning. Rehabilitation interventions that rely on the molecular and cellular pathways of these factors may be impacted by the presence of the polymorphism. For example, it has been hypothesized that individuals with the BDNF polymorphism may show a decreased response to neuroplasticity-based interventions, decreased rate of learning, and overall less recovery after stroke. However, research to date has been limited and additional work is needed to fully understand the role of genetic variation in learning and recovery.

RECOMMENDATIONS FOR CLINICAL PRACTICE

Genetic polymorphisms should be considered as possible predictors or covariates in studies that investigate neuroplasticity, motor learning, or motor recovery after stroke. Future predictive models of stroke recovery will likely include a combination of genetic factors and other traditional factors (eg, age, lesion type, corticospinal tract integrity) to determine an individual's expected response to a specific rehabilitation intervention.

Advances in understanding genomic markers and pharmacogenetics of Parkinson's disease

INTRODUCTION

The inheritance pattern of Parkinson's disease (PD) is likely multifactorial (owing to the interplay of genetic predisposition and environmental factors). Many pharmacogenetic studies have tried to establish a possible role of candidate genes in PD risk. Several studies have focused on the influence of genes in the response to antiparkinsonian drugs and in the risk of developing side-effects of these drugs.

AREAS COVERED

This review presents an overview of current knowledge, with particular emphasis on the most recent advances, both in case-control association studies on the role of candidate genes in the risk for PD as well as pharmacogenetic studies on the role of genes in the development of side effects of antiparkinsonian drugs. The most reliable results should be derived from meta-analyses of case-control association studies on candidate genes involving large series of PD patients and controls, and from genome-wide association studies (GWAS).

EXPERT OPINION

Prospective studies of large samples involving several genes with a detailed history of exposure to environmental factors in the same cohort of subjects, should be useful to clarify the role of genes in the risk for PD. The results of studies on the role of genes in the development of side-effects of antiparkinsonian drugs should, at this stage, only be considered preliminary.

Acute effects of cocaine and cannabis on reversal learning as a function of COMT and DRD2 genotype

RATIONALE

Long-term cannabis and cocaine use has been associated with impairments in reversal learning. However, how acute cannabis and cocaine administration affect reversal learning in humans is not known.

OBJECTIVE

In this study, we aimed to establish the acute effects of administration of cannabis and cocaine on valence-dependent reversal learning as a function of DRD2 Taq1A (rs1800497) and COMT Val108/158Met (rs4680) genotype.

METHODS

A double-blind placebo-controlled randomized 3-way crossover design was used. Sixty-one regular poly-drug users completed a deterministic reversal learning task under the influence of cocaine, cannabis, and placebo that enabled assessment of both reward- and punishment-based reversal learning.

RESULTS

Proportion correct on the reversal learning task was increased by cocaine, but decreased by cannabis. Effects of cocaine depended on the DRD2 genotype, as increases in proportion correct were seen only in the A1 carriers, and not in the A2/A2 homozygotes. COMT genotype did not modulate drug-induced effects on reversal learning.

CONCLUSIONS

These data indicate that acute administration of cannabis and cocaine has opposite effects on reversal learning. The effects of cocaine, but not cannabis, depend on interindividual genetic differences in the dopamine D2 receptor gene.

Individual predictors of sensorimotor adaptability

There are large individual variations in strategies and rates of sensorimotor adaptation to spaceflight. This is seen in both the magnitude of performance disruptions when crewmembers are first exposed to microgravity, and in the rate of re-adaptation when they return to Earth's gravitational environment. Understanding the sources of this variation can lead to a better understanding of the processes underlying adaptation, as well as provide insight into potential routes for facilitating performance of "slow adapters". Here we review the literature on brain, behavioral, and genetic predictors of motor learning, recovery of motor function following neural insult, and sensorimotor adaptation. For example, recent studies have identified specific genetic polymorphisms that are associated with faster adaptation on manual joystick tasks and faster recovery of function following a stroke. Moreover, the extent of recruitment of specific brain regions during learning and adaptation has been shown to be predictive of the magnitude of subsequent learning. We close with suggestions for forward work aimed at identifying predictors of spaceflight adaptation success. Identification of "slow adapters" prior to spaceflight exposure would allow for more targeted preflight training and/or provision of booster training and adaptation adjuncts during spaceflight.

Association of COMT val158met and DRD2 G>T genetic polymorphisms with individual differences in motor learning and performance in female young adults

Individuals learn new skills at different rates. Given the involvement of corticostriatal pathways in some types of learning, variations in dopaminergic transmission may contribute to these individual differences. Genetic polymorphisms of the catechol-O-methyltransferase (COMT) enzyme and dopamine receptor D2 (DRD2) genes partially determine cortical and striatal dopamine availability, respectively. Individuals who are homozygous for the COMT methionine (met) allele show reduced cortical COMT enzymatic activity, resulting in increased dopamine levels in the prefrontal cortex as opposed to individuals who are carriers of the valine (val) allele. DRD2 G-allele homozygotes benefit from a higher striatal dopamine level compared with T-allele carriers. We hypothesized that individuals who are homozygous for COMT met and DRD2 G alleles would show higher rates of motor learning. Seventy-two young healthy females (20 ± 1.9 yr) performed a sensorimotor adaptation task and a motor sequence learning task. A nonparametric mixed model ANOVA revealed that the COMT val-val group demonstrated poorer performance in the sequence learning task compared with the met-met group and showed a learning deficit in the visuomotor adaptation task compared with both met-met and val-met groups. The DRD2 TT group showed poorer performance in the sequence learning task compared with the GT group, but there was no difference between DRD2 genotype groups in adaptation rate. Although these results did not entirely come out as one might predict based on the known contribution of corticostriatal pathways to motor sequence learning, they support the role of genetic polymorphisms of COMT val158met (rs4680) and DRD2 G>T (rs 1076560) in explaining individual differences in motor performance and motor learning, dependent on task type.

Dopamine overdose hypothesis: evidence and clinical implications

About a half a century has passed since dopamine was identified as a neurotransmitter, and it has been several decades since it was established that people with Parkinson's disease receive motor symptom relief from oral levodopa. Despite the evidence that levodopa can reduce motor symptoms, there has been a developing body of literature that dopaminergic therapy can improve cognitive functions in some patients but make them worse in others. Over the past two decades, several laboratories have shown that dopaminergic medications can impair the action of intact neural structures and impair the behaviors associated with these structures. In this review, we consider the evidence that has accumulated in the areas of reversal learning, motor sequence learning, and other cognitive tasks. The purported inverted-U shaped relationship between dopamine levels and performance is complex and includes many contributory factors. The regional striatal topography of nigrostriatal denervation is a critical factor, as supported by multimodal neuroimaging studies. A patient's individual genotype will determine the relative baseline position on this inverted-U curve. Dopaminergic pharmacotherapy and individual gene polymorphisms can affect the mesolimbic and prefrontal cortical dopaminergic functions in a comparable, inverted-U dose-response relationship. Depending on these factors, a patient can respond positively or negatively to levodopa when performing reversal learning and motor sequence learning tasks. These tasks may continue to be relevant as our society moves to increased technological demands of a digital world that requires newly learned motor sequences and adaptive behaviors to manage daily life activities.

Striatal denervation pattern predicts levodopa effects on sequence learning in Parkinson's disease

Mild to moderate Parkinson's disease shows more denervation in the posterodorsal striatum and sparing of the anteroventral striatum. Dopaminergic medications can interfere with anteroventral striatum function by overdosing this relatively intact structure. The authors determined how regional striatal denervation affects medication-associated sequence learning impairment in Parkinson's disease. Eighteen Parkinson's patients performed motor sequence learning on and off levodopa. Patients underwent (11)C-dihydrotetrabenazine positron emission tomography scans to measure nigrostriatal denervation. Patients with more preserved putamen were more likely to exhibit levodopa-associated sequence learning impairments. Furthermore, the ratio of denervation in the anterior to posterior dorsal putamen predicted the level of learning differences on and off levodopa. These results demonstrate that the spatial pattern of nigrostriatal dopaminergic denervation predicts medication responsiveness for motor sequence learning.

The effects of clinical motor variables and medication dosage on working memory in Parkinson's disease

In this study, we investigate the interrelationship between clinical variables and working memory (WM) in Parkinson's disease (PD). Specifically, the aim of the study was to investigate the relationship between disease duration, dopaminergic medication dosage, and motor disability (UPDRS score) with WM in individuals with PD. Accordingly, we recruited three groups of subjects: unmedicated PD patients, medicated PD patients, and healthy controls. All subjects were tested on three WM tasks: short-delay WM, long-delay WM, and the n-back task. Further, PD encompasses a spectrum that can be classified either into akinesia/rigidity or resting tremor as the predominant motor presentation of the disease. In addition to studying medication effects, we tested WM performance in tremor-dominant and akinesia-dominant patients. We further correlated WM performance with disease duration and medication dosage. We found no difference between medicated and unmedicated patients in the short-delay WM task, but medicated patients outperformed unmedicated patients in the long-delay WM and n-back tasks. Interestingly, we also found that akinesia-dominant patients were more impaired than tremor-dominant patients at various WM measures, which is in agreement with prior studies of the relationship between akinesia symptom and basal ganglia dysfunction. Moreover, the results show that disease duration inversely correlates with more demanding WM tasks (long-delay WM and n-back tasks), but medication dosage positively correlates with demanding WM performance. In sum, our results show that WM impairment in PD patients depend on cognitive domain (simple vs. demanding WM task), subtype of PD patients (tremor- vs. akinesia-dominant), as well as disease duration and medication dosage. Our results have implications for the interrelationship between motor and cognitive processes in PD, and for understanding the role of cognitive training in treating motor symptoms in PD.