The Association between LRRK2 G2385R and Phenotype of Parkinson's Disease in Asian Population: A Meta-Analysis of Comparative Studies

Numerous studies have investigated the relationship between the LRRK2 G2385R variant and clinical characteristics in Parkinson's disease (PD), but the results have been inconsistent. This study investigated whether the LRRK2 G2385R variant was associated with a unique clinical phenotype of PD in the Asian population, using a meta-analysis. The PubMed, Web of Science, EMBASE, CNKI, and WANFANG databases were searched until September 2017. The strict selection criteria and exclusion criteria were determined, and mean differences (MD) or odds ratios (OR) with 95% confidence intervals (CI) were used to assess the strength of associations. Statistical analyses and graphics were performed using Review Manager 5.3. Sixteen related case-control studies were included in the meta-analysis. The LRRK2 G2385R carriers significantly more often presented a family history (OR: 1.98; 95% CI: 1.16−3.39; P=0.01) and had a longer disease duration (MD = 0.47, 95% CI: 0.01−0.93, P=0.04) and a higher MMSE score (MD = 1.02, 95% CI: 0.43–1.62 P=0.0007) than LRRK2 G2385R noncarriers. There were no significant differences in sex distribution, age at onset, initial symptoms, motor symptoms, depression, levodopa-equivalent dose, and related complications between LRRK2 G2385R-carrier and LRRK2 G2385R-noncarrier PD patients. Our results suggested that most of the clinical characteristics of PD patients with LRRK2 G2385R mutations are similar to those of LRRK2 G2385R noncarriers among Asian PD patients, except for the more common family history, relatively longer disease duration, and higher MMSE scores in the former group.


Introduction
Parkinson's disease (PD) is the second most common neurodegenerative disorder. PD is defined by the presence of bradykinesia and at least one of the following symptoms: muscular rigidity, rest tremor, or postural instability. It is characterized by the cardinal motor symptoms of resting tremor, rigidity, bradykinesia, and postural instability, and a variety of nonmotor symptoms, such as olfactory dysfunction, psychiatric disorders, autonomic disturbances, and cognitive decline, among others. Although the etiology of PD has not been fully elucidated, it is thought to be the result of the interaction between genetic and environmental factors [1].
A number of candidate genes involved in PD etiology have been identified. Among them, mutations in the leucine-rich repeat kinase 2 gene (LRRK2) have been consistently reported to be the most frequent known cause of both sporadic and familial PD [2]. In particular, the G2385R polymorphism in LRRK2 is an important genetic risk factor for PD in Asian individuals, as evidenced by various independent studies.
In recent years, genotype-phenotype correlation studies have suggested that PD patients with the LRRK2 G2385R variant may exhibit some unique clinical characteristics, although some results were contradictory. Available studies have found that LRRK2 G2385R carriers had a higher frequency of family history [3], longer disease duration [4], a lower age at onset [5], a higher proportion of postural instability and gait disorder (PIGD) phenotype [6], and a higher Mini-Mental State Examination (MMSE) score than PD patients who do not carry the LRRK2 G2385R [6]. Moreover, a higher levodopa-equivalent dose (LED) and a higher proportion of levodopa-induced complications, including motor fluctuations and dyskinesia, were also observed in PD patients with the LRRK2 G2385R variant in some studies [6,7]. However, several other findings indicated that neither demographic data nor clinical presentation differed significantly between LRRK2 G2385R carriers and noncarriers [8][9][10].
Taken together, there is no consensus about the relationship between LRRK2 G2385R and clinical manifestations of PD patients. Here, we performed a meta-analysis to investigate whether LRRK2 G2385R was associated with the clinical presentation of PD.

Methods
e literature search strategies, inclusion and exclusion criteria, outcome measurements, and methods of statistical analysis were completed according to the Preferred Reporting Items for Systematic Reviews and Meta-analysis and Meta-analysis of Observational Studies in Epidemiology recommendations for study reporting [11][12][13]. All analyses were based on previously published studies; thus, ethical approval and patient consent were not required.

Inclusion and Exclusion
Criteria. Studies were selected when they met all the following criteria and the following studies were included if they met any of the following criteria: (1) observational studies, such as case-control studies and cohort studies; (2) comparative studies, which had to include an LRRK2 G2385R-carrier PD group and an LRRK2 G2385R-noncarrier PD group; (3) studies of association between specific clinical features (motor symptoms and nonmotor symptoms) and the LRRK2 G2385R mutation; and (4) the diagnosis of PD had to be made according to the United Kingdom Parkinson Disease Society Brain Bank criteria. e following studies were excluded: (1) review articles, case reports, editorials; (2) duplicated reports (when multiple reports studying the same participants were published, the latest or most complete report was included); (3) studies with incomplete data, which included studies that did not compare LRRK2 G2385R-carrier and LRRK2 G2385R-noncarrier PD patients; and (4) functional studies, such as animal experiments and cell experiments studies.

Data Extraction and Outcomes of Interest.
Data from the included studies were extracted and summarized independently by two authors (Wei Di and Zhi-yong Zeng). Any disagreements were resolved by the senior authors (Jing-yan Li and Hua Lv). e primary outcomes were the comparison of clinical characteristics including motor symptoms and nonmotor symptoms in the two study groups (LRRK2 G2385R carriers and noncarriers).

Quality Assessment and Statistical Analysis.
e quality of selected studies was evaluated using the Newcastle−Ottawa Quality Assessment Scale (NOS) [14], which included three factors: patient selection, comparability of the study groups, and assessment of outcome. A score of 0-9 was allocated to each study. Studies that achieved six or more points were considered to be of high quality. e assessments were conducted by two authors (Wei Di and Zhi-yong Zeng).
All statistical analyses and graphics were generated using Review Manager 5.3 (Cochrane Collaboration, Oxford, England). e weighted mean difference (MD) and odds ratios (OR) were used to compare continuous and dichotomous variables, respectively. All results were reported with 95% confidence intervals (CIs). Statistical heterogeneity between studies was assessed using the chi-square test with significance set at P ≤ 0.10, and heterogeneity was quantified using the I 2 statistic. e random effect model was used if there was heterogeneity between studies; alternatively, the fixed effect model was used [15]. Z tests were conducted to assess the association between the LRRK2 G2385R variant and clinical characteristics. P values < 0.05 indicated statistically significant differences. Sensitivity analysis was performed in each comparison. Funnel plot analyses were used to screen for potential publication bias.
e detailed flow chart of study selection and reasons for exclusion are shown in Figure 1. All publications were full-text articles. Agreement between the two reviewers was 97% for study selection and 94% for quality assessment of trials. e detailed characteristics of all included studies are summarized in Table 1. All studies were performed on Asian individuals. e NOS score of all the included studies were no less than 6, indicating that none of the included studies were of low quality.

Family
History. PD patients with family history are defined as having at least one first-or second-degree relative with a diagnosis of PD. Five studies assessed the relationship between LRRK2 G2385R status and family history of PD, and the data showed that a family history of PD was significantly more common in the LRRK2 G2385R carriers than in the LRRK2 G2385R noncarriers (OR: 1.98; 95% CI: 1.16−3.39; P � 0.01) (Supplementary Figure 1).

Sex Distribution.
Twelve studies included a sex distribution analysis between LRRK2 G2385R-carrier PD group and LRRK2 G2385R-noncarrier PD group.

Disease Duration.
ere were seven studies that mentioned the difference in disease duration between LRRK2 G2385R-carrier PD patients and LRRK2 G2385Rnoncarrier PD patients. e average disease duration in carriers of the LRRK2 G2385R was slightly longer than that in noncarriers (MD � 0.47, 95% CI: 0.01−0.93, P � 0.04, Supplementary Figure 3).

Nonmotor Symptoms.
Only MMSE scores and the proportion of patients with depression were assessed owing to incomplete data about other nonmotor symptoms. ree studies included an MMSE score analysis in both groups and showed a significantly higher MMSE score in the LRRK2 G2385R-carrier PD group than in the LRRK2 G2385R-noncarrier PD group (MD � 1.02, 95% CI: 0.43-1.62, P � 0.0007, Supplementary Figure 8A). Two studies included a depression analysis, but there was no significant association between depression and LRRK2 G2385R status (OR � 1.46, 95% CI: 0.90−2.37, P � 0.13, Supplementary Figure 8B).

Levodopa erapy and Related Complications.
ere were some studies that mentioned the LED and related complications, including motor fluctuation and dyskinesia. e meta-analysis results of these studies showed no significant differences for LED, motor fluctuation, or dyskinesia between the LRRK2 G2385R-carrier PD group and the LRRK2 G2385R-noncarrier PD group (LED: MD � 0.14, 95% CI: −0.06 to 0.33, P � 0.17, Supplementary Figure 9A Taken together, the LRRK2 G2385R-carrier PD group more often presented a family history of PD and had a longer disease duration and a higher MMSE score than the LRRK2 G2385R-noncarrier PD group. However, LRRK2 G2385R carriers showed no significant differences in sex distribution, AAO, initial symptoms, motor symptoms and grade of severity, depression, or LED and levodopa-related complications compared to LRRK2 G2385R noncarriers. e results of the meta-analysis are summarized in Table 2.

Sensitivity Analysis and Publication Bias.
A sensitivity analysis was made in comparison with significant heterogeneity among the studies. When the heterogeneity among the studies was significant, we performed sensitivity analyses by excluding the relatively low-quality studies; the same results were obtained. erefore, we can conclude that the sensitivity is low and that the results are reliable. As there were fewer than 10 high-quality studies in most comparisons in our meta-analysis, an accurate publication bias assessment could not be performed. However, we completed a funnel plot to estimate publication bias. ere were no asymmetries in the funnel plot and no significant publication biases of the meta-analysis.

Discussion
In this meta-analysis, we extensively analyzed the relationship between LRRK2 G2385R carrier status and clinical manifestations of PD in an Asian population. e LRRK2 G2385Rcarrier PD group significantly more often had a family history of PD than did the LRRK2 G2385R-noncarrier PD group. e average disease duration in the LRRK2 G2385R-carrier PD group was slightly longer than that in the LRRK2 G2385Rnoncarrier PD group. e mean MMSE score was significantly lower in the LRRK2 G2385R-noncarrier PD group than in the LRRK2 G2385R-carrier PD group. Additionally, we observed that there were no significant differences in most of the clinical characteristics, including sex distribution, AAO, initial symptoms, motor symptoms and grade of severity, depression, and LED and levodopa-related complications between LRRK2 G2385R-carrier and LRRK2 G2385R-noncarrier PD patients. ese results indicated that PD patients carrying a LRRK2 G2385R mutation is associated with a significantly overlapping phenotype when compared with idiopathic PD. is finding is in line with those of previous studies reporting that G2019S carrier patients exhibit clinical features quite similar to those of noncarriers, with some mild differences [24][25][26]. One possible reason for this was that a broad spectrum of clinical characteristics may be caused by gene-gene interaction or gene-environment interactions, affecting the genotype and progression of the disease.
Nonmotor symptoms in PD are quite common and contribute to the patient's disability [27]. Associations between the genotype and nonmotor symptoms in PD patients have attracted research attention in recent years. To date, a few studies have focused mainly on several specific nonmotor symptoms, rather than on the general nonmotor symptom profile. erefore, we failed to find sufficient data to assess the nonmotor symptoms of PD patients in the context of LRRK2 G2385R status. For MMSE scores, given that only three published papers were included in the meta-analysis, a publication bias is likely to be present. A detailed clinical characterization of PD patients carrying LRRK2 variants is warranted.
e NOS scores of all included publications were rated from 6 points to 9 points, providing a reliable bottom for the current analysis. No statistically significant publication bias or heterogeneity between different studies was detected in our meta-analysis. However, several limitations may have influenced the precision of our results. First, given that the number of included studies was limited, publication bias could potentially occur, even though we used various searching approaches and statistical analyses to minimize the publication bias. Second, although this meta-analysis included all the published case-control cohorts, some unpublished results may have been neglected.

Conclusion
In conclusion, our study suggested that most of the clinical characteristics of PD patients with the LRRK2 G2385R mutation were similar to those of LRRK2 G2385R noncarriers among Asian individuals, except for a more common family history, relatively longer disease duration, and higher MMSE scores in the LRRK2 G2385R carriers. ese findings strengthen our understanding of the clinical and genetic heterogeneity of PD and may have implications for diagnosis and therapy of PD. However, further larger samples and multicenter cooperative studies in different populations are warranted to clarify the relevance between the LRRK2 G2385R genotype and clinical phenotype of PD.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.