Central China is relatively characterized by the six provinces of Hunan, Hubei, Jiangxi, Henan, Shanxi and Anhui, all largely agricultural. Epidemiological study suggests that living in a rural area, drinking well water, farming and exposure to pesticides may be significant risk factors in the development of PD . Central China is considered to be the cradle of agriculture in China. Importantly, the population density in this region is very high. Previous epidemiological study also showed that the prevalence of PD in South-Central (including Central China area) China is higher compared to other areas of China . In this study, we investigated the association of 3 GBA gene mutations (L444P, N370S and R120W) with central Chinese PD patients. The frequency of the GBA mutations in PD patients was 3.4%. This statistically significant difference in GBA mutations between PD patients and control subjects suggests that the GBA gene is a susceptibility gene in central China and GBA mutation may increase the risk of PD. High frequency of GBA mutations was found in LOPD cases. There was no significant difference due to gender between mutation carriers and non-carriers. Additionally, the average age at onset of PD carriers between carriers and non-carriers was insignificant.
GBA L444P mutation has reportedly been associated with PD in various ethnic groups [7, 9–16, 19, 20, 26]. In a previous study, L444P was identified as the most frequent mutation in non-Ashkenazi Jewish patients . However, there was no significant difference in PD patients and controls in other ethnic groups diagnosed with the L444P mutation (Toft and others 2006; Eblan and others 2006). In this study, the GBA mutations of PD subjects were all heterozygous for L444P, and there was a significant difference between the results of PD patients and controls. Together with the results of two other studies in Southern and Southwestern regions of China [10, 13], these data provide evidence that the L444P mutation confers risk for PD in China. Furthermore, following age-stratification, this study revealed a higher frequency of the GBA L444P mutations in LOPD patients compared with that in the control group. This result is different from that of other studies which showing GBA gene mutations occurred in EOPD rather than LOPD [10, 21]. The discrepancy may be attributed to different genetic backgrounds or environmental factors among others.
The N370S mutation, a common mutation in Jewish and non-Jewish GD patients, is rarely presented in Asian patients . Similar to previous findings, our result showed that N370S mutation was absent in both PD patients and control subjects [13, 21]. A recent study indicated that N370S might affect age of onset and may also contribute to LOPD susceptibility in Chinese Han populations . There were several explanations for the various research results, including the large influx of a Jewish population to China during World War II, and the possible introduction of the gene mutation by traders using the Silk Road in the distant past . It is also conceivable that the N370S mutation may exist only in specific areas of China and/or some other Asian regions. More research will be necessary to address the question of whether the mutations exist in different areas of China.
R120W is considered as the third most common mutation of non-neuropathic GD. A previous Japanese study determined that R120W was prevalent and significantly associated with PD (15/534, P < 0.001) . A Taiwan study found one R120W mutation in PD patients as well . However, the association between PD and the GBA R120W mutation was not significant. No R120W mutation has been found in the Ashkenazi Jewish PD patient population. Our study was the first to explore the association between the R120W mutation and PD in Mainland China. Similar to the case of the Ashkenazi Jews, no R120W mutation was found in our PD patients or controls [11, 15]. Different ethnic groups and environmental factors among different areas may explain the discrepancy.
The L444P mutation, resulting from a T to C transition in nucleotide 6092 of the GBA gene, can make leucine into proline, which leads to the imperfection of glucocerebrosidase. The mechanism of GBA mutations conferring risk for PD is not yet completely understood. First, decrease in glucocerebrosidase activities has a role in the pathogenesis of PD . Second, GBA mutations contribute to protein misfolding , leading to lysosomes and ubiquitin-proteasome system dysfunction which cause neurodegeneration . Third, GBA mutations may potentially accumulate the substrates glucocerebroside and/or glucosylsphingosine, disrupting membrane binding of alpha-synuclein, which prevents the formation of fibrillar protein structures of alpha-synuclein [29, 30]. Finally, the fluctuation in the levels of ceramide, the result of homozygous or heterozygous GBA mutations, is considered to cause Parkinsonism as well . More research evidence is needed to confirm the mechanism of these hypotheses. Potential molecular therapies and genetic counseling of patients and family members may be developed for PD patients with GBA mutations once the mechanism for the association is determined.
Additionally, we acknowledge that our case–control result may be susceptible to bias because of the test method and sample size. In our study, we chose to screen mainly by restriction digest rather than by sequencing. It has been reported that as many as 45% of mutant alleles can be missed among non-Ashkenazi Jewish patients (20% among Ashkenazi Jewish patients) when screening for only the N370S and L444P mutations instead of the whole gene . Replication of genetic associations in various populations is essential to establish specific genetic contribution to the disease. The purpose of this study is to confirm the contribution of GBA mutations to PD etiology, although we may miss some mutant alleles by restriction digest for only three mutations. Moreover, given the small sample size (208 cases and 298 controls, 136 LOPD and 72 EOPD), replications of the positive associations in the Chinese population is warranted. We will increase the sample size and explore new GBA gene mutations by sequencing the whole gene in the next step.