br Introduction The identification of genetic

Introduction The identification of genetic variants that influence susceptibility to Parkinson's disease (PD) determines functional studies, the generation of model systems and directs therapeutic strategies. To date, classical candidate gene association studies and genome-wide approaches have failed to find reproducible genetic risk factors in sporadic PD. However, the identification of Lrrk2 G2385R and R1628P demonstrates that common variants which can increase risk by as much as twofold do exist [1], [2]. Although these two variants appear to be specific for individuals of Asian descent, consistent association between genetic variability within the SNCA locus and susceptibility to PD is observed (OR=∼1.5) in diverse ethnicities, however, the functional pathogenic variant has yet to be determined [3], [4], [5]. These findings support the hypothesis that complex disorders like sporadic PD may be the result of multiple factors with relatively small individual effect sizes. A single nucleotide polymorphism (SNP) in the promoter region of dopamine β-hydroxylase (DBH) gene has been reported to substantially affect plasma activity of the dopamine β-hydroxylase enzyme [6], [7]. Two large association studies have examined the role of this functional SNP (DBH −1021C>T; rs1611115) with PD susceptibility [8], [9]. Healy et al. (2004) reported a significant increase of the TT genotype in controls, thus protective against PD in a UK Caucasian population [9]. Chun et al. (2007) attempted to replicate these findings in a US Caucasian series of 1244 PD patients and 1186 control subjects. In Pentamidine dihydrochloride australia with previous data, this study showed no evidence of association for DBH −1021C>T with either PD susceptibility or age at onset (AAO) in patients [8]. Dopamine β-hydroxylase converts dopamine to noradrenaline and subjects with low DBH expression may subsequently have protection against PD symptoms [9]. DBH is, therefore, a plausible biological candidate for PD susceptibility and highlights dopamine β-hydroxylase levels as a preclinical biomarker and a potential therapeutic target. Therefore, to elucidate the role of the DBH −1021C>T in PD, we have investigated its association in four PD patient–control series.
Subjects and methods Four independent Caucasian PD patient–controls series from the US, Ireland, Norway and Poland were examined. The combined series provides a total sample size similar to the two previous studies, with 1037 PD patients and 1659 control subjects. The demographics for each individual series are displayed in Table 1. All patients were examined and observed longitudinally by a movement disorders neurologist and diagnosed with PD according to published criteria [10]. For the Irish and US series each patient was individually selected and matched based on age (+/−4 years), gender, and ethnicity to an unrelated control without evidence of neurological disease. Patients were consecutively selected in the Norwegian and Polish series and controls were not individually matched for age and gender. All subjects are negative for Lrrk2 G2019S. The ethical review boards at each institution approved the study, and all participants provided informed consent. Genotyping of DBH −1021C>T was performed on a Sequenom MassArray iPLEX platform (San Diego, CA); all primer sequences are available on request. For the US and Irish matched series, associations between PD and DBH −1021C>T were measured by odds ratios (OR) and corresponding 95% confidence intervals (CI) obtained from single variable conditional logistic regression models. For the Norwegian, Polish, and combined series, associations between PD and DBH −1021C>T were measured by OR and 95% CI obtained from logistic regression models adjusted for age, sex, and series (combined series only). In PD cases, linear regression models adjusted for sex and series (combined series only) were used to examine associations between AAO and DBH −1021C>T. The effect of DBH −1021C>T on plasma dopamine β-hydroxylase activity is reported to be additive [6], therefore, we used this model for all association tests. Statistical significance was determined at the 5% level.