Polymorphisms in the proteasomal subunit alpha4 are not associated with Parkinson's disease

Abstract

Sirs: In the last years, mutations in several genes have been described that cause familial Parkinson’s disease (PD) [1]. In addition, polymorphisms or intronic changes in some of these genes have been reported as risk factors for nonfamilial forms of PD, e.g. in the α-synuclein [2] or the parkin gene [3]. It is therefore tempting to speculate that also sequence variations in genes that interact with known “PD genes” might be risk factors for the disease, as examined for two parkin interactors, synaptotagmin [4] and synphilin [5]. We have recently reported a new parkin interacting protein, the proteasomal subunit alpha4, which has been proposed to be involved in the regulation of proteasomal activity [6]. Here we investigate whether sequence variations in the alpha4 gene are associated with PD risk. Single nucleotide polymorphism (SNP) selection in the 7 exon encompassing gene was performed on the basis of HapMap data (release 16c 0.1). Genotype data from the CEU population spanning the coding region of the alpha 4 gene and 10 kb of flanking sequence were downloaded from the International HapMap Project. The Tagger Implementation of Haploview 3.2 [7] was used to select an optimal set of available tagging SNPs with a minor allele frequency (MAF) > 0.1 and an r2 threshold of 0.8. Three non-synonymous coding SNPs were added to the set of four tagging SNPs, which brought the total to seven. The fourth non-synonymous coding SNP known at the time of the conception of the study (rs6121906) could not be genotyped in the used system. Genotyping was performed using matrix-assisted laser desorption/ ionization time-of-flight (MALDITOF) mass spectrometry on a MassArray system (Sequenom). Cleaned extension products were analyzed by a mass spectrometer (Bruker Daltronik), and peaks were identified using the SpectroTYPER RT 3.3 software (Sequenom). Assays were designed by the AssayDesign software 3.0 (Sequenom) with the default parameters for the iPLEX chemistry. PD patients were recruited by participating institutions in Munich and Tuebingen after approval of the local ethics committees. Specialists in movement disorders examined the patients. Diagnosis was established according to U.K Brain Bank criteria [8]. After appropriate informed consent, blood samples were drawn for DNA extraction from 340 German sporadic PD patients. The median age of disease onset was 55.4 years. We also genotyped 340 healthy, ageand sex-matched subjects from the KORA (Cooperative Research in the Region of Augsburg) Survey 2000, which studied a large population-based sample. Association was tested by the Armitage trend test. We failed to detect an association between PD and polymorphisms in the parkin interactor alpha4 gene in a large case control study (Table 1). Our sample size meets current standards with a power of 80 % to detect a genotypic relative risk of 1.5 and 2.2 for heterozygous and homozygous SNP-carriers, respectively, at a 0.05 significance level. A clinically relevant risk of alpha4-polymorphisms for PD is therefore unlikely. Although yielding negative results for many genes, we consider the genetically and hypothesis driven search for PD risk factors by association studies as a necessary task to dissect the multifactorial etiology of PD.