SNCA and MAPT genes: Independent and joint effects in Parkinson disease in the Italian population

Background: Significant efforts have been focused on investigating the contribution of common variants to Parkinson disease (PD) risk. Several independent GWAS and metanalysis studies have shown a genome-wide significant association of single nucleotide polymorphisms (SNPs) in the alpha-synuclein (SNCA) and microtubule-associated protein tau (MAPT) regions. Here we investigated the role of SNCA and MAPT as PD susceptibility genes in a large Italian population of 904 patients and 891 controls. An evaluation of gene-gene and gene-environment interactions in association with PD was also attempted. Methods: The SNCA Rep1 microsatellite was genotyped by a fluorescent PCR assay, whereas the SNPlex genotyping system was used to genotype 12 additional markers across the SNCA gene, and 2 SNPs tagging the risk MAP H1 haplotype. Results: Single-marker analysis demonstrated nominal evidence of association for: i) the 261-bp-long allele of Rep1 ii) 7 SNPs in the SNCA region (top SNP: rs356186, P = 3.08 x 10(-04), intron 4); iii) both SNPs identifying the MAPT H1 haplotype (P = 4.63 x 10(-04) and P = 4.23 x 10(-04) for rs1800547 and rs9468, respectively). Moreover, we found a highly significant protective haplotype spanning similar to 83 kb from intron 4 to the 3' end of SNCA (P = 1.29 x 10(-05)). Conclusions: Our findings strongly confirm SNCA and MAPT as major PD susceptibility genes for idiopathic PD in the Italian population. Interaction analyses did not evidence either epistatic effects between the two loci or gene-environment interactions.