We read with interest the article by Grevle and colleagues, 1 reporting in Norway an association between the variant allele A1 of the TaqI A restriction fragment length polymorphism (RFLP) located in the dopamine D2 receptor (DRD2) gene and Parkinson's disease (PD). Similar findings have been reported in our previously published article investigating in a Southern Italian population several polymorphisms of the DRD2 gene. 2 Indeed, we studied 135 patients with PD and 202 age-matched normal control subjects and we found an association between the DRD2 TaqI A variant allele A1 and PD. We also showed an association between PD and another RFLP of the DRD2 gene, the variant allele B1 of the TaqI B polymorphism. Moreover, in agreement with Castiglione et al., 3 we found evidence of a strong linkage disequilibrium between the TaqI A and the TaqI B polymorphisms. Thus, it is likely that the TaqIA and the TaqIB convey the same information about the risk of presenting with PD, and both polymorphisms might be closely linked to a functional mutation which modifies the expression of the DRD2 gene.
Unfortunately, Grevle et al. 1 did not examine the TaqI B polymorphism, but concerning the TaqI A polymorphism, we are impressed about the similarities of the results between our study 2 and Grevle et al.'s study. 1 Indeed, the A1 allele frequencies in our study (PD: 20.7%; controls: 14.6%) 2 closely mirror the frequencies reported by Grevle et al. 1 (PD: 21%; controls: 12%), suggesting that Scandinavian and Southern Italian populations are highly homogenous regarding DRD2 TaqI A polymorphism. Although significant association does not always imply a causal relationship between the presence of the polymorphism and PD pathogenesis, since a great variability exists on polymorphism association studies in PD, 4 we retain that independent replication of data in samples from different geographic areas, such as the case for DRD2 TaqI A polymorphism, 1,2 yields strong evidence to support the hypothesis of a causal relationship between this polymorphism and PD.