We report two interesting and novel findings in this consortium study that comprised five U.S. case-control studies of PD. First, a polymorphism in the promoter region of ADORA2A (rs5996696) was associated with a 30% decreased risk of PD. Second, a newly identified polymorphism (rs71651683) in the 5’ transcription start region of ADORA2A was associated with a 49% decreased risk of PD. The associations of the 5’ and promoter ADORA2A variants with PD risk have not been previously reported. Since these associations remain after adjusting the p-values for multiple comparisons, they are less likely to represent false-positive findings.
In advance of the study, we hypothesized that any ADORA2A
polymorphism resulting in reduced expression or function of the receptor would be protective. This hypothesis was based on findings from animal models of PD: knockout mice with non-functioning ADORA2A
receptor showed protection against MPTP toxicity, and the effect was similar to those related to receptor blockade by caffeine or a pharmacologic agent (e.g., KW-600) [2
]. While the functional importance of rs5996696 and rs71651683 ADORA2A
SNPs is not currently known, they are likely to reduce protein expression by affecting transcription [26
]. Therefore, our finding that these two ADORA2A
SNPs are inversely associated with PD is consistent with the role of the ADORA2A
receptor in caffeine-associated neuroprotection.
A previous study showed that rs3032740 reduces protein expression [23
], therefore, we expected the presence of this variant to be protective for PD. We did not, however, find any suggestion of a protective effect of this polymorphism in any of the race/ethnicity groups, a finding that is consistent with two other reports that did not find associations of rs3032740 (or rs5751876, a SNP is strong LD with rs3032740) with PD risk [8
Metabolism by CYP1A2
is the primary pathway for the conversion of caffeine to paraxanthine. For the most frequently studied intronic variant, rs762551, we expected the risk of PD to be lower among slow metabolizers (AC or CC) compared to fast metabolizers (AA) as the former would have higher caffeine levels [24
] resulting in greater neuroprotection. However, consistent with other reports [8
], in our study, slow metabolizer status did not by itself render any protection against risk of PD. The other CYP1A2
SNPs genotyped, rs2470890 (exon) and rs2472304 (intron), were in strong LD; their associations with PD risk have not been previously reported. Interestingly, the “C” allele for rs2470890, the minor allele among non-Hispanic whites, was the more common allele among African-Americans, Asians, and Hispanics. We observed an increased PD risk associated with the “C” allele among Hispanics, but the permutation-adjusted per allele effect was not statistically significant at alpha=0.05; hence this finding should be interpreted with caution, especially since the functional impact of this exonic variant is not known.
Pooled analysis from the five case-control studies supported the inverse association of caffeinated coffee consumption with PD risk. A primary objective of this study was to evaluate whether the coffee-PD association was modified by ADORA2A
polymorphisms. Since variants that would result in a non-functioning ADORA2A receptor would probably not be influenced by caffeine, we hypothesized that caffeine would be more protective among homozygous carriers of the wildtype allele. Our findings do not support this hypothesis, however. For the two ADORA2A
polymorphisms in strong LD, rs3030274 and rs5751876, although the coffee-genotype interaction was stronger with cups consumed than with ever/never consumption, neither provided convincing evidence of interaction. These results are consistent with two other reports that did not find any effect modification of caffeine-PD association with these SNPs [8
]. We were unable to adequately evaluate interactions of rs5996696 and rs71651683 ADORA2A
polymorphisms with coffee consumption since the variant allele frequencies for these SNPs were relatively small (<6%).
For the CYP1A2
rs76551variant, we hypothesized that the inverse coffee-PD association would be stronger among slow metabolizers compared to rapid metabolizers who carry two copies of the wildtype allele. We did observe that the coffee-PD association was strongest among subjects homozygous for the variant allele, however, it was somewhat weaker for heterozygotes, who are also considered physiologically to be slow metabolizers. Furthermore, although the interaction was statistically significant at the alpha=0.05 level, interaction p
-values were not adjusted for multiple comparisons, and hence must be interpreted with caution. Similar to our results, Tan et al
] found that among Asian subjects, the caffeine-PD association was also stronger in slow compared to fast metabolizers (OR 0.19 vs. 0.40); however, the caffeine-genotype interaction was not statistically significant in multivariable analysis [10
]. Fascheris et al [8
] did not find any effect of rs762551 variant on caffeine-PD association; however, in their study caffeinated coffee consumption was not associated with PD risk.
For the CYP1A2
exonic variant rs2470890, subjects homozygous for the variant allele also showed the strongest coffee-PD inverse association. The functional significance of this synonymous variant is not known and it is possible that it has no effect on protein structure or function. A possible explanation for the minimal modification of the coffee-PD association by CYP1A2 polymorphisms might be that paraxanthine, the primary metabolite produced from caffeine breakdown, also non-selectively inhibits ADORA2A receptor in vitro
, and preliminary studies in mice show that, like caffeine, paraxanthine can also reduce MPTP toxicity [28
Our consortium study had several strengths. The five constituent case-control studies in the consortium were methodologically rigorous and included careful selection of well characterized cases, a majority of whom were newly diagnosed with PD, as well as population or community based controls. For genotype-PD associations, we used a permutation approach to adjust p-values for multiple comparisons, thereby minimizing type I error. Our study had some limitations as well. Although we included subjects from diverse racial/ethnic groups, we did not have sufficient numbers in all subgroups (e.g., African-Americans, n=95) to estimate genotypic effects with precision or to have sufficient power to evaluate caffeine-genotype interactions. Methodology for ascertaining caffeine exposure information varied between studies; however, the methods were comparable enough to allow construction of relevant caffeine related variables for our analyses.
This consortium study characterized ADORA2A and CYP1A2 SNPs in Whites (non-Hispanic and Hispanic), Asians, and African-Americans. Two ADORA2A SNPs, which have not been previously studied, were inversely associated with PD risk. While the results of our study do not support the hypothesis that the inverse coffee-PD association was modified by putative functional polymorphisms in ADORA2A, two CYP1A2 variants appeared to modify the protective effects of coffee on PD risk.