We report a marginally increased risk of PD with increasing number of minor alleles (C allele) of the CYP1A2 rs762551 polymorphism. The CYP1A2 enzyme is part of the cytochrome P450 system. It is a hepatic enzyme primarily responsible for the metabolism of a number of substances, including drugs, caffeine, environmental toxins and estrogens.[20
] This enzyme is responsible for more than 90 percent of caffeine metabolism.[21
] In humans, the first step in the biotransformation of caffeine, the microsomal caffeine 3-demethylation in the liver, is catalyzed by CYP1A2.[22
] CYP1A2 also metabolizes MPTP, a neurotoxin that induces Parkinsonism.[23
] The rs762551 polymorphism of CYP1A2 is believed to either be a direct cause of decreased CYP1A2 activity, or to be linked to other genetic variants conferring lower inducibility and reduced caffeine clearance.[7
] CYP1A2 rs762551 has also been linked to decreased serum estradiol levels,[20
] suggesting that, given an association between this polymorphism and risk of PD, this enzyme may be implicated in the interaction between caffeine and estrogen use and risk of PD. Cornelis et al.[24
] reported that this polymorphism modified the relationship between coffee intake and risk of myocardial infarction (MI). In carriers of at least one mutant allele of CYP1A2 rs762551, caffeine was associated with an increased risk of MI, whereas no association with caffeine was observed in non-carriers; the interaction between caffeine and PMH was not modified by CYP1A2. However, because we did not observe an interaction between CYP1A2 and either caffeine or PMH in this study, it is difficult to make a connection between this enzyme and the role of caffeine or estrogen in Parkinson disease,
Only one previous study has examined the association between this polymorphism and risk of PD. Tan et al.,[25
] examined the effect of the CYP1A2 rs762551 polymorphism on risk of PD and the interaction of this SNP with caffeine intake in men and women. They observed an OR for PD for allele A vs. allele C of 0.827 (0.675, 1.104), p = 0.068, which correspond to an OR of 1.21 for allele C vs. allele A. This association is in the same direction as that observed among women in our study. Tan et al did not report significant interactions between caffeine and CYP1A2.
Forsyth et al[26
] measured activity of CYP1A2 in PD patients and controls, in both treated and untreated PD patients, and in controls and did not find significant differences between these groups; genotypes were not reported.
We did not observe associations between the NAT2 acetylator phenotype and risk of PD. In some prior studies, NAT2 slow acetylators were reported to have an increased risk of PD,[9
] whereas other studies have been null.[28
] In addition to combining the NAT2 SNPs into a slow/fast acetylator phenotype, we examined the effect of each individual NAT2 SNP (rs1799931, rs180280, and rs1799930) and did not observe significant associations with risk of PD or interactions with caffeine or PMH.
The interaction of ESR2 rs1256049 with PMH and the marginal interactions of ESR1 rs3798577 and ESR2 rs1255998 with caffeine deserve further investigation. The rs1256049 polymorphisms is a silent synonymous polymorphism and has been hypothesized to lead to changes in mRNA folding and differences in mRNA translation or stability. [29
] To our knowledge, this polymorphism has not been previously associated with altered risk of Parkinson disease. Both ESR1 rs3798577 and ESR2 rs1255998 have previously been associated with lower serum estrogen concentrations.[31
] Therefore, the observation that, in women carrying two mutant alleles of either of these polymorphisms, caffeine has a stronger protective effect then it does among women with no mutant alleles () supports the hypothesis that the protective effect of caffeine appears to be strongest in the relative absence of estrogen.
Important strengths of this study are the nested case-control design, which reduces bias due to selection of controls, the reliance on well-established cohorts with high rates of follow-up (response rates over 90%), and the availability of prospectively collected, updated, and validated information on caffeine consumption, PMH use or smoking.[2
] Furthermore, participants in the NHS and the HPFS have excellent access to health care, and the large majority of participants diagnosed with PD were followed by a neurologist or movement disorder specialist. This does not preclude diagnostic errors, and some level of misclassification is almost inevitable, but this is likely to be small, because using a neuropathological diagnosis as the standard, the positive predictive values for a clinical diagnosis of PD made by neurologists range from 85 to 98%.[15
The main limitation of our study is the relatively small sample size. Whereas power was good for gene main effects – we estimated 80% power to detect odds ratios of greater than 1.18 for SNP with a minor allele frequency of 10% or greater. -- the power to detect interactions of moderate strength was modest, and therefore null results should be interpreted cautiously. Furthermore, some of the results may be due to chance. None of the p-values for the reported associations reached the statistical significance level that would be required if we apply a Bonferroni correction taking into account the 13 tests performed in women (significance p-value = 0.0038) and the 4 tests done in men (significance p-value = 0.0125).
In summary, we did not find convincing evidence that variations in CYP1A2, NAT2, ESR1 and ESR2 predict risk of PD or modify the association of caffeine with PD. The minor C allele of the rs762551 polymorphism of CYP1A2 was associated with an increased risk of PD among women, but not among men, but, even among women, this association was of marginal significance and could have occurred by chance. Possible interactions between PMH and ESR2 rs1256049 and between caffeine and ESR1 rs3798577 and ESR2 rs1256049 were of marginal significance.