Healy and colleagues1 recently reported that dopamine β‐hydroxylase (DBH) activity could mediate predisposition to Parkinson's disease through its role in catalysing the conversion of dopamine to noradrenaline (norepinephrine). By studying a promoter variant (−1021 C/T) in the DBH gene which has been shown to influence plasma DBH activity, they showed that homozygosity for the low DBH expressing T allele was protective against Parkinson's disease, and proposed that lower levels of DBH protein might lead to increased ratios of dopamine to noradrenaline.1 In Alzheimer's disease, significant reductions in noradrenergic neurones within locus coeruleus, as well as reduced brain noradrenaline levels, have often been reported.2 In addition, there is a causative link between reduced noradrenaline content and the potentiation of β‐amyloid (Aβ) induced cortical inflammation.3 We hypothesised that relative noradrenaline deficiency associated with homozygosity for the (−1021) DBH T allele might lead to increased risk for Alzheimer's disease, either through an independent effect or through interaction with the proinflammatory interleukin (IL) 1A and IL6 genes.
The study involved 266 patients with Alzheimer's disease (68% women; mean (SD) age at the time of study, 75.4 (9.1) years, range 50 to 98; mean age at onset, 71.6 (9.0) years, range 48 to 95) who met NINCDS/ADRDA criteria for probable Alzheimer's disease, and 273 controls (71% women; mean age 80.3 (7.8) years, range 63 to 100). The controls underwent a complete neurological and medical examination which showed that they were free from significant illness and had mini‐mental state examination scores of 28 or more. Cases and controls were not related. The Alzheimer's disease and control samples were white and originated from a homogeneous population in a limited geographical area in northern Spain. Blood samples were taken after written informed consent had been obtained from the subjects or their representatives. The study was approved by the ethics committee of the University Hospital “Marqués de Valdecilla”.
(−1021) DBH,1 (−889) IL1A,4 and (−174) IL65 polymorphisms were determined as described previously (we used the Taqman assay to genotype DBH, as stated in the paper by Healy et al1), and all genotypes were in Hardy–Weinberg equilibrium. Association between dichotomous variables was analysed with the odds ratio, and 95% confidence intervals were estimated by the Cornfield method or the exact method. Probability (p) values were estimated by χ2 or Fisher exact tests. Interrelations were analysed by stratification. Odds ratios for interaction of DBH with interleukins were adjusted for variables including age, sex, and APOE genotype; APOE was included as the APOE ε4 allele is the main recognised genetic risk factor for sporadic Alzheimer's disease, and not because it might fit a “proinflammatory” hypothesis.
When the risk was considered for a single polymorphism, the presence of the DBH T/T genotype was not associated with Alzheimer's disease. However, on evaluation of the interactive effects of DBH and either (−889) IL1A or (−174) IL6 polymorphisms by stratification (table 1), the subjects carrying both the DBH T/T genotype and the (−889) IL1A allele 2 (1/2 and 2/2 genotypes) or the (−174) IL6 G/G genotype had a higher risk of developing Alzheimer's disease than subjects without these risk genotypes. We chose to look only at minor allele homozygous patients for DBH and interleukin genes, because the heterozygous alleles showed no association. An additional synergistic effect of the these three loci (IL1A and IL6 in conjunction with DBH) on the risk of Alzheimer's disease could not be analysed, as stratification of the sample resulted in rather small groups.
Table 1 Odds ratios for Alzheimer's disease risk according to interaction of dopamine β‐hydroxylase (DBH) genotypes with interleukin (IL) 1A (−889) and IL6 (−174) genotypes.
| Genotype | DBH (T/T) | Alzheimer | Controls | OR (95% CI)∗ | p Value | OR (95% CI)† | p Value |
|---|---|---|---|---|---|---|---|
| IL1A (1/2+2/2) | |||||||
| − | − | 134 (50.3%) | 158 (57.9%) | 1 (reference) | 1 (reference) | ||
| − | + | 10 (3.8%) | 15 (5.5%) | 0.79 (0.34 to 1.81) | 0.571 | 0.73 (0.29 to 1.83) | 0.498 |
| + | − | 112 (42.1%) | 98 (35.9%) | 1.35 (0.94 to 1.93) | 0.100 | 1.34 (0.90 to 1.99) | 0.153 |
| + | + | 10 (3.8%) | 2 (0.7%) | 5.90 (1.25 to 27.90) | 0.011 | 2.93 (0.56 to 15.25) | 0.203 |
| Total | 266 | 273 | |||||
| IL6 (G/G) | |||||||
| − | − | 116 (47.9%) | 121 (54.3%) | 1 (reference) | 1 (reference) | ||
| − | + | 8 (3.3%) | 10 (4.5%) | 0.83 (0.32 to 2.19) | 0.713 | 0.54 (0.19 to 1.54) | 0.249 |
| + | − | 106 (43.8%) | 89 (39.9%) | 1.24 (0.85 to 1.82) | 0.263 | 1.36 (0.89 to 2.08) | 0.154 |
| + | + | 12 (5.0%) | 3 (1.3%) | 4.17 (1.13 to 15.42) | 0.020 | 5.66 (1.37 to 23.35) | 0.017 |
| Total | 242 | 223 |
*Crude odds ratio.
†Odds ratio adjusting by age, sex, and APOE genotype (using multiple logistic regression).
CI, confidence interval; DBT (T/T) (−), dopamine β‐hydroxylase C/C+C/T; IL1A (1/2+2/2) (−), interleukin 1A 1/1; IL6 (G/G) (−): interleukin 6 G/C+C/C; OR, odds ratio.
Our findings suggest, for the first time, that DBH allelic variations interact synergistically with the IL1A and IL6 polymorphisms on the development of Alzheimer's disease. The (−889) IL1A allele 2 and (−174) IL6 G allele have a functional effect; they are associated with increased production of IL1A and IL6, respectively, and could confer increased risk for Alzheimer's disease,[4. 5] probably by inducing an increase in cytokine mediated inflammation in the Alzheimer brain. Recent data implicate a role for noradrenaline in attenuating the cortical inflammatory response to Aβ protein: in control animals, Aβ induced IL1 and IL6 expression in microglial cells, and following noradrenaline depletion by systemic treatment of animals with the selective noradrenergic neurotoxin DSP4, there was a robust increase and prolonged expression of both IL1 and IL6.3 In the light of recent findings that inflammatory processes contribute to Alzheimer's disease pathogenesis, it is tempting to speculate that loss of noradrenaline mediated anti‐inflammatory protection associated with the (−1021) DBH T/T genotype might increase the risk of Alzheimer's disease by exacerbating inflammatory events linked to IL1 and IL6 polymorphisms. Our sample size had 70% power to detect an odds ratio of 5 or greater for a risk effect of IL1A interacting with DBH, and 63% power to detect an odds ratio of 4 or greater for a risk effect of IL6 interacting with DBH. This synergistic interaction needs to be confirmed by additional studies using different sets of patients and control subjects.
Acknowledgements
This work was supported by “Centro de Enfermedades Neurológicas (CIEN) (C03/C06)”.
Footnotes
Competing interests: none declared
References
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