Cross-sectional assessment of the consequences of a GTP cyclohydrolase 1 haplotype for specialized tertiary outpatient pain care.

OBJECTIVES: Reduced-function variants of the guanosine triphosphate cyclohydrolase gene (GCH1) have been associated with reduced pain in well-defined cohorts of patients and healthy volunteers. We addressed the question whether this genetic association plays a role in outpatient pain therapy. METHODS: In a cross-sectional observational study, 523 patients were enrolled in 3 different tertiary care outpatient pain centers at German University hospitals. Of the 519 Caucasian patients, data from 424 could be analyzed for functional associations of the formerly named "pain-protective" GCH1 haplotype with the key characteristics of pain therapy being (1) actual pain, (2) opioid dosing, and (3) pain therapy duration. RESULTS: With an allelic frequency of 14.2% the pain-protective haplotype was not rarer among pain patients than in the general population (P=0.344). However, a tendency toward gene dose-dependent effects of the GCH1 haplotype was observed in all the 3 therapy parameters. Carriers of the haplotype tended to have lower actual 24-hour pain scores (n=424; P=0.18), require lower opioid doses (P=0.096), and were significantly shorter on specialized pain therapy (P=0.004). The latter applied predominantly to differences between homozygous carriers and heterozygous (alpha-corrected t test: P=0.06) or non-carriers (P=0.011) of the haplotype. CONCLUSIONS: The results strength the support for a modest yet reproducible and consistent pain-protective effect associated with a GCH1 haplotype known to reduce GCH1 and thus BH4 up-regulation. Pending independent verification, the results might point to a prophylactic role of decreased GCH1 up-regulation delaying the need for pain therapy.

Genetic variants altering dopamine D2 receptor expression or function modulate the risk of opiate addiction and the dosage requirements of methadone substitution.

AIM: Addictive behavior is importantly mediated by mesolimbic dopaminergic signaling. Here, we comprehensively analyzed the DRD2 gene locus, and in addition, the ANKK1 rs1800497C>T single nucleotide polymorphism (SNP), formerly known as 'dopamine D2 receptor Taq1A C>T polymorphism', for associations with the risk of opiate addiction and the methadone dosage requirements. METHODS: Allelic frequencies of DRD2/ANKK1 polymorphisms were compared between 85 methadone-substituted Caucasian patients and a random sample of 99 healthy Caucasian controls. Within patients, the average and maximum daily methadone dose during the first year of treatment and the time when that maximum dose was reached were analyzed for an association with DRD2/ANKK1 genetics. RESULTS: Compared with the control group, drug users carried more frequently the minor allele of DRD2 SNP rs1076560G>T SNP (P=0.022, odds ratio 2.343) or the ATCT haplotype of DRD2 rs1799978A>G, rs1076560G>T, rs6277C>T, ANKK1 rs1800497C>T (P=0.048, odds ratio 2.23), with similar tendencies for ANKK1 rs1800497C>T (P=0.056, odds ratio 2.12) and the TCCTCTT haplotype of DRD2 rs12364283T>C, rs1799732C del, rs4648317C>T, rs1076560G>T, rs6275C>T, rs6277C>T, and ANKK1 rs1800497C>T (P=0.059, odds ratio 2.31). The average and maximum daily methadone doses were significantly associated with the DRD2 rs6275C>T SNP (P=0.016 and 0.005 for average and maximum dose, respectively). Carriers of the variant rs6275T allele needed higher methadone doses than noncarriers. In addition, this variant was associated with a longer time to reach the maximum methadone dose (P=0.025). CONCLUSION: On the basis of an analysis spanning the whole gene locus, from the DRD2 promoter to the ANKK1 rs1800497C>T polymorphism, DRD2 genetic polymorphisms modulate both the risk of opiate addiction, leading to the necessity of methadone substitution therapy, and the course of this therapy in terms of dosage requirements.