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Technical Briefs |
1 pharmazentrum frankfurt, Institute of Clinical Pharmacology, Johann Wolfgang Goethe-University, Theodor Stern Kai 7, 60590 Frankfurt, Germany
aauthor for correspondence: fax 49-69-6301-7636, e-mail skarke{at}em.uni-frankfurt.de
The human µ-opioid receptor, encoded by the OPRM1 gene (1)(2), is the major site for the analgesic action of opioids. The OPRM1 gene is therefore a first-line candidate for evaluating the role of mutations on the clinical effects of opioids. The mutant allele of the 118A>G single-nucleotide polymorphism (SNP) in the OPRM1 gene, which codes for an Asn40Asp µ-opioid receptor, has been associated with decreased opioid activity in carriers of the 118G allele. Morphine 6-glucuronide (M6G) and morphine have lower potencies for pupil constriction in carriers of the mutation, who also vomit less often after treatment with M6G than noncarriers (3). Carriers of the 118G allele need more alfentanil for postoperative analgesia but have less pain relief than noncarriers (4). The 118G allele has also been associated with a greater cortisol response to opioid receptor blockade with naloxone (5).
Regarding opioid addiction, the mutant allele of the 17C>T SNP was found more frequently in drug addicts than in nonaddicts (6)(7). An association between the mutant alleles of the 118A>G (exon 1) and 691C>G (intron 2) SNPs and opioid dependence was reported for Chinese heroin addicts, although this was based on a small study group (8). The frequency of the mutated 118G allele was higher in Indian heroin addicts than in controls (9). Addicted individuals carrying both the mutated 118G allele and the mutated 31A allele in intron 2 consumed higher doses of heroin than individuals who did not carry these mutations (10). The simultaneous presence of the mutated alleles for SNPs -1793T>A, -1699-(-1698)insT, -1320A>G, -111C>T, and 17C>T is associated with substance dependence (11). In European Americans, allele -2044A and haplotypes that include -2044A were found to be associated with susceptibility for substance dependence (12).
To promote further investigation of an association of OPRM1 mutations with altered opioid effects or substance dependence, we describe a rapid screening method for several mutations in the OPRM1 gene. SNPs in the OPRM1 gene that qualify for large-scale screening in patients were selected to become part of the screening method when they met one of the following three criteria: (a) in vitro or human studies had revealed a functional consequence; (b) the mutation causes an amino acid exchange, encoding an altered opioid receptor protein; or (c) the SNP has a high reported allelic frequency, which implies that it could have immediate clinical relevance for the administration of opioids in a large part of the population. On that basis, a total of 23 SNPs in the promoter region; in exons 1, 2, and 3; and in the second intron were chosen for screening (see Table 1
for details). To this we added the SNPs -54G>T in the promoter, 24G>A in exon 1 and 942G>A in exon 3 because their close proximity to SNPs -38C>A, 17C>T and 877G>A, respectively, allowed their detection by use of the already available PCR templates with a small extension of the respective assays (see the Data Supplement that accompanies the online version of this Technical Brief at http://www.clinchem.org/content/vol50/issue3/). Thus, our method includes a total of 26 SNPs.
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To detect the 26 selected SNPs in OPRM1, we developed a set of 14 assays (Table 1
) based on the real-time pyrophosphate detection method PyrosequencingTM (13). DNA from 116 healthy Caucasians who had given written informed consent was used to generate OPRM1 DNA fragments. Genotyping was approved by the University of Frankfurt Medical Faculty Ethics Review Board. Primers were designed with "Oligo" primer analysis software (Molecular Biology Insight, Inc.) with use of the OPRM1 nucleotide sequences provided with Ensembl Gene ID ENSG00000112038 (Table 2 in the online Data Supplement). A standard thermal cycling protocol (see Table 2 in the online Data Supplement for the number of cycles) was used. Cycling was for 30 s at 95 °C, 60 s at primer-specific temperatures (Table 2 in the online Data Supplement), and 30 s at 72 °C. Pyrosequencing primers (Table 2 in the online Data Supplement) were designed with use of SNP Primer Design software (Pyrosequencing AB;http://www.pyrosequencing.com). A series of simplex, duplex, and triplex assays were generated on a PSQ 96MA (Pyrosequencing AB; Table 1
, Fig. 1
, and online Data Supplement). In most cases, reverse assays were identified to be best suited for detection. A forward assay was applied in the case of the SNPs -1793T>A and -1698-(-1699)insT, and the duplex assay covering 440C>G and 454A>G (see the online Data Supplement). For each pyrosequencing assay, PCR template for each OPRM1 SNP was incubated in a shaker (10 min) with streptavidin-coated Sepharose beads (Amersham Pharmacia Biotech) and prepared with 672 mL/L 96% ethanol and denaturation buffer in a Vacuum Prep Workstation (Pyrosequencing AB) for transfer of the biotinylated templates into 55 µL of the corresponding 0.35 µmol/L sequencing primer (Table 2 in the online Data Supplement). Sequencing took place in a PSQ 96MA after incubation for 2 min at 80 °C. For confirmation of the pyrosequencing results, a total of 48 samples [4 randomly chosen samples for each of the 12 DNA templates (Table 1
), including 2 mutated DNAs where available] were sequenced by conventional methods on an ABI PRISM 310 Genetic Analyzer (PE/Applied Biosystems). For haplotype analysis, all pyrosequencing results were submitted to HAPLOTYPER computer software for Linux (Harvard University) (14). As an alternative to haplotyping by software, molecular haplotyping would be possible with pyrosequencing using allele-specific PCR templates (15) in an otherwise identical assay.
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The observed pyrograms corresponded to the predicted theoretical assay outcomes, allowing clear identification of the genotypes in all SNPs. Pyrosequencing results agreed with the results obtained with conventional sequencing of random samples. A pyrogram exemplifying the assay for the 118A>G SNP is shown in Fig. 1
; more examples are available in the online Data Supplement. The observed allelic frequencies of the mutations are given in Table 1
. The most frequent SNP was 691C>G in intron 2 (frequency of the mutated allele, 53.4%). The 118A>G SNP in exon 1 was found at a frequency of 12.1% (mutated allele). The observed allelic frequencies corresponded to those published previously (Table 1
) except for the -692C and -38A alleles, which had previously been reported to have frequencies of 4.3% and 1.4–5% (11), respectively, but which we did not find in any of the 116 participants in our study. We found fewer carriers than reported previously for the mutated allele -172T [3.4% vs 11.4% (11)], whereas we found the mutated 691G allele at a higher frequency in our study population [53.4% vs 42.9% (11)(16)]. The most frequent haplotype (39%) contained an IVS2 691G>C mutation and no mutation at any of the other 25 analyzed loci. The second most frequent haplotype had no mutations in any of the 26 positions (frequency, 33%). A haplotype with a mutation at only 118A>G was found at a frequency of 10%, and a combination of IVS2 mutations 31G>A and 691G>C was found at a frequency of 7%. The 118G allele was found together with the IVS2 691G>C allele, without any other mutations, at a frequency of 1.3%.
Receptors encoded by the mutated allele of the 118A>G SNP were reported to have a threefold higher binding affinity for ß-endorphin compared with wild-type receptors in AV-12 cells transfected with the 118G cDNA (6). However, the binding in HEK293 cells transfected with mutated 118G cDNA was not affected (17). Clinical data have indicated decreased opioid activity in heterozygous carriers of the 118A>G SNP (3)(4)(18). In a previous report, a patient not responding to morphine therapy was heterozygous for the 118G allele, whereas a patient who responded was wild type at this position (19). In another report, a 118G carrier with renal failure tolerated high plasma M6G, whereas a patient who did not have the mutation was sedated as a consequence of M6G accumulation (20). The mutated allele of SNP 802T>C SNP has been shown to produce altered receptor desensitization and receptor signaling with decreased G-protein coupling (21). The affinity of µ-opioid-receptor agonists such as morphine; diprenorphine; D-Ala(2),N-MePhe(4),Gly(5)-ol-enkephalin (DAMGO); ß-endorphin; metenkephalin; and dynorphine was not changed, but the potency and efficacy of DAMGO, ß-endorphin, and morphine were decreased (22). Mutated alleles containing SNPs 779A>G and 877G>A are possibly involved in altered desensitization of the µ-opioid receptor (23). Finally, mutated alleles carrying the -T1793A SNP and an inserted thymine at position -1699 were found to potentially influence transcriptional regulation (24).
Thus, evidence from in vitro experiments and clinical studies points to a functional importance of several OPRM1 SNPs. Research on SNPs in the OPRM1 gene is, therefore, of immediate interest for assessing the clinical effects of opioid analgesics and for studying the epidemiology of substance addiction. We provide a screening method suitable for large-scale genetic diagnosis of 26 SNPs in OPRM1. The method includes SNPs with reported high allelic frequencies as well as rare SNPs with demonstrated or potential functional relevance. The method presented could facilitate identification of OPRM1 mutations with clinical relevance and thus enable individualized opioid pharmacotherapy in the near future.
Acknowledgments
This work was supported by Dr. Robert Pfleger Stiftung, Bamberg, Germany.
References
The following articles in journals at HighWire Press have cited this article:
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  B. G. Oertel, M. Kettner, K. Scholich, C. Renne, B. Roskam, G. Geisslinger, P. H. Schmidt, and J. Lotsch A Common Human {micro}-Opioid Receptor Genetic Variant Diminishes the Receptor Signaling Efficacy in Brain Regions Processing the Sensory Information of Pain J. Biol. Chem., March 6, 2009; 284(10): 6530 - 6535. [Abstract] [Full Text] [PDF]
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