This study found that:
- A single nucleotide polymorphism (SNP) in the messenger RNA of the µ-opioid receptor gene was associated with patients’ responses to methadone treatment for opioid dependence.
- The association may occur because one variant of the SNP produces fewer µ-opioid receptors than the alternate form.
Methadone helps people remain abstinent from opioids by reducing withdrawal symptoms and craving for the drugs. However, the medication doesn’t work equally well for all patients. New research indicates that a genetic difference may be one reason why.
Drs. Richard C. Crist and Glenn A. Doyle from the Center for Neurobiology and Behavior at the University of Pennsylvania School of Medicine and colleagues focused on the OPRM1 gene as a locus where variation in the sequence of DNA might affect responses to methadone and buprenorphine. OPRM1 encodes the μ-opioid receptor, where all opioids, licit and illicit, including the two opioid agonist medications, exert their effects on the brain. If a variant in OPRM1 were to suppress the gene’s expression, reducing the quantity of receptors available, those effects might be reduced accordingly.
Drs. Crist and Doyle and colleagues hypothesized that if such an OPRM1 variant existed, its suppressive effect might occur in the messenger RNA (MOR-1) transcribed from the gene. The researchers observed that MOR-1 contains an unusually long 3’ untranslated region (3’ UTR) (see Figure 1), and other research had linked lengthy 3’ UTRs in other genes to expression-suppressing mechanisms.
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Contrasting Treatment Responses
The researchers analyzed DNA and treatment-response data from patients who participated in the Starting Treatment with Agonist Replacement Therapy (START) clinical trial. START randomly assigned first-time seekers of treatment for opioid dependence to receive maintenance treatment with either methadone or buprenorphine for 24 weeks at federally licensed opioid treatment programs.
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The analysis turned up one SNP (rs10485058) in the START participants’ MOR-1 3’ UTR that appeared to affect how they fared in methadone maintenance (see Figure 2). Methadone-maintained patients of European descent who had at least one copy of the guanine variant of rs10485058 produced more illicit-opioid-positive urine samples during the trial than patients who had only the adenine variant (see Table). Three other SNPs in the patients’ MOR-1 3’UTR (rs671531, rs558948, rs645027) were not associated with methadone treatment response. No SNP was associated with responses to buprenorphine.
A second analysis, this one of data from 1,215 Australians of European descent who were treated with opioid agonist therapy in the Comorbidity and Trauma Study (CTS), reinforced these findings. CTS participants who had at least one copy of the guanine variant of rs10485058 had significantly higher rates of self-reported relapse than those with only the adenine variant.
|SNP combination #||SNP variants in the combination||Treatment failure with methadone (P-value)||Treatment failure with buprenorphine (P-value)|
A Receptor Disadvantage
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Drs. Crist and Doyle and colleagues next addressed why patients with the guanine variant of rs10485058 might benefit less from methadone therapy than those with the adenine variant. One possible explanation was that the guanine variant produces fewer μ-opioid receptors than the adenine variant (see Figure 3). If that were the case, methadone and other opioids would have fewer sites to attach to and exert their therapeutic effects on cells.
Prior research had identified a mechanism that produces such differences and pertains especially to genes whose messenger RNA, like that of OPRM1, contains lengthy 3’ UTRs. Such messenger RNA can contain sequences that bind strongly with very short strips of RNA called microRNAs. When protein complexes containing microRNAs attach to messenger RNA, they impede the production of protein from the messenger RNA.
Dr. Crist, Dr. Doyle, and colleagues hypothesized that the guanine variant of rs10485058 binds to microRNAs more readily than the adenine variant, and hence produces fewer μ-opioid receptors. To test the hypothesis, the researchers cloned the two rs10485058 variants into the 3’ UTR of the luciferase gene, which produces the light-generating protein found in fireflies. Cells were cultured with these cloned genes and molecules that mimic microRNAs. A readout of the light emitted from the cultures showed that the microRNA mimic reduced the genetic activity of the guanine rs10485058 RNA more than it did that of the adenine variant.
The researchers hypothesized that differences between methadone and buprenorphine might explain their finding that variation in rs10485058 affected patients’ responses to methadone, but not to buprenorphine. Reduced μ-opioid receptor availability might be expected to inhibit methadone’s effects more than those of buprenorphine because methadone exerts its effects entirely through that receptor, while buprenorphine does so only partially.
Drs. Crist and Doyle and colleagues consider their results to be promising and to warrant additional replication in other populations. If such studies support the observed pharmacogenetic effect of rs10485058 on methadone treatment outcomes, physicians may have a new tool to guide treatment decisions for optimal patient outcomes.
The study was supported by NIH grant number DA036751.
Crist, R.C.; Doyle, G.A.; Nelson, E.C. et al. A polymorphism in the OPRM1 3'-untranslated region is associated with methadone efficacy in treating opioid dependence. Pharmacogenomics Journal doi: 10.1038/tpj.2016.89, 2016. Full text
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NIDA. (2017, August 23). Variation in the Gene for the μ-Opioid Receptor May Influence Responses to Methadone. Retrieved from https://www.drugabuse.gov/news-events/nida-notes/2017/08/variation-in-gene-m-opioid-receptor-may-influence-responses-to-methadone