The μ-opioid receptor (OPRM1) is the major target of endogenous opioid peptides and opioid analgesic agents. An important single nucleotide polymorphism (SNP) in this gene is the functional SNP, rs1799971, leading to a substitution of asparagine (Asn) to aspartic acid (Asp) at codon 40 in exon 1. Previous studies have suggested that this SNP may give different phenotypes in males and females. In the present study we therefore investigated whether the OPRM1 Asn40Asp SNP, grouped by gender, could predict clinical outcome regarding progression of pain intensity and disability in patients with discogenic low back pain and sciatica.
Patients (n = 252) with lumbar disc herniation and sciatic pain, all European-caucasian, were recruited from Oslo University Hospital Ullevål and Haukeland University Hospital. Blood samples were drawn, genomic DNA isolated and the OPRM1 Asn40Asp SNP was detected by TaqMan methodology. Pain intensity and functional consequences were rated on a visual analogue scale (VAS), by McGill Sensory and by Oswestry Disability Index (ODI) over a 12 months period (inclusion, 6 weeks, 6 months and 12 months).
The genotype */Asp was associated with more pain in women, but seemed to protect the men from pain after lumbar disc herniation. Wildtype Asn/Asn women and men reported similar pain ratings. Analysis of the recovery for the four groups; women Asn/Asn, women */Asp, men Asn/Asn and men */Asp, showed that the */Asp women had a significantly slower recovery, i.e., pain intensity over time than the */Asp men (VAS activity score p = 0.002, McGill sense score p = 0.021, ODI p = 0.205, rmANOVA including covariates smoke, treatment and age with p ≤ 0.1).
The present data suggest that the OPRM1 Asp variant increases the pain intensity in women, but have the opposite effect on men the first year after a disc herniation.
© 2012 Scandinavian Association for the Study of Pain