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Keywords:

  • analgesia;
  • environment;
  • epigenetics;
  • genetic association studies;
  • genetics;
  • inter-individual variability;
  • pain;
  • SNP

Abstract

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

Pain severity ratings and the analgesic dosing requirements of patients with apparently similar pain conditions may differ considerably between individuals. Contributing factors include those of genetic and environmental origin with epigenetic mechanisms that enable dynamic gene–environment interaction, more recently implicated in pain modulation. Insight into genetic factors underpinning inter-patient variability in pain sensitivity has come from rodent heritability studies as well as familial aggregation and twin studies in humans. Indeed, more than 350 candidate pain genes have been identified as potentially contributing to heritable differences in pain sensitivity. A large number of genetic association studies conducted in patients with a variety of clinical pain types or in humans exposed to experimentally induced pain stimuli in the laboratory setting, have examined the impact of single-nucleotide polymorphisms in various target genes on pain sensitivity and/or analgesic dosing requirements. However, the findings of such studies have generally failed to replicate or have been only partially replicated by independent investigators. Deficiencies in study conduct including use of small sample size, inappropriate statistical methods and inadequate attention to the possibility that between-study differences in environmental factors may alter pain phenotypes through epigenetic mechanisms, have been identified as being significant.

The sensation of pain produced by an acute insult such as trauma or surgery is an adaptive response characterized by guarding behavior to facilitate healing and is an important survival mechanism. In contrast, chronic pain that persists long after complete healing is widely regarded as a maladaptive response and a ‘disease’ entity in its own right [1]. Globally, the prevalence of chronic pain at 15–20% of the adult population [1] not only adversely affects patient quality of life but it also imposes a high socioeconomic cost encompassing work days lost, reduced productivity and increased healthcare costs [2].

In the clinical setting, subjective patient self-reported pain severity ratings that encompass both nociceptive (pain) stimulus intensity and the patient's response to the stimulus are used to guide analgesic drug treatment, as there are no objective ‘pain tests’ that can be readily applied at the bedside or in the primary care physician's office. The validity of using patient self-reports of pain intensity to guide clinical pain management has been shown based on observations in healthy subjects where functional magnetic resonance imaging showed that a standardized acute noxious nociceptive stimulus evoked marked inter-individual variability in pain severity ratings as well as levels of cortical activation [3]. Indeed, subjects who reported high pain scores exhibited more robust nociceptive stimulus-evoked cortical activation vis-à-vis individuals who reported comparatively low pain scores [3]. Such marked inter-individual differences in pain severity ratings by healthy subjects exposed to noxious acute nociceptive stimuli in the laboratory setting are replicated in patients with clinical pain [4].

Hence, in the past 15 years, a concerted effort has been directed at identifying genetic factors that may explain inter-individual differences in pain sensitivity and analgesic drug dosing requirements. Genetic research in the pain field encompasses rodent heritability studies, familial aggregation and twin studies in humans, genetic association studies conducted in healthy individuals exposed to a variety of noxious stimuli in the laboratory setting as well as patients with various clinical pain conditions. The effect of genotype on the pharmacokinetics and pharmacodynamics of analgesic drugs has also been assessed in numerous genetic association studies. Such studies have generally been designed to assess the influence of single-nucleotide polymorphisms (SNPs) in candidate pain genes encoding receptors and ion channels implicated in pain modulation [4], and/or the effect of genes encoding drug metabolizing enzymes and transporters on analgesic drug pharmacokinetics [5]. It was envisaged that this information would enable point-of-care genotyping devices to be developed to assist clinicians to tailor analgesic drug therapy to the individual patient [6, 7]. However, the findings of most genetic association studies in the pain field have either failed to replicate or have been only partially replicated by independent investigators [8, 9]. This lack of robustness is currently a major impediment to development of point-of-care genotyping devices for personalized analgesic drug therapy.

In the following sections, recent research on genetic factors potentially contributing to inter-individual variability in pain sensitivity as well as the efficacy, toxicity and pharmacokinetics of commonly prescribed analgesic agents, is reviewed (Fig. 1).

image

Figure 1. Schematic diagram illustrating that the considerable inter-individual variability in pain sensitivity and analgesic drug dosing requirements is underpinned by genetic and environmental factors and their interaction.

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Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

The relative ease of generating transgenic and knockout mice for specific candidate pain genes and their shorter generation time compared with rats means that mice are used widely in research on the neurobiology of pain [6]. However, drawbacks include the fact that mouse pain models cannot fully recapitulate human clinical pain conditions and there is divergence in mechanisms for alternative gene splicing events between mice and humans [6].

Systematic investigation of the sensitivity of 11 different mouse strains to applied acute noxious thermal, mechanical and chemical stimuli across 12 different testing modalities, revealed marked (1.2–54-fold range) between-strain differences in the heritability of pain sensitivity that had moderate-to-high heritability (h2 = 0.30–0.76) [10-12]. Subsequent extension of this work to mouse models of chronic inflammatory and neuropathic (nerve damage) pain showed there was also considerable genetic variability in levels of pain hypersensitivity in these models [12-14], as well as in the extent to which various analgesic drugs alleviated pain [14-16]. The number of candidate ‘pain genes’ identified using rodent pain models currently stands at 358 with these cataloged in a database that is readily accessible (http://www.jbldesign.com/jmogil/enter.html; accessed 11 June 2012) [17].

An inherent assumption in using quantitative trait loci (QTL) mapping to identify specific genes underlying complex traits such as pain sensitivity and for estimating the heritability of those traits in mice, is that disassociation of environmental from genetic factors influencing pain behavior phenotype, is possible. However this assumption may be inadvertently violated by subtle between-study differences in environmental factors. This point is well-illustrated by work involving simultaneous testing of several mouse strains by three laboratories where interactions between mouse genetic strain and the laboratory environment were important sources of variability despite adherence of the investigators to standardized behavioral testing protocols [18]. Subsequently, other reports on gene × environment interactions that significantly affect animal behavior have been published [19-21]. Environmental factors that can affect apparent pain sensitivity in mice include vendor, experimenter, cage density, housing, humidity, testing procedures, season and time of day [6, 22]. As environmental factors may modulate genetic factors via epigenetic mechanisms to potentially affect pain phenotypes in rodents as well as pain relief outcomes, such factors need to be well-controlled at least within a single study.

There are also well-documented sex differences in pain and analgesia in animals and humans, and these are reviewed in detail elsewhere [23].

Genetics of pain and analgesia in humans

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

Human twin studies

Twin studies enable the heritability of a trait to be estimated. However, in the pain field it is often difficult to distinguish between heritability of a painful condition and genetic factors involved in pain perception and/or analgesia [6]. The proportion of trait variance due to inherited genetic factors in a number of pain conditions including back pain, spinal pain, sciatica, musculoskeletal pain and irritable bowel syndrome have been estimated using a human twin study paradigm [24-26]. The main limitation of this approach is that although twin studies can show whether or not variability in a specific pain phenotype is genetically linked, they are unable to aid in identifying potential genes involved.

The heritability of responses to noxious stimuli applied to human subjects in a standardized manner in a research laboratory setting has been investigated in several twin studies [27-29]. For example, testing of 98 pairs of healthy Caucasian female twins [51 monozygotic (MZ) and 47 dizygotic (DZ)] with acute noxious thermal and chemical stimuli showed that genetic factors explained 22–55% of inter-individual variability in pain sensitivity [27]. However, in other work involving 53 identical MZ, 39 DZ twins and 4 single twins, there was 26% and 60% inter-subject variability in pain responses to noxious heat and cold-pressor stimuli respectively [28], but only 7% of the variance in cold-pressor pain and 3% of the variance in heat pain were explainable by genetic factors [28]. More recently, the contribution of genetic factors to inter-individual differences in levels of analgesia produced by an infusion of the strong opioid analgesic, alfentanil, relative to placebo for the relief of noxious acute heat and cold pain applied in a research laboratory setting was assessed in 81 pairs of MZ and 31 pairs of DZ twins in a randomized, double-blind study design [29]. There was significant heritability for cold-pressor pain tolerance and opioid analgesic-mediated increases in heat and cold pain thresholds with genetic and familial effects accounting for 12–60% and 24–32% of the response variance, respectively [29]. Significant covariates included age, gender, race, education and anxiety [29].

Collectively, the afore-mentioned twin studies show that the impact of genetic factors is not generalizable from one pain modality to another.

Genetic association studies in humans

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

Drugs that are currently used to treat chronic inflammatory and neuropathic pain conditions in patients often lack efficacy, have dose-limiting side-effects and/or are contraindicated due to co-morbid conditions [30]. For neuropathic pain, first-line treatments are anticonvulsants (e.g. gabapentin and pregabalin) or antidepressants [e.g. tricyclic antidepressants (TCAs) or serotonin norepinephrine re-uptake inhibitors (SNRIs)]. However, the extent to which genetic factors contribute to the wide inter-patient variability in the efficacy and side-effect profiles of these agents for the relief of neuropathic pain is poorly understood.

Strong opioid analgesics (e.g. morphine) are the drugs of choice for the management of moderate-to-severe pain following surgery or trauma [31]. However, there is considerable inter-patient variability in opioid analgesic dosing requirements as well as opioid-related side-effects with genetic factors proposed as a possible explanation [31].

Hence, multiple genetic association studies have been undertaken with the aim of identifying SNPs in genes encoding receptors, enzymes and ion channels involved in pain modulation and analgesia with a view to using this information to tailor analgesic drug dosing regimens as a means of optimizing pain control for individual patients. In the following sections, the outcomes of genetic association studies on selected genes relevant to pain sensitivity and analgesic drug outcomes are briefly reviewed. More extensive reviews of genetic factors affecting pain sensitivity have been published elsewhere [3, 4, 6, 8, 22].

Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

OPRM1 (μ-opioid receptor)

Strong opioid analgesics produce analgesia and many of their side-effects by interacting as agonists at the μ-opioid receptor [31] that is encoded by the gene OPRM1 (chromosome 6q24-q25) in humans [32]. OPRM1 spans over 200 kb and has at least 9 exons and 19 different splice variants [32]. The A118G SNP (Asp40Asn) in exon 1 has a rare allelic frequency at ∼20–30% in the general population and so its functional significance has been investigated in numerous genetic association studies. Despite several reports of positive associations between the A118G SNP in OPRM1 and pain sensitivity [33, 34], opioid dosing [35-41] and/or opioid-related side-effects, meta-analysis of published genetic association studies concluded that the A118G SNP is inconsistently associated with pain-related phenotypes [42].

Genetic determinants: ion channels × pain sensitivity

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

Voltage-gated ion channels

Voltage-gated sodium (Nav) and potassium (Kv) ion channels are key regulators of membrane potential in excitable tissues such as sensory neurons [43]. However, Nav and Kv channels have broadly opposing actions with Nav channels transmitting local membrane depolarization generated by stimulus transduction along the axon [44] whereas activation of Kv channels results in cell membrane hyperpolarization and a decrease in neuronal excitability [45].

SCN9A (Nav1.7 sodium channel)

After peripheral nerve injury or exposure of peripheral nociceptors to chronic inflammation, persistent hyper-excitability and ectopic discharge of primary afferent nerve fibers develops that is associated with abnormal sodium channel regulation [46]. Of the nine voltage-gated sodium channel subtypes identified to date, Nav1.3, Nav1.7, Nav1.8 and Nav1.9 are expressed primarily in sensory nerves [44]. The Nav1.7 sodium channel encoded by the SCN9A gene is widely expressed in dorsal root ganglion neurons [47] with Nav1.7 knockout mice showing that it has an essential role in nociceptive neurotransmission [48, 49]. Furthermore, as individuals with rare loss-of-function or gain-of-function mutations in the SCN9A gene have one of two extreme pain phenotypes, viz an inability to sense pain [50] or inherited erythromelalgia [51] respectively, Nav1.7 is regarded as a target with human validity for discovery of novel analgesic agents. In a mixed cohort of patients with chronic pain due to sciatica, osteoarthritis, pancreatitis, lumbar discectomy and phantom limb pain, the rs6746030 SNP in SCN9A encoding the R1150W variant in the α-subunit of Nav1.7 was associated significantly with pain perception [59]. However, this conclusion was partially contradicted in a much larger subsequent study involving patients with osteoarthritis or multiple region pain whereby the rs6746030 SNP was associated significantly only with multiple region pain but not osteoarthritis pain [52].

KCNS1 (Kv9.1 potassium channel)

Kv channels are tetramers of α- and β-subunits. In rats with peripheral nerve injury-induced neuropathic pain, KCNS1, the gene encoding the Kv9.1 α-subunit was identified by gene expression profiling as a putative pain gene [53]. This is of interest as Kv9.1 is constitutively expressed in sensory neurons and it suppresses currents mediated by the Kv2 and Kv3 α-subunit families despite it not forming functional homomeric channels itself [53-56]. The validity of KCNS1 as a ‘pain gene’ in humans was assessed by a genetic association study in five independent cohorts of patients with a range of clinical pain conditions as well as a group of healthy subjects administered standardized acute noxious nociceptive stimuli in a laboratory setting (total = 1359 individuals). In five of the six cohorts, the ‘valine risk allele’ of the non-synonymous SNP, rs734784 (I489V) in KCNS1 was associated with higher pain severity ratings; two copies of this allele conferred the highest risk, one copy an intermediate risk and none the least risk [53]. As 18–22% of the population are homozygous and 50% are heterozygous for the ‘valine risk allele’ of KCNS1, it is proposed as a prognostic indicator for chronic pain risk [53]. However, additional investigation by independent groups in much larger patient cohorts of diverse ethnic background and a greater breadth of clinical pain conditions is required to assess the robustness of this SNP in KCNS1 as a prognostic indicator of chronic pain risk.

Ionotropic ATP-gated receptors

P2RX7 (P2X purinoceptor 7)

The P2X7 receptor is a member of the ionotropic ATP-gated receptor family. It is implicated in the etiology of chronic pain as its genetic deletion in mice reduces pain sensitivity [57] and P2X7 receptor antagonists produce pain relief in rodent models of chronic inflammatory and neuropathic pain [58]. P2X7 receptor function is transduced via its cation channel or by formation of non-selective pores that allow passage of molecules up to 900 Da in mass [59]. The highly polymorphic P2RX7 gene (12q24.31 chromosomal region) spans 53 kb and has 13 exons [60] and SNPs in this gene appear to affect chronic pain sensitivity [59]. Genome-wide linkage analyses in multiple strains of mice with peripheral nerve injury-induced neuropathic pain showed that strains expressing the non-synonymous P451L SNP (rs48804829) in P2rx7 had impaired P2X7 receptor pore formation and reduced pain behavior compared with strains carrying the pore-forming Pro451 P2rx7 allele [59]. In cohorts of patients with post-mastectomy or osteoarthritis pain, individuals with the hypofunctional His270 (rs7958311) allele of P2RX7 had significantly lower pain intensity scores [59]. However, the robustness of these findings needs to be further evaluated by independent investigators in different patient cohorts.

Genetic determinants: neurotransmitters × pain sensitivity and analgesia

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

The pathobiology of neuropathic pain is underpinned by augmented excitatory neurotransmission and/or impaired inhibitory mechanisms in the spinal cord [61]. TCAs and SNRIs that boost descending inhibitory noradrenergic and serotoninergic signaling in the spinal cord by blocking the re-uptake of norepinephrine and serotonin by their corresponding transporters (NET and SERT, respectively) have efficacy for the treatment of neuropathic pain [62]. Hence, SNPs in genes encoding NET and SERT as well as genes encoding enzymes involved in the biosynthesis [e.g. guanosine triphosphate cyclohydrolase 1 (GCH1)] or degradation [e.g. catechol-O-methyl transferase (COMT) and monoamine oxidase] of norepinephrine and serotonin, may contribute to inter-individual variability in pain sensitivity as well as analgesic drug responses.

Monoamine re-uptake transporters

NET (SLC6A2)

Although SNPs in the NET gene (SLC6A2) are weakly associated with pain sensitivity in patients with post-operative pain [63], there are no clinical trials as yet that have assessed their influence on levels of pain relief produced by TCAs and SNRIs in patients with neuropathic pain.

SERT (SLC6A4)

The SERT gene (5HTT also known as SERT or SLC6A4) has two main functional variants, namely 5HTTLPR and STin2 VNTR [64]. For the 5HTTLPR variant, there is a 44 base pair (bp) insertion/deletion in the 5′ promoter region to generate a long (l) or a short (s) allele [64]. Systematic review and meta-analysis of 10 studies concluded that this polymorphism is not a significant risk factor for migraine in individuals of European or Asian descent [65]. Reports of an association of 5-HTTLPR polymorphisms with a higher risk for painful conditions such as fibromyalgia [66] and tension-headache [67] require verification by other groups. Similarly, the reported association of the s-allele with lower sensitivity to heat, cold and pressure pain relative to the l-allele in healthy subjects [68, 69] needs independent verification. For the STin2 VNTR polymorphism, a 17 bp variable number of tandem repeats in intron 2 of the 5HTT gene produces alleles carrying 9, 10 or 12 repeats. Meta-analysis of five studies in individuals of European descent suggested that the 10/12 and 10/10 genotypes had a protective effect against migraine compared with the 12/12 genotype [70].

Monoamine biosynthesis

GCH1

The enzyme GCH1 is upregulated in neuropathic pain [71] and it catalyzes the rate-limiting step in the synthesis of tetrahydrobiopterin (BH4), a co-factor essential for serotonin and norepinephrine biosynthesis. Hence, polymorphisms in GCH1, the gene encoding GCH1 have potential to contribute to inter-individual variability in pain sensitivity. Supporting this notion, a GCH1 haplotype and 15 SNPs were associated with reduced experimental pain sensitivity in two separate cohorts of patients with neuropathic pain due to lumbar nerve root compression [72]. However, this was not the case for patients with acute post-surgical pain after third molar extraction [73]. Although a difference in haploblock architecture between the study populations may explain these discrepant findings [74], the fact that patients with chronic nerve root pain have ongoing sensitization of the somatosensory system whereas those with acute post-surgical pain do not, is likely to be a major contributing factor. Several SNPs in non-coding and non-splicing sites of GCH1 are associated with reduced upregulation of GCH1 and BH4 expression [72, 74-76]. Of particular note, patients homozygous for these non-coding and non-splice site GCH1 variants had a longer mean period between cancer diagnosis and need for initiation of opioid therapy when compared with heterozygous patients or non-carriers [77]. By extrapolation, use of agents that provide partial blockade of GCH1 activity or that inhibit BH4 formation has potential to prevent or delay the development of cancer-related pain.

Catecholamine metabolism

COMT

Initial reports of a significant association between the valine-to-methionine SNP at position 158 (V158M) in the COMT gene (rs4680) and common pain conditions such as fibromyalgia [78], migraine [79] and temporomandibular joint disorder [80] were not replicated by others [63, 81-84]. Similarly, although the rs4680 SNP appeared to be associated significantly with inter-individual variability in morphine dosing requirements for satisfactory control of cancer pain [85], this finding failed to replicate [38]. Instead, there was a significant association between morphine doses to produce satisfactory analgesia and the combinatorial effect of the COMT Met/Met and OPRM1 A/A SNPs [38]. Overall, the use of COMT SNPs for guiding day–day management of clinical pain is not supported.

Monoamine oxidases

Monoamine oxidases (MAO) metabolize serotonin and norepinephrine that modulate descending inhibitory pain mechanisms in the central nervous system (CNS). The two MAO isoforms, MAO-A and MAO-B, are encoded by the genes MAOA and MAOB respectively and share 70% amino acid sequence homology [63]. In female patients with post-operative pain [63], a weak association was found between pain intensity and SNPs in MAOA but not MAOB. In contrast, post-operative pain intensity in male patients was significantly correlated with an A/G polymorphism in intron 13 of MAOB [86]. These findings reflect reports of inconsistent relationships between other human behaviors and SNPs in MAOA and MAOB [87]. In patients with migraine, SNPs in MAOA and MAOB do not appear to be linked to migraine risk [88-92].

Genetics association studies and analgesic drug pharmacokinetics

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

In the gastrointestinal mucosa and the liver, there is an array of metabolic enzymes that evolved as endogenous detoxification mechanisms to protect the body against dietary and environmental toxins (xenobiotics). These enzymes are classified into two broad groups, so-called phase 1 and phase 2. Phase 1 enzymes (e.g. the cytochrome P450 superfamily) alter the chemical structure of molecules to generate functional groups that serve as ‘handles’ or conjugation sites for phase 2 enzymes [e.g. UDP-glucuronosyltransferase (UGT)] that add water solubility enhancing groups such as glucuronic acid and sulfate.

After oral administration, analgesic drugs are absorbed across the gastrointestinal mucosa and delivered to the liver via the portal vein with the fraction of the dose surviving the first pass through the liver then entering the systemic circulation [93]. The net effect of metabolism on small molecules including analgesic drugs is to produce water-soluble metabolites that are generally pharmacologically inactive and readily excreted from the body via the kidney [93]. However, exceptions include the bioactivation of codeine-to-morphine and the metabolism of morphine to two major pharmacologically active glucuronide metabolites, viz morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) [93]. Additionally, the drug metabolizing activity of many CYP450s can be induced or inhibited by concomitant administration of other drugs, smoking, alcohol consumption and ingestion of some dietary substances (e.g. grape fruit juice). Drug metabolizing enzyme activity is also altered by advanced or very young age and by the presence of liver disease [7].

For analgesic agents that produce pain relief and/or side-effects via interaction with receptors in the CNS, they passively diffuse from the systemic circulation across the tight junctions of the blood–brain barrier (BBB) and enter the brain. Efflux transporters that reside in the BBB such as P-glycoprotein (P-gp) reduce the amount of drug entering the brain for molecules that are P-gp substrates.

The apparent potency of analgesic drugs is thus potentially affected by SNPs in genes that encode drug metabolizing enzymes and efflux transporters in the BBB by influencing analgesic drug disposition and pharmacokinetics [7]. A brief overview of the effect of SNPs on the pharmacokinetics of opioid analgesic drugs is provided in the following sections. A review of genetic factors potentially altering the pharmacokinetic properties of other analgesic drug classes is found elsewhere [94-96].

Genetic determinants: phase I metabolic enzymes

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

In humans, the cytochrome P450 (CYP 450) superfamily of phase I drug metabolizing enzymes is encoded by at least 57 functional CYP genes within 18 families (i.e. CYPs 1–5, 7, 8, 11, 17, 19–21, 24, 26, 27, 39, 46 and 51) with another 58 CYP pseudogenes also identified [97, 98]. Six CYPs contribute significantly to the metabolism of medications used clinically and their relative abundance is in the following order: CYP3A4>2C>1A2>2E1>2D6>2B6 [99]. Most CYP family members are polymorphic and information on known alleles compiled by the CYP Allele Nomenclature Committee is publicly available at http://www.cypalleles.ki.se. CYP variants alter protein expression or activity to produce one of four phenotypes, viz poor metabolizers (PM, two non-functional alleles), intermediate metabolizers (IM, at least one non-functional or variant allele), extensive metabolizers (EM, normal individuals with two functional alleles) and ultra-rapid metabolizers (UM, multiple copies of a functional allele or where a mutation results in increased gene transcription) [98].

CYP2D6

CYPD26 has more than 100 allelic variants resulting in considerable phenotypic diversity within populations and between ethnic groups [95, 100]. Despite its low relative abundance in human liver at 2–4% of CYP proteins, CYP2D6 is involved in the metabolism of ∼25% of clinically utilized medications including analgesics such as codeine and tramadol as well as antidepressants and anti-arrhythmics that are adjuvant drugs used to treat neuropathic pain [95]. In the Caucasian population, the frequency of the CYP2D6 PM phenotype is 7–11% with another 7% having the UM phenotype [100]. Individuals with the CYP2D6 PM phenotype do not derive pain relief from codeine as they are unable to metabolize (O-demethylate) it to morphine [98].

Codeine-induced toxicity due to excessive amounts of metabolically derived morphine has been reported in adult [101-103] and pediatric patients with the UM phenotype [104-108]. It is recommended that codeine not be prescribed to breast-feeding mothers to avoid the risk of respiratory depression and excessive sedation associated with formation of large amounts of metabolically derived morphine in UM mothers [105].

Tramadol is metabolized by CYP2D6 to its opioid analgesic metabolite, O-desmethyltramadol (M1) to augment the descending noradrenergic and serotoninergic inhibitory effects of tramadol itself [109, 110]. Hence, the analgesic effects of tramadol are reduced rather than abolished in individuals with the PM phenotype [109, 110]. After oral dosing in humans, PMs require ∼30% more tramadol to achieve satisfactory analgesia relative to other phenotypes and there is increased formation of M1 in UMs who are at higher risk for toxicities [111-113].

CPY3A (cytochrome P450 3A)

In humans, the CYP3A gene encodes two major CYP3As expressed in human liver, viz CYP3A4 and CYP3A5 [114] that catalyze the metabolism of a broad range of structurally diverse molecules including up to 60% of all drugs used clinically with ∼10-fold inter-subject variability in their metabolism [115]. Opioid analgesics such as codeine, dihydrocodeine, fentanyl, tramadol, alfentanil, methadone, oxycodone, buprenorphine and dextromethorphan are metabolized at least in part by CYP3A4 [95]. In the Caucasian population, common allelic variants are CYP3A4*2, CYP3A4*10 and CYP3A4*17 whereas in the Asian population CYP3A4*1G, CYP3A4*4, CYP3A4*5 and CYP3A4*18 are more common [116-120]. It is widely known by front-line clinicians that the doses of fentanyl required to produce satisfactory post-operative pain relief vary widely between individuals [121]. Fentanyl dosing requirements for satisfactory relief of pain following either gynecological or abdominal surgery were correlated significantly with the CYP3A4*1G genotype (2023 G>A) such that individuals homozygous for the GG genotype required significantly lower doses compared with individuals homozygous for the AA genotype [122-124].

UGT2B7

In humans, there are two major UGT classes, viz UGT1A and UGT2 that comprise at least eight and seven isoforms respectively [125]. UGT2B7 has a major role in the metabolism of commonly used pain-relievers including several opioid analgesics (morphine, hydromorphone, and oxymorphone), non-steroidal anti-inflammatory drugs, and anticonvulsants [126, 127]. Morphine is metabolized to two major glucuronide metabolites, viz M3G and M6G that account for >50% and ∼10% of morphine doses, respectively [126]. As M6G is analgesically active and the neuro-excitatory actions of M3G attenuate the analgesic effects of morphine in rodent studies; it is plausible that polymorphisms in UGTB7 may significantly impact M3G: morphine and M6G: morphine plasma concentration ratios in patients and thus overall levels of pain relief achieved. However, in a study involving 175 patients with cancer-related pain that investigated this notion, the M3G: morphine and M6G: morphine plasma concentration ratios were not significantly correlated with any of ten polymorphic SNPs in UGT2B7 [128].

Epigenetics: a bridge connecting gene and environment

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References

A major learning from mouse heritability and human genetic association studies in the pain field is that the heritability of pain-related phenotypes is influenced by dynamic gene × environment interactions and this constitutes the field of pain epigenetics. Although the potential for epigenetic changes such as DNA methylation, histone modification, chromatin remodeling and the action of small regulatory non-coding RNAs that alter gene expression without concomitant changes in the DNA sequence [129] to modulate pain is now recognized [130], assessment of the impact of epigenetic processes on gene repression or activation in chronic pain states is in its relative infancy. Additionally, epigenetic modulation of receptor targets and drug metabolizing enzymes may alter the pharmacodynamics and pharmacokinetics of analgesic drugs respectively [131-134]. The emerging field of pain epigenetics has been reviewed recently [129].

In conclusion, genetic and environmental factors as well as their interaction contribute significantly to the considerable inter-individual variability in pain sensitivity and analgesic drug dosing requirements of patients with apparently similar clinical pain states. Although numerous genetic association studies have assessed the impact of SNPs in various genes encoding receptors, enzymes and ion channels involved in pain modulation and analgesia, the findings have generally failed to replicate or been only partially replicated. Deficiencies in study conduct include small sample size, use of inappropriate statistical methods and inadequate attention to the control of environmental factors that may alter pain phenotypes through epigenetic mechanisms. The lack of robustness in genetic association study outcomes is a major impediment to development of point-of-care genotyping devices to assist clinicians to personalize drug dosing regimens for individual patients.

References

  1. Top of page
  2. Abstract
  3. Genetics of pain and antinociception (analgesia) in rodents – what have we learnt?
  4. Genetics of pain and analgesia in humans
  5. Genetic association studies in humans
  6. Genetic determinants: pain sensitivity and opioid analgesic pharmacodynamic effects
  7. Genetic determinants: ion channels × pain sensitivity
  8. Genetic determinants: neurotransmitters × pain sensitivity and analgesia
  9. Genetics association studies and analgesic drug pharmacokinetics
  10. Genetic determinants: phase I metabolic enzymes
  11. Epigenetics: a bridge connecting gene and environment
  12. Acknowledgement
  13. References