A GTP cyclohydrolase 1 genetic variant delays cancer pain

Mechanisms of pain in aging and age-related conditions: Focus on caregivers

Pain is a complex, subjective experience that can significantly impact quality of life, particularly in aging individuals, by adversely affecting physical and emotional well-being. Whereas acute pain usually serves a protective function, chronic pain is a persistent pathological condition that contributes to functional deficits, cognitive decline, and emotional disturbances in the elderly. Despite substantial progress that has been made in characterizing age-related changes in pain, complete mechanistic details of pain processing mechanisms in the aging patient remain unknown. Pain is particularly under-recognized and under-managed in the elderly, especially among patients with Alzheimer's disease (AD), Alzheimer's disease-related dementias (ADRD), and other age-related conditions. Furthermore, difficulties in assessing pain in patients with AD/ADRD and other age-related conditions may contribute to the familial caregiver burden. The purpose of this article is to discuss the mechanisms and risk factors for chronic pain development and persistence, with a particular focus on age-related changes. Our article also highlights the importance of caregivers working with aging chronic pain patients, and emphasizes the urgent need for increased legislative awareness and improved pain management in these populations to substantially alleviate caregiver burden.

Genome-Wide Association Study Identifies Candidate Loci Associated with Opioid Analgesic Requirements in the Treatment of Cancer Pain

Considerable individual differences have been widely observed in the sensitivity to opioids. We conducted a genome-wide association study (GWAS) in patients with cancer pain to identify potential candidate single-nucleotide polymorphisms (SNPs) that contribute to individual differences in opioid analgesic requirements in pain treatment by utilizing whole-genome genotyping arrays with more than 650,000 markers. The subjects in the GWAS were 428 patients who provided written informed consent and underwent treatment for pain with opioid analgesics in a palliative care unit at Higashi-Sapporo Hospital. The GWAS showed two intronic SNPs, rs1283671 and rs1283720, in the ANGPT1 gene that encodes a secreted glycoprotein that belongs to the angiopoietin family. These two SNPs were strongly associated with average daily opioid requirements for the treatment of pain in both the additive and recessive models (p < 5.0000 × 10−8). Several other SNPs were also significantly associated with the phenotype. In the gene-based analysis, the association was significant for the SLC2A14 gene in the additive model. These results indicate that these SNPs could serve as markers that predict the efficacy of opioid analgesics in cancer pain treatment. Our findings may provide valuable information for achieving satisfactory pain control and open new avenues for personalized pain treatment.

Phenotypic drug screen uncovers the metabolic GCH1/BH4 pathway as key regulator of EGFR/KRAS-mediated neuropathic pain and lung cancer

Increased tetrahydrobiopterin (BH4) generated in injured sensory neurons contributes to increased pain sensitivity and its persistence. GTP cyclohydrolase 1 (GCH1) is the rate-limiting enzyme in the de novo BH4 synthetic pathway, and human single-nucleotide polymorphism studies, together with mouse genetic modeling, have demonstrated that decreased GCH1 leads to both reduced BH4 and pain. However, little is known about the regulation of Gch1 expression upon nerve injury and whether this could be modulated as an analgesic therapeutic intervention. We performed a phenotypic screen using about 1000 bioactive compounds, many of which are target-annotated FDA-approved drugs, for their effect on regulating Gch1 expression in rodent injured dorsal root ganglion neurons. From this approach, we uncovered relevant pathways that regulate Gch1 expression in sensory neurons. We report that EGFR/KRAS signaling triggers increased Gch1 expression and contributes to neuropathic pain; conversely, inhibiting EGFR suppressed GCH1 and BH4 and exerted analgesic effects, suggesting a molecular link between EGFR/KRAS and pain perception. We also show that GCH1/BH4 acts downstream of KRAS to drive lung cancer, identifying a potentially druggable pathway. Our screen shows that pharmacologic modulation of GCH1 expression and BH4 could be used to develop pharmacological treatments to alleviate pain and identified a critical role for EGFR-regulated GCH1/BH4 expression in neuropathic pain and cancer in rodents.

Analgesic Effect of Tranilast in an Animal Model of Neuropathic Pain and Its Role in the Regulation of Tetrahydrobiopterin Synthesis

Trigeminal neuralgia is unilateral, lancinating, episodic pain that can be provoked by routine activities. Anticonvulsants, such as carbamazepine, are the drugs of choice; however, these possess side-effects. Microvascular decompression is the most effective surgical technique with a higher success rate, although occasionally causes adverse effects. The potential treatment for this type of pain remains unmet. Increased tetrahydrobiopterin (BH4) levels have been reported in association with axonal injury. This study aimed to evaluate the effect of tranilast on relieving neuropathic pain in animal models and analyze the changes in BH4 synthesis. Neuropathic pain was induced via infraorbital nerve constriction. Tranilast, carbamazepine, or saline was injected intraperitoneally to assess the rat's post-intervention pain response. In the von Frey's test, the tranilast and carbamazepine groups showed significant changes in the head withdrawal threshold in the ipsilateral whisker pad area. The motor coordination test showed no changes in the tranilast group, whereas the carbamazepine group showed decreased performance, indicating impaired motor coordination. Trigeminal ganglion tissues were used for the PCR array analysis of genes that regulate the BH4 pathway. Downregulation of the sepiapterin reductase (Spr) and aldoketo reductase (Akr) genes after tranilast injection was observed compared to the pain model. These findings suggest that tranilast effectively treats neuropathic pain.

Genetic Factors Associated With Pain Severity, Daily Opioid Dose Requirement, and Pain Response Among Advanced Cancer Patients Receiving Supportive Care

BACKGROUND

Current understanding of genetic factors associated with pain severity, and improvement of pain with opioids in advanced cancer patients (AC) is inadequate for delivery of personalized pain therapy (PPT). Therefore, the aim of this study was to determine the genetic factors associated with pain severity, daily opioid dose, and pain response in AC patients receiving supportive care.

METHODS

In this prospective study, AC patients were eligible if they had cancer pain ≥4/10 on Edmonton Symptom Assessment Scale (ESAS) - Pain Item and needed opioid rotation for pain control by specialist at the outpatient supportive care center. Association of genetic factors with pain phenotype was assessed using logistic regression models and SKATO (Gene-block) analysis.

RESULTS

About 174/178 (98%) patient samples were analyzed. After adjustment for demographic and clinical variables, pain severity was negatively associated with intron variant alleles in OPRM1 rs9322446, P = 0.02; rs2270459, P = 0.038; rs62052210, P = 0.038. Opioid daily dose was positively associated NFKBIA rs2233419, P = 0.008; rs2233417, P = 0.007; rs3138054, P = 0.008; rs1050851, P = 0.015; ORPM1 rs9479759, P = 0.046; rs2003185, P = 0.047; rs636433, P = 0.044; COMT (rs9306234, P = 0.014; rs165728, P = 0.014; rs2020917, P = 0.036; rs165728, P = 0.034); ARRB2 (rs1045280, P = 0.045); and pain response to opioids was negatively associated OPRM1 rs1319339, P = 0.024; rs34427887, P = 0.048; and COMT rs4646316, P = 0.03; rs35478083, P = 0.028, respectively. SKATO analysis showed association between pain severity and CXCL8 (P = 0.0056), and STAT6 (P = 0.0297) genes respectively, and pain response with IL-6 (P = 0.00499).

CONCLUSIONS

This study identified that SNPs of OPRM1, COMT, NFKBIA, CXCL8, IL-6, STAT6, and ARRB2 genes were associated with pain severity, opioid daily dose, and pain response in AC receiving supportive care. Additional studies are needed to validate our findings for PPT.

GTP Cyclohydrolase I as a Potential Drug Target: New Insights into Its Allosteric Modulation via Normal Mode Analysis

Guanosine triphosphate (GTP) cyclohydrolase I (GCH1) catalyzes the conversion of GTP into dihydroneopterin triphosphate (DHNP). DHNP is the first intermediate of the folate de novo biosynthesis pathway in prokaryotic and lower eukaryotic microorganisms and the tetrahydrobiopterin (BH4) biosynthesis pathway in higher eukaryotes. The de novo folate biosynthesis provides essential cofactors for DNA replication, cell division, and synthesis of key amino acids in rapidly replicating pathogen cells, such as Plasmodium falciparum (P. falciparum), a causative agent of malaria. In eukaryotes, the product of the BH4 biosynthesis pathway is essential for the production of nitric oxide and several neurotransmitter precursors. An increased copy number of the malaria parasite P. falciparum GCH1 gene has been reported to influence antimalarial antifolate drug resistance evolution, whereas mutations in the human GCH1 are associated with neuropathic and inflammatory pain disorders. Thus, GCH1 stands as an important and attractive drug target for developing therapeutics. The GCH1 intrinsic dynamics that modulate its activity remains unclear, and key sites that exert allosteric effects across the structure are yet to be elucidated. This study employed the anisotropic network model to analyze the intrinsic motions of the GCH1 structure alone and in complex with its regulatory partner protein. We showed that the GCH1 tunnel-gating mechanism is regulated by a global shear motion and an outward expansion of the central five-helix bundle. We further identified hotspot residues within sites of structural significance for the GCH1 intrinsic allosteric modulation. The obtained results can provide a solid starting point to design novel antineuropathic treatments for humans and novel antimalarial drugs against the malaria parasite P. falciparum GCH1 enzyme.

Peripheral tetrahydrobiopterin is involved in the pathogenesis of mechanical hypersensitivity in a rodent postsurgical pain model

Because treatment for postsurgical pain (PSP) remains a major unmet medical need, the emergence of safe and innovative nonopioid drugs has been strongly coveted. Tetrahydrobiopterin (BH4) is an interesting molecule for gaining a better understanding the pathological mechanism of neuropathic pain. However, whether BH4 and its pathway are involved in the pathogenesis of PSP remains unclear. In this study, we found that early in a rat paw incision model, the gene expression of GTP cyclohydrolase 1 (GTPCH) and sepiapterin reductase (SPR), BH4-producing enzymes in the de novo pathway, were significantly increased in incised compared with naive paw skin. Although a significant increase in GTPCH protein levels was observed in incised paw skin until only 1 day after incision, a significant increase in BH4 levels was observed until 7 days after incision. In vivo, Spr-knockout mice showed an antinociceptive phenotype in the hind paw incision compared with the wild-type and Spr heterozygote groups. Furthermore, QM385, the SPR inhibitor, showed a significant dose-dependent, antinociceptive effect, which was supported by a reduction in BH4 levels in incised skin tissues, with no apparent adverse effects. Immunohistochemical analysis demonstrated that macrophages expressing GTPCH protein were increased around the injury site in the rat paw incision model. These results indicate that BH4 is involved in the pathogenesis of PSP, and that inhibition of the BH4 pathway could provide a new strategy for the treatment of acute PSP.

Can Implementation of Genetics and Pharmacogenomics Improve Treatment of Chronic Low Back Pain?

Etiology of back pain is multifactorial and not completely understood, and for the majority of people who suffer from chronic low back pain (cLBP), the precise cause cannot be determined. We know that back pain is somewhat heritable, chronic pain more so than acute. The aim of this review is to compile the genes identified by numerous genetic association studies of chronic pain conditions, focusing on cLBP specifically. Higher-order neurologic processes involved in pain maintenance and generation may explain genetic contributions and functional predisposition to formation of cLBP that does not involve spine pathology. Several genes have been identified in genetic association studies of cLBP and roughly, these genes could be grouped into several categories, coding for: receptors, enzymes, cytokines and related molecules, and transcription factors. Treatment of cLBP should be multimodal. In this review, we discuss how an individual's genotype could affect their response to therapy, as well as how genetic polymorphisms in CYP450 and other enzymes are crucial for affecting the metabolic profile of drugs used for the treatment of cLBP. Implementation of gene-focused pharmacotherapy has the potential to deliver select, more efficacious drugs and avoid unnecessary, polypharmacy-related adverse events in many painful conditions, including cLBP.

RNA-binding proteins in tumor progression

RNA-binding protein (RBP) has a highly dynamic spatiotemporal regulation process and important biological functions. They are critical to maintain the transcriptome through post-transcriptionally controlling the processing and transportation of RNA, including regulating RNA splicing, polyadenylation, mRNA stability, mRNA localization, and translation. Alteration of each process will affect the RNA life cycle, produce abnormal protein phenotypes, and thus lead to the occurrence and development of tumors. Here, we summarize RBPs involved in tumor progression and the underlying molecular mechanisms whereby they are regulated and exert their effects. This analysis is an important step towards the comprehensive characterization of post-transcriptional gene regulation involved in tumor progression.

Repurposing of Tranilast for Potential Neuropathic Pain Treatment by Inhibition of Sepiapterin Reductase in the BH4 Pathway

Tetrahydrobiopterin (BH₄) is a cofactor in the production of various signaling molecules including nitric oxide, dopamine, adrenaline, and noradrenaline. BH₄ levels are critical for processes associated with cardiovascular function, inflammation, mood, pain, and neurotransmission. Increasing pieces of evidence suggest that BH₄ is upregulated in chronic pain. Sepiapterin reductase (SPR) catalyzes both the reversible reduction of sepiapterin to dihydrobiopterin (BH₂) and 6-pyruvoyl-tetrahydrobiopterin to BH₄ within the BH₄ pathway. Therefore, inhibition of SPR by small molecules can be used to control BH₄ production and ultimately alleviate chronic pain. Here, we have used various in silico and in vitro experiments to show that tranilast, licensed for use in bronchial asthma, can inhibit sepiapterin reduction by SPR. Docking and molecular dynamics simulations suggest that tranilast can bind to human SPR (hSPR) at the same site as sepiapterin including S157, one of the catalytic triad residues of hSPR. Colorimetric assays revealed that tranilast was nearly twice as potent as the known hSPR inhibitor, N-acetyl serotonin. Tranilast was able to inhibit hSPR activity both intracellularly and extracellularly in live cells. Triple quad mass spectrophotometry of cell lysates showed a proportional decrease of BH₄ in cells treated with tranilast. Our results suggest that tranilast can act as a potent hSPR inhibitor and therefore is a valid candidate for drug repurposing in the treatment of chronic pain.

Genetic Variants Associated with Cancer Pain and Response to Opioid Analgesics: Implications for Precision Pain Management

OBJECTIVE

To review the current knowledge on the association of genetic variants with cancer pain.

DATA SOURCES

Data-based publications and review articles retrieved from PubMed, CINAHL, and Web of Science, as well as an additional search in Google Scholar.

CONCLUSION

Genetic variability can influence differential pain perception and response to opioids in cancer patients, which will have implications in the optimal personalized treatment of cancer pain. More studies are warranted to replicate findings.

IMPLICATIONS FOR NURSING PRACTICE

Nurses are poised to educate patients on biomarker testing and interpretation and to use precision pain management strategies based on this information.

Genetics of perioperative pain management

PURPOSE OF REVIEW

The current review will discuss the current literature on genetics of pain and analgesia, with special emphasis on perioperative setting. We will also discuss pharmacogenetics-based management guidelines, current clinical status and future perspectives.

RECENT FINDINGS

Recent literature suggests that the interindividual variability in pain and postoperative analgesic response is at least in part because of one's genetic make-up. Some of the well characterized polymorphisms that are associated with surgical pain and opioid-related postoperative adverse outcomes are described in catechol-O-methyl transferase, CYP2D6 and μ-opioid receptor (OPRM1), ATP-binding cassette subfamily B member 1, ABCC3, organic cation transporter 1 genes. Clinical Pharmacogenetics Implementation Consortium has put forth recommendations on CYP2D6 genotype-based opioid selection and dosing. The list of drug-gene pairs studied continue to expand.

SUMMARY

Pharmacogenetic approach marks the dawn of personalized pain medicine both in perioperative and chronic pain settings.

Development of an AmpliSeqTM Panel for Next-Generation Sequencing of a Set of Genetic Predictors of Persisting Pain

Background: Many gene variants modulate the individual perception of pain and possibly also its persistence. The limited selection of single functional variants is increasingly being replaced by analyses of the full coding and regulatory sequences of pain-relevant genes accessible by means of next generation sequencing (NGS).

Methods: An NGS panel was created for a set of 77 human genes selected following different lines of evidence supporting their role in persisting pain. To address the role of these candidate genes, we established a sequencing assay based on a custom AmpliSeq^TM panel to assess the exomic sequences in 72 subjects of Caucasian ethnicity. To identify the systems biology of the genes, the biological functions associated with these genes were assessed by means of a computational over-representation analysis.

Results: Sequencing generated a median of 2.85 ⋅ 10⁶ reads per run with a mean depth close to 200 reads, mean read length of 205 called bases and an average chip loading of 71%. A total of 3,185 genetic variants were called. A computational functional genomics analysis indicated that the proposed NGS gene panel covers biological processes identified previously as characterizing the functional genomics of persisting pain.

Conclusion: Results of the NGS assay suggested that the produced nucleotide sequences are comparable to those earned with the classical Sanger sequencing technique. The assay is applicable for small to large-scale experimental setups to target the accessing of information about any nucleotide within the addressed genes in a study cohort.

Genetic variants of GCH1 associate with chronic and acute crisis pain in African Americans with sickle cell disease

The multidimensional nature of pain in sickle cell disease (SCD) has rendered its therapeutic management extremely challenging. In this study, we explored the role of five single nucleotide polymorphisms (SNPs) of candidate gene GCH1 in SCD pain. Composite pain index (CPI) scores and acute care utilization rates were used as phenotype markers. Rs8007267 was associated with chronic pain (additive model: B = -3.76, p = 0.037; dominant model: B = -5.61, p = 0.021) and rs3783641 (additive model: incident rate ratio [IRR] = 1.37, p = 0.024; recessive model: IRR = 1.81, p = 0.018) with utilization rate. These associations persisted when subjects with HbSS and HbSβ° genotype only were analyzed. We also identified two haploblocks (rs10483639[G>C]-rs752688[C>T]-rs4411417[T>C] and rs3783641[T>A]-rs8007267[T>C]) with SNPs in high linkage disequilibrium. Of these, haplotype T-C of haploblock rs3783641-rs8007267 showed significant association with rate of utilization (odds ratio [OR] = 0.31, p = 0.001). Our study indicates potential contribution of GCH1 polymorphisms to the variability of pain in African Americans with SCD.

Opioids and the Blood-Brain Barrier: A Dynamic Interaction with Consequences on Drug Disposition in Brain

Background:

Opioids are widely used in pain management, acting via opioid receptors and/or Toll-like receptors (TLR) present at the central nervous system (CNS). At the blood-brain barrier (BBB), several influx and efflux transporters, such as the ATP-binding cassette (ABC)

P-glycoprotein (P-gp, ABCB1), Breast Cancer Resistance Protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRP, ABCC) transporters, and solute carrier transporters (SLC), are responsible for the transport of xenobiotics from the brain into the bloodstream or vice versa.

Objective:

ABC transporters export several clinically employed opioids, altering their neuro-

pharmacokinetics and CNS effects. In this review, we explore the interactions between opioids

and ABC transporters, and decipher the molecular mechanisms by which opioids can modify their expression at the BBB.

Results:

P-gp is largely implicated in the brain-to-blood efflux of opioids, namely morphine and oxycodone. Long-term ex-posure to morphine and oxycodone has proven to up-regulate the expression of ABC transporters, such as P-gp, BCRP and MRPs, at the BBB, which may lead to increased tolerance to the antinociceptive effects of such drugs. Recent studies uncov-er two mechanisms by which morphine may up-regulate P-gp and BCRP at the BBB: 1) via a glutamate, NMDA-receptor and COX-2 signaling cascade, and 2) via TLR4 activation, subsequent development of neuro-

inflammation, and activation of NF-κB, presumably via glial cells.

Conclusion:

The BBB-opioid interaction can culminate in bilateral consequences, since ABC transporters condition the brain disposition of opioids, while opioids also affect the expression of ABC transporters at the BBB, which may result in increased CNS drug pharmacoresistance.

Heterozygous mutations in GTP-cyclohydrolase-1 reduce BH4 biosynthesis but not pain sensitivity

Human studies have demonstrated a correlation between noncoding polymorphisms of "the pain protective" haplotype in the GCH1 gene that encodes for GTP cyclohydrolase I (GTPCH1)-which leads to reduced tetrahydrobiopterin (BH4) production in cell systems-and a diminished perception of experimental and clinical pain. Here, we investigate whether heterozygous mutations in the GCH1 gene which lead to a profound BH4 reduction in patients with dopa-responsive dystonia (DRD) have any effect on pain sensitivity. The study includes an investigation of GCH1-associated biomarkers and pain sensitivity in a cohort of 22 patients with DRD and 36 controls. The patients with DRD had, when compared with controls, significantly reduced levels of BH4, neopterin, biopterin, and GTPCH1 in their urine, blood, or cytokine-stimulated fibroblasts, but their pain response with respect to non-painful stimulation, (acute) stimulus-evoked pain, or pain response after capsaicin-induced sensitization was not significantly different. A family-specific cohort of 11 patients with DRD and 11 controls were included in this study. The patients with DRD were heterozygous for the pain protective haplotype in cis with the GCH1 disease-causing mutation, c.899T>C. No effect on pain perception was observed for this combined haplotype. In conclusion, a reduced concentration of BH4 is not sufficient to alter ongoing pain sensitivity or evoked pain responses.

Association analyses of variants of SIPA1L2, MIR4697, GCH1, VPS13C, and DDRGK1 with Parkinson's disease in East Asians

A recent large-scale European-originated genome-wide association data meta-analysis followed by a replication study identified 6 new risk loci for Parkinson's disease (PD), which include rs10797576/SIPA1L2, rs117896735/INPP5F, rs329648/MIR4697, rs11158026/GCH1, rs2414739/VPS13C, and rs8118008/DDRGK1. However, whether these new loci are associated with PD in Asian populations remain elusive. The INPP5F is nonpolymorphic in Asians. The present study aimed to understand the effects of the other 5 new loci in a Han Chinese population comprising 579 sporadic PD patients and 642 controls. Significant associations with PD were observed in the variants of SIPA1L2 (p = 0.001) and VPS13C (p = 0.007), where the T (odd ratio [OR] = 1.484, 95% confidence interval [CI] 1.186-1.858) and A (OR = 1.362, 95% CI 1.087-1.707) alleles serve as the risk alleles, respectively. The genotype distributions in the SIPA1L2 and VPS13C variants were also different between the patients and controls (p = 0.002 and p = 0.023, respectively). In contrast, no significant association with PD was found in the variants of MIR4697, GCH1, and DDRGK1 either in allele or genotype frequencies. Noteworthy, a followed meta-analysis of East Asian studies suggested an association of the GCH1 variant with PD (p = 0.04, OR 1.08, 95% CI 1.00-1.16), while the other results are in line with those of our cohort. In conclusion, our study together with meta-analyses demonstrates that the variants of SIPA1L2 and VPS13C, potentially GCH1, but not of MIR4697 and DDRGK1, are associated with PD susceptibility in East Asians.

Genetic studies of human neuropathic pain conditions: a review

Numerous studies have shown associations between genetic variants and neuropathic pain disorders. Rare monogenic disorders are caused by mutations of substantial effect size in a single gene, whereas common disorders are likely to have a contribution from multiple genetic variants of mild effect size, representing different biological pathways. In this review, we survey the reported genetic contributors to neuropathic pain and submit them for validation in a 150,000-participant sample of the U.K. Biobank cohort. Successfully replicated association with a neuropathic pain construct for 2 variants in IL10 underscores the importance of neuroimmune interactions, whereas genome-wide significant association with low back pain (P = 1.3e-8) and false discovery rate 5% significant associations with hip, knee, and neck pain for variant rs7734804 upstream of the MAT2B gene provide evidence of shared contributing mechanisms to overlapping pain conditions at the molecular genetic level.

Combining Human and Rodent Genetics to Identify New Analgesics

Most attempts at rational development of new analgesics have failed, in part because chronic pain involves multiple processes that remain poorly understood. To improve translational success, one strategy is to select novel targets for which there is proof of clinical relevance, either genetically through heritable traits, or pharmacologically. Such an approach by definition yields targets with high clinical validity. The biology of these targets can be elucidated in animal models before returning to the patients with a refined therapeutic. For optimal treatment, having biomarkers of drug action available is also a plus. Here we describe a case study in rational drug design: the use of controlled inhibition of peripheral tetrahydrobiopterin (BH4) synthesis to reduce abnormal chronic pain states without altering nociceptive-protective pain. Initially identified in a population of patients with low back pain, the association between BH4 production and chronic pain has been confirmed in more than 12 independent cohorts, through a common haplotype (present in 25% of Caucasians) of the rate-limiting enzyme for BH4 synthesis, GTP cyclohydrolase 1 (GCH1). Genetic tools in mice have demonstrated that both injured sensory neurons and activated macrophages engage increased BH4 synthesis to cause chronic pain. GCH1 is an obligate enzyme for de novo BH4 production. Therefore, inhibiting GCH1 activity eliminates all BH4 production, affecting the synthesis of multiple neurotransmitters and signaling molecules and interfering with physiological function. In contrast, targeting the last enzyme of the BH4 synthesis pathway, sepiapterin reductase (SPR), allows reduction of pathological BH4 production without completely blocking physiological BH4 synthesis. Systemic SPR inhibition in mice has not revealed any safety concerns to date, and available genetic and pharmacologic data suggest similar responses in humans. Finally, because it is present in vivo only when SPR is inhibited, sepiapterin serves as a reliable biomarker of target engagement, allowing potential quantification of drug efficacy. The emerging development of therapeutics that target BH4 synthesis to treat chronic pain illustrates the power of combining human and mouse genetics: human genetic studies for clinical selection of relevant targets, coupled with causality studies in mice, allowing the rational engineering of new analgesics.

Central sensitization associated with low fetal hemoglobin levels in adults with sickle cell anemia

BACKGROUND AND AIMS

Pain is the hallmark of sickle cell anemia (SCA), presenting as recurrent acute events or chronic pain. Central sensitization, or enhanced excitability of the central nervous system, alters pain processing and contributes to the maintenance of chronic pain. Individuals with SCA demonstrate enhanced sensitivity to painful stimuli however central mechanisms of pain have not been fully explored. We hypothesized that adults with SCA would show evidence of central sensitization as observed in other diseases of chronic pain.

METHODS

We conducted a prospective study of static and dynamic quantitative sensory tests in 30 adults with SCA and 30 matched controls.

RESULTS

Static thermal testing using cold stimuli showed lower pain thresholds (p=0.04) and tolerance (p=0.04) in sickle cell subjects, but not for heat. However, SCA subjects reported higher pain ratings with random heat pulses (p<0.0001) and change in scores with temporal summation at the heat pain threshold (p=0.002). Similarly, with the use of pressure pain stimuli, sickle cell subjects reported higher pain ratings (p=0.04), but not higher pressure pain tolerance/thresholds or allodynia to light tactile stimuli. Temporal summation pain score changes using 2 pinprick probes (256 and 512mN) were significantly greater (p=0.004 and p=0.008) with sickle cell, and delayed recovery was associated with lower fetal hemoglobin (p=0.002 and 0.003).

CONCLUSIONS

Exaggerated temporal summation responses provide evidence of central sensitization in SCA.

IMPLICATIONS

The association with fetal hemoglobin suggests this known SCA modifier may have a therapeutic role in modulating central sensitization.

SRSF2 Regulates Alternative Splicing to Drive Hepatocellular Carcinoma Development

Aberrant RNA splicing is recognized to contribute to cancer pathogenesis, but the underlying mechanisms remain mainly obscure. Here, we report that the splicing factor SRSF2 is upregulated frequently in human hepatocellular carcinoma (HCC), where this event is associated with poor prognosis in patients. RNA-seq and other molecular analyses were used to identify SRSF2-regulated alternative splicing events. SRSF2 binding within an alternative exon was associated with its inclusion in the RNA, whereas SRSF2 binding in a flanking constitutive exon was associated with exclusion of the alternative exon. Notably, cancer-associated splice variants upregulated by SRSF2 in clinical specimens of HCC were found to be crucial for pathogenesis and progression in hepatoma cells, where SRSF2 expression increased cell proliferation and tumorigenic potential by controlling expression of these variants. Our findings identify SRSF2 as a key regulator of RNA splicing dysregulation in cancer, with possible clinical implications as a candidate prognostic factor in patients with HCC. Cancer Res; 77(5); 1168-78. ©2017 AACR.

Associations Between Neurotransmitter Genes and Fatigue and Energy Levels in Women After Breast Cancer Surgery

CONTEXT

Fatigue is a common problem in oncology patients. Less is known about decrements in energy levels and the mechanisms that underlie both fatigue and energy.

OBJECTIVES

In patients with breast cancer, variations in neurotransmitter genes between lower and higher fatigue latent classes and between the higher and lower energy latent classes were evaluated.

METHODS

Patients completed assessments before and monthly for six months after surgery. Growth mixture modeling was used to identify distinct latent classes for fatigue severity and energy levels. Thirty candidate genes involved in various aspects of neurotransmission were evaluated.

RESULTS

Eleven single-nucleotide polymorphisms or haplotypes (i.e., ADRB2 rs1042718, BDNF rs6265, COMT rs9332377, CYP3A4 rs4646437, GALR1 rs949060, GCH1 rs3783642, NOS1 rs9658498, NOS1 rs2293052, NPY1R Haplotype A04, SLC6A2 rs17841327, and 5HTTLPR + rs25531 in SLC6A4) were associated with latent class membership for fatigue. Seven single-nucleotide polymorphisms or haplotypes (i.e., NOS1 rs471871, SLC6A1 rs2675163, SLC6A1 Haplotype D01, SLC6A2 rs36027, SLC6A3 rs37022, SLC6A4 rs2020942, and TAC1 rs2072100) were associated with latent class membership for energy. Three of 13 genes (i.e., NOS1, SLC6A2, and SLC6A4) were associated with latent class membership for both fatigue and energy.

CONCLUSIONS

Molecular findings support the hypothesis that fatigue and energy are distinct, yet related symptoms. Results suggest that a large number of neurotransmitters play a role in the development and maintenance of fatigue and energy levels in breast cancer patients.

MAPK1/ERK2 as novel target genes for pain in head and neck cancer patients

Background

Genetic susceptibility plays an important role in the risk of developing pain in individuals with cancer. As a complex trait, multiple genes underlie this susceptibility. We used gene network analyses to identify novel target genes associated with pain in patients newly diagnosed with squamous cell carcinoma of the head and neck (HNSCC).

Results

We first identified 36 cancer pain-related genes (i.e., focus genes) from 36 publications based on a literature search. The Ingenuity Pathway Analysis (IPA) analysis identified additional genes that are functionally related to the 36 focus genes through pathway relationships yielding a total of 82 genes. Subsequently, 800 SNPs within the 82 IPA-selected genes on the Illumina HumanOmniExpress-12v1 platform were selected from a large-scale genotyping effort. Association analyses between the 800 candidate SNPs (covering 82 genes) and pain in a patient cohort of 1368 patients with HNSCC (206 patients with severe pain vs. 1162 with non-severe pain) showed the highest significance for MAPK1/ERK2, a gene belonging to the MAP kinase family (rs8136867, p value = 8.92 × 10⁻⁴; odds ratio [OR] = 1.33, 95 % confidence interval [CI]: 1.13–1.58). Other top genes were PIK3C2G (a member of PI3K [complex], rs10770367, p value = 1.10 × 10⁻³; OR = 1.46, 95 % CI: 1.16–1.82), TCRA (the alpha chain of T-cell receptor, rs6572493, p value = 2.84 × 10⁻³; OR = 0.70, 95 % CI: 0.55–0.88), PDGFC (platelet-derived growth factor C, rs6845322, p value = 4.88 × 10⁻³; OR = 1.32, 95 % CI: 1.09–1.60), and CD247 (a member of CD3, rs2995082, p value = 7.79 × 10⁻³; OR = 0.76, 95 % CI: 0.62–0.93).

Conclusions

Our findings provide novel candidate genes and biological pathways underlying pain in cancer patients. Further study of the variations of these candidate genes could inform clinical decision making when treating cancer pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0348-7) contains supplementary material, which is available to authorized users.

Human pain and genetics: some basics

Human pain causes untold misery and suffering, with major impact on functioning and resources. Recent advances in genetics have revealed that subtle changes in DNA could partly explain the variation in individual differences in pain. Various genes encoding for receptors are now known to play a major role in the sensitivity, perception and expression of pain. The fields of epigenetics and proteomics hold promises in the way pain could be treated and managed in future.

Evidence of a role for GTP cyclohydrolase-1 in visceral pain

BACKGROUND

The enzyme guanosine triphosphate-cyclohydrolase-1 (GCH-1) is a rate limiting step in the de novo synthesis of tetrahydrobiopterin (BH4) a co-factor in monoamine synthesis and nitric oxide production. GCH-1 is strongly implicated in chronic pain based on data generated using the selective GCH-1 inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP), and studies which have identified a pain protective GCH-1 haplotype associated with lower BH4 production and reduced pain.

METHODS

To investigate the role for GCH-1 in visceral pain we examined the effects of DAHP on pain behaviors elicited by colorectal injection of mustard oil in rats, and the pain protective GCH-1 haplotype in healthy volunteers characterized by esophageal pain sensitivity before and after acid injury, and assessed using depression and anxiety questionnaires.

KEY RESULTS

In rodents pretreatment with DAHP produced a substantial dose related inhibition of pain behaviors from 10 to 180 mg/kg i.p. (p < 0.01 to 0.001). In healthy volunteers, no association was seen between the pain protective GCH-1 haplotype and the development of hypersensitivity following injury. However, a substantial increase in baseline pain thresholds was seen between first and second visits (26.6 ± 6.2 mA) in subjects who sensitized to esophageal injury and possessed the pain protective GCH-1 haplotype compared with all other groups (p < 0.05). Furthermore the same subjects who sensitized to acid and possessed the haplotype, also had significantly lower depression scores (p < 0.05).

CONCLUSIONS & INFERENCES

The data generated indicate that GCH-1 plays a role in visceral pain processing that requires more detailed investigation.

Translational pain research: Lessons from genetics and genomics

Pharmacological, surgical, psychological, and alternative medicine approaches for the treatment of chronic pain, including neuropathic pain, provide only partial relief for most patients, with the efficacy of existing medications often blunted by dose-limiting side effects arising from drug actions on cells outside the pain-signaling axis. The development of more effective treatments for pain--particularly chronic pain states such as neuropathic pain--has been hampered by lack of predictive animal models and biomarkers, variation in pain characteristics between patients or on a day-to-day basis for single patients, patient stratification on the basis of symptoms rather than mechanism, and a high rate of placebo responses. We discuss genetic and genomic approaches to translational pain research. We review examples of the identification and validation of human pain targets through rodent genome-wide association studies (GWAS) and global mRNA expression studies, functional screening in flies and mice, human GWAS and whole-exome sequencing studies, and the targeted candidate gene approach. These and other emerging genetic and genomic strategies are likely to facilitate the development of new, more effective pain therapeutics.

GCH1 variants, tetrahydrobiopterin and their effects on pain sensitivity

Background A great proportion of the variation in pain experience and chronicity is caused by heritable factors. Within the last decades several candidate genes have been discovered either increasing or decreasing pain sensitivity or the risk of chronic pain in humans. One of the most studied genes is the GCH1 gene coding for the enzyme GTP cyclohydrolase 1 (GCH1). GCH1 catalyses the initial and rate-limiting step in the biosynthesis of tetrahydrobiopterin (BH4). The main function of BH4 is regulation of monoamine and nitric oxide biosynthesis, all involved in nociceptive signalling. Methods In this topical review we focus on the implication of the GCH1 gene and BH4 in painful conditions. We discuss experimental evidence from our group in relation to relevant research publications evaluating the BH4 pathway in pain. Studies assessing the role of GCH1 and BH4 in pain consist of human and animal studies, including DOPA-responsive dystonia (DRD) patients and hph-1 mice (a genetic mouse model of DRD) having mutations in the GCH1 gene as well as preclinical studies with the GCH1 inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP). The hypothesis is that genetic and pharmacological reduction of GCH1 would result in lower pain sensitivity. Results Previous studies have demonstrated that a particular "pain protective" GCH1 haplotype, found in 15% of the general human population, is linked to decreased pain sensitivity. We further support these findings in DRD patients, showing normal thresholds to mechanical and thermal stimuli, whereas a trend towards lower pain sensitivity is seen following chemical pain sensitisation. Consistent with these observations, non-injured hph-1 mice displayed normal mechano- and thermosensation compared to wild-type mice. After peripheral inflammation with Complete Freund' Adjuvant or sensitisation with capsaicin the mutant mice exhibited lower sensitivity to mechanical and heat stimuli. Moreover, hph-1 mice showed decreased nociception in the first phase of the formalin test. Several studies report analgesic effects of GCH1 inhibition with 90-270 mg/kg DAHP in rat models of inflammatory and neuropathic pain. However, we could not completely replicate these findings in mice. Fairly higher doses of DAHP (≥270 mg/kg) were needed to reduce inflammatory pain in mice, but the window between antinociception and toxic effects was small, since 400 mg/kg DAHP affected motor performance and general appearance. Also, the analgesic effects were marginal in mice compared to that observed in rats. Conclusions Variations in the GCH1 gene in both humans and mice appear to regulate pain sensitivity and pain behaviours, particularly after pain sensitisation, whereas pain sensitivity to phasic mechanical and thermal stimuli is normal. Moreover, pharmacological inhibition of GCH1 shows antinociceptive effects in preclinical pain studies, though our studies imply that GCH1 inhibition may have a small therapeutic index. Implications The implication of the GCH1 gene in pain may increase our understanding of the risk factors of chronic pain development and improve current pain therapy by personalised medicine. In addition, inhibition of GCH1 provides a potential target for analgesic drug development, though GCH1 inhibitors should possess local or partial effects to avoid serious side-effects to the central nervous system and cardiovascular system.

A GCH1 haplotype confers sex-specific susceptibility to pain crises and altered endothelial function in adults with sickle cell anemia

GTP cyclohydrolase (GCH1) is rate limiting for tetrahydrobiopterin (BH4) synthesis, where BH4 is a cofactor for nitric oxide (NO) synthases and aromatic hydroxylases. GCH1 polymorphisms are implicated in the pathophysiology of pain, but have not been investigated in African populations. We examined GCH1 and pain in sickle cell anemia where GCH1 rs8007267 was a risk factor for pain crises in discovery (n = 228; odds ratio [OR] 2.26; P = 0.009) and replication (n = 513; OR 2.23; P = 0.004) cohorts. In vitro, cells from sickle cell anemia subjects homozygous for the risk allele produced higher BH4. In vivo physiological studies of traits likely to be modulated by GCH1 showed rs8007267 is associated with altered endothelial dependent blood flow in females with SCA (8.42% of variation; P = 0.002). The GCH1 pain association is attributable to an African haplotype with where its sickle cell anemia pain association is limited to females (OR 2.69; 95% CI 1.21-5.94; P = 0.01) and has the opposite directional association described in Europeans independent of global admixture. The presence of a GCH1 haplotype with high BH4 in populations of African ancestry could explain the association of rs8007267 with sickle cell anemia pain crises. The vascular effects of GCH1 and BH4 may also have broader implications for cardiovascular disease in populations of African ancestry.

Association between variants of 5-hydroxytryptamine receptor 3C (HTR3C) and chemotherapy-induced symptoms in women receiving adjuvant treatment for breast cancer

Administration of chemotherapy is associated with a wide array of symptoms affecting quality of life. Genetic risk factors for severity of chemotherapy-induced symptoms have not been determined. The present study aimed to explore the associations between polymorphisms in candidate genes and chemotherapy-induced symptoms. Women treated with at least two cycles of adjuvant doxorubicin and cyclophosphamide, with or without paclitaxel for early breast cancer (n = 105) completed the memorial symptom assessment scale and provided blood for genotyping. DNA was extracted from peripheral blood leukocytes and assayed for single nucleotide polymorphisms (SNPs) in GTP cyclohydrolase 1 (GCH1, rs10483639, rs3783641, and rs8007267), catecholamine-o-methyltransferase (COMT, rs4818), and 5-hydroxytryptamine (serotonin) receptor 3C (HTR3C, rs6766410, and rs6807362). Genotyping of HTR3C revealed a significant association between the presence of rs6766410 and rs6807362 SNPs (K163 and G405 variants) and increased severity of symptoms (p = 0.0001 and p = 0.007, respectively). Multiple regressions revealed that rs6766410 and rs6807362, but not age or stage at diagnosis, predicted severity of symptoms (p = 0.001 and p = 0.006, respectively) and explained 12 % of the variance in each regression model. No association was found between the genetic variants of CGH1 or COMT and symptom score. Our study indicates, for the first time, an association between variants of HTR3C and severity of chemotherapy-induced symptoms. Analyzing these genetic variants may identify patients at increased risk for the development of chemotherapy-induced symptoms and targeting the serotonin pathway may serve as a novel treatment strategy for these patients.

Nature and nurture of human pain

Humans are very different when it comes to pain. Some get painful piercings and tattoos; others can not stand even a flu shot. Interindividual variability is one of the main characteristics of human pain on every level including the processing of nociceptive impulses at the periphery, modification of pain signal in the central nervous system, perception of pain, and response to analgesic strategies. As for many other complex behaviors, the sources of this variability come from both nurture (environment) and nature (genes). Here, I will discuss how these factors contribute to human pain separately and via interplay and how epigenetic mechanisms add to the complexity of their effects.

Structures and reaction mechanisms of GTP cyclohydrolases

GTP cyclohydrolases generate the first committed intermediates for the biosynthesis of certain vitamins/cofactors (folic acid, riboflavin, deazaflavin, and tetrahydrobiopterin), deazapurine antibiotics, some t-RNA bases (queuosine, archaeosine), and the phytotoxin, toxoflavin. They depend on divalent cations for hydrolytic opening of the imidazole ring of the substrate, guanosine triphosphate (GTP). Surprisingly, the ring opening reaction is not the rate-limiting step for GTP cyclohydrolases I and II whose mechanism have been studied in some detail. GTP cyclohydrolase I, Ib, and II are potential targets for novel anti-infectives. Genetic factors modulating the activity of human GTP cyclohydrolase are highly pleiotropic, since the signal transponders whose biosyntheses require their participation (nitric oxide, catecholamines) impact a very wide range of physiological phenomena. Recent studies suggest that human GTP cyclohydrolase may become an oncology target.

Impaired behavioural pain responses in hph-1 mice with inherited deficiency in GTP cyclohydrolase 1 in models of inflammatory pain

BACKGROUND

GTP cyclohydrolase 1 (GTP-CH1), the rate-limiting enzyme in the synthesis of tetrahydrobiopterin (BH4), encoded by the GCH1 gene, has been implicated in the development and maintenance of inflammatory pain in rats. In humans, homozygous carriers of a "pain-protective" (PP) haplotype of the GCH1 gene have been identified exhibiting lower pain sensitivity, but only following pain sensitisation. Ex vivo, the PP GCH1 haplotype is associated with decreased induction of GCH1 after stimulation, whereas the baseline BH4 production is not affected. Contrary, loss of function mutations in the GCH1 gene results in decreased basal GCH1 expression, and is associated with DOPA-responsive dystonia (DRD). So far it is unknown if such mutations affect acute and inflammatory pain.

RESULTS

In the current study, we examined the involvement of the GCH1 gene in pain models using the hyperphenylalaninemia 1 (hph-1) mouse, a genetic model for DRD, with only 10% basal GTP-CH1 activity compared to wild type mice. The study included assays for determination of acute nociception as well as models for pain after sensitisation. Pain behavioural analysis of the hph-1 mice showed reduced pain-like responses following intraplantar injection of CFA, formalin and capsaicin; whereas decreased basal level of GTP-CH1 activity had no influence in naïve hph-1 mice on acute mechanical and heat pain thresholds. Moreover, the hph-1 mice showed no signs of motor impairment or dystonia-like symptoms.

CONCLUSIONS

In this study, we demonstrate novel evidence that genetic mutations in the GCH1 gene modulate pain-like hypersensitivity. Together, the present data suggest that BH4 is not important for basal heat and mechanical pain, but they support the hypothesis that BH4 plays a role in inflammation-induced hypersensitivity. Our studies suggest that the BH4 pathway could be a therapeutic target for the treatment of inflammatory pain conditions. Moreover, the hph-1 mice provide a valid model to study the consequence of congenital deficiency of GCH1 in painful conditions.

Inhibition of GTP cyclohydrolase attenuates tumor growth by reducing angiogenesis and M2-like polarization of tumor associated macrophages

GTP cyclohydrolase (GCH1) is the key-enzyme to produce the essential enzyme cofactor, tetrahydrobiopterin. The byproduct, neopterin is increased in advanced human cancer and used as cancer-biomarker, suggesting that pathologically increased GCH1 activity may promote tumor growth. We found that inhibition or silencing of GCH1 reduced tumor cell proliferation and survival and the tube formation of human umbilical vein endothelial cells, which upon hypoxia increased GCH1 and endothelial NOS expression, the latter prevented by inhibition of GCH1. In nude mice xenografted with HT29-Luc colon cancer cells GCH1 inhibition reduced tumor growth and angiogenesis, determined by in vivo luciferase and near-infrared imaging of newly formed blood vessels. The treatment with the GCH1 inhibitor shifted the phenotype of tumor associated macrophages from the proangiogenic M2 towards M1, accompanied with a shift of plasma chemokine profiles towards tumor-attacking chemokines including CXCL10 and RANTES. GCH1 expression was increased in mouse AOM/DSS-induced colon tumors and in high grade human colon and skin cancer and oppositely, the growth of GCH1-deficient HT29-Luc tumor cells in mice was strongly reduced. The data suggest that GCH1 inhibition reduces tumor growth by (i) direct killing of tumor cells, (ii) by inhibiting angiogenesis, and (iii) by enhancing the antitumoral immune response.

GCH1-polymorphism and pain sensitivity among women with provoked vestibulodynia

Background

Provoked vestibulodynia (PVD) is a pain disorder localized in the vestibular mucosa. It is the most common cause of dyspareunia among young women and it is associated with general pain hypersensitivity and other chronic pain conditions. Polymorphism in the guanosine triphosphate cyclohydrolase (GCH1) gene has been found to influence general pain sensitivity and the risk of developing a longstanding pain condition. The aim of this study was to investigate GCH1-polymorphism in women with PVD and healthy controls, in correlation to pain sensitivity.

Results

We found no correlation between the previously defined pain-protective GCH1-SNP combination and the diagnosis of PVD. Nor any correlation with pain sensitivity measured as pressure pain thresholds on the arm, leg and in the vestibule, coital pain scored on a visual analog scale and prevalence of other bodily pain conditions among women with PVD (n = 98) and healthy controls (n = 102). However, among patients with current treatment (n = 36), there was a significant interaction effect of GCH1-gene polymorphism and hormonal contraceptive (HC) therapy on coital pain (p = 0.04) as well as on pressure pain thresholds on the arm (p = 0.04). PVD patients carrying the specified SNP combination and using HCs had higher pain sensitivity compared to non-carriers. In non-HC-users, carriers had lower pain sensitivity.

Conclusions

The results of this study gave no support to the hypothesis that polymorphism in the GCH1-gene contributes to the etiology of PVD. However, among patients currently receiving treatment an interaction effect of the defined SNP combination and use of hormonal contraceptives on pain sensitivity was found. This finding offers a possible explanation to the clinically known fact that some PVD patients improve after cessation of hormonal contraceptives, indicating that PVD patients carrying the defined SNP combination of GCH1 would benefit from this intervention.

Pharmacogenetics of pain and analgesia

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.

Inhibition of GTP cyclohydrolase reduces cancer pain in mice and enhances analgesic effects of morphine

Noncoding polymorphisms of the GTP cyclohydrolase gene (GCH1) reduce the risk for chronic pain in humans suggesting GCH1 inhibitors as analgesics. We assessed the effects of the GCH1 inhibitor diaminohydroxypyrimidine (DAHP) on nociception and inflammation in a mouse melanoma and a sarcoma cancer pain model, and its co-effects with morphine in terms of analgesic efficacy and respiratory depression. GCH1 inhibition did not reduce the tumor-evoked nociceptive hypersensitivity of the tumor-bearing paw. However, DAHP reduced melanoma- and sarcoma-evoked systemic hyperalgesia as determined by analyzing contralateral paws. GCH1 inhibition increased the inflammatory edema and infiltration with polymorphonuclear leukocytes surrounding the tumor but reduced the tumor-evoked microglia activation in the spinal cord suggesting that an increase of the local immune attack against the tumor may avoid general pain hypersensitivity. When used in combination with morphine at high or low doses, GCH1 inhibition increased and prolonged the analgesic effects of the opioid. It did not, however, increase the respiratory depression caused by morphine. Conversely, the GCH1-product, tetrahydrobiopterin, caused hyperalgesia, antagonized antinociceptive effects of morphine, and aggravated morphine-evoked respiratory depression, the latter mimicked by a cGMP analog suggesting that respiratory effects were partly mediated through the BH4-NO-cGMP pathway. The observed effects of GCH1 inhibition in the tumor model and its enhancement of morphine-evoked antinociception without increase of morphine toxicity suggest that GCH1 inhibitors might be useful as co-therapeutics for opioids in cancer patients.

Opioid genetics in the context of opioid switching

PURPOSE OF REVIEW

On a population level, there is no difference in terms of efficacy or side-effects between any of the strong opioids. On an individual level, however, there is marked variation in response to opioids. This review presents some of the recent advances in opioid pharmacogenetic studies.

RECENT FINDINGS

A growing number of genes have been studied in a number of different patient populations. Most data have come from candidate-gene studies. There have been two genome-wide association studies in pain and opioid response. The clinical and genetic complexity of response to opioids has limited the clinical applicability of the genetic results. Currently, interindividual variation in opioid response is managed clinically through a process known as opioid switching. The evidence supporting the efficacy of opioid switching is poor, mainly because randomized controlled trials in this area are lacking.

SUMMARY

Adequately powered studies to allow identification of genetic variants with small effect size and exploration of gene-gene interaction are needed. Integration of genetic analysis in clinical studies with carefully defined outcome measures will increase the likelihood of identifying clinical and genetic factors which can be used to predict opioid response.

Analgesia by inhibiting tetrahydrobiopterin synthesis

Physiological control of the co-factor tetrahydrobiopterin (BH4) is tight in normal circumstances but levels increase pathologically in the injured somatosensory system. BH4 is an essential co-factor in the production of serotonin, dopamine, epinephrine, norepinephrine and nitric oxide. Excess BH4 levels cause pain, likely through excess production of one or more of these neurotransmitters or signaling molecules. The rate limiting step for BH4 production is GTP Cyclohydrolase 1 (GCH1). A human GCH1 gene haplotype exists that leads to less GCH1 transcription, translation, and therefore enzyme activity, following cellular stress. Carriers of this haplotype produce less BH4 and therefore feel less pain, especially following nerve injury where BH4 production is pathologically augmented. Sulfasalazine (SSZ) an FDA approved anti-inflammatory agent of unknown mechanism of action, has recently been shown to be a sepiapterin reductase (SPR) inhibitor. SPR is part of the BH4 synthesis cascade and is also upregulated by nerve injury. Inhibiting SPR will reduce BH4 levels and therefore should act as an analgesic. We propose SSZ as a novel anti-neuropathic pain medicine.

Advancements in pain research

After the publication of the First Edition of this book in the series of Methods in Molecular Medicine (volume 99 in the series) in 2004, pain research continues its rapid acceleration until 2009, during which it experienced a plateau of growth that likely resulted from the economic downturn started in 2008 (Fig. 1.1). This rapid growth in pain research could be the driving force for an impressive 66% increase in new randomized, double-blind, placebo-control trials for neuropathic pain medications in the past 5 years compared with the last four decades. Unfortunately, little improvement in pain medications has been obtained [1] due to primarily our limited understanding of mechanisms mediating different pain states, especially that for chronic pain. It is highly possible that the growth in pain research will continue for decades to come due to three main reasons. First, there is an urgent need for more efficacious and safer pain medications that are necessary for better and individualized pain management. The increase in life expectancy of the general population and patients due to improvements in quality of health care and medicine is likely to increase the demand for better pain medications for improving the quality of daily life of those living with pain. It is estimated that the continuous increase in percentage of patients suffering from chronic pain (pain conditions lasting more than 6 months) arranges from 11 to 47% between 40 and 75 years of age [2], which will inevitably and continually increase the demand for better pain medications. Second, the cost of pain conditions to our society is high, estimated $55 billion per year in loss of productivity from full-time workers alone [3], so better pain management can significantly help economic growth and stability. Third, the swift advancement in technologies and our better understanding of sensory circuitries and pain pathways serves as a driving force for timely drug discovery research and development at an unprecedented pace to meet the demand for better pain medications.

Genetic basis of pain variability: recent advances

An estimated 15-50% of the population experiences pain at any given time, at great personal and societal cost. Pain is the most common reason patients seek medical attention, and there is a high degree of individual variability in reporting the incidence and severity of symptoms. Research suggests that pain sensitivity and risk for chronic pain are complex heritable traits of polygenic origin. Animal studies and candidate gene testing in humans have provided some progress in understanding the heritability of pain, but the application of the genome-wide association methodology offers a new tool for further elucidating the genetic contributions to normal pain responding and pain in clinical populations. Although the determination of the genetics of pain is still in its infancy, it is clear that a number of genes play a critical role in determining pain sensitivity or susceptibility to chronic pain. This review presents an update of the most recent findings that associate genetic variation with variability in pain and an overview of the candidate genes with the highest translational potential.

Pharmacogenetics of new analgesics

Patient phenotypes in pharmacological pain treatment varies between individuals, which could be partly assigned to their genotypes regarding the targets of classical analgesics (OPRM1, PTGS2) or associated signalling pathways (KCNJ6). Translational and genetic research have identified new targets, for which new analgesics are being developed. This addresses voltage-gated sodium, calcium and potassium channels, for which SCN9A, CACNA1B, KCNQ2 and KCNQ3, respectively, are primary gene candidates because they code for the subunits of the respective channels targeted by analgesics currently in clinical development. Mutations in voltage gated transient receptor potential (TRPV) channels are known from genetic pain research and may modulate the effects of analgesics under development targeting TRPV1 or TRPV3. To this add ligand-gated ion channels including nicotinic acetylcholine receptors, ionotropic glutamate-gated receptors and ATP-gated purinergic P2X receptors with most important subunits coded by CHRNA4, GRIN2B and P2RX7. Among G protein coupled receptors, δ-opioid receptors (coded by OPRD1), cannabinoid receptors (CNR1 and CNR2), metabotropic glutamate receptors (mGluR5 coded by GRM5), bradykinin B(1) (BDKRB1) and 5-HT(1A) (HTR1A) receptors are targeted by new analgesic substances. Finally, nerve growth factor (NGFB), its tyrosine kinase receptor (NTRK1) and the fatty acid amide hydrolase (FAAH) have become targets of interest. For most of these genes, functional variants have been associated with neuro-psychiatric disorders and not yet with analgesia. However, research on the genetic modulation of pain has already identified variants in these genes, relative to pain, which may facilitate the pharmacogenetic assessments of new analgesics. The increased number of candidate pharmacogenetic modulators of analgesic actions may open opportunities for the broader clinical implementation of genotyping information.

GCH1, BH4 and pain

Understanding and consequently treating neuropathic pain effectively is a challenge for modern medicine, as unlike inflammation, which can be controlled relatively well, chronic pain due to nerve injury is refractory to most current therapeutics. Here we define a target pathway for a new class of analgesics, tetrahydrobiopterin (BH4) synthesis and metabolism. BH4 is an essential co-factor in the synthesis of serotonin, dopamine, epinephrine, norepinephrine and nitric oxide and as a result, its availability influences many systems, including neurons. Following peripheral nerve damage, levels of BH4 are dramatically increased in sensory neurons, consequently this has a profound effect on the physiology of these cells, causing increased activity and pain hypersensitivity. These changes are principally due to the upregulation of the rate limiting enzyme for BH4 synthesis GTP Cyclohydrolase 1 (GCH1). A GCH1 pain-protective haplotype which decreases pain levels in a variety of settings, by reducing the levels of endogenous activation of this enzyme, has been characterized in humans. Here we define the control of BH4 homeostasis and discuss the consequences of large perturbations within this system, both negatively via genetic mutations and after pathological increases in the production of this cofactor that result in chronic pain. We explain the nature of the GCH1 reduced-function haplotype and set out the potential for a ' BH4 blocking' drug as a novel analgesic.

Pain, analgesia and genetics

OBJECTIVES

In the clinical setting, there is marked intersubject variability in the intensity of pain reported by patients with apparently similar pain states, as well as widely differing analgesic dosing requirements between individuals to produce satisfactory pain relief with tolerable side-effects. Genetic and environmental factors as well as their interaction are implicated, and these are discussed in this review.

KEY FINDINGS

Pioneering work undertaken in mice more than a decade ago, showed a strong genetic contribution to levels of nociception/hypersensitivity as well as levels of antinociception produced by commonly available analgesic agents. To date more than 300 candidate 'pain' genes have been identified as potentially contributing to heritable differences in pain sensitivity and analgesic responsiveness in animals and humans, with this information available in a publicly accessible database http://www.jbldesign.com/jmogil/enter.html. Since then, many genetic association studies have been conducted in humans to investigate the possibility that single nucleotide polymorphisms (SNPs) in an individual gene may explain drug inefficacy or excessive toxicity experienced by a small subset of the whole population who have the rare allele for a particular SNP.

SUMMARY

Despite the fact that SNPs in more than 20 genes that affect pain sensitivity or contribute to interindividual variability in responses to analgesic medications have been identified in the human genome, much of the data is conflicting. Apart from deficiencies in the design and conduct of human genetic association studies, recent research from other fields has implicated epigenetic mechanisms that facilitate dynamic gene-environment communication, as a possible explanation.

A yeast-based screen reveals that sulfasalazine inhibits tetrahydrobiopterin biosynthesis

We introduce an approach for detection of drug-protein interactions that combines a new yeast three-hybrid screening for identification of interactions with affinity chromatography for their unambiguous validation. We applied the methodology to the profiling of clinically approved drugs, resulting in the identification of previously known and unknown drug-protein interactions. In particular, we were able to identify off-targets for erlotinib and atorvastatin, as well as an enzyme target for the anti-inflammatory drug sulfasalazine. We demonstrate that sulfasalazine and its metabolites, sulfapyridine and mesalamine, are inhibitors of the enzyme catalyzing the final step in the biosynthesis of the cofactor tetrahydrobiopterin. The interference with tetrahydrobiopterin metabolism provides an explanation for some of the beneficial and deleterious properties of sulfasalazine and furthermore suggests new and improved therapies for the drug. This work thus establishes a powerful approach for drug profiling and provides new insights in the mechanism of action of clinically approved drugs.

Genomics and pain research in sickle cell disease: an explanation of heterogeneity?

Sickle cell disease (SCD) is a chronic illness, and the major complication, pain, results in complex multidimensional problems that affect an individual's ability to maintain adequate quality of life in multiple areas. Chronic SCD pain is inadequately treated, because it is not well understood, and the degree of chronic pain, clinical presentation, and sequela complications can vary from patient to patient, even among individuals with the same SCD genotype. The reason for this variation is unknown, but the underlying cause might be genetic. Researchers have not explored the contribution of a genomic variable to the occurrence of heterogeneous chronic SCD pain. Previous research on the guanosine triphosphate cyclohydrolase (GCH1) gene suggests that in some cases, phenotypic heterogeneity in human sensitivity to pain correlates with underlying genotypic variations in the GCH1 gene. These findings imply that genotypic variations might also explain why some SCD patients experience more chronic pain than others.

[Predictors of chronic pain following surgery. What do we know?]

Chronic postoperative pain is known to be a significant clinical and economic problem. The estimated mean incidence is high and varies between 10 and 50%, with variations mostly related to procedure-specific conditions. High-risk types of surgeries are e.g. thoracotomy, breast or inguinal hernia surgery and amputations. Although there is increasing knowledge about the incidence of chronic postoperative pain after certain types of surgical procedures, there are only limited data related to the mechanisms and pathophysiology leading to chronic pain after surgery. Neuropathic pain components have been discussed, especially following operations with a high incidence of nerve damage (for example axillary lymphadenectomy). Besides surgical factors it seems that there are a number of other factors which likely increase the risk of chronic postoperative pain. These predictors for the development of chronic postoperative pain are multiple and include individual genetic factors, age and sex of the individual patient, preoperative chronic pain, psychosocial factors, neurophysiological factors, intraoperative nerve and muscle damage, postoperative complications and acute pain in the early postoperative period. Quantitative sensory testing including tests of inhibitory circuits like DNIC might help to predict the risk of individual patients even before surgery has started. The perioperative identification of patients who are at high risk for developing chronic pain after surgery is therefore a major goal for the future. This may help to develop preventive treatment strategies and avoid treatments with side effects for patients who are not at risk for developing chronic pain after surgery. Due to a lack of appropriate data for sufficient preventive approaches an effective postoperative acute pain management and a nerve-conserving surgical technique are the major keys in the prophylaxis of chronic postoperative pain.

Influence from genetic variability on opioid use for cancer pain: a European genetic association study of 2294 cancer pain patients

Cancer pain patients need variable opioid doses. Preclinical and clinical studies suggest that opioid efficacy is related to genetic variability. However, the studies have small samples, findings are not replicated, and several candidate genes have not been studied. Therefore, a study of genetic variability with opioid doses in a large population using a confirmatory validation population was warranted. We recruited 2294 adult European patients using a World Health Organization (WHO) step III opioid and analyzed single nucleotide polymorphisms (SNPs) in genes with a putative influence on opioid mechanisms. The patients' mean age was 62.5 years, and the average pain intensity was 3.5. The patients' primary opioids were morphine (n=830), oxycodone (n=446), fentanyl (n=699), or other opioids (n=234). Pain intensity, time on opioids, age, gender, performance status, and bone or CNS metastases predicted opioid dose and were included as covariates. The patients were randomly divided into 1 development sample and 1 validation sample. None of 112 SNPs in the 25 candidate genes OPRM1, OPRD1, OPRK1, ARRB2, GNAZ, HINT1, Stat6, ABCB1, COMT, HRH1, ADRA2A, MC1R, TACR1, GCH1, DRD2, DRD3, HTR3A, HTR3B, HTR2A, HTR3C, HTR3D, HTR3E, HTR1, or CNR1 showed significant associations with opioid dose in both the development and the validation analyzes. These findings do not support the use of pharmacogenetic analyses for the assessed SNPs to guide opioid treatment. The study also demonstrates the importance of validating findings obtained in genetic association studies to avoid reporting spurious associations as valid findings. To elicit knowledge about new genes that influence pain and the need for opioids, strategies other than the candidate gene approach is needed.