Genetically determined P2X7 receptor pore formation regulates variability in chronic pain sensitivity.

Chronic pain is highly variable between individuals, as is the response to analgesics. Although much of the variability in chronic pain and analgesic response is heritable, an understanding of the genetic determinants underlying this variability is rudimentary. Here we show that variation within the coding sequence of the gene encoding the P2X7 receptor (P2X7R) affects chronic pain sensitivity in both mice and humans. P2X7Rs, which are members of the family of ionotropic ATP-gated receptors, have two distinct modes of function: they can function through their intrinsic cationic channel or by forming nonselective pores that are permeable to molecules with a mass of up to 900 Da. Using genome-wide linkage analyses, we discovered an association between nerve-injury-induced pain behavior (mechanical allodynia) and the P451L mutation of the mouse P2rx7 gene, such that mice in which P2X7Rs have impaired pore formation as a result of this mutation showed less allodynia than mice with the pore-forming P2rx7 allele. Administration of a peptide corresponding to the P2X7R C-terminal domain, which blocked pore formation but not cation channel activity, selectively reduced nerve injury and inflammatory allodynia only in mice with the pore-forming P2rx7 allele. Moreover, in two independent human chronic pain cohorts, a cohort with pain after mastectomy and a cohort with osteoarthritis, we observed a genetic association between lower pain intensity and the hypofunctional His270 (rs7958311) allele of P2RX7. Our findings suggest that selectively targeting P2X7R pore formation may be a new strategy for individualizing the treatment of chronic pain.

Association between polymorphisms in serotonin and dopamine-related genes and endogenous pain modulation.

UNLABELLED: Genetic studies have become indispensable in understanding pain mechanisms, shedding light on the role of monoamine pathways in pain modulation. The present study was aimed to explore the relationship between functional polymorphisms in serotonin and dopamine-related genes and pain modulation. Two paradigms of pain modulation were administered to 191 healthy participants in a random order: Conditioned Pain Modulation in response to painful stimuli (CPM(painful)) tested by the coadministration of repeated short painful heat stimuli and a conditioning tonic cold pain stimulation; and Conditioned Pain Modulation in response to nonpainful stimuli (CPM(nonpainful)) tested similarly, except for using a painless conditioning stimulation. Using the Transmission Disequilibrium Test (TDT), functional variable number of tandem repeat (VNTR) polymorphisms of the following candidate genes were studied: 1) serotonin transporter (5-HTTLPR); 2) dopamine transporter (DAT1); 3) dopamine receptor 4 (DRD4); and 4) monoamine oxidase A (MAOA). DNA samples from both participants and their parents were analyzed. A significant association was found between CPM(nonpainful) and the 5-HTTLPR polymorphism (P = .001). More specifically, carriers of the long allele exhibited a significantly higher magnitude of CPM(nonpainful) than carriers of the short allele. No associations were found between the dopamine-related genes and both types of pain modulation. These results highlight the importance of serotonin in endogenous analgesia. PERSPECTIVE: This article presents an association between the serotonin transporter gene polymorphism (5-HTTLPR) and pain modulation derived by nonpainful conditioned pain modulation (CPM(nonpainful)), rather than painful conditioned pain modulation (CPM(painful)). These findings emphasize the complex role of serotonin in pain modulation, and highlight the importance of genetic studies in the understanding of interindividual differences in sensitivity to pain.

Multiple chronic pain states are associated with a common amino acid-changing allele in KCNS1.

Not all patients with nerve injury develop neuropathic pain. The extent of nerve damage and age at the time of injury are two of the few risk factors identified to date. In addition, preclinical studies show that neuropathic pain variance is heritable. To define such factors further, we performed a large-scale gene profiling experiment which plotted global expression changes in the rat dorsal root ganglion in three peripheral neuropathic pain models. This resulted in the discovery that the potassium channel alpha subunit KCNS1, involved in neuronal excitability, is constitutively expressed in sensory neurons and markedly downregulated following nerve injury. KCNS1 was then characterized by an unbiased network analysis as a putative pain gene, a result confirmed by single nucleotide polymorphism association studies in humans. A common amino acid changing allele, the 'valine risk allele', was significantly associated with higher pain scores in five of six independent patient cohorts assayed (total of 1359 subjects). Risk allele prevalence is high, with 18-22% of the population homozygous, and an additional 50% heterozygous. At lower levels of nerve damage (lumbar back pain with disc herniation) association with greater pain outcome in homozygote patients is P = 0.003, increasing to P = 0.0001 for higher levels of nerve injury (limb amputation). The combined P-value for pain association in all six cohorts tested is 1.14 E-08. The risk profile of this marker is additive: two copies confer the most, one intermediate and none the least risk. Relative degrees of enhanced risk vary between cohorts, but for patients with lumbar back pain, they range between 2- and 3-fold. Although work still remains to define the potential role of this protein in the pathogenic process, here we present the KCNS1 allele rs734784 as one of the first prognostic indicators of chronic pain risk. Screening for this allele could help define those individuals prone to a transition to persistent pain, and thus requiring therapeutic strategies or lifestyle changes that minimize nerve injury.

Factors affecting - and relationships between-different modes of endogenous pain modulation in healthy volunteers.

UNLABELLED: Endogenous analgesia (EA) can be reflected by diffuse noxious inhibitory control (DNIC), non-noxious inhibitory control (NNIC) and habituation to repeated painful stimuli. However, the coexistence of these phenomena in a given individual and the degree to which various factors predict their magnitudes have not been fully investigated. Using experimental paradigms of DNIC, NNIC and habituation, the present study explored the relationships between - and the contribution factors to - the magnitude of EA exhibited by healthy volunteers (n=191; 104 F, 87 M) exposed to these three experimental paradigms. Each subject was assigned to all three paradigms (DNIC-tested by co-administering repeated short painful heat stimuli and a conditioning tonic cold pain stimulation; NNIC - tested similarly with the exception of using a painless conditioning stimulation; habituation - tested by applying repeated painful heat stimuli only) in a random order. Pain intensities decreased from baseline in all three paradigms. However, DNIC produced significantly more pain reduction than the other two modes (RM-ANOVA). The magnitude of pain reduction of DNIC was found to be highly correlated with that of NNIC and habituation (r=0.56, p<0.001 for both correlations). A hierarchical regression analysis showed that baseline (p<0.001) and conditioning pain scores (p=0.043) predicted the magnitude of DNIC. A gender split analysis showed that conditioning pain scores served as a predictive factor for men only. CONCLUSIONS: Under these experimental conditions, different EA conditions seem to be related to each other. High initial pain intensities predict 'effective' DNIC and habituation, whereas intensity of the conditioning stimulus determines the magnitude of DNIC in men only.

Associations between polymorphisms in dopamine neurotransmitter pathway genes and pain response in healthy humans.

Although evidence shows that several dopamine neurotransmission pathway genes are associated with specific clinical pain syndromes, such as fibromyalgia, chronic headache, and postoperative pain, the exact role of dopamine in pain processing is not fully understood. The aim of this study was to explore the relationship between functional polymorphisms in dopaminergic candidate genes and sensitivity to pain in healthy subjects. Healthy subjects (n=192; 105 F, 87 M) were exposed to experimental tonic cold pain (1 degrees C) and phasic heat pain (47 degrees C) stimuli. DNA samples were obtained from both participants and their parents. The relationships between pain response (intensity in response to heat and cold; threshold and tolerance in response to cold only) and the functional Variable Number of Tandem Repeat (VNTR) polymorphisms of three dopamine-related genes were investigated using a Transmission Disequilibrium Test (TDT). Specifically, 30-bp repeat in the promoter region of the monoamine oxidase-A gene (MAO-A), 40-bp repeat in the 3'-untranslated region of the dopamine transporter gene (DAT-1), and 48-bp repeat in the exon 3 of the dopamine receptor 4 gene (DRD4) were examined. Significant associations between cold pain tolerance and DAT-1 (p=0.008) and MAO-A (p=0.024) polymorphisms were found. Specifically, tolerance was shorter for carriers of allele 10 and the rarer allele 11, as compared to homozygous for allele 9, and for carriers of allele 4 as compared to homozygous for allele 3, respectively. These results, together with the known function of the investigated candidate gene polymorphisms, suggest that low dopaminergic activity can be associated with high pain sensitivity and vice versa.

Lowest neonatal serum sodium predicts sodium intake in low birth weight children.

Forty-one children aged 10.5 +/- 0.2 years (range, 8.0-15.0 yr), born with low birth weight of 1,218.2 +/- 36.6 g (range, 765-1,580 g) were selected from hospital archives on the basis of whether they had received neonatal diuretic treatment or as healthy matched controls. The children were tested for salt appetite and sweet preference, including rating of preferred concentration of salt in tomato soup (and sugar in tea), ratings of oral spray (NaCl and sucrose solutions), intake of salt or sweet snack items, and a food-seasoning, liking, and dietary questionnaire. Results showed that sodium appetite was not related to neonatal diuretic treatment, birth weight, or gestational age. However, there was a robust inverse correlation (r = -0.445, P < 0.005) between reported dietary sodium intake and the neonatal lowest serum sodium level (NLS) recorded for each child as an index of sodium loss. The relationship of NLS and dietary sodium intake was found in both boys and girls and in both Arab and Jewish children, despite marked ethnic differences in dietary sources of sodium. Hence, low NLS predicts increased intake of dietary sodium in low birth weight children some 8-15 yr later. Taken together with other recent evidence, it is now clear that perinatal sodium loss, from a variety of causes, is a consistent and significant contributor to long-term sodium intake.