Association of genetic variation in cannabinoid mechanisms and gastric motor functions and satiation in overweight and obesity

Food intake and associated behavioral factors are important determinants of weight status (1), and the stomach signals satiation during feeding in response to the volume and calories ingested (2,3). Previous studies in our laboratory showed that the postprandial change in gastric volume is an independent predictor of maximum tolerated volume and satiation (4); in addition, when we applied pharmacological perturbations with i.v. erythromycin and atropine to induce postprandial discomfort including fullness and bloating, we observed in a multivariate analysis that accelerated gastric emptying of solids at 1 hour, and delayed gastric emptying at 4 hours, as well as fasting gastric volume were associated with postprandial symptom scores reflecting satiation (5,6). Postprandial gastric volume and change in gastric volume during fasting were univariately associated, but was not significant if fasting gastric volume was included in the model. The endocannabinoid system, a lipid-derived signaling system, has central and peripheral physiologic functions (7). The endocannabinoids, anandamide and 2-arachidonyl glycerol (2-AG), bind to the cannabinoid (CB) receptor and regulate appetite. The cannabinoid receptor type 1 (CB1) is located centrally in the hypothalamus and nucleus accumbens and in peripheral tissues (8,9). There is evidence that gastric emptying is physiologically regulated in rodents by the endocannabinoid system, which is down regulated following a high fat diet leading to overweight (10). In a pharmacodynamic study in healthy human volunteers, we showed that the non-selective cannabinoid agonist dronabinol retards gastric emptying in females and increases fasting gastric volumes in males (11). The endocannabinoid anandamide is hydrolyzed by the enzyme fatty acid amide hydrolase (FAAH), and the polymorphism rs324420 (FAAH C385A, located on chromosome 1 p35–p34) results in an amino acid change at position 129, P129T. This produces a variant that displays normal catalytic properties but an enhanced sensitivity to proteolytic degradation (12). Both cannabinoid receptor 1 (CNR1, located on 6q14–15) and FAAH play an integral part in the control of the endocannabinoid system and the response to endocannabinoids; these mechanisms also play a role in the development of obesity (12,13). Cannabinoids also modulate motor activity in the gastrointestinal tract; for example, non-selective cannabinoid agonist dronabinol inhibits gastric and colonic motility (11,14,15). In humans, studies using a CB1 receptor antagonist, rimonabant, have shown modest weight reduction and improvement in cardiovascular risk factors in obese subjects (16,17). Common variations in genes controlling cannabinoid function or metabolism have been associated with the development of obesity, diabetes as well as with potential alterations in gastric function. FAAH rs324420 results in a mutant form of FAAH with reduced expression and cellular stability (18). Sipe and colleagues suggested that the FAAH rs324420 polymorphism is associated with overweight and obese individuals (19). Sipe et al. also reported that, in severe obesity (BMI>40kg/m2), significantly increased levels of the endocannabinoid anandamide and related N-acylethanolamines were documented in carriers of the FAAH 385 A mutant alleles compared with wild-type FAAH controls (20). This suggests that FAAH 385 A mutant alleles have a direct effect on elevated plasma endocannabinoid levels which may be biomarkers of risk for severe obesity (20). In addition, there is evidence that this missense mutation influences weight loss in response to a hypocaloric diet (21). Polymorphisms in CNR1 are also associated with obesity. Thus, CNR1 gene variation rs2023239 has been shown to predispose to weight gain in Europeans (22), rs1049353 (G1359A) is associated with fat distribution (23) and abdominal adiposity in men (24), and T allele carriers for the CNR1 polymorphism rs806378 (C4558T) have increased odds ratio of childhood obesity (22) and experience increased antipsychotic-induced weight gain compared to CC carriers (25). rs1049353 is a synonymous change that does not alter the amino acid. The T allele associated with CNR1 polymorphism rs806378 is associated with altered nuclear protein binding in a functional (electron shift mobility) assay, suggesting it is a functional polymorphism (25). Therefore, we focused our study on the CNR1 rs806378 polymorphism and studied its association with gastric function and satiation. The aim of this study was to assess the association of allelic variations in the rs324420 (nearest gene FAAH) and rs806378 (nearest gene CNR1) single nucleotide polymorphisms (SNPs) with gastric motor and sensory functions, and satiation (that could predispose to the development of obesity) in a cohort of overweight and obese human participants.