Anorexia nervosa (AN) is an eating disorder with unknown etiology. The multifactorial pathogenesis of AN has been emphasized in various studies [1-3]. Accordingly, heritability estimates derived from twin studies revealed that 58–76% of the variance of AN can be explained by genetic factors [4]. The highest incidence for the development of AN is around puberty and patients with AN are typically characterized by an abnormal eating behaviour with disturbances of attitudes towards body weight and shape [5]. Therefore, it is reasonable that genetic factors regulating food intake and body weight are implicated in the pathogenesis of AN [1,2,6,7].

One of the endogenous systems that, due to its therapeutic potential in the treatment of obesity, recently reached scientific interest is the endocannabinoid system. Both exogenous (e.g. Δ⁹-tetrahydrocannabinol) and endogenous cannabinoids (e.g. anandamide and 2-arachidonoylglycerol (2-AG)) stimulate food intake through activation of the cannabinoid receptor 1 (CNR1) [8]. In contrast, inhibition of CNR1 signalling through administration of the selective inverse agonist rimonabant (Acomplia^®) decreases food intake in both rodents [9-11] and humans [12,13]. The endocannabinoid system further interacts with the leptinergic system; obese rodents with disturbed leptin signal transduction (ob/ob and db/db mice as well as fa/fa rats) show elevated levels of anandamide and 2-AG in the hypothalamus. Vice versa, leptin treatment of ob/ob mice decreased hypothalamic levels of both, anandamide and 2-AG [11]. Accordingly, compared to age matched normal weight controls, serum levels of anandamide are increased in patients with AN. Additionally, plasma levels of leptin are negatively correlated with anandamide in both, patients with AN and normal weight healthy controls [2]. In light of these observations, it has previously been suggested that the endocannabinoid system might be implicated in the etiology of AN, in particular through its interaction with the leptinergic system [2,7].

Hypoleptinemia is a cardinal feature of prolonged semi-starvation that entails various metabolic and neuroendocrine alterations which are typically observed in patients with acute AN [5,14] The most prominent neuroendocrine alterations mediated by semi-starvation induced hypoleptinemia include amenorrhea, osteopenia/osteoporosis, and alterations of the hypothalamic-pituitary-gonadal (HPG) and -adrenal (HPA) axis [5,14]. Additionally, several lines of evidence indicate that hypoleptinemia entails development of hyperactivity in patients with AN [5,14-18]. However, the implication of the endocannabinoid system in body weight regulation together with its interaction with the leptinergic system makes it a plausible system implicated in the pathogenesis of AN [2,7].

The most prominent endocannabinoids are N-arachidonoylethanolamine (anandamide) [19] and 2-arachidonoylglycerol (2-AG) [20]. Both are synthesised through cells on demand and undergo a rapid degradation through specific hydrolases and lipases [8,21,22]. The most prominent endocannabinoid degrading enzymes are the fatty acid amide hydrolase (FAAH), the N-acylethanolamine-hydrolyzing acid amidase (NAAA) and the monoglyceride lipase (MGLL) [23]. FAAH is a membrane-bound 60–65 kDa protein that is widely distributed throughout the brain and the periphery [23,24]. Under alkaline conditions, FAAH rapidly inhibits the orexigenic effects of anandamide by degrading it to ethanolamine and arachidonic acid [25,26]. NAAA is an enzyme with similar function but has, in contrast to FAAH, its pH optimum at 4.5–5 [27,28]. The monoglyceride lipase (MGLL) is a serine hydrolase that hydrolyses 2-AG in glycerol and arachidonic acid [22]. As endocannabinoids stimulate food intake through activation of CNR1 and as FAAH, NAAA and MGLL counteract the orexigenic effects of endocannabinoids through their rapid degradation, genetic variation of CNR1 leading to decreased receptor signalling as well as genetic variation of FAAH, NAAA and MGLL leading to increased enzyme activity might be implicated in the etiology of AN.

We performed association studies to analyse whether allelic variation in CNR1, FAAH, NAAA and MGLL is related to the AN phenotype. We therefore genotyped the previously described (AAT)_n repeat in the 3' flanking region of CNR1 as well as a total of 15 SNPs representative of regions with restricted haplotype diversity in CNR1 (rs2180619, rs806379, rs1535255, rs2023239 and rs1049353), FAAH (rs932816, rs324420, rs324419, rs2295632 and rs873978), NAAA (rs2292534, rs6532046, rs10518142 and rs874546) and MGLL (rs893294) in up to 91 German AN trios (patient with AN and both biological parents). Genotyping was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry or allele specific polymerase chain reaction (ARMS-PCR). The AAT₁₄ repeat allele of CNR1 has recently been found to be associated with the binge eating/purging type of AN whereby the AAT₁₃ repeat allele tended to be preferentially transmitted to patients with the restricting type of AN [29]. Furthermore, SNP rs324420 (c.385C/A) in the FAAH gene, leading to decreased enzyme activity and thus increased levels of endocannabinoids and presumably increased food intake, was recently found to be associated with obesity [30] and drug abuse [31]. The CNR1 haplotype comprising the minor alleles of SNP rs806379 (T-allele), rs1535255 (G-allele) and rs2023239 (C-allele) has further been shown to be associated with drug and alcohol abuse in European and African Americans [32].