LINKAGE AND ASSOCIATION STUDIES BETWEEN THE MELANOCORTIN RECEPTOR-4 AND RECEPTOR-5 GENES AND OBESITY-RELATED PHENOTYPES IN THE QUEBEC FAMILY STUDY

Background: The agouti yellow mouse shows adult onset of moderate obes ity and diabetes. A depressed basal lipolytic rate in adipocytes or a decreased adrenergic tone arising from antagonizing cr-melanocyte-stim ulating hormone (MSH) activation of melanocortin receptors (MCR) could be at the origin of the obesity phenotype. Materials and Methods: MCR 4 and 5 (MC4R, MCSR) genes were studied in the Quebec Family Study. S equence variations were detected by Southern blot probing of restricte d genomic DNA, and mRNA tissue expression was detected by RT-PCR. Subj ects with a wide range of weight were used for single-point sib-pair l inkage studies (maximum of 289 sibships from 124 nuclear families). An alysis of variance across genotypes in unrelated males (n = 143) and f emales (n = 156) was also undertaken. Body mass index (BMI), sum of si x skinfolds (SF6), fat mass (FM), percent body fat (%FAT), respiratory quotient (RQ), resting metabolic rate (RMR), fasting glucose and insu lin, and glucose and insulin area during an oral glucose tolerance tes t were analyzed. Results: MC4R showed polymorphism with NcoI, and MCSR , with PstI and PvuII, with a heterozygosity of 0.38, 0.10, and 0.20, respectively. Linkages were observed between MCSR and BMI (p = 0.001), SF6 (p = 0.005), FM (p = 0.001), and RMR (p = 0.002), whereas associa tions were observed in females between MCSR and BMI (p = 0.003), and b etween MC4R and FM (p = 0.002) and %FAT (p = 0.004). After correction for multiple tests, these p values are lowered by one tenth. MC4R and MC5R mRNAs have been detected in brain, adipose tissue, and skeletal m uscle. Conclusions: MC4R and MC5R exibit evidence of linkage or associ ation with obesity phenotypes, but this evidence is strongest for MCSR .