Transmission disequilibrium and sequence variants at the leptin receptor gene in extremely obese German children and adolescents

Genetic determinants of the degree of obesity and body fat distribution hav e been demonstrated by family studies. The heritability has been estimated to be in the range 0.2-0.7. Mutation leading to obesity in humans has been described for only two genes, one of them the leptin gene. The leptin gene codes for a cytokine secreted by fat cells that binds to the leptin recepto r (Lep-R), which exerts some of its biological functions by expression in t he brain. Hence, the Lep-R gene appears to be a promising candidate for the determination of obesity in humans. We isolated genomic DNA clones from th e Lep-R gene region and identified a new polymorphic microsatellite marker (OBR-CA) within XO kb of the translation start of Lep-R. We genotyped this and a second, intragenic microsatellite marker (D1S2852) in 130 nuclear fam ilies consisting of extremely obese children and adolescents and both paren ts. Using the most frequent parental allele of both markers, our analysis r evealed a significant transmission disequilibrium for the 266-bp allele of D1S2852(corrected P-value=0.042). No significant result was obtained with t he most frequent allele of OBR-CA (corrected P-value=1.0). However, two rar e alleles showed transmission disequilibrium and were subsequently used for constructing:: a haplotype with the 266-bp allele. This haplotype had; a t ransmission rate of 80% (nominal P-value=0.02). In order to identify the un derlying mutation, we sequenced all coding exons of Lep-R and the partially overlapping gene encoding the obese receptor gene-related protein (ob-rgrp ) in individuals carrying this haplotype. We found one new mutation (Ser675 Thr) in the Lep-R gene in one proband and several other mutations known to be nor associated with obesity in other study groups. As this new mutation cannot explain our positive linkage result, the transmission disequilibrium of the 266-bp allele and the high transmission rate of the identified hapl otype point towards a mutation in close proximity to marker D1S2852.