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Childhood and adult obesity have been widely associated with FTO genetic variability in different populations. This study aimed to investigate the linkage disequilibrium (LD) block structure of a region surrounding the candidate rs9939609 SNP and determine the best single nucleotide polymorphism (SNP) combination that explains the higher proportion of variability observed in children with severe obesity, including obese subjects from families with a very high occurrence of obesity. A sliding window approach pointed to a block containing the rs1477196/rs17817449/rs9939609 haplotype (P value 3.1 × 10(-8)). Carriers of the GGA combination had an increased risk of obesity (odds ratio (OR) 2.07, 95% confidence interval (CI) 1.41-3.04, P = 2.0 × 10(-4)) with respect to those individuals with the reference ATT haplotype. A further SNP, rs9921255, also showed association with obesity (P = 8.3 × 10(-4), OR 1.77; 95% CI 1.15-2.74 and OR 5.78; 95% CI 1.22-27.49 for heterozygotes and homozygotes, respectively) and did not segregate with the previously described risk haplotype. The calculation of risk score based on the GGA haplotype combined with the rs9921255 variant showed a much greater effect of the FTO gene on high BMI. This score yields an attributable risk of 34% for severe obesity, and the increased risk per risk allele was 1.71 (P = 1.0 × 10(-6)). We conclude that the description of this polymorphic combination in the FTO gene could be useful for the early identification of inherited susceptibility to weight-gain since childhood, with a higher sensitivity than considering the effect of a single marker.

Objective: To select individuals whose morbid obesity can be attributed mainly to their individual genetic profile. After excluding patients with potential monogenic syndromes or diseases associated with obesity, we evaluated the association of the single nucleotide polymorphisms (SNPs) rs1861868 and rs9939609 of the fat-mass and obesity-associated FTO gene with an inherited predisposition to morbid obesity., Patients and Methods: We evaluated 270 patients with morbid obesity and onset before the age of 14 years and selected 194 due to their phenotypes and family history; 289 control individuals were included. The rs1861868 and rs9939609 variants, located in the FTO gene, were genotyped. Genotype and haplotype frequencies were compared between cases and controls., Results: The A allele of rs9939609 was associated with severe obesity starting in childhood among the Spanish population. The rs1861868 G/rs9939609 A haplotype of the FTO gene was also significantly associated with severe obesity in our population, with an odds ratio of 3.03 (95% confidence interval, 1.74-5.27)., Conclusion: Analysis of the genetic basis of obesity requires rigorous selection of cases. In this study, the association of the rs9939609 SNP with obesity widely described in distinct populations was confirmed among overweight Spanish children. Genotyping rs1861868 allowed us to identify the first risk haplotype in the FTO gene, which is located in the adjacent haplotype block containing rs9939609. In-depth study of the variability of the FTO gene is essential to define its deleterious capacity., (Copyright (c) 2009 SEEN. Published by Elsevier Espana. All rights reserved.)