CORDIS - EU research results

Functional characterization of TrkB and Ob-R mutations identified in severely obese children

Final Activity Report Summary - TRKB AND OB-R (Functional characterization of TrkB and Ob-R mutations identified in severely obese children)

Genome wide association studies have recently revealed the importance of fat mass and obesity associated gene (FTO) as an obesity-susceptibility gene. Based on in silico structural and sequence analysis, we have suggested that FTO is part of the family of Fe(II) 2-oxoglutarate (2-OG) oxygenases. Murine Fto can demethylate 3-methylthymine in vitro, in a 2-OG-dependent manner, suggesting a possible role in DNA repair. Fto is highly expressed in the hypothalamus of mice, including in the arcuate nucleus, where it is nutritionally regulated. However, nothing is known about its role in the regulation of energy balance.

A key objective was to further investigate the role of FTO in humans, we asked whether there is a difference in the number of functionnaly significant mutations in FTO in lean and obese individuals. As a result we sequenced FTO coding exons and intron-exon boundaries in 1433 lean and 1433 severely obese individuals. Results: We identified 33 heterozygous non-synonymous variants in lean (2.3%) and 35 in obese (2.4%) individuals, with 8 mutations unique to the obese and 11 unique to the lean. Three mutations replace absolutely conserved residues; R316Q and R322Q in the catalytic domain, and R96H in the predicted substrate recognition lid of the protein. R316Q and R322Q were unable to catalyse the conversion of 2-OG to succinate in the presence or absence of 3-methylthymidine. R96H retained some basal activity, which was not enhanced by 3-methylthymidine.

However, these mutations were found in both lean and obese individuals. The other missense mutations did not significantly alter FTO function. While these studies were on-going, we ascertained a large consanguineous multiplex family in which nine individuals presented with a previously unreported polymalformative syndrome including intra-uterine and post-natal growth retardation, severe psychomotor delay, structural and functional brain malformations, cardiac defects, genital anomalies, cleft palate and characteristic facial dysmorphism. All affected children died before the age of 3 years. Genetic screening revealed that all affected individuals were homozygous for R316Q, one of the mutations that ablate FTO function.

While body composition has not yet been investigated, it is worth noting that an obese phenotype was NOT reported in the homozygous affected individuals or heterozygous carrier relatives. Thus, we have reported the first case of FTO deficiency in humans and found that FTO is absolutely necessary for normal development of the central nervous and cardiovascular systems. We can also conclude from our genetics studies that heterozygosity for a severely dysfunctional FTO allele is compatible with being either lean or obese in humans.