FTO gene

Fat mass and obesity-associated protein also known as alpha-ketoglutarate-dependent dioxygenase FTO is an enzyme that in humans is encoded by the FTO gene located on chromosome 16. As one homolog in the AlkB family proteins, it is the first mRNA demethylase that has been identified. Certain variants of the FTO gene appear to be correlated with obesity in humans.

Function

The amino acid sequence of the transcribed FTO protein shows high similarity with the enzyme AlkB which oxidatively demethylates DNA. Recombinant FTO protein was first discovered to catalyze demethylation of 3-methylthymine in single-stranded DNA, and 3-methyluridine in single-stranded RNA, with low efficiency. The nucleoside N6-methyladenosine, an abundant modification in RNA, was then found to be a major substrate of FTO. The FTO gene expression was also found to be significantly upregulated in the hypothalamus of rats after food deprivation and strongly negatively correlated with the expression of orexigenic galanin-like peptide which is involved in the stimulation of food intake.

Increases in hypothalamic expression of FTO are associated with the regulation of energy intake but not feeding reward.

People with two copies of the risk allele for the rs9939609 single nucleotide polymorphism (SNP) showed differing neural responses to food images via fMRI. However, rs9939609's association with FTO is controversial, and may actually affect another gene, called iroquious homeobox protein 3 (IRX3).

FTO demethylates m6A in mRNA

N6-methyladenosine (m⁶A) is an abundant modification in mRNA and is found within some viruses, and most eukaryotes including mammals, insects, plants, and yeast. It is also found in tRNA, rRNA, and small nuclear RNA (snRNA) as well as several long non-coding RNA, such as Xist. Adenosine methylation is directed by a large m⁶A methyltransferase complex containing METTL3 as the SAM-binding sub-unit. In vitro, this methyltransferase complex preferentially methylates RNA oligonucleotides containing GGACU and a similar preference was identified in vivo in mapped m⁶A sites in Rous sarcoma virus genomic RNA and in bovine prolactin mRNA. In plants, the majority of the m⁶A is found within 150 nucleotides before the start of the poly(A) tail.

Mapping of m⁶A in human and mouse RNA has identified over 18,000 m⁶A sites in the transcripts of more than 7,000 human genes with a consensus sequence of [G/A/U][G>A]m⁶AC[U>A/C] consistent with the previously identified motif. Sites preferentially appear in two distinct landmarks—around stop codons and within long internal exons—and are highly conserved between human and mouse. A subset of stimulus-dependent, dynamically modulated sites has been identified. Silencing the m⁶A methyltransferase significantly affects gene expression and alternative RNA splicing patterns, resulting in modulation of the p53 (also known as TP53) signalling pathway and apoptosis.

FTO demethylates m⁶A containing RNA efficiently in vitro. FTO knockdown with siRNA led to increased amounts of m⁶A in polyA-RNA, whereas overexpression of FTO resulted in decreased amounts of m⁶A in human cells. FTO partially co-localizes with nuclear speckles, which supports the notion that m⁶A in nuclear RNA is a major physiological substrate of FTO. Function of FTO likely affects the processing of pre-mRNA, other nuclear RNAs, or both. The discovery of the FTO-mediated oxidative demethylation of m⁶A in nuclear RNA may initiate further investigations on biological regulation based on reversible chemical modification of RNA.

Tissue distribution

The FTO gene is widely expressed in both fetal and adult tissues.

Clinical significance

Association with obesity

A study of 38,759 Europeans for variants of FTO identified an obesity risk allele. In particular, carriers of one copy of the allele weighed on average 1.2 kilograms (2.6 lb) more than people with no copies. Carriers of two copies (16% of the subjects) weighed 3 kilograms (6.6 lb) more and had a 1.67-fold higher rate of obesity than those with no copies. The association was observed in ages 7 and upwards. This gene is not directly associated with diabetes; however, increased body-fat also increases the risk of developing type 2 diabetes.

Simultaneously, a study in 2,900 affected individuals and 5,100 controls of French descent, together with 500 trios (confirming an association independent of population stratification) found association of SNPs in the very same region of FTO (rs1421085). The authors found that this variation, or a variation in strong LD with this variation explains 1% of the population BMI variance and 22% of the population attributable risk of obesity. The authors of this study claim that while obesity was already known to have a genetic component (from twin studies), no replicated previous study has ever identified an obesity risk allele that was so common in the human population. The risk allele is a cluster of 10 single nucleotide polymorphism in the first intron of FTO called rs9939609. According to HapMap, it has population frequencies of 45% in the West/Central Europeans, 52% in Yorubans (West African natives) and 14% in Chinese/Japanese. Furthermore, morbid obesity is associated with a combination of FTO and INSIG2 single nucleotide polymorphisms.

In 2009, variants in the FTO gene were further confirmed to associate with obesity in two very large genome wide association studies of body mass index (BMI).

In adult humans, it was shown that adults bearing the at risk AT and AA alleles at rs9939609 consumed between 500 and 1250 kJ more each day than those carrying the protective TT genotype (equivalent to between 125 and 280 kcal per day more intake). The same study showed that there was no impact of the polymorphism on energy expenditure. This finding of an effect of the rs9939609 polymorphism on food intake or satiety has been independently replicated in five subsequent studies (in order of publication). Three of these subsequent studies also measured resting energy expenditure and confirmed the original finding that there is no impact of the polymorphic variation at the rs9939609 locus on energy expenditure. A different study explored the effects of variation in two different SNPs in the FTO gene (rs17817449 and rs1421085) and suggested there might be an effect on circulating leptin levels and energy expenditure, but this latter effect disappeared when the expenditure was normalised for differences in body composition. The accumulated data across seven independent studies therefore clearly implicates the FTO gene in humans as having a direct impact on food intake but no effect on energy expenditure.

The obesity-associated noncoding region within the FTO gene interacts directly with the promoter of IRX3, a homeobox gene, and IRX5, another homeobox gene. The noncoding region of FTO interacts with the promoters of IRX3 and FTO in human, mouse and zebrafish, and with IRX5. Results suggest that IRX3 and IRX5 are linked with obesity and determine body mass and composition. This is further supported by the fact that obesity-associated single nucleotide polymorphisms, in which cytosine is substituted for thymine, are involved in the expression of IRX3 and IRX5 (not FTO) in human brains. The enhanced expression of IRX3 and IRX5 resulting from this single nucleotide alteration promoted a shift from energy-dissipating beige adipocytes to energy-storing white adipocytes and a subsequent reduction in mitochondrial thermogenesis by a factor of 5. Another study found indications that the FTO allele associated with obesity represses mitochondrial thermogenesis in adipocyte precursor cells in a tissue-autonomous manner, and that there is a pathway for adipocyte thermoregulation which involves the proteine ARID5B, the single-nucleotide variant rs1421085, and the IRX3 and IRX5 genes.

Association with Alzheimer's disease

Recent studies revealed that carriers of common FTO gene polymorphisms show both a reduction in frontal lobe volume of the brain and an impaired verbal fluency performance. Fittingly, a population-based study from Sweden found that carriers of the FTO rs9939609 A allele have an increased risk for incident Alzheimer disease.

Association with other diseases

The presence of the FTO rs9939609 A allele was also found to be positively correlated with other symptoms of the metabolic syndrome, including higher fasting insulin, glucose, and triglycerides, and lower HDL cholesterol. However all these effects appear to be secondary to weight increase since no association was found after correcting for increases in body mass index. Similarly, the association of rs11076008 G allele with the increased risk for degenerative disc disease was reported.

Model organisms

Model organisms have been used in the study of FTO function. In contrast to the findings in humans deletion, analysis of the Fto gene in mice showed loss of function is associated with no differences in energy intake but greater energy expenditure and this results in a reduction of body weight and fatness.

Fto^{tm1a(EUCOMM)Wtsi} phenotype

Characteristic	Phenotype
Homozygote viability	Normal
Fertility	Normal
Body weight	Normal
Anxiety	Normal
Neurological assessment	Normal
Grip strength	Normal
Hot plate	Normal
Dysmorphology	Normal
Indirect calorimetry	Normal
Glucose tolerance test	Normal
Auditory brainstem response	Normal
DEXA	Normal
Radiography	Abnormal
Body temperature	Normal
Eye morphology	Normal
Clinical chemistry	Normal
Plasma immunoglobulins	Normal
Haematology	Normal
Micronucleus test	Normal
Heart weight	Normal
Skin Histopathology	Normal
Brain histopathology	Normal
Eye Histopathology	Normal
Salmonella infection	Normal
Citrobacter infection	Normal
All tests and analysis from

Another conditional knockout mouse line, called Fto^{tm1a(EUCOMM)Wtsi} was generated as part of the International Knockout Mouse Consortium program — a high-throughput mutagenesis project to generate and distribute animal models of disease to interested scientists. Male and female animals from this line underwent a standardized phenotypic screen to determine the effects of deletion. Twenty five tests were carried out on mutant mice and only significant skeletal abnormalities were observed, including kyphosis and abnormal vertebral transverse processes, and only in female homozygous mutant animals.

The reasons for the differences in FTO phenotype between humans and different lines of mice is presently uncertain. However, many other genes involved in regulation of energy balance exert effects on both intake and expenditure.

Origin of name

By exon trapping, Peters et al. (1999) cloned a novel gene from a region of several hundred kb deleted by the mouse 'fused toes' (FT) mutation. They named the gene 'fatso' (Fto) due to its large size.

External links

• FTO protein, human at the US National Library of Medicine Medical Subject Headings (MeSH)