A number of factors contribute to whether you end up carrying some excess weight. Clearly too many cream buns, and too little exercise can contribute to you being a bit plump. But genes are also important. Some people are, due to their genes, a bit more prone to putting on weight than others.
Despite what some media outlets might say there is no ‘fat’ gene or ‘slim’ gene. Our metabolism is influenced by many many genes. But one gene in particular, a gene called FTO has been implicated in weight gain. It is clearly only one gene amongst many that influences our propensity to put on fat but it is a rare example of a gene that we have been able to show has a significant effect. But how it does this has been tricky to pin down.
This week in Nature, Smemo et al. showed that the FTO gene stretches across the nucleus and influences another gene, a gene called IRX3. It seems that FTO influences our tendency to pile on the pounds via this second gene. Now these two genes are quite far apart along the DNA, millions of basepairs of DNA separate them so if the DNA molecule is stretched out to its full length there two genes would be almost 1 mm apart. The nucleus is only a few micrometres (a few thousandth of a mm across) so they can never be that far apart, but we might still expect them to be about a micrometre apart — too far for one to influence the other directly. But Smemo et al. found that the DNA loops around so that the two genes are effectively in contact with each other and so able to directly interact and influence each other.
So inside each of your cells DNA is looping round and FTO is reaching out to IRX3, in a way that depends on which variant of the FTO gene we have, and this in term makes the possessor of one variant of FTO more prone to pile on the pounds than someone who inherited a different variant.
I should say that I first read about this in a lovely blog post of Ed Yong, over at his blog at National Geographic. The paper is not at all readable, his blog post is a whole lot better.