Obesity: Variation in DNA sequence increases fat storage

Genetic mechanism driving obesity identified

More than 500 million people worldwide suffer from obesity, including around 15 million in Germany. But what part does individual genetic makeup play? A team of researchers from TUM, MIT and Harvard Medical School in Boston joined forces to investigate the genetic causes of obesity.

In 2007, a region within a gene – known as the FTO gene – was identified as the key genetic candidate associated with obesity. People carrying this region had an increased risk of becoming overweight. Until now, though, the mechanism behind this link between the gene region and obesity could not be determined.

FTO affects fat cell progenitors

With the aid of the Roadmap Epigenomics Project, the researchers first used bioinformatic methods to establish the tissue types in which the FTO region was most active or actually showed epigenetic alteration – a sign of particular genetic activity. Their findings were surprising. “Many studies have tried to link the FTO region with areas of the brain controlling appetite or propensity for physical activity,” explains Dr. Melina Claussnitzer, TUM researcher and first author for this study. “We have now been able to show that the regulatory region within FTO has the greatest impact on adipocyte (or fat cell) progenitors, independent of circuits in the brain.”

This led the researchers to suppose that process dysregulation in these upstream progenitor cells could be responsible for the development of obesity. They examined samples of human fat tissue taken from participants carrying either the normal or the risk region of the FTO gene. The outcome was that two specific genes – IRX3 and IRX5 – were only expressed in the risk group.

Fat storage instead of burning

“As far as we were concerned, this was a highly significant finding. Further experiments showed that IRX3 and IRX5 activate a process that causes the progenitor cells to develop into fat storage cells and lose the ability to burn fat,” adds Prof. Hans Hauner, Chair of Nutritional Medicine at TUM, who was involved in the study. “This effect evidently changes the energy balance and can contribute to obesity,” he elaborates.

Once the researchers had understood the process, they were also able to selectively influence it. If they initiated IRX3 or IRX5 expression in cultures with human fat cell progenitors, the cells activated lipid storage. If, on the other hand, both genes were inactive, the cells burned fat and generated heat. The team was then also able to confirm these findings in animal experiments. Mice with IRX3 inhibited in fat cells displayed a higher metabolic rate and did not gain weight even on a high-fat diet.

DNA sequence alteration as potential obesity trigger

The researchers then went even further, revealing not only the mechanism but also the exact genetic cause in their study. They identified a single area of the FTO gene region that was altered in the risk variant. If the researchers then used the latest genetic engineering methods to repair this defect in human fat cells, they functioned normally again, increasing fat burning and heat generation instead of lipid storage.

Melina Claussnitzer compares this detection of the link between FTO and obesity to investigating a crime. “The prime suspect, FTO, turns out not to be the actual perpetrator. Our new methods have now convicted two offenders, IRX3 and IRX5, originally not under suspicion.” She adds: “Our biggest challenge was identifying three things: the instrument of crime – that is, a genetic variation in an elusive region – the scene of the crime, i.e. fat cell progenitors, and the facts of the case in terms of inhibited fat burning.”

New method also applicable to other gene regions

This meant developing a new methodology, which called for intensive efforts on the part of Claussnitzer and the study’s final author, MIT Professor Manolis Kellis. The two TUM and MIT researchers are now applying this fresh approach to a wide range of other diseases in collaboration with MIT and Harvard Medical School.

“There are thousands of genetic associations within the genome that have been linked to the most diverse diseases. Yet the mechanisms behind this remain completely unknown, since they are localized in genomic regions that do not code for proteins and were even dubbed “junk” in the past. Our method serves as a model to accelerate studies involved in decoding genetic signals in the future. This could pave the way for personalized medicine for obesity or type-2 diabetes,” concludes Claussnitzer.

First author Dr. Melina Claussnitzer leads a working group at TUM’s Chair of Nutritional Medicine, is a faculty member at the Beth Israel Deaconess Medical Center (Harvard Medical School) and holds a visiting professorship at MIT’s Computer Science and Artificial Intelligence Laboratory and at the Broad Institute of MIT and Harvard.

Publication
Claussnitzer M, Dankel SN, Kim K-H, et al. FTO obesity variant circuitry and adipocyte browning in humans, New England Journal of Medicine, August 2015.
DOI: 10.1056/NEJMoa1502214

Contact
Dr. Melina Claussnitzer, PhD
Researcher, TUM Chair of Nutritional Medicine
(Currently Harvard Medical School and MIT, Boston, US)
Phone: +1 617 852 1948
mclaussnspam prevention@bidmc.harvard.edu
melina.claussnitzerspam prevention@wzw.tum.de

Prof. Dr. Hans Hauner
Technical University of Munich
Chair of Nutritional Medicine
Else Kroener-Fresenius Center for Nutritional Medicine
hans.haunerspam prevention@tum.de
www.em.wzw.tum.de