While I took a month off from blogging, an international group of researchers published what may well become a landmark paper on the genetics of obesity in the New England Journal of Medicine.
As regular readers may be well aware, a number of previous genetic studies have pointed to the importance of the FTO gene for human obesity – however, what exactly this gene does to effect body weight was largely unclear.
The rs1421085 single-nucleotide variant of this gene has both a high frequency and a strong effect size, which suggests positive selection or bottlenecks (e.g., 44% frequency in European populations vs. 5% in African populations).
In the present paper, that included examination of epigenomic data, allelic activity, motif conservation, regulator expression, and gene coexpression patterns in mice and humans, the researchers showed that the FTO allele associated with obesity represses mitochondrial energy production in adipocyte precursor cells in a tissue-autonomous manner.
To be precise, the rs1421085 variant of this gene apparently disrupts a conserved motif for the ARID5B repressor, which leads to derepression of a potent preadipocyte enhancer and a doubling of IRX3 and IRX5 expression during early adipocyte differentiation. These molecules play key roles in thermogenic dissipation both through UCP-1 and UCP-1-independent pathways.
This change leads to a persistent and cell-autonomous developmental shift from energy-dissipating beige (brite) adipocytes to energy-storing white adipocytes, with a reduction in mitochondrial thermogenesis by a factor of 5. It is also associated with an increase in lipid storage and adipocyte cell size.
Inhibition of Irx3 in adipose tissue in mice reduced body weight and increased energy dissipation without a change in physical activity or appetite.
Knockdown of IRX3 or IRX5 in primary adipocytes from human subjects with the risk allele restored thermogenesis, increasing it by a factor of 7, and overexpression of these genes had the opposite effect in adipocytes from nonrisk-allele carriers.
Finally, repair of the ARID5B motif in primary cultured adipocytes from a patient with the risk allele restored IRX3 and IRX5 repression, activated browning expression programs, and restored thermogenesis, increasing it by a factor of 7.
These deep insights into the function of what is apparently a key pathway in human susceptibility (or resistance) to obesity, offers a number of potential targets for pharmacological interventions for obesity – something that we desperately need for patients struggling with this issue.
However, as an accompanying editorial is quick to point out,
“As yet, there is still no simple path to an anti-obesity drug that can be derived from this research.”
Then again, who expects finding new treatments for obesity to be simple?
The amygdala is a part of the so-called limbic system that performs a primary role in the processing of memory, decision-making, and emotional reactions. The amygdala has also been implicated in a variety of mental health problems including anxiety, binge drinking and post-traumatic stress syndrome.
A study by Xu and colleagues, published in the Journal of Clinical Investigation now shows that in mice, activity of the estrogen receptor–α (ERα) in the medial amygdala may have a profound influence on the development of obesity – an effect, which appears to me largely mediated through effects on physical activity.
Building on previous work showing that ERα activity in the brain prevents obesity in both males and female rats, the researchers used a series of complex experiments to demonstrate that specific deletion of the ERα gene from SIM1 neurons, which are highly expressed in the medial amygdala, cause a marked decrease in physical activity and weight gain in both male and female mice fed with regular chow, without any increase in food intake. In addition, this deletion caused increased susceptibility to diet-induced obesity in males but not in females.
Deletion of the ERα receptor also blunted the body weight-lowering effects of a glucagon-like peptide-1-estrogen (GLP-1-estrogen) conjugate.
In contrast, over-expression or stimulation of SIM1 neurons increased physical activity in mice and protected them from diet-induced obesity.
These findings point to a novel mechanism of neuronal control of physical activity, which in turn appears to have important effects on the susceptibility to weight gain.
If anyone ever tells you that the current obesity epidemic can have nothing to do with genetics because “genes don’t change in a couple of generations”, it is completely fair to let them know that they probably do not know what they are talking about.
Indeed, there is now overwhelming evidence showing that a variety of health problems, particularly related to metabolic diseases including obesity, can well be transmitted from generation to generation as a result of epigenetic modifications that persist in subsequent generations, even if these are no longer exposed to the “trigger” environment.
Anyone who is interested in learning about how much we know about these intergenerational mechanisms, will probably want to read a recent review article on this subject by Rachel Stegemann and David Buchner, published in Seminars in Cell & Developmental Biology.
In this papers the authors review examples of transgenerational inheritance of metabolic disease in both humans and model organisms and how these can be triggered by both genetic and environmental stimuli.ors
As the authors note,
“A diverse assortment of initial triggers can induce transgenerational inheritance including high-fat or high-sugar diets, low-protein diets, various toxins, and ancestral genetic variants. Although the mechanistic basis underlying the transgenerational inheritance of disease risk remains largely unknown, putative molecules mediating transmission include small RNAs, histone modifications, and DNA methylation.”
They also discuss example of therapeutically targeting the epigenome (e.g. through dietary modification or exercise) to prevent the transgenerational transmission of metabolic disease.
These findings have substantial implications for our attempts to prevent or even reverse the development of obesity in future generations.
Now a small study by Mojca Jensterle and colleagues from Ljubljana, published in the European Journal of Clinical Pharmacology, reports that genetic variability in the GLP-1 receptor gene may predict the variability to the human GLP-1 analogue liraglutide, now approved for obesity treatment in the US, Canada and Europe.
In their study, Jensterle and colleagues examine the realationship between two common alleles (variants) of the GLP-1 receptor in 57 women with obesity and polycystic ovary syndrome.
All women were treated with liraglutide 1.2 mg QD s.c (well under the 3.0 mg QD dose approved for obesity treatment) for 12 weeks.
Twenty of the participants were classified as strong responders (>5% weight loss), who lost about 7.4 Kg, whereas 37 were considered poor responders losing only 2.2 Kg.
Carriers of at least one rs10305420 allele were about 70% less likely to be a high responder than individuals with two wild-type alleles. Similarly, carriers of at least one rs6923761 allele were about three times as likely to high responders compared to homozygous carriers of the wild type.
Although my previous work in these type of genetic studies have made me highly critical (not to say sceptical) of these types of small studies, the notion that genetic variability in the GLP-1 receptor (the molecular target of liraglutide) may well lead to differences in response is not all that far fetched.
Thus, whether true or not, I have little doubt that indeed much of the variability in pharmacological response to liraglutide (or for that matter any other drug for anything) may well be determined by genetics.
Whether testing people for genetic markers before starting a specific treatment will ever become reality for obesity and whether or not, the genetic variability seen in this study will still be seen when lirglutide is used at the actual dose approved for obesity treatment remains to be seen.
In the meantime, the easiest way to see who responds and who does not is to try it. This why the regulatory approval of liraglutide for obesity comes with a simple stopping rule – if it doesn’t work for you – stop taking it!
Disclaimer: I have received consulting and speaking honoraria from Novo Nordisk, the maker of liraglutide.
While this is increasingly being appreciated in adults, data on childhood cancer survivors is rather sparse.
Thus, a study by Carmen Wilson and colleagues, published in Cancer, which follows the development of obesity in individuals treated for cancer as kids is of particular interest.
The study looks at 1996 cancer survivors who previously received treatment for cancer at a large Children’s Research Hospital, who survived ≥10 years from diagnosis (median age at diagnosis, 7.2 years; median age at follow-up, 32.4 years).
Interestingly, 47% of survivors, who received cranial radiation therapy developed obesity compared to only 30% of those who did not.
This risk was greatest in those who also received glucocorticoids or were the youngest at the time of treatment.
The researchers also found a significant modifying effect of genetic markers, some of which are known to be involved in neural growth, repair and connectivity.
Thus, this study shows that survivors of childhood cancer appear to be prone to developing obesity as adults particularly if they were treated with cranial radiation therapy and/or corticosteroids.
Clinicians should be aware of this increased risk and should consider measures to prevent excess weight gain in individuals with a history of childhood cancer.
Wilson CL, Liu W, Yang JJ, Kang G, Ojha RP, Neale GA, Srivastava DK, Gurney JG, Hudson MM, Robison LL, & Ness KK (2015). Genetic and clinical factors associated with obesity among adult survivors of childhood cancer: A report from the St. Jude Lifetime Cohort. Cancer PMID: 25963547