This is the first study to compare adipose tissue structure, gene expression and function in extreme obesity in both monogenic and polygenic cohorts with a view to elucidating the relationship between insulin resistance and obesity. Using the hyperinsulinemic euglycemic clamp as the gold standard for assessing insulin sensitivity, we were able to show the specific contribution of adipose tissue, skeletal muscle and liver to systemic insulin resistance in both cohorts. Surprisingly, we found that patients with obesity due to BBS had similar glucose tolerance and insulin sensitivity to polygenic BMI-matched controls irrespective of the duration of fat mass accumulation. Furthermore, their adipose tissue structure, transcriptomic finger prints and functions were similar.
There was no significant difference between BBS and controls with common obesity in most parameters of insulin sensitivity in our study, although it is important to note that the predominant phenotype in our study was BBS1. The results remained unchanged when dedicated subgroup analysis using BBS1 was performed. Insulin resistance and diabetes mellitus have historically been considered an important feature of BBS, but a number of recent studies have paradoxically suggested improved glucose tolerance with certain BBS subtypes23. In a study by Feuillan et al., carried out in children and young adults with a mean age of 14 years, a similar degree of impaired glucose tolerance was noted in 43 patients with BBS and 100 BMI-matched controls14. This finding was more apparent in the patients with BBS1 mutation who were similar to BMI-matched controls for all variables whereas the differences in certain variables between patients with BBS10 mutations and controls persisted on subgroup analysis14. Another study in the BBS12 phenotype showed normal glucose tolerance by OGTT. Further in-vitro analysis showed, improved glucose absorption and insulin sensitivity in BBS12 inactivated human adipocytes. These findings were reproduced in a Bbs12 knockout mouse (Bbs12−/−) model which showed improved insulin sensitivity23. In another study using zebrafish models, suppression of the BBS1 or BBS4 gene was associated with increased β-cell mass during development and a compensatory increased proliferation of β-cell when exposed to high glucose conditions24. Taken together, most mutations in BBS genesis is associated with improved or equal insulin sensitivity relative to the degree of their obesity, which is in concordance to our in-depth IR analysis of BBS cohort.
The adipose tissue in BBS showed both hyperplasia and hypertrophy, both of which phenomenon are a common adaptive response to obesity. Both structurally and functionally, the adipose tissue behaved similarly to controls with common obesity. Despite extreme obesity being an important phenotype, there is a paucity of studies assessing the structure and function of adipose tissue in BBS. Studies in pre-adipocytes have shown that BBS genes play a role in adipogenesis25,26. BBS12 inactivated human adipocytes and Bbs12−/− mice showed improved insulin sensitivity and enhanced adipogenesis23 These studies implicate this BBS gene in adipogenesis but direct evidence that this translates to impaired adipose tissue function is lacking. Similarly, there was a shift in the inflammatory profile of adipose tissue macrophages as reflected by an elevated CD63/CD168 ratio. This was not supported by other findings. Similar levels of macrophage crown-like structure abundance and macrophage amount cells and macrophage infiltration with CD63 and CD 168 labelling were noted in both groups. Furthermore, no significant changes were noted in an array of cytokines and pro-inflammatory markers.
Studies have suggested that patients with BBS have increased leptin systemic levels for their degree of adiposity, but BBS1 (27% of participants) had lower leptin than BBS10 (30% of participants)14 Our study with 78% of BBS1 and 11% of BBS10 show a positive trend in leptin level compared to control, although this did not reach statistical significance. BBS proteins are involved in leptin receptor (long LepR) signalling in the hypothalamus27. BBS knockout mice models are obese, recapitulating the human phenotype28. In previous animal studies, BBS knockout mice were hyperphagic but continued to develop increased adiposity despite pair-feeding, suggesting that reduced energy expenditure may play an important role in the development of obesity29. Disruption of the BBSome by deleting the BBS1 gene from the nervous system causes obesity in mice. This phenotype is reproduced by ablation of the BBS1 gene selectively in the LRb (long LepR) -expressing cells30. Disruptive leptin signalling is a critical determinant of energy balance through its role in the regulation of the trafficking of the long signalling form of the leptin receptor (long LepR)30. How ciliary dysfunction causes obesity in BBS remains unknown31 but imbalances in energy intake and energy expenditure continue to be the main hypothesis for obesity in BBS, mechanisms that have long been implicated in common polygenic obesity.
In general, based on the aetiology, obesity is divided in two broad categories: monogenic obesity, which is inherited in a Mendelian pattern and polygenic (common) obesity, which is the result of numerous polymorphisms that each has a small effect. Although often considered to be two distinct forms, gene discovery studies for both forms of obesity show that they have shared genetic and similar underlying biology, pointing to a key role for the brain in the control of energy homeostasis and ultimately body weight32. The findings in our study would be in keeping with this notion, showing no major structural or functional difference in adipose tissue in monogenic BBS and polygenic obese controls.
Limitations of the present study should be acknowledged. Firstly, even though our single-centre national service is one of the largest centres in the world for BBS, the rarity of the syndrome and the demands of this functional study of insulin resistance precluded our ability to recruit large numbers across currently known BBS genotypes. We believe however, that BBS1 as the most common form in Europe and North America is a representative model to study. In addition, our in-depth study using a gold standard of insulin sensitivity along with the in vivo and in vitro adipose tissue studies allowed us sufficient power to compare the two cohorts. Nevertheless, our study is the only study in the literature to perform in depth functional and structural adipose tissue study comparing extremely monogenic and polygenic forms of obesity in subjects matched for obesity.
In conclusion, our data shed new lights on the relationship between the duration of fat mass, genetic phenotype, insulin sensitivity and AT function. This study adds to the literature by suggesting that both polygenic and early-onset monogenic obesity share biological underpinning and lie on a spectrum rather than discrete entities. Hence, the metabolic dysfunction in BBS is proportionate to the degree of adiposity rather than the cause or duration of obesity.