Abstract
Obese patients who undergo gastric bypass surgery experience metabolic changes resulting in appetite loss and the apparent disappearance of type II diabetes within days to weeks, even before weight loss occurs. The biological mechanisms underlying these changes are not well-understood. This study aimed to investigate the change in circulating microRNAs that correspond to these metabolic effects. MicroRNAs are a recently-discovered class of small regulatory molecules that play a key role in diverse biological processes, including lipid metabolism and insulin regulation. It was hypothesised that the microRNAs that change uniquely in association with bariatric surgery will be involved with the modulation of expression of genes in metabolic pathways that produce the physiological changes observed post-operatively.
The overall study comprised two phases, a discovery and a validation phase. The discovery phase formed the basis of this thesis. Three groups were recruited, a Bariatric group, consisting of obese patients undergoing gastric bypass surgery, and an obese Control group and a lean Control group, consisting of patients undergoing non-bariatric surgery. For the discovery phase, both Control groups were combined into a single Control group. Fasting blood samples were taken from twelve patients in each of the Bariatric and combined Control groups at baseline, 5 days and 6 weeks post-operatively, for microRNA analysis. Patient demographics and markers of surgical response including anthropometric measures (weight, body mass index, waist-hip ratio), glycaemic parameters (insulin, HbA1c, fasting glucose), fasting lipid profile, hs-CRP, blood pressure and results of an appetite questionnaire were recorded at each time-point. Affymetrix GeneChip arrays were used to generate microRNA profiles that were compared between samples. The Qlucore software package was used to identify differentially-expressed microRNAs by principal component analysis and hierarchical clustering. MicroRNAs specific to the post-operative Bariatric group were determined, and linked to the unique obesity-related metabolic changes.
The validation phase involves replication of the specific microRNA changes detected in the discovery phase in a larger number of Bariatric and Control patients at all time-points previously assessed (baseline, 5 days and 6 weeks), as well as at 3 months and 1 year post-operatively, using microRNA-specific Taqman assays. This is currently ongoing.
Discovery phase results revealed 7 differentially-expressed microRNAs at 6 weeks in the Bariatric group, and 22 at the same time-point in the Bariatric diabetic sub-group. These were associated with significant clinical improvements in weight, glycaemic parameters and appetite scores at 6 weeks. In contrast, there were no differentially-expressed microRNAs at 5 days in the Bariatric group, or at either time-point in the Control group.
MicroRNA-associated gene ontology and predicted pathways analysis identified regulatory associations with genes known to be involved in potentially biologically-plausible roles encompassing adipogenesis, pancreatic β-cell regulation, insulin signalling and secretion, and appetite control. These roles appear consistent with the metabolic effects seen following gastric bypass surgery: weight and adipose tissue loss, improved insulin sensitivity and glucose metabolism and decreased appetite. This work requires further validation; however, it indicates the therapeutic potential of altering levels of these unique microRNAs as novel pharmacological mechanisms that may mimic the effects of bariatric surgery.