ESPE Abstracts

Background: Monogenic early onset obesity is mainly due to pathogenic mutations in a single gene involved in the leptin melanocortin signaling pathway. This pathway is essential for satiety and energy homeostasis. Adenylate cyclase 3 (ADCY3) is a transmembrane protein localized in the primary cilia of neurons and plays a vital role in converting ATP to cAMP, a secondary messenger that regulates various downstream signaling pathways that control carbohydrates and lipids metabolism. Homozygous pathogenic mutations in the ADCY3 gene lead to severe early onset obesity, and insulin resistance. Recently, a study has shown the association of ADCY3 gene mutations with obesity due to its subcellular localization with MC4R in the neural cilia.

Objective: To identify pathogenic variants implicated in early onset obesity and functionally characterize the variants in vitro and molecular modelling.

Results: Using gene panel sequencing we identified novel homozygous frameshift variant c. 2520C>G, p. Thr840X in ADCY3 gene in a four-years old girl who was two years old at the time of diagnosis. She presented with severe obesity (26 Kg, 100^th percentile, +2.1SD and BMI of 29.4 Kg/m²). She was born to first-cousin consanguineous parents who were heterozygous carriers for the variant. She had mild hepatomegaly and insulin resistance with other biochemical and hormonal tests being normal. Functional in-vitro analysis in cell culture using Human Embryonic Kidney 293 (HEK293) transfected with mutant Thr840X showed decreased cyclic-AMP activity compared to cells transfected with wild type ADCY3. Western blot showed a significant downregulation of the ADCY3 protein in the mutant cells. Protein-protein docking indicated stronger binding affinity of WT ADCY3 to G-proteins compared to the truncated protein. The G-proteins appeared to specifically interact with the intracellular region of ADCY3 that is missing in the truncated protein, suggesting that the mutant ADCY3 receptor cannot be activated by the stimulatory G-protein unit.

Conclusion: These findings contribute to the existing data that show the role of ADCY3 in the genetic pathogenesis of early-onset obesity. In-vitro and in-silico functional characterization of the p.Thr804X variant showed conformational change of the receptor impairing enzymatic activity leading to loss of function of the receptor. Furthermore, with functional and computational modelling of additional ADCY3 variants could open the door for personalized medicine approaches that could target ADCY3. Genetic testing is important in severe early onset obesity and early diagnosis may benefit patients with personalized treatment.