ESPE Abstracts

Background: The leptin-melanocortin pathway is pivotal in appetite and energy homeostasis. Pathogenic variants in genes involved in this pathway lead to severe early-onset monogenic obesity (MO). The MC4R gene plays a central role in the leptin-melanocortin, and variants predominantly heterozygous in this gene, are the most common cause of MO. We identified a novel heterozygous variant c.802T>C p.Tyr268His in the MC4R gene in two unrelated patients with morbid obesity and aimed to evaluate the functional impact of this novel variant on the pathogenicity of obesity.

Case 1: 17-year-old male Qatari patient with a birth weight of 4 Kg born to consanguineous parents was referred to our clinic for morbid obesity. He started to gain weight at the age of 2 years, and his current weight is 277 Kg and a height of 177 cm, with a BMI of 88.4 Kg/m2. The patient presented with morbid obesity, hyperphagia, difficulty breathing, elevated liver enzymes (ALT: 85U/L(5-30U/L), AST: 12U/L(0-39U/L), ALP: 191U/L (52-171U/L).

Case 2: A 11-year-old female Jordanian patient with morbid obesity with a birth weight of 3.5 Kg. She started to gain weight since she was a few months old, and her current weight is 72.5 kg, BMI of 30.5 Kg/m2, BMI Z-score of 2.88, and BMI percentile of 99.8th percentile. She has a strong history of obesity; both her mother and father had gastric bypass surgery for morbid obesity. All her baseline investigations were normal.

Methods: HEK293 cells were transfected with plasmid DNA encoding either wildtype or mutant p.Tyr268His MC4R variant. Cyclic AMP (cAMP) generation and MC4R gene expression were evaluated with and without stimulation with forskolin. Additionally, the effect of the variant on the stability of the MC4R gene was assessed using in silico molecular modeling.

Results: HEK293 cells transfected with mutant MC4R p Tyr268His showed impaired ability to generate cAMP compared to cells transfected with wildtype MC4R receptor. The molecular modeling data of p.Tyr268His suggest the variant destabilizes the MC4R structure and affects the overall dynamics of the MC4R gene.

Conclusion: in vitro functional analysis and in silico molecular modeling showed the pathogenicity of the variant p.Tyr268His. The variant does not affect total protein expression; however, it is predicted to affect the post-translational localization of MC4R protein to the cell surface. This finding might help our patients to benefit from the novel therapeutic advances for monogenic forms of obesity.