Objective: Leptin, a protein product of adipocytes, plays a critical role in the regulation of body weight, immune function, pubertal development, and fertility. So far, only three homozygous mutations in the leptin gene in a total of 13 individuals have been found leading to a phenotype of extreme obesity with marked hyperphagia and impaired immune function.
Design: Serum leptin was measured by ELISA. The leptin gene (OB) was sequenced in patient DNA. The effect of the identified novel mutation was assessed using HEK293 cells.
Results: We describe a 14-yr-old child of nonobese Austrian parents without known consanguinity. She had a body mass index of 31.5 kg/m(2) (+2.46 SD score) and undetectable leptin serum levels. Sequencing of the leptin gene revealed a hitherto unknown homozygous transition (TTA to TCA) in exon 3 of the LEP gene resulting in a L72S replacement in the leptin protein. RT-PCR, Western blot, and immunohistochemical analysis indicated that the mutant leptin was expressed in the patient's adipose tissue but retained within the cell. Using a heterologous cell system, we confirmed this finding and demonstrated that the side chain of Leu72 is crucial for intracellular leptin trafficking. Our patient showed signs of a hypogonadotropic hypogonadism. However, in contrast to the literature, she showed only mild obesity and a normal T cell responsiveness.
Conclusions: These findings shed a new light on the clinical consequences of leptin deficiency. Congenital leptin deficiency should be considered possible in pediatric patients with mild obesity even if parents are lean and unrelated.