Deletion of the MC4R gene in a 9-year-old obese boy.

Background: The most common monogenic form of obesity is caused by mutations in the melanocortin 4 receptor (MC4R) gene. More than 150 mutations have been reported in the MC4R gene, the majority being point mutations. Most individuals with MC4R gene mutations have early-onset obesity, hyperphagia, and increased longitudinal growth.

Methods: A 9-year-old Caucasian boy was referred to genetics for obesity, food-seeking behavior, and developmental delay. History and physical exam were not consistent with Prader Willi syndrome, but revealed several minor anomalies. Owing to significant obesity and hyperphagia, a Prader Willi syndrome methylation test and a microarray were requested.

Results: Methlylation testing for Prader Willi syndrome was normal. Microarray analysis revealed two changes: (1) A 2.6-Mb deletion at chromosome 18q21.31 was identified and contained several OMIM genes, including the MC4R gene, and (2) an 0.87-Mb duplication at chromosome region 16p13.3 was found and contained one gene. Parental samples revealed that the boy's father had the same deletion and duplication. This case appears to be the first with a deletion of 18q21.31 encompassing the MC4R gene presenting with features of hyperphagia and obesity.

Conclusions: Haploinsufficiency of the MC4R gene either through whole gene deletion or nonsense or missense mutations is associated with a significant risk of obesity. The case emphasizes both the role of the MC4R gene in obesity as well as the importance of looking for chromosomal microdeletions/duplications as a cause of obesity in children with minor anomalies or developmental delay.