Mild developmental delay and obesity in two patients with mosaic 1p36 deletion syndrome

We identified mosaic 1p36 deletions in two patients with developmental delay, distinctive features, and obesity, who can walk alone and communicate with others. Thus, their neurological defects are milder than those in typical patients with 1p36 deletion syndrome because most patients with 1p36 deletion cannot acquire expressive language. Chromosomal microarray testing revealed 3.0 and 4.5 Mb aberrations in the subtelomeric region of the short arm of chromosome 1. Mean signal ratios of the identified aberrations were -0.4 and -0.5, indicating mosaicism, which was confirmed by fluorescence in situ hybridization analysis with a mosaic ratio of 70% and 77%, respectively. Previous studies demonstrated that deletion of the distal 2-3 Mb region would be responsible for hyperphagia and obesity seen in patients. On the other hand, the severity of the neurological defect often correlates with the size of the terminal deletion of 1p36, and patients with larger deletions of 1p36 would usually show severely impaired developmental milestones and be immobile and aphasic. In such cases, hyperphagia and obesity could be clinically masked. In this study, two patients with mosaic deletions of 1p36 showed obesity as a consequence of hyperphagia. This study suggests that patients with 1p36 deletion would be at risk for hyperphagia and obesity when they have both risk factors, that is, (1) deletions including the 2-3 Mb critical region and (2) milder phenotypes that allow them to reach food on their own and to overeat.