Rai1 haploinsufficiency causes reduced Bdnf expression resulting in hyperphagia, obesity and altered fat distribution in mice and humans with no evidence of metabolic syndrome

Smith-Magenis syndrome (SMS) is a genetic disorder caused by haploinsufficiency of the retinoic acid induced 1 (RAI1) gene. In addition to intellectual disabilities, behavioral abnormalities and sleep disturbances, a majority of children with SMS also have significant early-onset obesity. To study the role of RAI1 in obesity, we investigated the growth and obesity phenotype in a mouse model haploinsufficient for Rai1. Data show that Rai1^+/- mice are hyperphagic, have an impaired satiety response and have altered abdominal and subcutaneous fat distribution, with Rai1^+/- female mice having a higher proportion of abdominal fat when compared with wild-type female mice. Expression analyses revealed that Bdnf (brain-derived neurotrophic factor), a gene previously associated with hyperphagia and obesity, is downregulated in the Rai1^+/- mouse hypothalamus, and reporter studies show that RAI1 directly regulates the expression of BDNF. Even though the Rai1^+/- mice are significantly obese, serum analyses do not reveal any evidence of metabolic syndrome. Supporting these findings, a caregiver survey revealed that even though a high incidence of abdominal obesity is observed in females with SMS, they did not exhibit a higher incidence of indicators of metabolic syndrome above the general population. We conclude that Rai1 haploinsufficiency represents a single-gene model of obesity with hyperphagia, abnormal fat distribution and altered hypothalamic gene expression associated with satiety, food intake, behavior and obesity. Linking RAI1 and BDNF provides a more thorough understanding of the role of Rai1 in growth and obesity and insight into the complex pathogenicity of obesity, behavior and sex-specific differences in adiposity.