Dose-dependent effect of risperidone treatment in a case of 22q13.3 deletion syndrome.

We describe a 18-year-old female with 22q13.3 deletion syndrome characterized by an alteration of SHANK3/PROSAP2 and severe mental retardation, intense psychomotor agitation and aggressive behaviour. When the patient was given risperidone, different therapeutic results were observed: at a 6 mg dose, risperidone had no therapeutic effect and the patient's behavioural problems increased; at a low dose (1mg), risperidone treatment resulted in rapid improvement of mood and behaviour. Recent findings suggest that risperidone exerts dose-dependent effects on Glu receptors in developing rats. An altered preset of the glutamate receptors, due to the presence of a haploinsufficiency of SHANK3/PROSAP2, could explain the different results of risperidone therapy observed in our patient with 22q13.3 deletion syndrome.