PWS is the result of the loss of expression of several imprinted genes located in the 15q11-q13 region, which are normally expressed on the paternally derived chromosome [17] (Figure 1). In 70% of PWS patients, a paternal 15q11-q13 deletion is found. The remaining have a uniparental maternal disomy 15 (~25%) or an imprinting defect (~5%). This neurodevelopmental disorder has an estimated prevalence of 1/10000 live births and is characterized by infantile hypotonia, neonatal feeding difficulties, hypogonadism, hyperphagia (leading to obesity in early childhood) and cognitive deficits. Many studies have also reported a range of obsessive-compulsive and ritualistic behaviors not related to food in about 50% of PWS patients, including skin picking, hoarding, concerns with symmetry, exactness, ordering and arranging, need to tell or ask, and insistence on routines [18-21]. Thus, paternally expressed genes within the PWS critical region (e.g., MKRN3, MAGEL2, NDN, and SNRPN-SNURF) could represent risk factors for OCD.
