Clinical aspects and genetics of Prader-Willi syndrome

Abstract

Prader-Willi syndrome (PWS) is considered to be a rare neurogenetic disorder, nevertheless it represents the most common syndromatic obesity. Main features are severe hypotonia in the newborn period with feeding difficulty and failure to thrive in the first few months, and improvement in later infancy. Between 1 and 6 years the development of a marked truncal obesity is observed, sometimes later on reaching a life-threatening degree. Apart from retarded motor development speech ability is also hampered due to dysfunction of oropharyngeal muscles. Moreover, intellectual impairment is observed that leads to mostly moderate learning difficulty due to deficits in short term memory and abstract thinking. The genetic background of PWS is loss of function of a paternally inherited gene cluster on chromosome 15q11.2, therefore representing a paradigm of an epigenetic phenomenon with silencing and activating of genes depending on their parental origin. Together with the Angelman-Syndrome (AS), genetically located in the same region but clinically different PWS was identified as one of the first human disorders to be caused by the mechanism of genomic imprinting. There are some different genotypes in PWS leading to a quite similar phenotype with small differences e.g. in the pigment expression. An early diagnosis is important because the neuromuscular dysfunction improves with appropriate physiotherapy. Even more, dietary programs with periodical calorie restricted meals can counteract development of morbid obesity with subsequent complications of cardiovascular disorders and diabetes. Clinical diagnosis remains difficult especially in the newborn period and is considered mostly because of the marked hypotonia. Today, the availability of molecular testing of loss of function in the paternal inherited PWS candidate gene makes a definitive diagnosis possible as a prerequisite of symptomatic therapy. One of the most recommended therapeutic interventions is the application of recombinant human growth hormone (rhGH) which has been shown to be useful for improvement of length, physical ability and favourable influence on respiratory problems by a lot of clinical studies. Early diagnosis by means of molecular methods is helpful for comprehensive genetic counseling. It may avoid unnecessary investigations like computed tomography of the brain and muscle biopsy and it enables parents and professionals to start a purposeful therapy. Although it remains a symptomatic tool, GH therapy appears to be useful for most of the patients. Considering the multimorbidity in PWS a multidisciplinary approach seems appropriate for this still mystical condition.