Prader–Willi syndrome (PWS) is a genetically determined disorder associated with the loss of the paternal contribution to the proximal part of the long arm of chromosome 15. Its pathophysiology is dominated by hypothalamic dysfunctions. The psychopathological phenotype comprises affective and psychotic symptoms as well as an increase of pre-existent obsessive-compulsive behaviors. The present study comprises 19 PWS patients who were referred for neuropsychiatric evaluation because of psychotic deterioration. Patients were assessed by using the elements of semistructured symptom checklists. In the majority a genetic analysis was performed to detect the underlying chromosomal defect. In 16 of the 19 patients a diagnosis of cycloid psychosis could be established. The other three showed a bipolar affective disorder. Of the psychotic patients, 11 were diagnosed as UPD and one as del 15q11–13. The remaining four patients were diagnosed clinically. For various reasons the genetic etiology could not be established. In PWS patients with a psychotic disorder (cycloid psychosis) a disproportional number of UPD is found.

We report on a Prader-Willi syndrome (PWS) patient carrier of a balanced 15q15q translocation and affected by a prolactin-secreting pituitary adenoma. Evidence provided by molecular studies indicates that the structural rearrangement is an isochromosome of maternal origin. According to the identification of isodisomy as the basis of the association of rare disorders and the recent report on chromosome 15 monosomy and nullisomy in pituitary adenoma, we suggest that in our case PWS and pituitary adenoma might be related.

Uniparental disomy (UPD), the inheritance of both homologues from one chromosome from the same parent, was first proposed in 1980 by Erik Engel [1] to be a potential cause of congenital developmental defects in hymans. First hints from the premolecular era towards its existence came from instances where a pericentric inversion was present on one homologue in a parent and on both in one offspring [2] and where there was transmission of an interhomologous Robertsonian translocation (of chromosome 22) from a healthy mother to healthy offspring [3-4]. In mice, UPD was experimentally produced by crossing two mice lines with different Robertsonian translocations both involving the same chromosome [for 2 review see ref. 5].

Through this approach, it was possible to define imprinted regions, chromosomes and chromosomal segments for which either maternal or paternal or both types of uniparental disomy led to phenotypic abnormalities. The latter are explained by genomic imprinting, the differential silencing of a gene or genes from one of the parents (the mother or the father) during any stage of embryogenesis or later in life. If, for example, the maternal homologue of a given gene is imprinted (and hence only the paternal allele is active), maternal UPD would lead to loss of the active allele and thus might cause consequences due to loss of function.

A male child has been identified with Angelman syndrome. He has been shown to carry a de novo Robertsonian 15/15 translocation where both chromosome 15s have been derived from the father. Consequently the disease in this instance is due to paternal uniparental disomy.

A girl carrying a de novo balanced 13-14 robertsonian translocation showed a clinical phenotype with severe hypotonia, hyperextensible joints, frontal bossing, asymmetric face, no mental retardation, severe scoliosis and motor delay. In situ hybridization analysis on chromosome spreads revealed the presence of the two centromeres in the rearranged chromosomes. Molecular analysis on genomic DNA showed the presence in the proposita of two chromosomes 14 of maternal origin and no chromosome 14 from the father indicating a maternal monocentric uniparental disomy for chromosome 14 (mUPD14). Our patient shows several similarities with other reported cases of mUPD14, suggesting imprinting of a region(s) of chromosome 14 and defining a possible mUPD14 Syndrome.