Intrachromosomal triplication of 15q11-q13.
A 7 year old girl with intrachromosomal triplication 46,XX,-15,+der(15)(pter-->q13::q13-->q11::q11-->qter) resulting in tetrasomy of 15q11-q13 is reported. Fluorescence in situ hybridisation confirmed that the tetrasomic region included the entire segment normally deleted in Prader-Willi and Angelman syndrome patients, and breakpoints were similar to those reported in two tandem duplications of 15q11-q13. The middle repeat was inverted, suggesting a possible origin through an inverted duplication intermediate. Microsatellite analysis showed that the rearrangement was of maternal origin and involved both maternal homologues. Clinical findings included multiple minor anomalies (a fistula over the glabella, epicanthic folds, downward slanting palpebral fissures, ptosis of the upper lids, strabismus, a broad and bulbous tip of the nose, and small hands and feet), motor and mental retardation, a seizure disorder, and limited verbal abilities. In addition, immunological examination disclosed a selective immunodeficiency. The overall phenotype did not clearly resemble that of cases with tetrasomy 15pter-q13 associated with an extra inv dup(15)(pter-->q13:q13-->pter) chromosome. The latter aberration causes more severe mental deficit and intractable seizures, but less marked phenotypic alterations, although some overlap in mild facial dysmorphic features is present. A number of features common to Angelman syndrome were also observed in the patient.