Mosaic tetrasomy 5p resulting from an isochromosome 5p marker chromosome: case report and review of literature.

We report on the fifth case, and oldest reported patient, of an individual affected with mosaic tetrasomy 5p resulting from an isochromosome 5p [i(5)(p10)] marker chromosome. A syndrome of mosaic tetrasomy 5p is defined, and includes the following features seen in the reported cases: developmental delay, seizures, ventriculomegaly (other brain anomalies), small stature/growth delay and mosaic pigmentary skin changes. Other findings include various dysmorphic facial features as well as hand and foot anomalies. This syndrome is likely more common than suggested in the literature, as the clinical presentation can be variable, and the chromosome anomaly is unlikely to be found on routine karyotype of peripheral blood lymphocytes. The i(5)(p10) marker chromosome is found only as a mosaic anomaly, with levels ranging from 0% to 10% in cultured lymphocytes to 12-85% in cultured skin fibroblasts. Microarray analysis performed on unstimulated lymphocytes from the patient in this report did not detect any evidence of the chromosome abnormality, indicating that this methodology may not be useful as a diagnostic tool in this disorder. Diagnosis of the mosaic tetrasomy 5p syndrome will rely on good clinical assessment, and appropriate cytogenetic studies, including analysis of skin fibroblasts. A child with unexplained developmental delay, seizures, hypotonia, and ventriculomegaly with or without dysmorphic features should be assessed carefully for pigmentary changes of the skin. If a diagnosis of mosaic 5p tetrasomy is suspected, karyotype of cultured fibroblasts in addition to routine cytogenetic analysis, to look for this marker chromosome is warranted.