A new neocentromere locus on chromosome 13 resulting in mosaic tetrasomy for distal 13q and an asymmetric phenotype.

An 8-year-old girl was referred to the Genetics Centre with mild developmental delay, mild dysmorphic features, and a head circumference on the 98th centile. She was noted to have large irregular ear lobes, torticollis, and mild hemihypertrophy. Karyotype analysis of cultured peripheral lymphocytes and skin fibroblasts revealed the presence of a symmetrical supernumerary marker chromosome in 13% of cells from both tissue types. Further analysis showed that this marker chromosome originated from the distal region of chromosome 13 and contained no centromeric alpha-satellite DNA. The marker chromosome was not found in blood from the parents. This case represents a novel symmetrical structure with a previously unreported neocentromere locus, leading to an unusual phenotype. Similar cases of individuals with a chromosome 13 with a neocentromere have been reported. They are reviewed and compared with the current case. The importance of scanning metaphases for abnormalities in individuals presenting with asymmetry is emphasized.