Mosaicism in a patient with down syndrome reveals post-fertilization formation of a Robertsonian translocation and isochromosome

It has been estimated that a few hundred children are born each year in the United States with translocation Down syndrome. About 5% of the cases with Down syndrome carry a Robertsonian translocation involving chromosome 21. The case described here is a patient with Down syndrome who showed mosaicism for two cell lines. Each cell line contains a different, de novo acrocentric rearrangement. We constructed somatic cell hybrids from the patient's cells and determined the parental origins of the rearrangements by molecular and fluorescence in situ hybridization (FISH) analyses. The analysis showed that the rob(14q21q) formed between a paternally inherited chromosome 21 and a maternally inherited chromosome 14, indicating that this rearrangement formed post-zygotically. Further molecular analysis also determined that the rea(21q21q) is an isochromosome of paternal origin. The cell line containing the isochromosome is unbalanced, resulting in trisomy 21. Because the same paternal chromosome 21 was involved in both the isochromosome and the Robertsonian translocation, we speculate that an unstable chromosome 21 was stabilized either through formation of a rob(14q21q) or through formation of an isochromosome. The mechanism proposed for the formation of the rob(14q21q) in this case is different from that for most de novo rob(14q21q), but similar to a previously reported mosaic case of Down syndrome. © 2002 Wiley-Liss, Inc.