Mosaic rearrangement of chromosome 18: Characterization by FISH mapping and DNA studies shows trisomy 18p and monosomy 18p both of paternal origin

Structural abnormalities of chromosome 18p mainly consist of isochromosomes of the short arm, which result in tetrasomy 18p. Trisomy 18p is much rarer , and less well characterized. We report on a 12-year-old girl with minor f acial anomalies, delayed development, abnormal hands, atopic dermatitis, an d hearing loss. She was mosaic for two abnormal cell lines in peripheral bl ood. In 90% of cells, a dicentric chromosome with duplication of the whole short arm of chromosome 18 resulted in trisomy 18p; 10% of cells had monoso my 18p, arising from a t(14;18)(p11;q11). FISH mapping, with multiple regio n specific and locus specific probes from the short and long arm of chromos ome 18, showed that the structure of the dicentric chromosome 18 was 18pter -->18q23::18q11-->18pter. DNA polymorphisms for chromosome 18 showed that t he abnormalities of chromosome 18 were paternal in origin. Combining all re sults, we could link the trisomy 18p and monosomy 18p to a common origin vi a a complex series of events in an early mitosis. Am. J. Med. Genet, 92:101 -106 2000, (C) 2000 Wiley-Liss, Inc.