TETRASOMY-5P MOSAICISM IN A BOY WITH DELAYED GROWTH, HYPOTONIA, MINORANOMALIES, AND AN ADDITIONAL ISOCHROMOSOME-5P [46,XY 47,XY,+I(5P)]/

We describe a 1-year-old boy with a rare de novo 46,XY/47,XY,+ i(5p) m osaicism (ratios 28/3 in peripheral blood lymphocytes and 2/12 in skin fibroblasts). The boy, born after a pregnancy of 34 weeks, had lung h ypoplasia, persistent hypotonia, and postnatal growth failure. Craniof acial anomalies were also present. His clinical manifestations corresp ond to those described in trisomy 5p patients. Prenatal diagnosis on m aternal age indication had shown normal male chromosomes in 16 cells i n the short term culture of a chorionic villus sampling. Retrospective ly, 1 out of 217 cells in this culture showed the i(5p). Several mecha nisms could have resulted in the formation of this 46/47, + i(5p) mosa ic. Postzygotic local incorrect ligation during chromatid replication, followed by a second replication offers an attractive model on theore tical grounds since it needs only one step to explain both isochromoso me formation and mosaicism. Differences between the various tissues in selection pressure on cells with the isochromosome might explain the different ratios of mosaicism found. (C) 1993 Wiley-Liss, Inc.