Isochromosome 12p mosaicism (Pallister-Killian syndrome): newborn diagnosis by direct bone marrow analysis

A patient who exhibited the phenotype of the Pallister mosaic aneuploid syndrome was cytogenetically diagnosed in the newborn period by bone marrow analysis. A 47,XY,i(12p) karyotype was observed in 100% of the metaphases from direct bone marrow preparations, while phytohemagglutinin (PHA)-stimulated bone marrow was 23% isochromosome positive. Initially, 10% of metaphases from a peripheral blood culture were isochromosome positive, but at 2 months of age all metaphases examined were cytogenetically normal. Serial fibroblast cultures were 75%, 100%, and 28% positive, respectively. The isochromosome was also present in all metaphases examined from lung tissue and testes. This karyotypic pattern supports a theory that tissue-limited mosaicism may result from selection due to differing developmental potentials of certain karyotypes in various tissues.