Mosaic tetrasomy 12p: Four new cases, and confirmation of the chromosomal origin of the supernumerary chromosome in one of the original Pallister‐Mosaic syndrome cases

Four new cases are reported in which mosaicism for a supernumerary chromosome interpreted as an isochromosome for 12p [i(12p)] is present. In 2 cases seen in early childhood the mosaicism was present at a low level in peripheral blood and was documented in one case to be present with a higher frequency in fibroblast cultures from skin. These cases have clinical features compatible with those in previously reported cases of the Teschler-Nicola/Killian syndrome, many of whom have now been found to be mosaic for a similar i(12p) chromosome in fibroblast cultures. One case was diagnosed prenatally from amniotic fluid culture. The fourth case was a neonatal death, in which fibroblast cultures were established from muscle and increased activity of LDH-B was demonstrated, supporting the theory that the origin of the additional chromosome was from 12p. Loss of the cell line with the supernumerary chromosome occurs after long-term fibroblast culture. Previously unpublished studies showing increased LDH-B activity in case 1 of Pallister-Mosaic syndrome originally reported in 1977 are also reported. It is of interest that our 2 cases which did not survive birth and one previously published case diagnosed prenatally had diaphragmatic herniae.