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

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Abstract
Four new cases are reported in which mosaicism for a supernumerary chromo‐some 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 Previously unpublished studies showing increased LDH‐B activity in case 1 of Pallister‐Mosaic syndrome originally reported in 1977 are also reported.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.