Monosomy of the entire short arm of chromosome 18 as a result of an 18;acrocentric whole arm translocation has been reported in over 20 patients, 3 of which were familial. The centromeric origin in de novo cases has not been characterized. We report molecular cytogenetic studies of two prenatallydetected de novo cases. Amniocenteses were performed because of sonographic findings of fetal holoprosencephaly. Cytogenetic studies and dual color fluorescence in situ hybridization using Oncor โฃ-satellite probes for D18Z1 and D13Z1/D21Z1 showed monosomy 18p with presence of a dicentric 18;21 chromosome in both cases [45,XY, dic(18;21)(p11.1;p11.1)]. In one case, a second cell line was found, which contained 46 chromosomes with a del(18)(p11.1) and an apparently telocentric 21 not present in either parent [46,XY,del(18)(p11.1),del(21) (p11.1)]. The del(18)(p11.1) contained only the 18 alphoid sequence and the telocentric 21 contained only the 21 alphoid sequence.
No centromeric break was detected. We propose that the second cell line arose from dissociation of the dic(18;21) with no centromeric DNA break. In addition to our case, there have been three previous reports of dissociation of dicentric 18;acrocentric chromosomes indicating that the translocation site can be unstable and dissociate. Am.