Symmetric replication of an unstable isodicentric Xq chromosome derived from isolocal maternal sister chromatid recombination

An amniocyte culture was found to be mosaic for 45,X/46,X,idic(X) (p11.2)/ 47,X, idic(X)(p11.2),idic(X)(p11.2) cell lines, reflecting mitotic nondisju nction of the idic(X)(p11.2) chromosome. Upon learning of abnormal karyotyp e and ultrasound findings, the parents decided to discontinue the pregnancy , Subsequent cultures of fetal skin, kidney, and lung were mosaic 45,X/ 46B ,X,idic(X)(p11.2) reflecting mitotic loss of the unstable idic(X)(p11.2) ch romosome. C-banding and in situ hybridization of X chromosome-specific cy-s atellite probe to metaphase fetal cells confirmed two centromeres on the id ic(X)(p11.2) chromosome with both centromeres appearing to be active in two -thirds of cells. This result was confirmed by centromere protein-E (CENP-E ) antibody staining which delineated 80% of scored cells with two active ce ntromeres and 20% with 1 active centromere. Bromodeoxyuridine (BrdU) incorp oration and acridine orange staining characterized the DNA replication patt ern of the idic(X)(p11.2) chromosome as late and symmetrically replicating, Polymerase chain reaction analysis of highly polymorphic loci determined t hat the normal X chromosome carried paternal alleles and the idic(X)(p11.2) chromosome carried maternal alleles from only one grandparental chromosome . Overall, the results suggest that recombination occurred between two mate rnal sister chromatids both in the same chromosome band Xp11.2 (isolocal) p rior to maternal meiosis II anaphase to generate an unstable maternal idic( X)(p11.2) chromosome, Additional factors that could contribute to i(Xq) and idic(X) formation and instability are discussed along with a mechanism to explain the high frequency of intrauterine loss in 45,X pregnancies. (C) 19 99 Wiley-Liss, Inc.