Prenatal diagnosis and molecular cytogenetic analysis of a de novo isodicentric chromosome 18.

Isodicentric chromosome 18 [idic(18)] is rare structural aberration. We report on a prenatal case described by conventional and molecular cytogenetic analyses. The sonography at 24 weeks of gestation revealed multiple fetal anomalies; radial aplasia and ventricular septal defect were significant features. Routine karyotyping showed a derivative chromosome replacing one normal chromosome 18. The parental karyotypes were normal, indicating that the derivative chromosome was de novo. Array comparative genomic hybridization (array-CGH) revealed 18p11.21→qter duplication and 18p11.21→pter deletion for genomic DNA of the fetus. The breakpoint was located at 18p11.21 (between 12104527 bp and 12145199 bp from the telomere of 18p). Thus, the derivative chromosome was ascertained as idic(18)(qter→p11.21::p11.21→qter). Fluorescent in situ hybridization (FISH) confirmed that the derivative chromosome was idic(18). Our report describes a rare isodicentric chromosome 18 and demonstrates that array-CGH is a useful complementary tool to cytogenetic analysis for reliable identifying derivative chromosome.