Postzygotic isochromosome formation as a cause for false‐negative results from chorionic villus chromosome examinations

OBJECTIVE: To investigate the origin and mechanisms of formation of isochromosomes 13q and 21q in instances where prenatal chromosome examination revealed a normal karyotype while postnatal chromosome examination from blood showed translocation trisomy 13 and 21. METHODS: G and/or Q-banded chromosome examinations from CVS cultures and lymphocyte chromosome examinations from two newborns. Microsatellite marker analysis of DNA from the probands and their parents. Prenatal ultrasonic examinations of the fetuses and postnatal clinical examinations of the probands. RESULTS: Short and long-term CVS examinations from two fetuses revealed normal karyotypes. Lymphocyte karyotypes of the newborns showed the karyotype 46,XY,i(21)(q10) in the first case and 46,XY,i(13)(q10) in the second. The isochromosomes 21q and 13q were shown, by microsatellite marker analysis of the patients and their parents, to be of maternal and paternal origin, respectively. CONCLUSION: Postzygotic isochromosome formation is one of the possible mechanisms that may lead to false-negative results of chorionic villus chromosome examinations, even if both short-term and long-term cultures are performed and give normal results.