Detection of trisomy 18 on formalin-fixed and paraffin-embedded material by fluorescence in situ hybridization

Formalin-fixed and paraffin-embedded autopsy material from 10 fetuses and infants with unknown karyotype and anomalies suggestive of trisomy 18 were subjected to fluorescence in situ hybridization (FISH). Nuclei were extracted from the tissues and hybridized with a chromosome 18-specific centromere probe. The hybridization was successful in 9 of 10 cases. Two cases showed three hybridization signals in most of the nuclei (74% and 85%). These had anomalies frequently occurring with trisomy 18 (congenital heart defect, omphalocele, and horseshoe kidney). Two cases showed a mixture of two and three signals (47%/49% and 59%/36%), suggesting the possibility of mosaicism. One of these cases had anomalies consistent with a trisomy 18 phenotype. In the other case intrauterine growth retardation and syndactylies suggested triploidy. Hybridization with a chromosome 8-specific probe gave a distribution of two and three signals (34% and 62%, respectively). This result strengthened the suspicion of a possible triploid mosaicism. In five of the cases most of the nuclei showed two signals (85% to 88%). However, as only one type of tissue was examined for enumeration of chromosome 18, the possibility of organ mosaicism or other chromosome aberrations cannot be excluded. The FISH technique is applicable on macerated and autolysed formalin-fixed tissue, making it possible to retrospectively analyze autopsy material from aborted and stillborn fetuses and infants. This analysis contributes to a better quality of perinatal autopsies and is helpful in parental counseling.