Parental origin and mechanisms of formation of triploidy: a study of 25 cases

Triploidy is one of the most frequently observed chromosome abnormalities in spontaneous abortions in humans. The parental origin of the additional chromosome set is known to have a major impact on the phenotype of the foetuses and to result in differences in size and structure of the placenta. Early studies based on cytogenetic polymorphisms indicated a preponderant diandric origin of the triploidies; such detection method, however, is known to be prone to error. Other studies revealed a predominant digynic origin in cases with longer intrauterine survival. It is now thought that, to some extent, a detection bias in favour of cases with associated partial hydatidiform moles may account for the high incidences of diandric cases reported in some studies. Furthermore, depending on the gestational age of the cases analysed there may indeed be differences in the proportion of diandric and digynic triploidies. We investigated the parental origin and mechanisms of formation of triploidy in a group of 25 probands with gestational ages ranging from 8 to 37 weeks. DNA samples were extracted from foetal material and from blood samples of the parents, and were analysed using microsatellite markers. The parental origin of the triploidies was found to be maternal in 20 cases and paternal in 5. Regarding the digynic cases, an error at meiosis I was inferred in 10 cases, whereas in the other half an error occurred at meiosis II. All five diandric cases included in this study were found to be due to dispermy. No significant differences in the average maternal ages were found amongst the different subgroups of patients.