Formation of uniparental disomy 7 delineated from new cases and a UPD7 case after trisomy 7 rescue. Presentation of own results and review of the literature

Maternal uniparental disomy for the entire chromosome 7 (matUPD7) has been reported several times in Silver-Russell syndrome (SRs) and growth-restrict ed patients. Here we present our results from the analysis of an abortion w ith confined placental mosaicism (CPM) for trisomy 7 which showed a materna l meiotic origin of the trisomy in the placenta and rescue to maternal UPD7 in foetal membrane. Furthermore, two newly detected SRS cases with materna l UPD7 revealed isodisomy and partial heterodisomy, respectively. Summarisi ng these results with those published previously on the origin of UPD7, sim ilar numbers of isodisomy (n = 11) and cases with complete or partial heter odisomy (n = 12) have been reported. In respect to the different formation mechanisms of UPD, complete isodisomy should be the result of a post-zygoti c mitotic segregation error, whereas heterodisomic UPDs should be caused by trisomic rescue after meiotic non-disjunction events. In maternal UPD7, 50 % of cases seem to be caused by post-zygotic mitotic segregation errors, w hich is similar to the situation in trisomy 7. This result corresponds to t he situation in trisomy 8 but is in contrast to observations in the frequen t aneuploidies. Thus, the different findings in these aberrations reflect t he presence of multiple factors that act to ensure normal segregation, vary ing in importance for each chromosome. (C) 2000 Editions scientifiques et m edicales Elsevier SAS.