TRISOMY-7 CVS MOSAICISM - PREGNANCY OUTCOME, PLACENTAL AND DNA ANALYSIS IN 14 CASES

Prenatal diagnosis by chorionic villus sampling (CVS) documents placen tal chromosomal mosaicism in approximately 2% of viable pregnancies at 9-12 weeks of gestation and can involve various chromosomes and place ntal cell lineages. Confined placental mosaicism (CPM) is the result o f postzygotic mitotic errors occurring in either diploid or trisomic z ygotes. With trisomic zygote rescue, depending on the parental origin of the chromosome which is lost, uniparental disomy (UPD) or biparenta l disomy (BPD) may arise [Kalousek et al., Am J Hum Genet 52: 8-16, 19 93]. In this paper, we present 14 pregnancies which were diagnosed by CVS as mosaic trisomy 7. All follow-up amniocenteses showed a normal d iploid karyotype. Using both classical cytogenetics and interphase ana lysis, studies of term placentae showed variable levels of trisomy 7. DNA analysis was performed in nine cases to determine whether the dipl oid fetus had BPD 7 or UPD 7. Fetal UPD 7 was present only in one case ; in eight other cases biparental inheritance was demonstrated. DNA an alysis to establish the origin of trisomy 7 in the placenta was fully informative in six cases. One trisomy resulted from a meiotic error an d was associated with fetal UPD 7, while the rest were somatic in orig in. It is difficult to compare the effect of CPM for trisomy 7 to othe r trisomies confined to the placenta, as for most chromosomes there ar e few available cases. It appears that intrauterine fetal growth is no t greatly affected by the presence of a trisomy 7 cell line in the pla centa. This finding is in contrast to the serious effect of high level s of trisomy 16 within the placenta on fetal intrauterine growth in a series of well-documented cases of CPM 16 [Kalousek et al. 1993]. (C) 1996 Wiley-Liss, Inc.