On the significance of true trisomy 20 mosaicism in amniotic fluid culture

Abstract

Nine new cases of prenatally detected true mosaic trisomy 20 (T20) are reported. In three instances the fetuses were aborted. One fetus showed multiple malformations associated with a high percentage of T20 cells among amniotic fluid (AF) cells and fibroblasts of different fetal tissues. In two other fetuses only a slight facial dysmorphy was seen which was accompanied by a low percentage of T20 cells among AF cells. In five instances the pregnancies were carried to term, and normal somatic and psychomotor development of the children has been observed, in one case up to the age of 24 months. In one case the pregnancy is continuing. The T20 cells were not detected among cultured lymphocytes of these children. A review of the hitherto known cases of prenatally detected mosaic T20 indicates a relationship between the prenatal findings and the fetal development. This may serve as a provisory basis for genetic counselling: in the case of a percentage above 50% of T20 cells among AF cells there seems to be a risk of about 50% for the fetus to be affected by severe anomalies. However, in cases of a prenatally detected mosaic T20 with a percentage equal to or less than 50, fetal or congenital malformations have not been observed among 23 individuals so far examined.

Expanding the phenotype of mosaic trisomy 20

“…A few reports were found that describe severely affected individuals, primarily fetuses or those that suffered neonatal death from lethal malformations. While they do share some of the features listed above, their differences lie in the additional anomalies noted that we have not included in our phenotype [Pallister et al, 1976;Pan et al, 1976;Djalali et al, 1985;Hsieh et al, 1992]. There is little consistency in phenotype among these cases, as they are described.…”

“…However, most of the cases in these studies had short follow-up times and much of the data is based on birth records. In some studies there is a trend increasing percent of trisomic cells detected prenatally, and risk of abnormal outcome; however, this relationship is not consistent enough to be useful in prenatal counseling [Djalali et al, 1985;Reish et al, 1998;Wallerstein et al, 2000;Robinson et al, 2005]. No pattern of malformation has been described in the literature.…”

Cytogenetic and Molecular Genetic Characterization of Trisomy 20 Mosaicism in Fetal Blood and Tissues

“…This abnormal chromosome, while it can be identified prenatally in amniotic fluid, is postnatally identified almost exclusively in cultured fibroblasts and is rarely observed in peripheral blood. Djalali et al (1985) have proposed that a relationship may exist between the risk of phenotypic abnormalities and the percentage of trisomy 20 cells identified in amniotic fluid cultures and have suggested that it may be possible to stratify patients into 'high-risk' and 'low-risk' categories based on the likely mechanism generating the trisomy. More recently, Hsu et al (1991) have re-examined the data on mosaic trisomy 20 and concluded that no relationship exists between the percentage of trisomy 20 cells and the frequency of an abnormal phenotype.…”

Prenatal diagnosis of mosaic trisomy 20 in New Zealand

“…1,5 It has been suggested that the percentage of trisomic cells in the culture or the presence of trisomic cells in foetal tissues may predict a poor outcome. 5,6 The literature, however, suggests that neither of these indicators is reliable. We aimed to identify cases of mosaic trisomy 20 found on amniocentesis or chorionic villus sampling (CVS) in New Zealand in the last 10 years to provide a representative review of the clinical features and outcomes associated with this finding.…”