Mosaicism in amniotic fluid cell cultures: classification and significance

Confined chorionic mosaicism in prenatal diagnosis

Confined chorionic mosaicism, detected commonly on chorionic villus sampling (CVS) and occasionally in cultured amniotic fluid cells, is described in five pregnancies that showed confined chorionic mosaicism for trisomies 12, 13, 14, 17 and a marker chromosome. Cytogenetic findings in these pregnancies support the conclusion that within chorion some chromosomal mosaicism are confined to the trophectoderm derivatives while others to the extra-embryonic mesoderm. The etiology of confined chorionic mosaicism is discussed in relation to a significant role of multiple cell lineages contributing to the early development of placenta. The need is indicated for the use of both direct and long-term cultures in CVS prenatal diagnosis, and for the confirmatory testing of fetal blood or amniotic fluid in cases where mosaicism is detected in chorionic villi.

Chorion in culture

This study is based on the processing of 115 samples of pure chorion. The different types of colony morphology, together with cytogenetic analysis, indicate the predisposition of chorion cells to artefacts of culture. Chorion survives longer than other foetal tissues and can be cultured when those are dead or contaminated (e.g. in macerated foetuses and missed abortions) making it a suitable and popular tissue for cytogenetic purposes. However, our results indicate that it is slow in culture and prone to chromosome variation so that pseudomosaicism frequently arises and may lead to misdiagnosis. We therefore suggest that cytogenetic analysis of spontaneous abortion material is interpreted with caution in cases where chorion is the only tissue cultured, especially when mosaicism is found.

Prenatal diagnosis of trisomy 20 mosaicism indicating esophageal and rectal origin

Trisomy 20 mosaicism in cultured amniotic fluid cells has in only a few cases been confirmed in fetal tissue. This may lead to the assumption that the trisomic cells are of extra-fetal origin and interruption of the pregnancy is not advisable. Chromosome analysis of numerous fetal tissues indicated in two cases the presence of one or more trisomy 20 cell clones in rectum and esophagus, respectively. The clinical significance of trisomy 20 mosaicism in single organs remains to be elucidated. Besides the karyotype, genetic counselling should take into account all accessible information of the pregnancy, e.g. ultra-sound, serum alpha-fetoprotein values and obstetrical history.

Cytogenetic diagnosis using midtrimester fetal blood samples: application to suspected mosaicism and other diagnostic problems

Cytogenetic studies on fetal blood cells obtained at 18-25 weeks gestation have provided information for decision making in 25 cases identified as being at high risk of having an abnormal fetus. In particular, in the 21 cases studied to consider the possibility of true mosaicism, confirmation in fetal blood was obtained in three, one of which presented as a pseudomosaic on the original amniotic fluid cell study. Fetal blood was also informative in two cases (one positive and the other negative) in which a diagnosis of the fragile X syndrome was being considered. Furthermore, when high risk pregnancies presented late in gestation (21-24 weeks), these methods allowed for a rapid cytogenetic diagnosis. The procedure has proved useful in most of these cases since the couples involved had indicated that they would probably have terminated the pregnancy without the reassurance of normal fetal lymphocyte studies. Since the technique carries a much higher risk of pregnancy loss than does amniocentesis, its use should only be considered when there are compelling indications.

Trisomy 20 mosaicism

A further case of trisomy 20 mosaicism found at amniocentesis is presented. Pregnancy was terminated, the fetus showed facial dysmorphia and minor cardial and renal anomalies. 19 published reports of true trisomy 20 mosaicism at amniocentesis are reviewed. Five pregnancies resulted in obviously normal newborns. The significance of mostly minor anomalies found at autopsy of 7 fetuses remains unclear. With regard to genetic counselling the significance of trisomy 20 mosaicism is summarized as follows: (1) true trisomy 20 mosaicism in amniotic fluid cells reflects mosaicism of the fetus; (2) severe malformation is not a major feature of trisomy 20 mosaicism; (3) the risk of mental retardation is still undetermined, due to limited experience. However, there is no definite proof that the condition is harmful at all.

European collaborative study on prenatal diagnosis: mosaicism, pseudomosaicism and single abnormal cells in amniotic fluid cell cultures

A report is given of the results of a European collaborative study on mosaicism, pseudomosaicism and single abnormal cells in amniotic fluid cell cultures. The mean frequency of cases with mosaicism was 0.10 per cent, with pseudomosaicism 0.64 per cent and with single abnormal cells 2.83 per cent in a series of 44 170 amniotic fluid samples. There was no significant difference between the colony (in situ) and the flask method with regard to the frequency of mosaicism. Pseudomosaicism and single abnormal cells were more frequent in cases studied with the flask method probably due to other factors than the method of cultivation of the cells. The frequency of maternal cell contamination was 0.17 per cent and the frequency of wrong sex assignment was 0.11 per cent. A more correct estimation is obtained if these frequencies are doubled. There was a considerable variation between laboratories with regard to the frequencies given above. One reason for this variation is that there are no sharp limits between mosaicism, pseudomosaicism and single abnormal cells. Thus the material contained cases diagnosed as having pseudomosaicism which turned out to be mosaics at birth and to have an abnormal phenotype. These cases were very rare but pose a definite problem in prenatal cytogenetic diagnosis.

Prenatal diagnosis of trisomy 9 mosaicism possibly limited to fetal membranes

In repeat amniotic fluid cultures mosaicism due to trisomy 9 was noted. Autopsy of the aborted female fetus showed a sinus urogenitalis and gonadal dysgenesis with absence of germ cells only. Fetal lymphocytes and skin fibroblasts had a normal karyotype but trisomy 9 was found in cells grown from placenta. It is likely that trisomic cells were limited to fetal membranes.

Discordance between prenatal cytogenetic diagnosis and outcome of pregnancy

From 1.3.73 to 30.9.80 5580 women had an amniocentesis performed here or elsewhere; fetal chromosome analyses were carried out in this laboratory. We found 112 abnormal karyotypes (2 per cent) out of 5591 chromosome analyses. In 40 women (0.7 per cent) no cytogenetic diagnosis was obtained. Follow-up was successful in 99.5 per cent. Nine cases are reported in detail: Three cases had discrepancy between the karyotype in amniotic fluid and peripheral blood after delivery, two of these cases turned out to be 46,XX (male) while the third was prenatally determined as trisomy 21, but had a 46,XX karyotype at birth. Six cases had discrepancy between the karyotype in amniotic fluid and the phenotypic outcome at birth/abortion. One case was a prenatally undetected 45,X/46,XY mosaicism; one case was an unexplained 45,X male fetus; two cases were prenatally determined as trisomy 21, but at abortion a normal karyotype was determined and in two cases maternal cells were probably examined. The incidence of cytogenetic errors in this study was very low.

Decreased oxygen supply enhances growth in culture of human mid-trimester amniotic fluid cells

Human mid-trimester amniotic fluid cells were cultivated under conditions of decreased oxygen supply. Compared to control cultures the low-oxygen group showed improved growth which was quantitated by three independent assays (1) direct cell counts, (2) bromodeoxyuridine (BrdU)-Hoechst flow-cytometry, and (3) cloning efficiency. The growth promoting effects of lowered oxygen hold for all major morphologic categories of amniotic fluid cells.

Chromosomal mosaicism confined to the placenta in human conceptions

Placental and fetal tissues from 46 human pregnancies were cultured and cytogenetically analyzed in an attempt to document the existence of chromosomal mosaicism confined strictly to tissues of extraembryonic origin. In two gestations in which chromosomal mosaicism was found, it was expressed exclusively in placental chorionic cells and was not detected in cells derived from the embryo proper. This demonstration of confined chorionic mosaicism may have implications for the understanding of the fetoplacental unit and for prenatal diagnosis.

Trisomy 20 mosaicism in amniotic fluid cells

The significance of trisomy 20 mosaicism in cultured amniotic fluid cells is still confusing. We report a case of amniotic cell normal/trisomy 20 mosaicism diagnosed prenatally. The pregnancy was carried to term and a normal baby girl was delivered. The authors consider that in cases of amniotic fluid cell normal/trisomy 20 mosaicism the termination of pregnancy may not be advised, however, the parents should be fully informed.

Autosomal mosaicism in amniotic fluid cells from a twin pregnancy

Amniocentesis was performed on a 39-year-old gravida 4 woman because of maternal age. Ultrasonography demonstrated a twin pregnancy, and two amniotic fluid specimens were obtained under direct ultrasound guidance. All cells analyzed from the first specimen were 46,XX. In the second specimen, one colony in each of two flasks had trisomy 9; the remaining 3 colonies analyzed were 46,XX. The total cell count in the second specimen was 14 (24%) of 58 cells which were trisomic for chromosome 9. The parents elected to continue the pregnancy, which resulted in two live-born infants, a male and a female, both chromosomally normal. The most likely explanation for the amniotic fluid finding is that fluid from one sac was sampled twice and that trisomic cells from the amnion were obtained during the second tap.

Antenatal diagnosis of mosaic trisomy 8 confirmed in fetal tissues

An aneuploid karyotype with an extra submetacentric C-group chromosome was observed in all metaphase cells in 5 of 24 primary amniotic fluid cell clones and in admixture with normal cells in two additional clones. Trisomy 8 was demonstrated by R-banding. The parents elected to terminate the pregnancy. Successful cultures were initiated from nine fetal tissues and aneuploid cells were observed in cultures derived from two separate sites in the skin.

Microenzymatic assays for lysosomal enzymes in primary amniotic fluid cell cultures

A study of three lysosomal enzymes (hexosaminidase, beta-galactosidase and alpha-galactosidase) in normal primary amniotic fluid cell cultures using a microenzymatic assay is presented. No difference in enzyme activity was found between primary and amniotic cell cultures in passage number one. A progressive change in the proportions of hexosaminidase A and hexosaminidase B with time was demonstrated in culture. The feasibility of this procedure for the early prenatal diagnosis of disorders due to lysosomal enzyme deficiency is discussed.