Trisomy/Tetrasomy 21 Mosaicism in CVS: Interpretation of Cytogenetic

J Med Genet 1999;36:333–334 333

Trisomy/tetrasomy 21 mosaicism in CVS: J Med Genet: first published as 10.1136/jmg.36.4.333 on 1 April 1999. Downloaded from interpretation of cytogenetic discrepancies between placental and fetal chromosome complements

Anna Soler, Ester Margarit, Ana Carrió, Dolors Costa, Rosa Queralt, Francisca Ballesta

Abstract Table 1 Cytogenetic results Trisomy/tetrasomy 21 mosaicism was found in chorionic villi (semidirect prepa- No of cells ration) obtained from a 40 year old 47,XY, 48,XY, pregnant woman. Since both cell lines Culture type 46,XY +21 +21,+ 21 were abnormal, the couple elected for Semidirect CVS — 10 10 pregnancy termination. Placenta and fetal Long term villi — 8 — tissue samples were obtained for cyto- Cord 13 — — Skin 17 — — genetic study. Long term cultured villi Kidney 64* 15* — showed a non-mosaic trisomy 21 karyo- Tissue mixture 137* 44* — type, while other tissues showed either a *Cells analysed by FISH. normal karyotype or normal/trisomy21 mosaicism. These discrepancies could be decided to terminate the pregnancy. Termina- explained by a modified “bottle neck” tion took place at 13 weeks’ gestation and pla- embryogenic model with a trisomic zygote cental and fetal tissue samples were obtained and a non-disjunction event taking place for cytogenetic analysis. Long term placental in one of the first divisions. Our case culture showed a non-mosaic trisomy 21 emphasises the need for confirmatory karyotype. Cord and skin cultures showed a studies in other tissues when mosaicism is normal karyotype in all cells examined. Other encountered in chorionic villi, even if all cultures established from kidney and a tissue cell lines are abnormal. mixture produced no metaphases and were ( 1999;36:333–334) J Med Genet analysed by fluorescence in situ hybridisation (FISH) using the LSI 21 Vysis probe. The

Keywords: conﬁned placental mosaicism; trisomy/ http://jmg.bmj.com/ tetrasomy 21; embryogenic models mixture of nuclei with two and three signals was interpreted as a mosaic 46/47,+21 in both cultures (table 1). Chorionic villus sampling (CVS) has been widely accepted as a technique for ﬁrst trimes- ter prenatal diagnosis. Cytogenetic analysis can Discussion be performed on direct or semidirect prepara- The predictive value of non-mosaic trisomy 21 found on CVS is accepted to be high.6 Only

tions of cytotrophoblast cells or after long term on September 24, 2021 by guest. Protected copyright. culture of mesodermal core cells. With regard one case of a non-mosaic false positive trisomy 21 was reported in the Eucromic series of to the accuracy of cytogenetic findings on 5 CVS, a serious problem is the occurrence of 62 865 CVS. However, confirmatory studies confined placental mosaicism (CPM), charac- on amniocytes or fetal blood cells are recom- terised by a discrepancy between the chromo- mended when a diagnosis of normal/trisomy 21 somal constitution of the fetus and the placenta mosaicism is obtained on CVS, owing to the (CVS material).1 CPM has been reported to possibility of a normal karyotype in the fetus. occur in 1-2% of pregnancies.2–5 In most cases In the present case, our first interpretation a normal cell line is involved, which represents was that the tetrasomic cell line was secondary Servei de Genètica, the chromosomal constitution of the fetus. to an initial trisomy 21 and probably confined Hospital Clínic i to the cytotrophoblast, or, alternatively, the ini- Provincial de We report on a case with trisomy/tetrasomy 21 mosaicism in chorionic villi (semidirect tial zygote could be tetrasomic and a secondary Barcelona, Villarroel trisomic cell line had appeared in the cytotro- 170, 08035 Barcelona, preparation), trisomy 21 in long term placental Spain culture, and normal/trisomy 21 mosaicism in phoblast. In either case, an abnormal fetal phe- A Soler fetal tissues. notype was assumed. E Margarit The discordance between the karyotypes A Carrió obtained from the trophoblast (semidirect D Costa Case report preparations), the chorionic mesodermal core R Queralt A 40 year old woman, G2, P1, was referred for (placenta long term culture), and the fetal F Ballesta CVS at 11.5 weeks’ gestation because of tissues did not fit into the embryogenic models 7 Correspondence to: advanced maternal age. proposed by Crane and Cheung and Bianchi et Dr Soler. A CVS karyotype was obtained by a semi- al.8 These models could explain the presence of direct method (overnight culture) and showed trisomic and tetrasomic cells in trophoblast, Received 4 March 1998 Revised version accepted for trisomy/tetrasomy 21 mosaicism (table 1). arising from an initial trisomic zygote, as well as publication 18 August 1998 Since both cell lines were abnormal, the couple the presence of normal cells in fetal tissues, but 334 Soler, Margarit, Carrió, et al

A 46 and 48 chromosomes, plus the initial

No. of cells trisomic cell line (fig 1B). By chance, the J Med Genet: first published as 10.1136/jmg.36.4.333 on 1 April 1999. Downloaded from 2 disomic cell line may have remained restricted to the inner cell mass and the tetrasomic line to 4 the cytotrophoblast. The proportion of normal/ trisomic cells in fetal tissues and extraembryonic structures can vary depending on the dif- 8 ferent cell lines. To our knowledge, this is the first case of a 16 discrepancy between (semi)direct CVS prepa- 32 rations, long term villi culture, and fetal tissues Fetus Extraembryonic Trophoblast involving mosaic trisomy/tetrasomy 21. There structures is a reported case of a 46/48,+21,+21 mosaicism found in long term villi culture with a normal karyotype in direct preparations and in B amniocytes.10 Hahnemann and Verjeslev5 in- clude one case of trisomy/tetrasomy 21 among No. of cells autosomal tetrasomy mosaic cases, but no 8 details were given for this particular case. The present case emphasises the need for confirmatory studies in other fetal tissues when mosaicism is encountered in chorionic villi, even if all the cell lines are abnormal.

Funded by a grant from Fondo de Investigaciones Sanitarias (F1598/0162) to AS. The FISH studies were carried out with 16 the aid of a grant from Fundació Catalana de la Síndrome de Down (FCSD-02).

1 Kalousek DK, Dill FJ. Chromosomal mosaicism conﬁned to the placenta in human conceptions. Science 1983;221:665- 7. 2 Vejerslev LO, Mikkelsen M. The European collaborative 32 study on mosaicism in chorionic villus sampling: data from 1986 to 1987. Prenat Diagn 1989;9:575-88. Fetus Extraembryonic Trophoblast 3 Ledbetter DH, Zachary JM, Simpson JL, et al. Cytogenetic structures results from the US collaborative study on CVS. Prenat Diagn 1992;12:317-45. Figure 1 Interpretation of fetal/placental chromosome discrepancies of the reported case 4 Association of Clinical Geneticists Working Party on Chori- according to the embryogenic models of Crane and Cheung7 and Bianchi et al 8 (A) and onic Villi in Prenatal Diagnosis. Cytogenetic analysis of Kennerknecht et al 9 (B). White, grey, and black circles represent cells with 46, 47, and 48 chorionic villi for prenatal diagnosis: an ACC collaborative chromosomes respectively. study of UK data. Prenat Diagn 1994;14:363-79.

5 Hahnemann JM, Verjeslev LO. Accuracy of cytogenetic http://jmg.bmj.com/ findings on chorionic villus sampling (CVS). Diagnostic not the trisomic cells found in long term consequences of CVS mosaicism and non-mosaic discrep- placental culture and fetal tissues (fig 1A). ancy in centres contributing to Eucromic 1986-1992. Pre- 9 nat Diagn 1997;17:801-20. Kennernecht et al proposed a modification 6 Breed ASPM, Mantingh A, Beekhuis JR, Kloosterman MD, of the embryogenic model of Crane and Ten Bolscher H, Anders GJPA. The predictive value of 7 cytogenetic diagnosis of CVS: 1500 cases. Prenat Diagn Cheung to explain certain fetoplacental dis- 1990;10:101-10. crepancies. According to their modified model, 7 Crane JP, Cheung SW. An embryogenic model to explain cytogenetic inconsistencies observed in chorionic villus the cells that contribute to the inner cell mass versus fetal tissue. Prenat Diagn 1988;8:119-29. are defined after the pluripotent eight cell 8 Bianchi DW, Wilkins-Haug LE, Enders AC, Hay DE. on September 24, 2021 by guest. Protected copyright. Origin of extraembryonic mesoderm in experimental stage. Following this model, we could imagine animals: relevance to chorionic mosaicism in humans. Am a trisomic zygote probably arising from mater- J Med Genet 1993;46:542-50. 9 Kennerknecht I, Vogel W, Mehnert K. A modified embryo- nal non-disjunction, and a second non- genic model to explain embryonic/extraembryonic chro- disjunction event that took place in the third mosomal inconsistencies. Prenat Diagn 1993;13:1156-9. 10 Best RG, Brooks KA, Clarkson KB. Tetrasomy 21 confined mitotic division of the morula pluripotent cells, placental mosaicism ascertained through chorionic villus resulting in two “complementary” cell lines of sampling. Am J Hum Genet Suppl 1996;59:A409.