TRISOMY 8 MOSAICISM: CELL CYCLE KINETICS AND ...

Origin of Trisomy 8 cells
DISCUSSION
T8m occurs due to mitotic nondisjunction within a normal fetus, therefore
introducing a second cell line (James and Jacobs 1996; Karadima et al. 1998; Nicolaidis
and Petersen 1998; Webb et al. 1998; Kalousek 1999). The presence of trisomic cells
throughout fetal tissues and placental tissues depends directly on the timing of the
nondisjunctional event (Webb et al. 1998). The earlier the event in embryonic
development, the more tissues will be affected. This includes CPM, which results in a
normal fetus and abnormal placenta, due to the nondisjunction occurring in a placental
lineage cell. Abnormal placental tissues are correlated to abnormal fetal development,
whereas normal to low placental mosaicism supports growth and intrauterine survival of
abnormal fetuses (Farra et al. 2000). T8m anomalies, however, are not correlated to the
level of mosaicism present within fetal tissues. A similar mosaic constitution within the
tissues has been reported for patients, who have had drastic differences in their
phenotypes, some being phenotypically normal, where others have severe mental
retardation and multiple skeletal anomalies (James and Jacobs 1996; Jordan et al. 1998).
The investigation of tissues collected from our patient lead to the determination of
an early nondisjunction event since both embryonic and extra-embryonic tissues were
mosaic. The embryonic samples, which included umbilical blood, peripheral blood
(T-lymphocytes), and umbilical cord each were scored for over 50% trisomy 8 cells in
G-banded metaphases, while no trisomic cells were found in the transformed blood
(B-lymphoblasts) (Table 1). This difference seen in the level of mosaicism between the
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