The Principles of Clinical Cytogenetics - Extra Materials - Springer

308 Linda Marie Randolph
Table 17
Outcome of Cases with Rare Autosomal Trisomy Mosaicism Diagnosed in Amniocytes
Abnormal outcomes/ Abnormal phenotype Fetal demise
Type total no. of cases (no. with IUGR) a or stillborn
46/47,+2 10/11 (90.9%) 7 (2) 3
46/47,+3 1/2 — 1 0
46/47,+4 1/2 — 1 0
46/47,+5 2/5 (40.0%) 2 (2) 0
46/47,+6 0/3 — 0 0
46/47,+7 1/8 (12.5%) 1 0
46/47,+8 1/14 (7.1%) 1 0
46/47,+9 14/25 (56.0%) 14 (2) 0
46/47,+11 0/2 — 0 0
46/47,+12 6/23 (26.1%) 4 2
46/47,+14 2/5 (40.0%) 2 0
46/47,+15 6/11 (54.5%) 6 (3) 0
46/47,+16 15/21 (71.4%) 15 (8) 0
46/47,+17 0/7 — 0 0
46/47,+19 0/1 — 0 0
46/47,+22 7/11 (63.6%) 6 (2) 1
a IUGR = intrauterine growth restriction
Source: Data from ref. 278.
In addition to the level of mosaicism, the chromosome involved is an important consideration.
True mosaicism has been reported in liveborns with almost all trisomies (37). However,
true mosaicism for trisomies 8, 9, 21, 18, 13, 16, X, and Y and for monosomies X and Y has
potentially great significance. For chromosomes 8 and 9, mosaicism is the most common form in
which trisomies occur in liveborns, perhaps because the full trisomy is not compatible with fetal
survival in the majority of cases (266,267). Even one cell with trisomy 8 could be significant.
For trisomies of chromosomes 13, 18, 21, X, and Y and monosomy X and Y, mosaicism has been
fairly commonly reported, and the clinical manifestations could vary from no apparent abnormality,
at least in the newborn period, to more characteristic features of the full trisomy. The
degree of mosaicism is not related to the outcome (30). See Table 17 for incidences of mosaicism
for specific chromosomes.
Schuring-Blom et al. (268) evaluated first-trimester cytotrophoblast cell preparations—direct
preparations—showing full or mosaic trisomy 13 or 18, with the purpose of determining how often
the result was a true positive in the fetus or newborn. Cultured mesenchymal tissue was available
only for about half of the cases. Of the 51 cases, five false positives were seen in those with full
trisomy 18 and three with mosaic trisomy 18. One false positive was seen in full trisomy 13, and two
false positives were seen in mosaic trisomy 13. Their conclusions were as follows:
• Full trisomy 13 or 18 in a short-term culture preparation is a reliable result only in combination
with abnormal ultrasound findings or trisomic cells in mesenchyme or amniotic fluid.
• Mosaic trisomy 13 or 18 in a short-term culture preparation merits further prenatal testing by
amniocentesis.
In a multicenter study evaluating karyotype–phenotype correlations when mosaic trisomy 13, 18, 20,
or 21 was seen at amniocentesis, Wallerstein et al. (269) found an abnormal outcome in 40% of mosaic
trisomy 13, 54% of mosaic trisomy 18, 6.5% of mosaic trisomy 20, and 50% of mosaic trisomy 21. The
risk of abnormal outcome in pregnancies with less than 50% trisomic cells and greater than 50% tri-