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Fruit of the Womb

Chromosomal Mosaicism Detected at the Time of Chorionic Villus Sampling

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Since the widespread acceptance of first trimester screening for fetal chromosomal abnormalities (aneuploidy), and following the recommendations that this should be offered to all pregnant women, not just those considered to be ‘at risk’ for indications such as ‘advanced maternal age’, early invasive testing by chorionic villus sampling (CVS) has become more widely employed as a means of diagnosing aneuploidy. Any diagnostic procedure has its risk and limitations. The comment from the reader below is related to one of the major ‘risks’ of CVS, that is the risk associated with the interpretation of the significance of the test results themselves when the diagnosis of ‘mosaicism’ returns from the laboratory…

Thu Mar 20, 08:27:00 PM 2008, Anonymous has left a new comment on your post "Invasive Diagnostic Testing after Reassuring First...":

Dear Dr. Trofatter:
I am a 35 yr old who is 13 weeks pregnant. This is my first pregnancy and I am seeing a high risk specialist since I had bleeding from week 4-6. Two weeks ago, I had CVS and the preliminary results (I assume FISH) results showed a healthy baby girl without any chromosomal abnormalities. A week later the genetic specialist called me and told me the final results showed mosaic Turner's syndrome and that I need to have an early amniocentesis to determine if this finding is limited to the placenta only. Why did the preliminary results show a false negative? Should I rely on the FISH results from amniocentesis in order to make my decision regarding termination or wait for the final amniocentesis results? Thank you!


Sat Mar 22, 03:23:00 PM 2008, Kenneth F. Trofatter, Jr., MD, PhD said...

Chromosomal mosaicism is defined as the presence of two or more populations of cells having different chromosomal complements in the same individual. Usually, this involves one population of cells that is chromosomally “normal” and another that is not, often with either one extra (trisomy) or one too few (monosomy) chromosomes. Mosaicism can involve both the fetus (true fetal mosaicism) and the placental tissues or the placental tissues alone. The latter is termed “confined placental mosaicism” and was first described by Kalousek and Dill in 1983 (Science 1983;221:665-667).

Placental mosaicism has been found at the time (9-12 weeks’ gestation) of chorionic villus sampling (CVS) to occur in approximately 1-2% of all pregnancies (Hahnemann and Vejerslev, Prenat Diagn 1997;17:801-20; Grati, et al., Eur J Human Genetics 2006;14:282–288). Under most circumstances, mosaicism is though to result from either a chromosomal nondisjunction event in the early stages (zygote/blastocyst) after conception, generating a trisomic cell line (single extra chromosome) in an initially normal baby, or as the result of the loss of one chromosome in an initially trisomic baby (“trisomy rescue”). When the latter occurs, the resulting fetal and/or placental tissues can end up with two identical chromosomes from one parent (rather than two slightly different ones from each). This is termed “uniparental disomy (UPD)” and is a condition that itself has associated risks which will not be included in today’s discussion.

When placental mosaicism is found at the time of CVS, there is a 10-12% chance the baby (not just the placental tissues) will share the mosaicism. However, even if the baby is affected, it is highly variable as to what percentage of fetal cells and even which organs will be involved in the mosaicism. Generally, the earlier the nondisjunction event occurs, the greater the likelihood that a higher percentage of placental and fetal cells will be involved in the chromosomal abnormality.

In the case of confined placental mosaicism (CPM), when the baby is not affected directly by the aneuploid cell line, the outcome of the pregnancy is somewhat influenced by which (and how many) placental cells are affected by the mosaicism. Three types of CPM have been described and are based on which placental cells are affected (Kalousek, et al., Hum Genet 1992;88:642–646; Simoni and Sirchia, Prenat Diagn 1994;14:1185–1189). In type I, the abnormal cell line is confined to the cytotrophoblast; in type II, it affects only the mesenchymal cells of the stromal villous core; and in type III, it involves both tissues. Although the distribution of these types differs somewhat by the study reported, as an aexample of the same, Grati and colleagues (Eur J Human Genetics 2006;14:282–288) found mosaicism confined to the placenta in 177 cases (87.2%): 39.9% type I, 40.4% type II and 6.9% type III.

When CVS is done, usually samples of both cells lines (cytotrophoblasts and mesenchymal cells) are obtained. Chromosomal analysis of these cell lines can be performed by means of direct preparations (such as fluorescent in situ hybridization, or FISH), short-term cultures (cytotrophoblasts), or long-term cultures (mesenchymal cells) of the chorionic villi. With reference to the mechanisms of mosaicism we have discussed above, type I and II CPMs are usually the consequence of postconceptional nondisjunction events, and type III is the result of “trisomy rescue” (from a meiotic error of maternal or paternal gamete formation prior to conception). The latter is associated with an increased risk of pregnancy complications and of UPD in the 'rescued' diploid fetus (Robinson, et al., Am J Hum Genet 1997;60:917–927). In the case of type I and II CPM, the risk of UPD is very low (Kalousek, Am J Med Genet 2000;91:39–45).

For the purposes of counseling our reader, the type of CPM is very important and to some degree, the risk to the baby of sharing in the mosaicism, or of having UPD, is often reflected in the laboratory test results. Mosaicism not detected in the cultures, but only in the direct preparation is more likely to represent CPM. Furthermore, certain mosaicisms , such as those involving sex chromosomes (such as Turner syndrome, 45,XO) or trisomies 8, 9, 12, 13, 15, 18, 20, and 21 often involve the baby and are therefore considered high-risk for true fetal mosaicism and the consequences of the same (Association of Clinical Cytogeneticists Working Party on chorionic villi in prenatal diagnosis Prenat Diagn 1994;14:363-379; Hahneann and Verjeslev, Am J Med Genet 1997;70:79-187). Grati and colleagues (Eur J Human Genetics 2006;14:282–288) found that type I CPM was associated with a 2.4% risk of fetal mosaicism, type II with 12.8%, and type III with 46.1%.

As is in the case of our reader, when mosaicism is found after CVS, either in the direct preparation (cytotrophoblasts) and/or in the cultures (mesenchymal cells), amniocentesis is usually recommended to determine if the abnormal cell line is confined to the placenta. Ninety percent of the time, the amniotic fluid results are “normal” and truly reflect a chromosomally normal baby. However, even when the amniotic fluid chromosome results return normal, there is a small chance of pregnancy complications as the result of a low level of fetal mosaicism (cryptic fetal mosaicism), UPD, and placental dysfunction as a consequence of the aneuploid cell line.

Although most pregnancies with CPM continue to term with no complications and are accompanied by normal fetal development, if a significant percentage of placental cells are aneuploid, this could result in impaired growth or even the loss of a chromosomally normal baby, and is more likely to be seen with type III CPM. Furthermore, if the baby has UPD, and this results in metabolic dysfunction, even though the total number of chromosomes is normal, a poor outcome might also result.

Returning to our reader’s questions, I would offer the following responses: First, the fact that the direct preparation (FISH) demonstrated a normal fetal karyotype and the cultured preparation suggested a Turner’s mosaicism (45,XO) speaks to the inherent weakness of rapid preparations in accurately establishing a diagnosis. Their positive predictive values are good if an abnormality is found, but due to the small number of cells obtained, resulting in 'sampling error', their false negative rate is relatively high. For the same reason, I do not think you should necessarily rely on the FISH results alone from amniocentesis when you have that done, particularly, if your plans are to terminate the pregnancy if a Turner’s mosaicism is found. I would strongly suggest you get some good genetic counseling prior to making any final decision in that regard as well.

Thank you for reading, for sharing your story, for your excellent questions and for giving me the opportunity to expound on a very important subject that may affect many of our readers.
Dr T
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