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Clinical Diagnosis |
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CHROMOSOMAL MOSAICISM FOUND IN PRENATAL DIAGNOSIS |
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Overview |
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Chromosomal mosaicism, most often involving trisomic cells (47 chromosomes) and typical cells (46 chromosomes), is detected in 1-2% of pregnancies undergoing chorionic villus sampling and in 0.1% of pregnancies undergoing amniocentesis. Chorionic villus sampling is a prenatal diagnosis procedure which involves analysis of the chromosomes in the placenta. Amniocentesis is a prenatal diagnosis procedure which involves chromosome analysis of cells in the amniotic fluid which are cells from the baby. The possibility of mosaicism should be discussed with parents before any prenatal testing procedure is performed.
The clinical outcome of chromosomal mosaicism is strongly dependent on the specific chromosome involved and the number of trisomic cells in both the placenta and the baby. When we see trisomic cells in amniotic fluid this often indicates that there are trisomic cells in the baby too. However, the true level and distribution of trisomic cells cannot be accurately assessed with any prenatal procedure. Therefore, ultrasound is often the best judge of how a baby is developing. What can ultrasound look for? |
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> Prenatal Diagnosis |
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Chorionic villus sampling |
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Amniocentesis |
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Ultrasound |
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Confined mosaicism |
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Uniparental disomy |
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Diagnosis in blood |
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Preimplantation diagnosis |
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What factors which should be considered when trying to predict the outcome of trisomy (or other types of) mosaicism? |
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1) the chromosome involved
Outcome is more serious for some chromosomes when compared to others. We explore this in detail in the section on "chromosome specific information". This is because trisomy of some chromosomes may not survive at all, unless the trisomic cells are present only in the placenta and not in the baby. A mosaic finding on CVS or amniocentesis involving trisomy 18 or 21 is likely to have far worse implications than a mosaic finding for trisomy 15 or 16. This is because we know that babies with trisomy 15 or 16 cells cannot survive, so if trisomy 15 or 16 is found at prenatal diagnosis it is less likely to indicate the presence of the abnormal trisomic cells in the baby.
2) the tissues affected and level of trisomy in those tissues
In theory, cases with a relatively high proportion of trisomic cells are more likely to be associated with an abnormal outcome than those with a low proportion of trisomic cells. Although knowing all of the tissues affected and the level of trisomic cells found in each tissue would be very helpful in predicting the clinical outcome, it is virtually impossible to determine which tissues are affected by the trisomy either prenatally or in a living person. The only way is on autopsy, where each tissue could be analyzed.
3) method of ascertainment
Was chromosomal mosaicism detected on chorionic villus sampling (CVS), amniocentesis, or in a sample of blood? The way that trisomic cells were detected gives some guidance as to what tissues might be affected. For example, the presence of trisomy in CVS shows that the placenta is affected. Generally, the presence of the trisomy in amniotic fluid suggests that at least one fetal tissue may be affected by the trisomy. However, it is possible to have high levels of the trisomy in amniotic fluid with no confirmation of trisomy in blood or skin of the baby at birth. Sometimes in cases which result in fetal death there is no evidence of the trisomy in multiple examined tissues. Likewise it is possible to have trisomy in the baby which is not detected on amniotic fluid sampling. When abnormal cells are discovered in the blood of a living affected individual, that person will show how the presence of the cells effects growth and development.
4) ultrasound findings
Because the outcome of trisomy mosaicism can vary widely, ultrasound often offers the best indication as to how the baby is growing and developing. Abnormal cells tend not to grow and develop properly.
5) presence/absence of uniparental disomy
For many chromosomes the effect of uniparental disomy is either unclear or not believed to affect the babies development. However, uniparental disomy for chromosomes 6, 7, 11, 14, and 15 can be of concern.
6) number of previous case reports
We often look to previous case reports to try and understand how a mosaic finding may affect an individual's health. It is much more difficult to predict how a chromosomal change will affect a developing individual if there have only been a few reports.
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