20.05.14 Prenatal mosaicism

Question 1

What is prenatal mosaicism?

Answer 1

Detection of multiple cells lines derived from a single amnio, CV or fetal blood sample

Question 2

What are the main issues with prenatal mosaicism?

Answer 2

1) Phenotypic variability/consequences

2) Is it constitutional mosaicism, CPM or pseudomosaicism (technical artefact of culturing)?

Question 3

How common is prenatal mosaicism?

Answer 3

1-2% for CVS (taken 8-12 weeks)

0. 1-0.3% for Amnio samples (taken 15-20 weeks)
   - lower rate in AF due to later gestation and some true mosaic conceptions would have been lost by then
   - also, mosaicism is more often found in the placenta rather than fetus

Question 4

Basic principles of embryology

Answer 4

• Formation of zygote
• Cells division occurs until early blastocyst has formed
• Contains two cell lines:
  1) Trophoblast - goes on to form the chorion and ‘extra-embryonic’ tissues
  2) Inner cell mass - goes on to form the amnion and the fetus itself
• During implantation, trophoblast cells penetrate and invade the endometrium, develop the mesoderm layers which later form blood vessels, and become the chorionic villus
• The inner cell mass develops into the epiblast (ectoderm) and the hypoblast (endoderm) - the mesoderm then forms between these two layers
• The hypoblasts extends around and the yolk sac develops in the cavity
• Epiblast develops to generate the pro-embryonic disc, that then develops into the embryo, and the pro-amniotic cavity
• The chorion rapidly divides and is therefore more likely to contain mitotic errors (hence high mosaicism freq)
• AF contains cels derived from the developing fetus (skin, urine, epithelial cells from the respiratory system) - therefore any mosaicism is more likely to be true representation of the fetal karyotype
• Mosicism found in different cells:
  1) trophoblast only - CPM
  2) both trophoblast and ICM - both fetus and placenta
  3) ICM only - fetus only (not in placenta)

Question 5

What types of mosaicism are there and how to we tell the difference?

Answer 5

• Non-disjunction of an originally disomic zygote (the monozygotic cell line is usually non-viable and lost) or trisomy rescue in an originally trisomic zygote may occur early in development
• in what cells it develops will determine whether it affects placenta and/or fetus
• Cultured CVS only looks at trophoblast cells - so any mosaicism detected here may be CPM or may be true fetal karyotype
• 3 types of prenatal mosaicism depending on number of abnormal cells and their spread across different cultures:
  1) Level 1 - single abnormal cell in 1 culture
  2) Level 2 - two or more abnormal cells but only in 1 culture
  3) Level 3 - two or more abnormal cells, across two or more cultures
• Levels 1 and 2 = pseudomosaicism
• Level 3 = true mosaicism and needs to be reported
• BPG state whether basic, moderate or extensive workup needs to be done depending on the abnormality detected (normally the more viable the abnormality is, the greater chance it has of being true and therefore the more extra workup is required)
• Normally requires 20 cells from a set number of cultures to be be examined, the number of cultures depends on level of workup
• If sample is fetal blood then a mosaicism screen of 30 cells is required
• professional judgement is also required, depending on the chromosome and abnormality detected

Question 6

How do we report mosaicism?

Answer 6

• Level 1 pseudomosaicism is a technical artefact so do not report this
• If level 2 mosaicism is detected, this is more complex. Could be pseudomosaicism, but as multilple abnormal cells were seen, must be cautious.
• If level 2 in CVS, mention on report and advise amnio follow up
• For Level 3 mosaicism, number of abnormal cells is included in karyotype.
• If it includes chromosomes involved in UPD (6, 7, 11, 14 and 15) this must be clearly stated and further studies suggested, plus recommend a detailed USS.
• If level 3 is seen in CVS then follow up amnio is recommended with an additional USS.
• Should also comment that percentage of abnormal cells in culture may not reflect tissue distribution in fetus

Question 7

What is CPM?

Answer 7

• Abnormal cells restricted to the extra-embryonic tissues
• Problem associated with CVS
• Most common trisomies mitotically derived in CPM are 2, 3, 7 and 8
• Most common trisomies meiotically derived in CPM are 16 and 22 - these can cause placental dysfunction (leading to IUGR) but normally still works well enough to support fetus
• > 80% of autosomal trisomies detected in CVS are CPM
• Abnormal cells lines are normally confined to trophoblast, villus mesenchyme or both of these
• If mosaicism is detected in CVS then recommend AF testing and details abnormality scan
• However, even if AF is then normal it is very hard to say that fetal mosaicism has been completely excluded
• May also want to test parents and refer to CG

Question 8

Pallister-Killian syndrome

Answer 8

• tissue specific mosaic isochromosome 12p
• Normal blood karyotype (extremely rare in dividing lymphocytes), high levels of abnormal cells in fibroblast, AF, CVS and BM samples
• Due to instability of i(12)p at mitosis