Confined Placental Mosaicism (CPM)

Question 1

Definition of CPM?

Answer 1

The presence of abnormal cells restricted to the extra embryonic tissues.

Question 2

% of cases of mosaicism from autosomal trisomy in CVS?

Answer 2

Majority (80%) cases of mosaicism for autosomal trisomy in CVS is CPM.

Abnormal cell lines confined to trophoblasts & mesenchyme.

Question 3

% of CPM at CVS?

Answer 3

CPM detected in 1-2% of ongoing preg at CVS.

Question 4

What % is fetus involved when CPM is found?

Answer 4

CPM found, most often represents trisomic cell line in placenta & diploid fetus. Fetus is involved in 10% cases!

Question 5

How does CPM occur?

Answer 5

(1) . Mitotic CPM: mitotic non-disjunction can occur in trophoblast cell. Trisomy 2,3,7& 8 are derived mitotically.
(2) . Meiotic CPM: triatomic rescue. Triatomic conception undergoes triatomic rescue in certain cells, incl those destined to become fetus, then remaining trisomy cellsmay be confined to placenta. Trisomy 16&22 are derived meiotically.

Question 6

Trisomic cells in CPM: which chromosome trisomy involve the fetus & are considered HIGH risk for true fetal mosaicism?

Answer 6

Trisomies 8, 9, 12, 13, 15, 18, 20, & 21

Question 7

What complications can CPM lead to in pregnancies?

Answer 7

Suboptimal placental function resulting in decreased intrauterine growth rates!

Question 8

Ways to minimise CPM?

Answer 8

• select more than one frond from placental biopsy.
• select fronds from different areas.
• year after enzyme digestion, which allows mesenchymal cells to be enriched.
• if mosaicism found on CVS, follow up with amniocentesis & detailed ultrasound scans!
• NEVER base decision to terminate entirely on CVS mosaic result!

Question 9

What are the 3 types of CPM?

Answer 9

Type I: Abnormal cell line confines to cytotrophoblast. (Maj. assoc with normal pregnancy outcome)

Type II: affects mesenchymal cells of strongly villus core.

Type III: involves cytotrophoblasts & mesenchymal cells. Can be either CPM or generalised to fetid (true feral mosaicism).

Type I & I: CPM consequence of nondisjunction.
Type III: result of trisomy rescue, assoc with increased risk of pregnancy complications & UPD in rescued fetus.

Question 10

How would you minimise CPM in prenatal testing?

Answer 10

• Mesenchymal vote culture results more likely to reflect true feral mosaicism than direct prep.
• More than 1 frond & select fronds from different areas.
• Enzymatic digestion allows mesenchymal cells to be enriched for testing.
• Follow up amniocentesis & detailed ultrasound scans if mosaicism detected in CVs!
• Termination NOT to be based entirely on CVS mosaic result.